Red Hat Enterprise Linux 3
Using the GNU Compiler
Collection (GCC)
Red Hat Enterprise Linux 3: Using the GNU Compiler Collection (GCC)
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Table of Contents
1. Introduction..................................................................................................................................... 1
2. Compile C, C++, Objective-C, Ada, Fortran, Java, or treelang ................................................. 3
3. Language Standards Supported by GCC ..................................................................................... 5
4. GCC Command Options ................................................................................................................7
4.1. Option Summary................................................................................................................ 7
4.2. Options Controlling the Kind of Output ..........................................................................12
4.3. Compiling C++ Programs ................................................................................................ 15
4.4. Options Controlling C Dialect .........................................................................................15
4.5. Options Controlling C++ Dialect..................................................................................... 19
4.6. Options Controlling Objective-C Dialect ........................................................................25
4.7. Options to Control Diagnostic Messages Formatting ...................................................... 26
4.8. Options to Request or Suppress Warnings....................................................................... 26
4.9. Options for Debugging Your Program or GCC ...............................................................38
4.10. Options That Control Optimization ...............................................................................47
4.11. Options Controlling the Preprocessor............................................................................64
4.12. Passing Options to the Assembler.................................................................................. 72
4.13. Options for Linking........................................................................................................ 72
4.14. Options for Directory Search.........................................................................................75
4.15. Specifying subprocesses and the switches to pass to them............................................76
4.16. Specifying Target Machine and Compiler Version ........................................................83
4.17. Hardware Models and Configurations ........................................................................... 83
4.17.1. SPARC Options............................................................................................... 84
4.17.2. IBM RS/6000 and PowerPC Options .............................................................87
4.17.3. Intel 386 and AMD x86-64 Options ............................................................... 95
4.17.4. IA-64 Options ...............................................................................................100
4.17.5. S/390 and zSeries Options ............................................................................102
4.18. Options for Code Generation Conventions.................................................................. 103
4.19. Environment Variables Affecting GCC ....................................................................... 108
4.20. Using Precompiled Headers......................................................................................... 110
4.21. Running Protoize .........................................................................................................111
5. C Implementation-defined behavior ......................................................................................... 115
5.1. Translation .....................................................................................................................115
5.2. Environment................................................................................................................... 115
5.3. Identifiers .......................................................................................................................115
5.4. Characters ...................................................................................................................... 115
5.5. Integers........................................................................................................................... 116
5.6. Floating point ................................................................................................................. 116
5.7. Arrays and pointers ........................................................................................................117
5.8. Hints...............................................................................................................................117
5.9. Structures, unions, enumerations, and bit-fields ............................................................117
5.10. Qualifiers...................................................................................................................... 118
5.11. Preprocessing directives............................................................................................... 118
5.12. Library functions.......................................................................................................... 118
5.13. Architecture.................................................................................................................. 118
5.14. Locale-specific behavior .............................................................................................. 119
6. Extensions to the C Language Family....................................................................................... 121
6.1. Statements and Declarations in Expressions..................................................................121
6.2. Locally Declared Labels ................................................................................................122
6.3. Labels as Values............................................................................................................. 122
6.4. Nested Functions............................................................................................................ 123
6.5. Constructing Function Calls .......................................................................................... 125
6.6. Referring to a Type with typeof .................................................................................. 125
6.7. Generalized Lvalues.......................................................................................................127
6.8. Conditionals with Omitted Operands............................................................................. 128
6.9. Double-Word Integers.................................................................................................... 128
6.10. Complex Numbers .......................................................................................................128
6.11. Hex Floats ....................................................................................................................129
6.12. Arrays of Length Zero ................................................................................................. 129
6.13. Structures With No Members ...................................................................................... 130
6.14. Arrays of Variable Length............................................................................................ 130
6.15. Macros with a Variable Number of Arguments. ..........................................................131
6.16. Slightly Looser Rules for Escaped Newlines...............................................................132
6.17. String Literals with Embedded Newlines ....................................................................132
6.18. Non-Lvalue Arrays May Have Subscripts ................................................................... 133
6.19. Arithmetic on void- and Function-Pointers ................................................................133
6.20. Non-Constant Initializers ............................................................................................. 133
6.21. Compound Literals....................................................................................................... 133
6.22. Designated Initializers ................................................................................................. 134
6.23. Case Ranges.................................................................................................................136
6.24. Cast to a Union Type....................................................................................................136
6.25. Mixed Declarations and Code......................................................................................137
6.26. Declaring Attributes of Functions................................................................................137
6.27. Attribute Syntax ...........................................................................................................143
6.28. Prototypes and Old-Style Function Definitions ........................................................... 146
6.29. C++ Style Comments...................................................................................................147
6.30. Dollar Signs in Identifier Names.................................................................................. 147
6.31. The Character [ESC] in Constants ............................................................................... 147
6.32. Inquiring on Alignment of Types or Variables ............................................................ 147
6.33. Specifying Attributes of Variables............................................................................... 147
6.33.1. i386 Variable Attributes ................................................................................ 151
6.34. Specifying Attributes of Types ....................................................................................151
6.35. Type Attributes.............................................................................................................154
6.35.1. i386 Type Attributes......................................................................................155
6.36. An Inline Function is As Fast As a Macro................................................................... 155
6.37. Assembler Instructions with C Expression Operands.................................................. 156
6.37.1. i386 floating point asm operands .................................................................. 160
6.38. Constraints for asmOperands .......................................................................................161
6.38.1. Simple Constraints........................................................................................ 161
6.38.2. Multiple Alternative Constraints...................................................................163
6.38.3. Constraint Modifier Characters.....................................................................164
6.38.4. Constraints for Particular Machines..............................................................165
6.39. Controlling Names Used in Assembler Code ..............................................................173
6.40. Variables in Specified Registers................................................................................... 174
6.40.1. Defining Global Register Variables ..............................................................174
6.40.2. Specifying Registers for Local Variables......................................................175
6.41. Alternate Keywords ..................................................................................................... 176
6.42. Incomplete enumTypes ................................................................................................ 176
6.43. Function Names as Strings........................................................................................... 176
6.44. Getting the Return or Frame Address of a Function....................................................177
6.45. Using vector instructions through built-in functions ................................................... 178
6.46. Other built-in functions provided by GCC ..................................................................179
6.47. Built-in Functions Specific to Particular Target Machines ..........................................184
6.47.1. X86 Built-in Functions ................................................................................. 184
6.47.2. PowerPC AltiVec Built-in Functions ............................................................ 188
6.48. Pragmas Accepted by GCC ......................................................................................... 205
6.48.1. RS/6000 and PowerPC Pragmas ...................................................................205
6.48.2. Solaris Pragmas............................................................................................. 205
6.48.3. Tru64 Pragmas.............................................................................................. 206
6.49. Unnamed struct/union fields within structs/unions......................................................206
6.50. Thread-Local Storage...................................................................................................206
6.50.1. ISO/IEC 9899:1999 Edits for Thread-Local Storage.................................... 207
6.50.2. ISO/IEC 14882:1998 Edits for Thread-Local Storage.................................. 208
7. Extensions to the C++ Language ...............................................................................................211
7.1. Minimum and Maximum Operators in C++..................................................................211
7.2. When is a Volatile Object Accessed?............................................................................. 211
7.3. Restricting Pointer Aliasing........................................................................................... 212
7.4. Vague Linkage ............................................................................................................... 213
7.5. Declarations and Definitions in One Header ................................................................. 214
7.6. Where’s the Template?...................................................................................................215
7.7. Extracting the function pointer from a bound pointer to member function ................... 217
7.8. C++-Specific Variable, Function, and Type Attributes.................................................. 217
7.9. Java Exceptions..............................................................................................................218
7.10. Deprecated Features.....................................................................................................218
7.11. Backwards Compatibility.............................................................................................219
8. GNU Objective-C runtime features........................................................................................... 221
8.1. +load: Executing code before main.............................................................................. 221
8.1.1. What you can and what you cannot do in +load ...........................................222
8.2. Type encoding................................................................................................................222
8.3. Garbage Collection ........................................................................................................224
8.4. Constant string objects ................................................................................................... 225
8.5. compatibility_alias.........................................................................................................226
9. Binary Compatibility .................................................................................................................. 227
10. gcov--a Test Coverage Program..............................................................................................231
10.1. Introduction to gcov .................................................................................................... 231
10.2. Invoking gcov...............................................................................................................231
10.3. Using gcovwith GCC Optimization............................................................................ 236
10.4. Brief description of gcovdata files ..............................................................................236
11. Known Causes of Trouble with GCC...................................................................................... 237
11.1. Actual Bugs We Haven’t Fixed Yet ............................................................................. 237
11.2. Cross-Compiler Problems ............................................................................................ 237
11.3. Interoperation...............................................................................................................237
11.4. Problems Compiling Certain Programs ....................................................................... 240
11.5. Incompatibilities of GCC.............................................................................................240
11.6. Fixed Header Files ....................................................................................................... 243
11.7. Standard Libraries........................................................................................................ 243
11.8. Disappointments and Misunderstandings ....................................................................244
11.9. Common Misunderstandings with GNU C++ ............................................................. 245
11.9.1. Declare andDefine Static Members .............................................................. 245
11.9.2. Name lookup, templates, and accessing members of base classes ...............245
11.9.3. Temporaries May Vanish Before You Expect............................................... 246
11.9.4. Implicit Copy-Assignment for Virtual Bases................................................247
11.10. Caveats of using protoize.......................................................................................248
11.11. Certain Changes We Don’t Want to Make................................................................. 249
11.12. Warning Messages and Error Messages..................................................................... 251
12. Reporting Bugs.......................................................................................................................... 253
12.1. Have You Found a Bug? .............................................................................................. 253
12.2. How and where to Report Bugs ................................................................................... 253
13. How To Get Help with GCC ....................................................................................................255
14. Contributing to GCC Development......................................................................................... 257
15. Funding Free Software ............................................................................................................. 259
16. The GNU Project and GNU/Linux .......................................................................................... 261
17. GNU GENERAL PUBLIC LICENSE ....................................................................................263
17.1. Preamble ...................................................................................................................... 263
17.2. How to Apply These Terms to Your New Programs.................................................... 266
18. GNU Free Documentation License..........................................................................................269
18.1. ADDENDUM: How to use this License for your documents ..................................... 273
19. Contributors to GCC................................................................................................................ 275
Option Index....................................................................................................................................287
Keyword Index ................................................................................................................................ 301
Chapter 1.
Introduction
This manual documents how to use the GNU compilers, as well as their features and incompatibilities,
and how to report bugs. It corresponds to GCC version 3.4. The internals of the GNU compilers,
including how to port them to new targets and some information about how to write front ends for
new languages, are documented in a separate manual. .
2 Chapter 1. Introduction
Chapter 2.
Compile C, C++, Objective-C, Ada, Fortran,
Java, or treelang
Several versions of the compiler (C, C++, Objective-C, Ada, Fortran, Java and treelang) are integrated;
this is why we use the name "GNU Compiler Collection". GCC can compile programs written in any
of these languages. The Ada, Fortran, Java and treelang compilers are described in separate manuals.
"GCC" is a common shorthand term for the GNU Compiler Collection. This is both the most general
name for the compiler, and the name used when the emphasis is on compiling C programs (as the
abbreviation formerly stood for "GNU C Compiler").
When referring to C++ compilation, it is usual to call the compiler "G++". Since there is only one
compiler, it is also accurate to call it "GCC" no matter what the language context; however, the term
"G++" is more useful when the emphasis is on compiling C++ programs.
Similarly, when we talk about Ada compilation, we usually call the compiler "GNAT", for the same
reasons.
We use the name "GCC" to refer to the compilation system as a whole, and more specifically to
the language-independent part of the compiler. For example, we refer to the optimization options as
affecting the behavior of "GCC" or sometimes just "the compiler".
Front ends for other languages, such as Mercury and Pascal exist but have not yet been integrated into
GCC. These front ends, like that for C++, are built in subdirectories of GCC and link to it. The result
is an integrated compiler that can compile programs written in C, C++, Objective-C, or any of the
languages for which you have installed front ends.
In this manual, we only discuss the options for the C, Objective-C, and C++ compilers and those of the
GCC core. Consult the documentation of the other front ends for the options to use when compiling
programs written in other languages.
G++ is a compiler, not merely a preprocessor. G++ builds object code directly from your C++ program source. There is no intermediate C version of the program. (By contrast, for example, some
other implementations use a program that generates a C program from your C++ source.) Avoiding
an intermediate C representation of the program means that you get better object code, and better
debugging information. The GNU debugger, GDB, works with this information in the object code to
give you comprehensive C++ source-level editing capabilities ().
4 Chapter 2. Compile C, C++, Objective-C, Ada, Fortran, Java, or treelang
Chapter 3.
Language Standards Supported by GCC
For each language compiled by GCC for which there is a standard, GCC attempts to follow one or
more versions of that standard, possibly with some exceptions, and possibly with some extensions.
GCC supports three versions of the C standard, although support for the most recent version is not yet
complete.
The original ANSI C standard (X3.159-1989) was ratified in 1989 and published in 1990. This standard was ratified as an ISO standard (ISO/IEC 9899:1990) later in 1990. There were no technical
differences between these publications, although the sections of the ANSI standard were renumbered
and became clauses in the ISO standard. This standard, in both its forms, is commonly known as
C89, or occasionally as C90, from the dates of ratification. The ANSI standard, but not the ISO standard, also came with a Rationale document. To select this standard in GCC, use one of the options
-ansi, -std=c89 or -std=iso9899:1990; to obtain all the diagnostics required by the standard,
you should also specify -pedantic (or -pedantic-errors if you want them to be errors rather
than warnings). Refer to Section 4.4 Options Controlling C Dialect.
Errors in the 1990 ISO C standard were corrected in two Technical Corrigenda published in 1994 and
1996. GCC does not support the uncorrected version.
An amendment to the 1990 standard was published in 1995. This amendment added digraphs and
__STDC_VERSION__ to the language, but otherwise concerned the library. This amendment is com-
monly known as AMD1; the amended standard is sometimes known as C94 or C95. To select this
standard in GCC, use the option -std=iso9899:199409 (with, as for other standard versions,
-pedantic to receive all required diagnostics).
A new edition of the ISO C standard was published in 1999 as ISO/IEC 9899:1999, and
is commonly known as C99. GCC has incomplete support for this standard version; see
http://gcc.gnu.org/c99status.html for details. To select this standard, use -std=c99 or
-std=iso9899:1999. (While in development, drafts of this standard version were referred to as
C9X.)
Errors in the 1999 ISO C standard were corrected in a Technical Corrigendum published in 2001.
GCC does not support the uncorrected version.
By default, GCC provides some extensions to the C language that on rare occasions conflict with the
C standard. Refer to Chapter 6 Extensions to the C Language Family. Use of the -std options listed
above will disable these extensions where they conflict with the C standard version selected. You may
also select an extended version of the C language explicitly with -std=gnu89 (for C89 with GNU
extensions) or -std=gnu99 (for C99 with GNU extensions). The default, if no C language dialect
options are given, is -std=gnu89; this will change to -std=gnu99 in some future release when the
C99 support is complete. Some features that are part of the C99 standard are accepted as extensions
in C89 mode.
The ISO C standard defines (in clause 4) two classes of conforming implementation. A conforming
hosted implementation supports the whole standard including all the library facilities; a conforming
freestanding implementation is only required to provide certain library facilities: those in
float.h
,
limits.h
,
stdarg.h
, and
stddef.h
; since AMD1, also those in
iso646.h
; and in C99, also those in
stdbool.h
and
stdint.h
. In addition, complex
types, added in C99, are not required for freestanding implementations. The standard also defines
two environments for programs, a freestanding environment, required of all implementations and
which may not have library facilities beyond those required of freestanding implementations,
where the handling of program startup and termination are implementation-defined, and a hosted
environment, which is not required, in which all the library facilities are provided and startup is
through a function int main (void) or int main (int, char *[]). An OS kernel would be
6 Chapter 3. Language Standards Supported by GCC
a freestanding environment; a program using the facilities of an operating system would normally be
in a hosted implementation.
GCC aims towards being usable as a conforming freestanding implementation, or as the compiler for
a conforming hosted implementation. By default, it will act as the compiler for a hosted implementation, defining __STDC_HOSTED__ as 1 and presuming that when the names of ISO C functions are
used, they have the semantics defined in the standard. To make it act as a conforming freestanding
implementation for a freestanding environment, use the option -ffreestanding; it will then define
__STDC_HOSTED__ to 0 and not make assumptions about the meanings of function names from the
standard library, with exceptions noted below. To build an OS kernel, you may well still need to make
your own arrangements for linking and startup. Refer to Section 4.4 Options Controlling C Dialect.
GCC does not provide the library facilities required only of hosted implementations, nor yet all the
facilities required by C99 of freestanding implementations; to use the facilities of a hosted environment, you will need to find them elsewhere (for example, in the GNU C library). Refer to Section 11.7
Standard Libraries.
Most of the compiler support routines used by GCC are present in libgcc, but there are a few exceptions. GCC requires the freestanding environment provide memcpy, memmove, memset and memcmp.
Some older ports of GCC are configured to use the BSD bcopy, bzero and bcmp functions instead,
but this is deprecated for new ports. Finally, if __builtin_trap is used, and the target does not
implement the trap pattern, then GCC will emit a call to abort.
For references to Technical Corrigenda, Rationale documents and information concerning the history
of C that is available online, see http://gcc.gnu.org/readings.html
There is no formal written standard for Objective-C. The most authoritative manual is
"Object-Oriented Programming and the Objective-C Language", available at a number of web sites
• http://developer.apple.com/techpubs/macosx/Cocoa/ObjectiveC/ is a recent version
• http://www.toodarkpark.org/computers/objc/ is an older example
• http://www.gnustep.org has additional useful information
There is no standard for treelang, which is a sample language front end for GCC. Its only purpose
is as a sample for people wishing to write a new language for GCC. The language is documented in
gcc/treelang/treelang.texi which can be turned into info or HTML format.
, for information on standard conformance and compatibility of the Ada compiler.
, for details of the Fortran language supported by GCC.
, for details of compatibility between gcj and the Java Platform.
Chapter 4.
GCC Command Options
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The
"overall options" allow you to stop this process at an intermediate stage. For example, the -c option says not to run the linker. Then the output consists of object files output by the assembler.
Other options are passed on to one stage of processing. Some options control the preprocessor and
others the compiler itself. Yet other options control the assembler and linker; most of these are not
documented here, since you rarely need to use any of them.
Most of the command line options that you can use with GCC are useful for C programs; when an
option is only useful with another language (usually C++), the explanation says so explicitly. If the
description for a particular option does not mention a source language, you can use that option with
all supported languages.
Section 4.3 Compiling C++ Programs, for a summary of special options for compiling C++ programs.
The gcc program accepts options and file names as operands. Many options have multi-letter names;
therefore multiple single-letter options may not be grouped: -dr is very different from -d -r.
You can mix options and other arguments. For the most part, the order you use doesn’t matter. Order
does matter when you use several options of the same kind; for example, if you specify -L more than
once, the directories are searched in the order specified.
Many options have long names starting with -f or with -W--for example, -fforce-mem,
-fstrength-reduce, -Wformat and so on. Most of these have both positive and negative forms;
the negative form of -ffoo would be -fno-foo. This manual documents only one of these two
forms, whichever one is not the default.
Option Index, for an index to GCC’s options.
4.1. Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
Overall Options
Refer to Section 4.2 Options Controlling the Kind of Output.
-c -S -E -o file -pipe -pass-exit-codes
-x language -v -### --help --target-help --version
C Language Options
Refer to Section 4.4 Options Controlling C Dialect.
-ansi -std=standard -aux-info filename
-fno-asm -fno-builtin -fno-builtin-function
-fhosted -ffreestanding -fms-extensions
-trigraphs -no-integrated-cpp -traditional -traditional-cpp
-fallow-single-precision -fcond-mismatch
-fsigned-bitfields -fsigned-char
-funsigned-bitfields -funsigned-char
-fwritable-strings
8 Chapter 4. GCC Command Options
C++ Language Options
Refer to Section 4.5 Options Controlling C++ Dialect.
-fabi-version=n -fno-access-control -fcheck-new
-fconserve-space -fno-const-strings
-fno-elide-constructors
-fno-enforce-eh-specs -fexternal-templates
-falt-external-templates
-ffor-scope -fno-for-scope -fno-gnu-keywords
-fno-implicit-templates
-fno-implicit-inline-templates
-fno-implement-inlines -fms-extensions
-fno-nonansi-builtins -fno-operator-names
-fno-optional-diags -fpermissive
-frepo -fno-rtti -fstats -ftemplate-depth-n
-fuse-cxa-atexit -fvtable-gc -fno-weak -nostdinc++
-fno-default-inline -Wabi -Wctor-dtor-privacy
-Wnon-virtual-dtor -Wreorder
-Weffc++ -Wno-deprecated
-Wno-non-template-friend -Wold-style-cast
-Woverloaded-virtual -Wno-pmf-conversions
-Wsign-promo -Wsynth
Objective-C Language Options
Refer to Section 4.6 Options Controlling Objective-C Dialect.
-fconstant-string-class=class-name
-fgnu-runtime -fnext-runtime -gen-decls
-Wno-protocol -Wselector -Wundeclared-selector
Language Independent Options
Refer to Section 4.7 Options to Control Diagnostic Messages Formatting.
-fmessage-length=n
-fdiagnostics-show-location=[once|every-line]
Warning Options
Refer to Section 4.8 Options to Request or Suppress Warnings.
-fsyntax-only -pedantic -pedantic-errors
-w -Wextra -Wall -Waggregate-return
-Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment
-Wconversion -Wno-deprecated-declarations
-Wdisabled-optimization -Wno-div-by-zero -Werror
-Wfloat-equal -Wformat -Wformat=2
-Wformat-nonliteral -Wformat-security
-Wimplicit -Wimplicit-int
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
-Wimport -Winline -Winvalid-pch -Wno-endif-labels
-Wno-invalid-offsetof
-Wlarger-than-len -Wlong-long
-Wmain -Wmissing-braces
-Wmissing-format-attribute -Wmissing-noreturn
-Wno-multichar -Wno-format-extra-args -Wno-format-y2k
-Wno-import -Wnonnull -Wpacked -Wpadded
-Wparentheses -Wpointer-arith -Wredundant-decls
-Wreturn-type -Wsequence-point -Wshadow
-Wsign-compare -Wstrict-aliasing
-Wswitch -Wswitch-default -Wswitch-enum
-Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized
-Wunknown-pragmas -Wunreachable-code
-Wunused -Wunused-function -Wunused-label -Wunused-parameter
Chapter 4. GCC Command Options 9
-Wunused-value -Wunused-variable -Wwrite-strings
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations
-Wmissing-prototypes -Wnested-externs
-Wstrict-prototypes -Wtraditional
Debugging Options
Refer to Section 4.9 Options for Debugging Your Program or GCC.
-dletters -dumpspecs -dumpmachine -dumpversion
-fdump-unnumbered -fdump-translation-unit[-n]
-fdump-class-hierarchy[-n]
-fdump-tree-original[-n]
-fdump-tree-optimized[-n]
-fdump-tree-inlined[-n]
-feliminate-dwarf2-dups -feliminate-unused-debug-types
-fmem-report -fprofile-arcs
-frandom-seed=string -fsched-verbose=n
-ftest-coverage -ftime-report
-g -glevel -gcoff -gdwarf -gdwarf-1 -gdwarf-1+ -gdwarf-2
-ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+
-p -pg -print-file-name=library -print-libgcc-file-name
-print-multi-directory -print-multi-lib
-print-prog-name=program -print-search-dirs -Q
-save-temps -time
Optimization Options
Refer to Section 4.10 Options That Control Optimization.
-falign-functions=n -falign-jumps=n
-falign-labels=n -falign-loops=n
-fbranch-probabilities -fcaller-saves -fcprop-registers
-fcse-follow-jumps -fcse-skip-blocks -fdata-sections
-fdelayed-branch -fdelete-null-pointer-checks
-fexpensive-optimizations -ffast-math -ffloat-store
-fforce-addr -fforce-mem -ffunction-sections
-fgcse -fgcse-lm -fgcse-sm -floop-optimize -fcrossjumping
-fif-conversion -fif-conversion2
-finline-functions -finline-limit=n -fkeep-inline-functions
-fkeep-static-consts -fmerge-constants -fmerge-all-constants
-fmove-all-movables -fnew-ra -fno-branch-count-reg
-fno-default-inline -fno-defer-pop
-fno-function-cse -fno-guess-branch-probability
-fno-inline -fno-math-errno -fno-peephole -fno-peephole2
-funsafe-math-optimizations -ffinite-math-only
-fno-trapping-math -fno-zero-initialized-in-bss
-fomit-frame-pointer -foptimize-register-move
-foptimize-sibling-calls -fprefetch-loop-arrays
-freduce-all-givs -fregmove -frename-registers
-freorder-blocks -freorder-functions
-frerun-cse-after-loop -frerun-loop-opt
-fschedule-insns -fschedule-insns2
-fno-sched-interblock -fno-sched-spec -fsched-spec-load
-fsched-spec-load-dangerous -fsched2-use-superblocks
-fsched2-use-traces -fsignaling-nans
-fsingle-precision-constant -fssa -fssa-ccp -fssa-dce
-fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps
-funroll-all-loops -funroll-loops -fpeel-loops
-funswitch-loops -fold-unroll-loops -fold-unroll-all-loops
--param name=value
-O -O0 -O1 -O2 -O3 -Os
10 Chapter 4. GCC Command Options
Preprocessor Options
Refer to Section 4.11 Options Controlling the Preprocessor.
-Aquestion=answer
-A-question[=answer]
-C -dD -dI -dM -dN
-Dmacro[=defn] -E -H
-idirafter dir
-include file -imacros file
-iprefix file -iwithprefix dir
-iwithprefixbefore dir -isystem dir
-M -MM -MF -MG -MP -MQ -MT -nostdinc -P -remap
-trigraphs -undef -Umacro -Wp,option
-Xpreprocessor option
Assembler Option
Refer to Section 4.12 Passing Options to the Assembler.
-Wa,option -Xassembler option
Linker Options
Refer to Section 4.13 Options for Linking.
object-file-name -llibrary
-nostartfiles -nodefaultlibs -nostdlib -pie
-s -static -static-libgcc -shared -shared-libgcc -symbolic
-Wl,option -Xlinker option
-u symbol
Directory Options
Refer to Section 4.14 Options for Directory Search.
-Bprefix -Idir -I- -Ldir -specs=file
Target Options
Refer to Section 4.16 Specifying Target Machine and Compiler Version.
-V version -b machine
Machine Dependent Options
Refer to Section 4.17 Hardware Models and Configurations.
SPARC Options
-mcpu=cpu-type
-mtune=cpu-type
-mcmodel=code-model
-m32 -m64
-mapp-regs -mbroken-saverestore -mcypress
-mfaster-structs -mflat
-mfpu -mhard-float -mhard-quad-float
-mimpure-text -mlive-g0 -mno-app-regs
-mno-faster-structs -mno-flat -mno-fpu
-mno-impure-text -mno-stack-bias -mno-unaligned-doubles
-msoft-float -msoft-quad-float -msparclite -mstack-bias
-msupersparc -munaligned-doubles -mv8
RS/6000 and PowerPC Options
-mcpu=cpu-type
-mtune=cpu-type
-mpower -mno-power -mpower2 -mno-power2
-mpowerpc -mpowerpc64 -mno-powerpc
-maltivec -mno-altivec
-mpowerpc-gpopt -mno-powerpc-gpopt
Chapter 4. GCC Command Options 11
-mpowerpc-gfxopt -mno-powerpc-gfxopt
-mnew-mnemonics -mold-mnemonics
-mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc
-m64 -m32 -mxl-call -mno-xl-call -mpe
-malign-power -malign-natural
-msoft-float -mhard-float -mmultiple -mno-multiple
-mstring -mno-string -mupdate -mno-update
-mfused-madd -mno-fused-madd -mbit-align -mno-bit-align
-mstrict-align -mno-strict-align -mrelocatable
-mno-relocatable -mrelocatable-lib -mno-relocatable-lib
-mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian
-mdynamic-no-pic
-mcall-sysv -mcall-netbsd
-maix-struct-return -msvr4-struct-return
-mabi=altivec -mabi=no-altivec
-mabi=spe -mabi=no-spe
-misel=yes -misel=no
-mspe=yes -mspe=no
-mfloat-gprs=yes -mfloat-gprs=no
-mprototype -mno-prototype
-msim -mmvme -mads -myellowknife -memb -msdata
-msdata=opt -mvxworks -mwindiss -G num -pthread
i386 and x86-64 Options
-mtune=cpu-type -march=cpu-type
-mfpmath=unit
-masm=dialect -mno-fancy-math-387
-mno-fp-ret-in-387 -msoft-float -msvr3-shlib
-mno-wide-multiply -mrtd -malign-double
-mpreferred-stack-boundary=num
-mmmx -msse -msse2 -m3dnow
-mthreads -mno-align-stringops -minline-all-stringops
-mpush-args -maccumulate-outgoing-args -m128bit-long-double
-m96bit-long-double -mregparm=num -momit-leaf-frame-pointer
-mno-red-zone -mno-tls-direct-seg-refs
-mcmodel=code-model
-m32 -m64
IA-64 Options
-mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic
-mvolatile-asm-stop -mb-step -mregister-names -mno-sdata
-mconstant-gp -mauto-pic -minline-float-divide-min-latency
-minline-float-divide-max-throughput
-minline-int-divide-min-latency
-minline-int-divide-max-throughput -mno-dwarf2-asm
-mfixed-range=register-range
S/390 and zSeries Options
-mtune=cpu-type -march=cpu-type
-mhard-float -msoft-float -mbackchain -mno-backchain
-msmall-exec -mno-small-exec -mmvcle -mno-mvcle
-m64 -m31 -mdebug -mno-debug -mesa -mzarch
Code Generation Options
Refer to Section 4.18 Options for Code Generation Conventions.
-fcall-saved-reg -fcall-used-reg
-ffixed-reg -fexceptions
-fnon-call-exceptions -funwind-tables
-fasynchronous-unwind-tables
-finhibit-size-directive -finstrument-functions
-fno-common -fno-ident -fno-gnu-linker
-fpcc-struct-return -fpic -fPIC -fpie -fPIE
12 Chapter 4. GCC Command Options
-freg-struct-return -fshared-data -fshort-enums
-fshort-double -fshort-wchar
-fverbose-asm -fpack-struct -fstack-check
-fstack-limit-register=reg -fstack-limit-symbol=sym
-fargument-alias -fargument-noalias
-fargument-noalias-global -fleading-underscore
-ftls-model=model
-ftrapv -fwrapv -fbounds-check
4.2. Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking,
always in that order. The first three stages apply to an individual source file, and end by producing an
object file; linking combines all the object files (those newly compiled, and those specified as input)
into an executable file.
For any given input file, the file name suffix determines what kind of compilation is done:
file.c
C source code which must be preprocessed.
file.i
C source code which should not be preprocessed.
file.ii
C++ source code which should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the library libobjc.a to make an
Objective-C program work.
file.mi
Objective-C source code which should not be preprocessed.
file.h
C or C++ header file to be turned into a precompiled header.
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code which must be preprocessed. Note that in .cxx, the last two letters must both
be literally x. Likewise, .C refers to a literal capital C.
file.hh
file.H
C++ header file to be turned into a precompiled header.
Chapter 4. GCC Command Options 13
file.f
file.for
file.FOR
Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP
Fortran source code which must be preprocessed (with the traditional preprocessor).
file.r
Fortran source code which must be preprocessed with a RATFOR preprocessor (not included
with GCC).
, for more details of the handling of Fortran input files.
file.ads
Ada source code file which contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package,
generic, or subprogram renaming declaration). Such files are also called specs.
file.adb
Ada source code file containing a library unit body (a subprogram or package body). Such files
are also called bodies.
file.s
Assembler code.
file.S
Assembler code which must be preprocessed.
other
An object file to be fed straight into linking. Any file name with no recognized suffix is treated
this way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files (rather than letting the compiler
choose a default based on the file name suffix). This option applies to all following input files
until the next -x option. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objc-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input ratfor
java
treelang
-x none
Turn off any specification of a language, so that subsequent files are handled according to their
file name suffixes (as they are if -x has not been used at all).
14 Chapter 4. GCC Command Options
-pass-exit-codes
Normally the gcc program will exit with the code of 1 if any phase of the compiler returns
a non-success return code. If you specify -pass-exit-codes, the gcc program will instead
return with numerically highest error produced by any phase that returned an error indication.
If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc
where to start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some
combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all.
-c
Compile or assemble the source files, but do not link. The linking stage simply is not done. The
ultimate output is in the form of an object file for each source file.
By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc.,
with .o.
Unrecognized input files, not requiring compilation or assembly, are ignored.
-S
Stop after the stage of compilation proper; do not assemble. The output is in the form of an
assembler code file for each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc.,
with .s.
Input files that don’t require compilation are ignored.
-E
Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of
preprocessed source code, which is sent to the standard output.
Input files which don’t require preprocessing are ignored.
-o file
Place output in file file. This applies regardless to whatever sort of output is being produced,
whether it be an executable file, an object file, an assembler file or preprocessed C code.
Since only one output file can be specified, it does not make sense to use -o when compiling
more than one input file, unless you are producing an executable file as output.
If -o is not specified, the default is to put an executable file in a.out, the object file for
source.suffix in source.o, its assembler file in source.s, and all preprocessed C source on
standard output.
-v
Print (on standard error output) the commands executed to run the stages of compilation. Also
print the version number of the compiler driver program and of the preprocessor and the compiler
proper.
-###
Like -v except the commands are not executed and all command arguments are quoted. This is
useful for shell scripts to capture the driver-generated command lines.
Chapter 4. GCC Command Options 15
-pipe
Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but
the GNU assembler has no trouble.
-help
Print (on the standard output) a description of the command line options understood by gcc. If
the -v option is also specified then -help will also be passed on to the various processes invoked
by gcc, so that they can display the command line options they accept. If the -Wextra option
is also specified then command line options which have no documentation associated with them
will also be displayed.
-target-help
Print (on the standard output) a description of target specific command line options for each tool.
-version
Display the version number and copyrights of the invoked GCC.
4.3. Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx;
C++ header files often use .hh or .H; and preprocessed C++ files use the suffix .ii. GCC recognizes
files with these names and compiles them as C++ programs even if you call the compiler the same
way as for compiling C programs (usually with the name gcc).
However, C++ programs often require class libraries as well as a compiler that understands the C++
language--and under some circumstances, you might want to compile programs or header files from
standard input, or otherwise without a suffix that flags them as C++ programs. You might also like to
precompile a C header file with a .h extension to be used in C++ compilations. g++ is a program that
calls GCC with the default language set to C++, and automatically specifies linking against the C++
library. On many systems, g++ is also installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line options that you
use for compiling programs in any language; or command-line options meaningful for C and related
languages; or options that are meaningful only for C++ programs. Section 4.4 Options Controlling C
Dialect, for explanations of options for languages related to C. Section 4.5 Options Controlling C++
Dialect, for explanations of options that are meaningful only for C++ programs.
4.4. Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C++ and
Objective-C) that the compiler accepts:
-ansi
In C mode, support all ISO C90 programs. In C++ mode, remove GNU extensions that conflict
with ISO C++.
This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C
code), or of standard C++ (when compiling C++ code), such as the asm and typeof keywords,
and predefined macros such as unix and vax that identify the type of system you are using. It
also enables the undesirable and rarely used ISO trigraph feature. For the C compiler, it disables
recognition of C++ style // comments as well as the inline keyword.
16 Chapter 4. GCC Command Options
The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue
to work despite -ansi. You would not want to use them in an ISO C program, of course, but
it is useful to put them in header files that might be included in compilations done with -ansi.
Alternate predefined macros such as __unix__ and __vax__ are also available, with or without
-ansi.
The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that,
-pedantic is required in addition to -ansi. Refer to Section 4.8 Options to Request or
Suppress Warnings.
The macro __STRICT_ANSI__ is predefined when the -ansi option is used. Some header files
may notice this macro and refrain from declaring certain functions or defining certain macros
that the ISO standard doesn’t call for; this is to avoid interfering with any programs that might
use these names for other things.
Functions which would normally be built in but do not have semantics defined by ISO C (such
as alloca and ffs) are not built-in functions with -ansi is used. Refer to Section 6.46 Other
built-in functions provided by GCC, for details of the functions affected.
-std=
Determine the language standard. This option is currently only supported when compiling C or
C++. A value for this option must be provided; possible values are
c89
iso9899:1990
ISO C90 (same as -ansi).
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully supported; see
http://gcc.gnu.org/c99status.html for more information. The names c9x and
iso9899:199x are deprecated.
gnu89
Default, ISO C90 plus GNU extensions (including some C99 features).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC, this will
become the default. The name gnu9x is deprecated.
c++98
The 1998 ISO C++ standard plus amendments.
gnu++98
The same as -std=c++98 plus GNU extensions. This is the default for C++ code.
Even when this option is not specified, you can still use some of the features of newer standards in so far as they do not conflict with previous C standards. For example, you may use
__restrict__ even when -std=c99 is not specified.
Chapter 4. GCC Command Options 17
The -std options specifying some version of ISO C have the same effects as -ansi, except that
features that were not in ISO C90 but are in the specified version (for example, // comments and
the inline keyword in ISO C99) are not disabled.
Chapter 3 Language Standards Supported by GCC, for details of these standard versions.
-aux-info filename
Output to the given filename prototyped declarations for all functions declared and/or defined in
a translation unit, including those in header files. This option is silently ignored in any language
other than C.
Besides declarations, the file indicates, in comments, the origin of each declaration (source file
and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O
for old, respectively, in the first character after the line number and the colon), and whether it
came from a declaration or a definition (C or F, respectively, in the following character). In the
case of function definitions, a K&R-style list of arguments followed by their declarations is also
provided, inside comments, after the declaration.
-fno-asm
Do not recognize asm, inline or typeof as a keyword, so that code can use these words as
identifiers. You can use the keywords __asm__, __inline__ and __typeof__ instead. -ansi
implies -fno-asm.
In C++, this switch only affects the typeof keyword, since asm and inline are standard keywords. You may want to use the -fno-gnu-keywords flag instead, which has the same effect. In
C99 mode (-std=c99 or -std=gnu99), this switch only affects the asm and typeof keywords,
since inline is a standard keyword in ISO C99.
-fno-builtin
-fno-builtin-function
Do not recognize built-in functions that do not begin with __builtin_ as prefix. Refer to Section 6.46 Other built-in functions provided by GCC, for details of the functions affected, including those which are not built-in functions when -ansi or -std options for strict ISO C
conformance are used because they do not have an ISO standard meaning.
GCC normally generates special code to handle certain built-in functions more efficiently; for
instance, calls to alloca may become single instructions that adjust the stack directly, and calls
to memcpy may become inline copy loops. The resulting code is often both smaller and faster,
but since the function calls no longer appear as such, you cannot set a breakpoint on those calls,
nor can you change the behavior of the functions by linking with a different library.
With the -fno-builtin-function option only the built-in function function is disabled.
function must not begin with __builtin_. If a function is named this is not built-in in this
version of GCC, this option is ignored. There is no corresponding -fbuiltin-function
option; if you wish to enable built-in functions selectively when using -fno-builtin or
-ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted
Assert that compilation takes place in a hosted environment. This implies -fbuiltin. A hosted
environment is one in which the entire standard library is available, and in which main has
a return type of int. Examples are nearly everything except a kernel. This is equivalent to
-fno-freestanding.
18 Chapter 4. GCC Command Options
-ffreestanding
Assert that compilation takes place in a freestanding environment. This implies -fno-builtin.
A freestanding environment is one in which the standard library may not exist, and program
startup may not necessarily be at main. The most obvious example is an OS kernel. This is
equivalent to -fno-hosted.
Chapter 3 Language Standards Supported by GCC, for details of freestanding and hosted environments.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C conformance)
implies -trigraphs.
-no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling. This option allows a user
supplied "cc1", "cc1plus", or "cc1obj" via the -B option. The user supplied compilation step can
then add in an additional preprocessing step after normal preprocessing but before compiling.
The default is to use the integrated cpp (internal cpp)
The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-standard C compiler. They are
now only supported with the -E switch. The preprocessor continues to support a pre-standard
mode. See the GNU CPP manual for details.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and third arguments. The
value of such an expression is void. This option is not supported for C++.
-funsigned-char
Let the type char be unsigned, like unsigned char.
Each kind of machine has a default for what char should be. It is either like unsigned char
by default or like signed char by default.
Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char
and expect it to be signed, or expect it to be unsigned, depending on the machines they were
written for. This option, and its inverse, let you make such a program work with the opposite
default.
The type char is always a distinct type from each of signed char or unsigned char, even
though its behavior is always just like one of those two.
-fsigned-char
Let the type char be signed, like signed char.
Note that this is equivalent to -fno-unsigned-char, which is the negative form
of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to
-funsigned-char.
Chapter 4. GCC Command Options 19
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the declaration does not
use either signed or unsigned. By default, such a bit-field is signed, because this is consistent:
the basic integer types such as int are signed types.
-fwritable-strings
Store string constants in the writable data segment and don’t uniquize them. This is for compatibility with old programs which assume they can write into string constants.
Writing into string constants is a very bad idea; "constants" should be constant.
4.5. Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++ programs; but you
can also use most of the GNU compiler options regardless of what language your program is in. For
example, you might compile a file firstClass.C like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs; you can use the other options
with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. Version 1 is the version of the C++ ABI that first appeared
in G++ 3.2. Version 0 will always be the version that conforms most closely to the C++ ABI
specification. Therefore, the ABI obtained using version 0 will change as ABI bugs are fixed.
The default is version 1.
-fno-access-control
Turn off all access checking. This switch is mainly useful for working around bugs in the access
control code.
-fcheck-new
Check that the pointer returned by operator new is non-null before attempting to modify the
storage allocated. This check is normally unnecessary because the C++ standard specifies that
operator new will only return 0 if it is declared throw(), in which case the compiler will
always check the return value even without this option. In all other cases, when operator
new has a non-empty exception specification, memory exhaustion is signalled by throwing
std::bad_alloc. See also new (nothrow).
-fconserve-space
Put uninitialized or runtime-initialized global variables into the common segment, as C does.
This saves space in the executable at the cost of not diagnosing duplicate definitions. If you
compile with this flag and your program mysteriously crashes after main() has completed, you
may have an object that is being destroyed twice because two definitions were merged.
20 Chapter 4. GCC Command Options
This option is no longer useful on most targets, now that support has been added for putting
variables into BSS without making them common.
-fno-const-strings
Give string constants type char * instead of type const char *. By default, G++ uses type
const char * as required by the standard. Even if you use -fno-const-strings,you cannot
actually modify the value of a string constant, unless you also use -fwritable-strings.
This option might be removed in a future release of G++. For maximum portability, you should
structure your code so that it works with string constants that have type const char *.
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a temporary which is only used to
initialize another object of the same type. Specifying this option disables that optimization, and
forces G++ to call the copy constructor in all cases.
-fno-enforce-eh-specs
Do not check for violation of exception specifications at runtime. This option violates the C++
standard, but may be useful for reducing code size in production builds, much like defining
NDEBUG. The compiler will still optimize based on the exception specifications.
-fexternal-templates
Cause #pragma interface and implementation to apply to template instantiation; template
instances are emitted or not according to the location of the template definition. Refer to Section
7.6 Where’s the Template?, for more information.
This option is deprecated.
-falt-external-templates
Similar to -fexternal-templates, but template instances are emitted or not according to the
place where they are first instantiated. Section 7.6 Where’s the Template?, for more information.
This option is deprecated.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to
the for loop itself, as specified by the C++ standard. If -fno-for-scope is specified, the scope
of variables declared in a for-init-statement extends to the end of the enclosing scope, as was the
case in old versions of G++, and other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but to allow and give a warning for
old-style code that would otherwise be invalid, or have different behavior.
-fno-gnu-keywords
Do not recognize typeof as a keyword, so that code can use this word as an identifier. You can
use the keyword __typeof__ instead. -ansi implies -fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates which are instantiated implicitly (that is, by use); only
emit code for explicit instantiations. Section 7.6 Where’s the Template?, for more information.
Chapter 4. GCC Command Options 21
-fno-implicit-inline-templates
Don’t emit code for implicit instantiations of inline templates, either. The default is to handle
inlines differently so that compiles with and without optimization will need the same set of
explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions controlled by #pragma
implementation. This will cause linker errors if these functions are not inlined everywhere
they are called.
-fms-extensions
Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a
pointer to member function via non-standard syntax.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include
ffs, alloca, _exit, index, bzero, conjf, and other related functions.
-fno-operator-names
Do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as
synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only
such diagnostic issued by G++ is the one for a name having multiple meanings within a class.
-fpermissive
Downgrade messages about nonconformant code from errors to warnings. By default, G++ effectively sets -pedantic-errors without -pedantic; this option reverses that. This behavior
and this option are superseded by -pedantic, which works as it does for GNU C.
-frepo
Enable automatic template instantiation at link time. This option also implies
-fno-implicit-templates. Refer to Section 7.6 Where’s the Template?, for more
information.
-fno-rtti
Disable generation of information about every class with virtual functions for use by the C++
runtime type identification features (dynamic_cast and typeid). If you don’t use those parts
of the language, you can save some space by using this flag. Note that exception handling uses
the same information, but it will generate it as needed.
-fstats
Emit statistics about front-end processing at the end of the compilation. This information is
generally only useful to the G++ development team.
-ftemplate-depth-n
Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO
C++ conforming programs must not rely on a maximum depth greater than 17.
22 Chapter 4. GCC Command Options
-fuse-cxa-atexit
Register destructors for objects with static storage duration with the __cxa_atexit function
rather than the atexit function. This option is required for fully standards-compliant handling
of static destructors, but will only work if your C library supports __cxa_atexit.
-fvtable-gc
Emit special relocations for vtables and virtual function references so that the linker can identify unused virtual functions and zero out vtable slots that refer to them. This is most useful
with -ffunction-sections and -Wl,-gc-sections, in order to also discard the functions
themselves.
This optimization requires GNU as and GNU ld. Not all systems support this option.
-Wl,-gc-sections is ignored without -static.
-fno-weak
Do not use weak symbol support, even if it is provided by the linker. By default, G++ will use
weak symbols if they are available. This option exists only for testing, and should not be used
by end-users; it will result in inferior code and has no benefits. This option may be removed in a
future release of G++.
-nostdinc++
Do not search for header files in the standard directories specific to C++, but do still search the
other standard directories. (This option is used when building the C++ library.)
In addition, these optimization, warning, and code generation options have meanings only for C++
programs:
-fno-default-inline
Do not assume inline for functions defined inside a class scope. Section 4.10 Options That
Control Optimization. Note that these functions will have linkage like inline functions; they just
won’t be inlined by default.
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with the vendor-neutral C++
ABI. Although an effort has been made to warn about all such cases, there are probably some
cases that are not warned about, even though G++ is generating incompatible code. There may
also be cases where warnings are emitted even though the code that is generated will be compatible.
You should rewrite your code to avoid these warnings if you are concerned about the fact that
code generated by G++ may not be binary compatible with code generated by other compilers.
The known incompatibilities at this point include:
• Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data into the same
byte as a base class. For example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place B::f2 into the same byte asA::f1; other compilers will not. You
can avoid this problem by explicitly padding A so that its size is a multiple of the byte size on
your platform; that will cause G++ and other compilers to layout B identically.
• Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when laying
out virtual bases. For example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
Chapter 4. GCC Command Options 23
struct C : public A, public virtual B {};
In this case, G++ will not place B into the tail-padding for A; other compilers will. You can
avoid this problem by explicitly padding A so that its size is a multiple of its alignment (ignoring virtual base classes); that will cause G++ and other compilers to layout C identically.
• Incorrect handling of bit-fields with declared widths greater than that of their underlying types,
when the bit-fields appear in a union. For example:
union U { int i : 4096; };
Assuming that an int does not have 4096 bits, G++ will make the union too small by the
number of bits in an int.
• Empty classes can be placed at incorrect offsets. For example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the A base class of C at a nonzero offset; it should be placed at offset zero. G++
mistakenly believes that the A data member of B is already at offset zero.
• Names of template functions whose types involve typename or template template parameters
can be mangled incorrectly.
template
typename Q
void f(typename Q::X) {}
templatetemplatetypenameclass Q
void f(typename Qint::X) {}
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy (C++ only)
Warn when a class seems unusable, because all the constructors or destructors in a class are
private and the class has no friends or public static member functions.
-Wnon-virtual-dtor (C++ only)
Warn when a class declares a non-virtual destructor that should probably be virtual, because it
looks like the class will be used polymorphically. This warning is enabled by -Wall.
-Wreorder (C++ only)
Warn when the order of member initializers given in the code does not match the order in which
they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
Here the compiler will warn that the member initializers for i and j will be rearranged to match
the declaration order of the members. This warning is enabled by -Wall.
The following -W... options are not affected by -Wall.
24 Chapter 4. GCC Command Options
-Weffc++ (C++ only)
Warn about violations of the following style guidelines from Scott Meyers’ Effective C++ book:
• Item 11: Define a copy constructor and an assignment operator for classes with dynamically
allocated memory.
• Item 12: Prefer initialization to assignment in constructors.
• Item 14: Make destructors virtual in base classes.
• Item 15: Have operator= return a reference to *this.
• Item 23: Don’t try to return a reference when you must return an object.
and about violations of the following style guidelines from Scott Meyers’ [More Effective C++]
book:
• Item 6: Distinguish between prefix and postfix forms of increment and decrement operators.
• Item 7: Never overload &&, ||, or ,.
If you use this option, you should be aware that the standard library headers do not obey all of
these guidelines; you can use grep -v to filter out those warnings.
-Wno-deprecated (C++ only)
Do not warn about usage of deprecated features. Refer to Section 7.10 Deprecated Features.
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are declared within a template. With
the advent of explicit template specification support in G++, if the name of the friend is an
unqualified-id (i.e., friend foo(int)), the C++ language specification demands that the friend
declare or define an ordinary, nontemplate function. (Section 14.5.3). Before G++ implemented
explicit specification, unqualified-ids could be interpreted as a particular specialization of a
templatized function. Because this non-conforming behavior is no longer the default behavior for G++, -Wnon-template-friend allows the compiler to check existing code for potential trouble spots, and is on by default. This new compiler behavior can be turned off with
-Wno-non-template-friend which keeps the conformant compiler code but disables the
helpful warning.
-Wold-style-cast (C++ only)
Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The newstyle casts (static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects, and much easier to grep for.
-Woverloaded-virtual (C++ only)
Warn when a function declaration hides virtual functions from a base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the A class version of f is hidden in B, and code like this:
B* b;
b-
f();
will fail to compile.
Chapter 4. GCC Command Options 25
-Wno-pmf-conversions (C++ only)
Disable the diagnostic for converting a bound pointer to member function to a plain pointer.
-Wsign-promo (C++ only)
Warn when overload resolution chooses a promotion from unsigned or enumeral type to a signed
type over a conversion to an unsigned type of the same size. Previous versions of G++ would try
to preserve unsignedness, but the standard mandates the current behavior.
-Wsynth (C++ only)
Warn when G++’s synthesis behavior does not match that of cfront. For instance:
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default A& operator = (const A&);, while cfront
will use the user-defined operator =.
4.6. Options Controlling Objective-C Dialect
This section describes the command-line options that are only meaningful for Objective-C programs;
but you can also use most of the GNU compiler options regardless of what language your program is
in. For example, you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, only -fgnu-runtime is an option meant only for Objective-C programs; you can
use the other options with any language supported by GCC.
Here is a list of options that are only for compiling Objective-C programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each literal string specified with the
syntax @"...". The default class name is NXConstantString.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C runtime. This is the default
for most types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. The macro __NEXT_RUNTIME__ is predefined if (and only if) this option is used.
-gen-decls
Dump interface declarations for all classes seen in the source file to a file named
sourcename.decl.
26 Chapter 4. GCC Command Options
-Wno-protocol
If a class is declared to implement a protocol, a warning is issued for every method in the protocol
that is not implemented by the class. The default behavior is to issue a warning for every method
not explicitly implemented in the class, even if a method implementation is inherited from the
superclass. If you use the -Wno-protocol option, then methods inherited from the superclass
are considered to be implemented, and no warning is issued for them.
-Wselector
Warn if multiple methods of different types for the same selector are found during compilation.
The check is performed on the list of methods in the final stage of compilation. Additionally, a
check is performed that for each selector appearing in a @selector(...) expression, a corresponding method with that selector has been found during compilation. Because these checks
scan the method table only at the end of compilation, these warnings are not produced if the final
stage of compilation is not reached, for example because an error is found during compilation,
or because the -fsyntax-only option is being used.
-Wundeclared-selector
Warn if a @selector(...) expression referring to an undeclared selector is found. A selector is considered undeclared if no method with that name has been declared (explicitly, in an
@interface or @protocol declaration, or implicitly, in an @implementation section) be-
fore the @selector(...) expression. This option always performs its checks as soon as a
@selector(...) expression is found (while -Wselector only performs its checks in the final
stage of compilation), and so additionally enforces the coding style convention that methods and
selectors must be declared before being used.
4.7. Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of the output device’s aspect (for
example, its width, . . . ). The options described below can be used to control the diagnostic messages
formatting algorithm, for example, how many characters per line, how often source location information should be reported. Right now, only the C++ front end can honor these options. However it is
expected, in the near future, that the remaining front ends would be able to digest them correctly.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n characters. The default is 72
characters for g++ and 0 for the rest of the front ends supported by GCC. If n is zero, then no
line-wrapping will be done; each error message will appear on a single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit once
source location information; that is, in case the message is too long to fit on a single physical line
and has to be wrapped, the source location won’t be emitted (as prefix) again, over and over, in
subsequent continuation lines. This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the
same source location information (as prefix) for physical lines that result from the process of
breaking a message which is too long to fit on a single line.
Chapter 4. GCC Command Options 27
4.8. Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are not inherently erroneous but
which are risky or suggest there may have been an error.
You can request many specific warnings with options beginning -W, for example -Wimplicit to
request warnings on implicit declarations. Each of these specific warning options also has a negative
form beginning -Wno- to turn off warnings; for example, -Wno-implicit. This manual lists only
one of the two forms, whichever is not the default.
The following options control the amount and kinds of warnings produced by GCC; for further,
language-specific options also refer to Section 4.5 Options Controlling C++ Dialect and Section
4.6 Options Controlling Objective-C Dialect.
-fsyntax-only
Check the code for syntax errors, but don’t do anything beyond that.
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use
forbidden extensions, and some other programs that do not follow ISO C and ISO C++. For ISO
C, follows the version of the ISO C standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or without this option (though
a rare few will require -ansi or a -std option specifying the required version of ISO C). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected.
-pedantic does not cause warning messages for use of the alternate keywords whose names
begin and end with __. Pedantic warnings are also disabled in the expression that follows
__extension__. However, only system header files should use these escape routes;
application programs should avoid them. Section 6.41 Alternate Keywords.
Some users try to use -pedantic to check programs for strict ISO C conformance. They soon
find that it does not do quite what they want: it finds some non-ISO practices, but not all--only
those for which ISO C requires a diagnostic, and some others for which diagnostics have been
added.
A feature to report any failure to conform to ISO C might be useful in some instances, but would
require considerable additional work and would be quite different from -pedantic. We don’t
have plans to support such a feature in the near future.
Where the standard specified with -std represents a GNU extended dialect of C, such as gnu89
or gnu99, there is a corresponding base standard, the version of ISO C on which the GNU
extended dialect is based. Warnings from -pedantic are given where they are required by the
base standard. (It would not make sense for such warnings to be given only for features not in
the specified GNU C dialect, since by definition the GNU dialects of C include all features the
compiler supports with the given option, and there would be nothing to warn about.)
-pedantic-errors
Like -pedantic, except that errors are produced rather than warnings.
-w
Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
28 Chapter 4. GCC Command Options
-Wchar-subscripts
Warn if an array subscript has type char. This is a common cause of error, as programmers often
forget that this type is signed on some machines.
-Wcomment
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a
Backslash-Newline appears in a // comment.
-Wformat
Check calls to printf and scanf, etc., to make sure that the arguments supplied have types
appropriate to the format string specified, and that the conversions specified in the format string
make sense. This includes standard functions, and others specified by format attributes (refer to
Section 6.26 Declaring Attributes of Functions), in the printf, scanf, strftime and strfmon
(an X/Open extension, not in the C standard) families.
The formats are checked against the format features supported by GNU libc version 2.2. These
include all ISO C90 and C99 features, as well as features from the Single Unix Specification
and some BSD and GNU extensions. Other library implementations may not support all these
features; GCC does not support warning about features that go beyond a particular library’s
limitations. However, if -pedantic is used with -Wformat, warnings will be given about format
features not in the selected standard version (but not for strfmon formats, since those are not in
any version of the C standard). Refer to Section 4.4 Options Controlling C Dialect.
Since -Wformat also checks for null format arguments for several functions, -Wformat also
implies -Wnonnull.
-Wformat is included in -Wall. For more control over some aspects of format checking, the
options -Wno-format-y2k, -Wno-format-extra-args, -Wno-format-zero-length,
-Wformat-nonliteral, -Wformat-security, and -Wformat=2 are available, but are not
included in -Wall.
-Wno-format-y2k
If -Wformat is specified, do not warn about strftime formats which may yield only a two-digit
year.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a printf or scanf format
function. The C standard specifies that such arguments are ignored.
Where the unused arguments lie between used arguments that are specified with $ operand number specifications, normally warnings are still given, since the implementation could not know
what type to pass to va_arg to skip the unused arguments. However, in the case of scanf formats, this option will suppress the warning if the unused arguments are all pointers, since the
Single Unix Specification says that such unused arguments are allowed.
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that
zero-length formats are allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a string literal and so cannot be
checked, unless the format function takes its format arguments as a va_list.
Chapter 4. GCC Command Options 29
-Wformat-security
If -Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to printf and scanf functions where the
format string is not a string literal and there are no format arguments, as in printf (foo);.
This may be a security hole if the format string came from untrusted input and contains %n. (This
is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may
be added to -Wformat-security that are not included in -Wformat-nonliteral.)
-Wformat=2
Enable -Wformat plus format checks not included in -Wformat. Currently equivalent to
-Wformat -Wformat-nonliteral -Wformat-security.
-Wnonnull
Enable warning about passing a null pointer for arguments marked as requiring a non-null value
by the nonnull function attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull
option.
-Wimplicit-int
Warn when a declaration does not specify a type.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being declared.
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration.
-Wmain
Warn if the type of main is suspicious. main should be a function with external linkage, returning
int, taking either zero arguments, two, or three arguments of appropriate types.
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In the following example, the
initializer for a is not fully bracketed, but that for b is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a
context where a truth value is expected, or when operators are nested whose precedence people
often get confused about.
Also warn about constructions where there may be confusion to which if statement an else
branch belongs. Here is an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
30 Chapter 4. GCC Command Options
In C, every else branch belongs to the innermost possible if statement, which in this example
is if (b). This is often not what the programmer expected, as illustrated in the above example
by indentation the programmer chose. When there is the potential for this confusion, GCC will
issue a warning when this flag is specified. To eliminate the warning, add explicit braces around
the innermost if statement so there is no way the else could belong to the enclosing if. The
resulting code would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
-Wsequence-point
Warn about code that may have undefined semantics because of violations of sequence point
rules in the C standard.
The C standard defines the order in which expressions in a C program are evaluated in terms
of sequence points, which represent a partial ordering between the execution of parts of the
program: those executed before the sequence point, and those executed after it. These occur
after the evaluation of a full expression (one which is not part of a larger expression), after the
evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is
called (but after the evaluation of its arguments and the expression denoting the called function),
and in certain other places. Other than as expressed by the sequence point rules, the order of
evaluation of subexpressions of an expression is not specified. All these rules describe only a
partial order rather than a total order, since, for example, if two functions are called within one
expression with no sequence point between them, the order in which the functions are called is
not specified. However, the standards committee have ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take effect.
Programs whose behavior depends on this have undefined behavior; the C standard specifies that
"Between the previous and next sequence point an object shall have its stored value modified at
most once by the evaluation of an expression. Furthermore, the prior value shall be read only to
determine the value to be stored.". If a program breaks these rules, the results on any particular
implementation are entirely unpredictable.
Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;.
Some more complicated cases are not diagnosed by this option, and it may give an occasional
false positive result, but in general it has been found fairly effective at detecting this sort of
problem in programs.
The present implementation of this option only works for C programs. A future implementation
may also work for C++ programs.
The C standard is worded confusingly, therefore there is some debate over the precise meaning of
the sequence point rules in subtle cases. Links to discussions of the problem, including proposed
formal definitions, may be found on our readings page, at http://gcc.gnu.org/readings.html.
-Wreturn-type
Warn whenever a function is defined with a return-type that defaults to int. Also warn about any
return statement with no return-value in a function whose return-type is not void.
For C++, a function without return type always produces a diagnostic message, even when
-Wno-return-type is specified. The only exceptions are main and functions defined in system
headers.
Chapter 4. GCC Command Options 31
-Wswitch
Warn whenever a switch statement has an index of enumeral type and lacks a case for one or
more of the named codes of that enumeration. (The presence of a default label prevents this
warning.) case labels outside the enumeration range also provoke warnings when this option is
used.
-Wswitch-default
Warn whenever a switch statement does not have a default case.
-Wswitch-enum
Warn whenever a switch statement has an index of enumeral type and lacks a case for one or
more of the named codes of that enumeration. case labels outside the enumeration range also
provoke warnings when this option is used.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the program (trigraphs
within comments are not warned about).
-Wunused-function
Warn whenever a static function is declared but not defined or a non\-inline static function is
unused.
-Wunused-label
Warn whenever a label is declared but not used.
To suppress this warning use the unused attribute (refer to Section 6.33 Specifying Attributes of
Variables).
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the unused attribute (refer to Section 6.33 Specifying Attributes of
Variables).
-Wunused-variable
Warn whenever a local variable or non-constant static variable is unused aside from its declaration
To suppress this warning use the unused attribute (refer to Section 6.33 Specifying Attributes of
Variables).
-Wunused-value
Warn whenever a statement computes a result that is explicitly not used.
To suppress this warning cast the expression to void.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you must either specify -Wextra
-Wunused (note that -Wall implies -Wunused), or separately specify -Wunused-parameter.
32 Chapter 4. GCC Command Options
-Wuninitialized
Warn if an automatic variable is used without first being initialized or if a variable may be clobbered by a setjmp call.
These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing. If you don’t specify -O, you simply won’t get
these warnings.
These warnings occur only for variables that are candidates for register allocation. Therefore,
they do not occur for a variable that is declared volatile, or whose address is taken, or whose
size is other than 1, 2, 4 or 8 bytes. Also, they do not occur for structures, unions or arrays, even
when they are in registers.
Note that there may be no warning about a variable that is used only to compute a value that
itself is never used, because such computations may be deleted by data flow analysis before the
warnings are printed.
These warnings are made optional because GCC is not smart enough to see all the reasons why
the code might be correct despite appearing to have an error. Here is one example of how this
can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of y is always 1, 2 or 3, then x is always initialized, but GCC doesn’t know this. Here
is another common case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because save_y is used only if it is set.
This option also warns when a non-volatile automatic variable might be changed by a call to
longjmp. These warnings as well are possible only in optimizing compilation.
The compiler sees only the calls to setjmp. It cannot know where longjmp will be called; in
fact, a signal handler could call it at any point in the code. As a result, you may get a warning
even when there is in fact no problem because longjmp cannot in fact be called at the place
which would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you use that never return
as noreturn. Refer to Section 6.26 Declaring Attributes of Functions.
-Wunknown-pragmas
Warn when a #pragma directive is encountered which is not understood by GCC. If this command
line option is used, warnings will even be issued for unknown pragmas in system header files.
This is not the case if the warnings were only enabled by the -Wall command line option.
Chapter 4. GCC Command Options 33
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It warns about code which might
break the strict aliasing rules that the compiler is using for optimization. The warning does not
catch all cases, but does attempt to catch the more common pitfalls. It is included in -Wall.
-Wall
All of the above -W options combined. This enables all the warnings about constructions that
some users consider questionable, and that are easy to avoid (or modify to prevent the warning),
even in conjunction with macros. This also enables some language-specific warnings described in
Section 4.5 Options Controlling C++ Dialect and Section 4.6 Options Controlling Objective-C
Dialect.
The following -W... options are not implied by -Wall. Some of them warn about constructions that
users generally do not consider questionable, but which occasionally you might wish to check for;
others warn about constructions that are necessary or hard to avoid in some cases, and there is no
simple way to modify the code to suppress the warning.
-Wextra
(This option used to be called -W. The older name is still supported, but the newer name is more
descriptive.) Print extra warning messages for these events:
• A function can return either with or without a value. (Falling off the end of the function body is
considered returning without a value.) For example, this function would evoke such a warning:
foo (a)
{
if (a
0)
return a;
}
• An expression-statement or the left-hand side of a comma expression contains no side effects.
To suppress the warning, cast the unused expression to void. For example, an expression such
as x[i,j] will cause a warning, but x[(void)i,j] will not.
• An unsigned value is compared against zero with
or
=.
• A comparison like x
=y
=z appears; this is equivalent to (x=y ? 1 : 0)= z, which
is a different interpretation from that of ordinary mathematical notation.
• Storage-class specifiers like static are not the first things in a declaration. According to the
C Standard, this usage is obsolescent.
• The return type of a function has a type qualifier such as const. Such a type qualifier has no ef-
fect, since the value returned by a function is not an lvalue. (But don’t warn about the GNU extension of volatile void return types. That extension will be warned about if -pedantic
is specified.)
• If -Wall or -Wunused is also specified, warn about unused arguments.
• A comparison between signed and unsigned values could produce an incorrect result when
the signed value is converted to unsigned. (But don’t warn if -Wno-sign-compare is also
specified.)
• An aggregate has an initializer which does not initialize all members. For example, the follow-
ing code would cause such a warning, because x.h would be implicitly initialized to zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
• A function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }
34 Chapter 4. GCC Command Options
• An empty body occurs in an if or else statement.
• A pointer is compared against integer zero with
,=,
, or
=.
• A variable might be changed by longjmp or vfork.
• Any of several floating-point events that often indicate errors, such as overflow, underflow,
loss of precision, etc.
• (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
• (C++ only) A non-static reference or non-static const member appears in a class without
constructors.
• (C++ only) Ambiguous virtual bases.
• (C++ only) Subscripting an array which has been declared register.
• (C++ only) Taking the address of a variable which has been declared register.
• (C++ only) A base class is not initialized in a derived class’ copy constructor.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating point division by zero is not
warned about, as it can be a legitimate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems
and would only make the compiler output harder to read. Using this command line option tells
GCC to emit warnings from system headers as if they occurred in user code. However, note that
using -Wall in conjunction with this option will not warn about unknown pragmas in system
headers--for that, -Wunknown-pragmas must also be used.
-Wfloat-equal
Warn if floating point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the programmer) to consider floatingpoint values as approximations to infinitely precise real numbers. If you are doing this, then you
need to compute (by analyzing the code, or in some other way) the maximum or likely maximum
error that the computation introduces, and allow for it when performing comparisons (and when
producing output, but that’s a different problem). In particular, instead of testing for equality, you
would check to see whether the two values have ranges that overlap; and this is done with the
relational operators, so equality comparisons are probably mistaken.
-Wtraditional (C only)
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about
ISO C constructs that have no traditional C equivalent, and/or problematic constructs which
should be avoided.
• Macro parameters that appear within string literals in the macro body. In traditional C macro
replacement takes place within string literals, but does not in ISO C.
• In traditional C, some preprocessor directives did not exist. Traditional preprocessors would
only consider a line to be a directive if the # appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C understands but would ignore be-
cause the # does not appear as the first character on the line. It also suggests you hide directives
like #pragma not understood by traditional C by indenting them. Some traditional implementations would not recognize #elif, so it suggests avoiding it altogether.
Chapter 4. GCC Command Options 35
• A function-like macro that appears without arguments.
• The unary plus operator.
• The U integer constant suffix, or the F or L floating point constant suffixes. (Traditional C
does support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, for example, the _MIN/_MAX macros in
limits.h
. Use of these macros in user code might normally lead to spurious warnings,
however gcc’s integrated preprocessor has enough context to avoid warning in these cases.
• A function declared external in one block and then used after the end of the block.
• A switch statement has an operand of type long.
• A non-static function declaration follows a static one. This construct is not accepted by
some traditional C compilers.
• The ISO type of an integer constant has a different width or signedness from its traditional
type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal
values, which typically represent bit patterns, are not warned about.
• Usage of ISO string concatenation is detected.
• Initialization of automatic aggregates.
• Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
• Initialization of unions. If the initializer is zero, the warning is omitted. This is done under
the assumption that the zero initializer in user code appears conditioned on for example,
__STDC__ to avoid missing initializer warnings and relies on default initialization to zero
in the traditional C case.
• Conversions by prototypes between fixed/floating point values and vice versa. The absence of
these prototypes when compiling with traditional C would cause serious problems. This is a
subset of the possible conversion warnings, for the full set use -Wconversion.
• Use of ISO C style function definitions. This warning intentionally is not issued for prototype
declarations or variadic functions because these ISO C features will appear in your code when
using libiberty’s traditional C compatibility macros, PARAMS and VPARAMS. This warning is
also bypassed for nested functions because that feature is already a gcc extension and thus not
relevant to traditional C compatibility.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wendif-labels
Warn whenever an #else or an #endif are followed by text.
-Wshadow
Warn whenever a local variable shadows another local variable, parameter or global variable or
whenever a built-in function is shadowed.
-Wlarger-than-len
Warn whenever an object of larger than len bytes is defined.
-Wpointer-arith
Warn about anything that depends on the "size of" a function type or of void. GNU C assigns
these types a size of 1, for convenience in calculations with void * pointers and pointers to
functions.
36 Chapter 4. GCC Command Options
-Wbad-function-cast (C only)
Warn whenever a function call is cast to a non-matching type. For example, warn if int
malloc() is cast to anything *.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example,
warn if a const char * is cast to an ordinary char *.
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of the target is increased. For
example, warn if a char * is cast to an int * on machines where integers can only be accessed
at two- or four-byte boundaries.
-Wwrite-strings
When compiling C, give string constants the type const char[length] so that copying the
address of one into a non-const char * pointer will get a warning; when compiling C++, warn
about the deprecated conversion from string constants to char *. These warnings will help you
find at compile time code that can try to write into a string constant, but only if you have been very
careful about using const in declarations and prototypes. Otherwise, it will just be a nuisance;
this is why we did not make -Wall request these warnings.
-Wconversion
Warn if a prototype causes a type conversion that is different from what would happen to the same
argument in the absence of a prototype. This includes conversions of fixed point to floating and
vice versa, and conversions changing the width or signedness of a fixed point argument except
when the same as the default promotion.
Also, warn if a negative integer constant expression is implicitly converted to an unsigned type.
For example, warn about the assignment x = -1 if x is unsigned. But do not warn about explicit
casts like (unsigned) -1.
-Wsign-compare
Warn when a comparison between signed and unsigned values could produce an incorrect result
when the signed value is converted to unsigned. This warning is also enabled by -Wextra; to get
the other warnings of -Wextra without this warning, use -Wextra -Wno-sign-compare.
-Waggregate-return
Warn if any functions that return structures or unions are defined or called. (In languages where
you can return an array, this also elicits a warning.)
-Wstrict-prototypes (C only)
Warn if a function is declared or defined without specifying the argument types. (An old-style
function definition is permitted without a warning if preceded by a declaration which specifies
the argument types.)
-Wmissing-prototypes (C only)
Warn if a global function is defined without a previous prototype declaration. This warning is
issued even if the definition itself provides a prototype. The aim is to detect global functions that
fail to be declared in header files.
Chapter 4. GCC Command Options 37
-Wmissing-declarations (C only)
Warn if a global function is defined without a previous declaration. Do so even if the definition
itself provides a prototype. Use this option to detect global functions that are not declared in
header files.
-Wmissing-noreturn
Warn about functions which might be candidates for attribute noreturn. Note these are only
possible candidates, not absolute ones. Care should be taken to manually verify functions actually
do not ever return before adding the noreturn attribute, otherwise subtle code generation bugs
could be introduced. You will not get a warning for main in hosted C environments.
-Wmissing-format-attribute
If -Wformat is enabled, also warn about functions which might be candidates for format attributes. Note these are only possible candidates, not absolute ones. GCC will guess that format
attributes might be appropriate for any function that calls a function like vprintf or vscanf,
but this might not always be the case, and some functions for which format attributes are appropriate may not be detected. This option has no effect unless -Wformat is enabled (possibly
by -Wall).
-Wno-multichar
Do not warn if a multicharacter constant (’FOOF’) is used. Usually they indicate a typo in the
user’s code, as they have implementation-defined values, and should not be used in portable code.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as deprecated by using the
deprecated attribute. (Refer to Section 6.26 Declaring Attributes of Functions, Section 6.33
Specifying Attributes of Variables, and Section 6.34 Specifying Attributes of Types.)
-Wpacked
Warn if a structure is given the packed attribute, but the packed attribute has no effect on the
layout or size of the structure. Such structures may be mis-aligned for little benefit. For instance,
in this code, the variable f.x in struct bar will be misaligned even though struct bar does
not itself have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpadded
Warn if padding is included in a structure, either to align an element of the structure or to align
the whole structure. Sometimes when this happens it is possible to rearrange the fields of the
structure to reduce the padding and so make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even in cases where multiple
declaration is valid and changes nothing.
-Wnested-externs (C only)
Warn if an extern declaration is encountered within a function.
38 Chapter 4. GCC Command Options
-Wunreachable-code
Warn if the compiler detects that code will never be executed.
This option is intended to warn when the compiler detects that at least a whole line of source
code will never be executed, because some condition is never satisfied or because it is after a
procedure that never returns.
It is possible for this option to produce a warning even though there are circumstances under which part of the affected line can be executed, so care should be taken when removing
apparently-unreachable code.
For instance, when a function is inlined, a warning may mean that the line is unreachable in only
one inlined copy of the function.
This option is not made part of -Wall because in a debugging version of a program there is often
substantial code which checks correct functioning of the program and is, hopefully, unreachable
because the program does work. Another common use of unreachable code is to provide behavior
which is selectable at compile-time.
-Winline
Warn if a function can not be inlined and it was declared as inline.
-Wno-invalid-offsetof (C++ only)
Suppress warnings from applying the offsetof macro to a non-POD type. According to the
1998 ISO C++ standard, applying offsetof to a non-POD type is undefined. In existing C++
implementations, however, offsetof typically gives meaningful results even when applied to
certain kinds of non-POD types. (Such as a simple struct that fails to be a POD type only
by virtue of having a constructor.) This flag is for users who are aware that they are writing
nonportable code and who have deliberately chosen to ignore the warning about it.
The restrictions on offsetof may be relaxed in a future version of the C++ standard.
-Winvalid-pch
Warn if a precompiled header (refer to Section 4.20 Using Precompiled Headers) is found in the
search path but cannot be used.
-Wlong-long
Warn if long long type is used. This is default. To inhibit the warning messages, use
-Wno-long-long. Flags -Wlong-long and -Wno-long-long are taken into account only
when -pedantic flag is used.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning does not generally indicate that
there is anything wrong with your code; it merely indicates that GCC’s optimizers were unable
to handle the code effectively. Often, the problem is that your code is too big or too complex;
GCC will refuse to optimize programs when the optimization itself is likely to take inordinate
amounts of time.
-Werror
Make all warnings into errors.
4.9. Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your program or GCC:
Chapter 4. GCC Command Options 39
-g
Produce debugging information in the operating system’s native format (stabs, COFF, XCOFF,
or DWARF). GDB can work with this debugging information.
On most systems that use stabs format, -g enables use of extra debugging information that only
GDB can use; this extra information makes debugging work better in GDB but will probably make other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff,
-gdwarf-1+, -gdwarf-1, or -gvms (see below).
Unlike most other C compilers, GCC allows you to use -g with -O. The shortcuts taken by
optimized code may occasionally produce surprising results: some variables you declared may
not exist at all; flow of control may briefly move where you did not expect it; some statements
may not be executed because they compute constant results or their values were already at hand;
some statements may execute in different places because they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the
optimizer for programs that might have bugs.
The following options are useful when GCC is generated with the capability for more than one
debugging format.
-ggdb
Produce debugging information for use by GDB. This means to use the most expressive format
available (DWARF 2, stabs, or the native format if neither of those are supported), including
GDB extensions if at all possible.
-gstabs
Produce debugging information in stabs format (if that is supported), without GDB extensions.
This is the format used by DBX on most BSD systems.
-gstabs+
Produce debugging information in stabs format (if that is supported), using GNU extensions
understood only by the GNU debugger (GDB). The use of these extensions is likely to make
other debuggers crash or refuse to read the program.
-gcoff
Produce debugging information in COFF format (if that is supported).
-gxcoff
Produce debugging information in XCOFF format (if that is supported). This is the format used
by the DBX debugger on IBM RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is supported), using GNU extensions
understood only by the GNU debugger (GDB). The use of these extensions is likely to make
other debuggers crash or refuse to read the program, and may cause assemblers other than the
GNU assembler (GAS) to fail with an error.
-gdwarf
Produce debugging information in DWARF version 1 format (if that is supported).
This option is deprecated.
40 Chapter 4. GCC Command Options
-gdwarf+
Produce debugging information in DWARF version 1 format (if that is supported), using GNU
extensions understood only by the GNU debugger (GDB). The use of these extensions is likely
to make other debuggers crash or refuse to read the program.
This option is deprecated.
-gdwarf-2
Produce debugging information in DWARF version 2 format (if that is supported). This is the
format used by DBX on IRIX 6.
-gvms
Produce debugging information in VMS debug format (if that is supported). This is the format
used by DEBUG on VMS systems.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gvmslevel
Request debugging information and also use level to specify how much information. The default level is 2.
Level 1 produces minimal information, enough for making backtraces in parts of the program
that you don’t plan to debug. This includes descriptions of functions and external variables, but
no information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro definitions present in the program.
Some debuggers support macro expansion when you use -g3.
Note that in order to avoid confusion between DWARF1 debug level 2, and DWARF2, neither
-gdwarf nor -gdwarf-2 accept a concatenated debug level. Instead use an additional -glevel
option to change the debug level for DWARF1 or DWARF2.
-feliminate-dwarf2-dups
Compress DWARF2 debugging information by eliminating duplicated information about each
symbol. This option only makes sense when generating DWARF2 debugging information with
-gdwarf-2.
-p
Generate extra code to write profile information suitable for the analysis program prof. You
must use this option when compiling the source files you want data about, and you must also use
it when linking.
-pg
Generate extra code to write profile information suitable for the analysis program gprof. You
must use this option when compiling the source files you want data about, and you must also use
it when linking.
-Q
Makes the compiler print out each function name as it is compiled, and print some statistics about
each pass when it finishes.
Chapter 4. GCC Command Options 41
-ftime-report
Makes the compiler print some statistics about the time consumed by each pass when it finishes.
-fmem-report
Makes the compiler print some statistics about permanent memory allocation when it finishes.
-fprofile-arcs
Add code so that program flow arcs are instrumented. During execution the program records how
many times each branch and call is executed and how many times it is taken or returns. When
the compiled program exits it saves this data to a file called auxname.da for each source file.
The data may be used for profile-directed optimizations (-fbranch-probabilities), or for
test coverage analysis (-ftest-coverage). Each object file’s auxname is generated from the
name of the output file, if explicitly specified and it is not the final executable, otherwise it is the
basename of the source file. In both cases any suffix is removed (for example, foo.da for input
file dir/foo.c, or dir/foo.da for output file specified as -o dir/foo.o).
• Compile the source files with -fprofile-arcs plus optimization and code generation op-
tions. For test coverage analysis, use the additional -ftest-coverage option. You do not
need to profile every source file in a program.
• Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
• Run the program on a representative workload to generate the arc profile information. This
may be repeated any number of times. You can run concurrent instances of your program, and
provided that the file system supports locking, the data files will be correctly updated. Also
fork calls are detected and correctly handled (double counting will not happen).
• For profile-directed optimizations, compile the source files again with the same optimization
and code generation options plus -fbranch-probabilities (refer to Section 4.10 Options
That Control Optimization).
• For test coverage analysis, use gcov to produce human readable information from the .bbg
and .da files. Refer to the gcov documentation for further information.
With -fprofile-arcs, for each function of your program GCC creates a program flow graph,
then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be
instrumented: the compiler adds code to count the number of times that these arcs are executed.
When an arc is the only exit or only entrance to a block, the instrumentation code can be added
to the block; otherwise, a new basic block must be created to hold the instrumentation code.
-ftest-coverage
Produce a graph file that the gcov code-coverage utility (refer to Chapter 10 gcov --a Test
Coverage Program) can use to show program coverage. Each source file’s data file is called
auxname.bbg. Refer to the -fprofile-arcs option above for a description of auxname and
instructions on how to generate test coverage data. Coverage data will match the source files
more closely, if you do not optimize.
-dletters
Says to make debugging dumps during compilation at times specified by letters. This is used
for debugging the compiler. The file names for most of the dumps are made by appending a pass
number and a word to the dumpname. dumpname is generated from the name of the output file,
if explicitly specified and it is not an executable, otherwise it is the basename of the source file.
In both cases any suffix is removed (for example, foo.00.rtl or foo.01.sibling). Here are
the possible letters for use in letters, and their meanings:
A
Annotate the assembler output with miscellaneous debugging information.
42 Chapter 4. GCC Command Options
b
Dump after computing branch probabilities, to file.15.bp.
B
Dump after block reordering, to file.31.bbro.
c
Dump after instruction combination, to the file file.21.combine.
C
Dump after the first if conversion, to the file file.16.ce1.
d
Dump after delayed branch scheduling, to file.36.dbr.
D
Dump all macro definitions, at the end of preprocessing, in addition to normal output.
e
Dump after SSA optimizations, to file.04.ssa and file.07.ussa.
E
Dump after the second if conversion, to file.32.ce3.
f
Dump after life analysis, to file.20.life.
F
Dump after purging ADDRESSOF codes, to file.10.addressof.
g
Dump after global register allocation, to file.26.greg.
h
Dump after finalization of EH handling code, to file.02.eh.
k
Dump after reg-to-stack conversion, to file.34.stack.
o
Dump after post-reload optimizations, to file.27.postreload.
G
Dump after GCSE, to file.11.gcse.
i
Dump after sibling call optimizations, to file.01.sibling.
j
Dump after the first jump optimization, to file.03.jump.
Chapter 4. GCC Command Options 43
k
Dump after conversion from registers to stack, to file.34.stack.
l
Dump after local register allocation, to file.25.lreg.
L
Dump after loop optimization passes, to file.12.loop and file.18.loop2.
M
Dump after performing the machine dependent reorganization pass, to file.35.mach.
n
Dump after register renumbering, to file.30.rnreg.
N
Dump after the register move pass, to file.23.regmove.
r
Dump after RTL generation, to file.00.rtl.
R
Dump after the second scheduling pass, to file.33.sched2.
s
Dump after CSE (including the jump optimization that sometimes follows CSE), to
file.09.cse.
S
Dump after the first scheduling pass, to file.24.sched.
t
Dump after the second CSE pass (including the jump optimization that sometimes follows
CSE), to file.19.cse2.
u
Dump after null pointer elimination pass to file.08.null.
w
Dump after the second flow pass, to file.28.flow2.
X
Dump after SSA dead code elimination, to file.06.ssadce.
z
Dump after the peephole pass, to file.29.peephole2.
a
Produce all the dumps listed above.
44 Chapter 4. GCC Command Options
H
Produce a core dump whenever an error occurs.
m
Print statistics on memory usage, at the end of the run, to standard error.
p
Annotate the assembler output with a comment indicating which pattern and alternative was
used. The length of each instruction is also printed.
P
Dump the RTL in the assembler output as a comment before each instruction. Also turns on
-dp annotation.
v
For each of the other indicated dump files (except for file.00.rtl), dump a representation
of the control flow graph suitable for viewing with VCG to file.pass.vcg.
x
Just generate RTL for a function instead of compiling it. Usually used with r.
y
Dump debugging information during parsing, to standard error.
-fdump-unnumbered
When doing debugging dumps (see -d option above), suppress instruction numbers and line
number note output. This makes it more feasible to use diff on debugging dumps for compiler
invocations with different options, in particular with and without -g.
-fdump-translation-unit (C and C++ only)
-fdump-translation-unit-options (C and C++ only)
Dump a representation of the tree structure for the entire translation unit to a file. The file name is
made by appending .tu to the source file name. If the -options form is used, options controls
the details of the dump as described for the -fdump-tree options.
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
Dump a representation of each class’s hierarchy and virtual function table layout to a file. The
file name is made by appending .class to the source file name. If the -options form is used,
options controls the details of the dump as described for the -fdump-tree options.
-fdump-tree-switch (C++ only)
-fdump-tree-switch-options (C++ only)
Control the dumping at various stages of processing the intermediate language tree to a file.
The file name is generated by appending a switch specific suffix to the source file name. If
the -options form is used, options is a list of - separated options that control the details of
the dump. Not all options are applicable to all dumps, those which are not meaningful will be
ignored. The following options are available
Chapter 4. GCC Command Options 45
address
Print the address of each node. Usually this is not meaningful as it changes according to
the environment and source file. Its primary use is for tying up a dump file with a debug
environment.
slim
Inhibit dumping of members of a scope or body of a function merely because that scope has
been reached. Only dump such items when they are directly reachable by some other path.
all
Turn on all options.
The following tree dumps are possible:
original
Dump before any tree based optimization, to file.original.
optimized
Dump after all tree based optimization, to file.optimized.
inlined
Dump after function inlining, to file.inlined.
-frandom-seed=string
This option provides a seed that GCC uses when it would otherwise use random numbers. At
present, this is used to generate certain symbol names that have to be different in every compiled
file.
The string should be different for every file you compile.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls the amount of debugging output
the scheduler prints. This information is written to standard error, unless -dS or -dR is specified, in which case it is output to the usual dump listing file, .sched or .sched2 respectively.
However for n greater than nine, the output is always printed to standard error.
For n greater than zero, -fsched-verbose outputs the same information as -dRS. For n greater
than one, it also output basic block probabilities, detailed ready list information and unit/insn
info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And
for n over four, -fsched-verbose also includes dependence info.
-save-temps
Store the usual "temporary" intermediate files permanently; place them in the current directory
and name them based on the source file. Thus, compiling foo.c with -c -save-temps would
produce files foo.i and foo.s, as well as foo.o. This creates a preprocessed foo.i output file
even though the compiler now normally uses an integrated preprocessor.
-time
Report the CPU time taken by each subprocess in the compilation sequence. For C source files,
this is the compiler proper and assembler (plus the linker if linking is done). The output looks
like this:
# cc1 0.12 0.01
46 Chapter 4. GCC Command Options
# as 0.00 0.01
The first number on each line is the "user time," that is time spent executing the program itself.
The second number is "system time," time spent executing operating system routines on behalf
of the program. Both numbers are in seconds.
-print-file-name=library
Print the full absolute name of the library file library that would be used when linking--and
don’t do anything else. With this option, GCC does not compile or link anything; it just prints
the file name.
-print-multi-directory
Print the directory name corresponding to the multilib selected by any other switches present in
the command line. This directory is supposed to exist in GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler switches that enable them. The
directory name is separated from the switches by ;, and each switch starts with an @ instead of
the -, without spaces between multiple switches. This is supposed to ease shell-processing.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with
libgcc.a. You can do
gcc -nostdlib files... ‘gcc -print-libgcc-file-name‘
-print-search-dirs
Print the name of the configured installation directory and a list of program and library directo-
ries gcc will search--and don’t do anything else.
This is useful when gcc prints the error message installation problem, cannot exec
cpp0: No such file or directory. To resolve this you either need to put cpp0 and the
other compiler components where gcc expects to find them, or you can set the environment
variable GCC_EXEC_PREFIX to the directory where you installed them. Don’t forget the trailing
’/’. Section 4.19 Environment Variables Affecting GCC.
-dumpmachine
Print the compiler’s target machine (for example, i686-pc-linux-gnu)--and don’t do anything else.
-dumpversion
Print the compiler version (for example, 3.0)--and don’t do anything else.
-dumpspecs
Print the compiler’s built-in specs--and don’t do anything else. (This is used when GCC itself is
being built.. Refer to Section 4.15 Specifying subprocesses and the switches to pass to them.
-feliminate-unused-debug-types
Normally, when producing DWARF2 output, GCC will emit debugging information for all types
declared in a compilation unit, regardless of whether or not they are actually used in that compi-
Chapter 4. GCC Command Options 47
lation unit. Sometimes this is useful, such as if, in the debugger, you want to cast a value to a type
that is not actually used in your program (but is declared). More often, however, this results in a
significant amount of wasted space. With this option, GCC will avoid producing debug symbol
output for types that are nowhere used in the source file being compiled.
4.10. Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler’s goal is to reduce the cost of compilation and to make
debugging produce the expected results. Statements are independent: if you stop the program with a
breakpoint between statements, you can then assign a new value to any variable or change the program
counter to any other statement in the function and get exactly the results you would expect from the
source code.
Turning on optimization flags makes the compiler attempt to improve the performance and/or code
size at the expense of compilation time and possibly the ability to debug the program.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed.
-O
-O1
Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a
large function.
With -O, the compiler tries to reduce code size and execution time, without performing any
optimizations that take a great deal of compilation time.
-O turns on the following optimization flags:
-fdefer-pop
-fmerge-constants
-fthread-jumps
-floop-optimize
-fcrossjumping
-fif-conversion
-fif-conversion2
-fdelayed-branch
-fguess-branch-probability
-fcprop-registers
-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with
debugging.
-O2
Optimize even more. GCC performs nearly all supported optimizations that do not involve a
space-speed tradeoff. The compiler does not perform loop unrolling or function inlining when
you specify -O2. As compared to -O, this option increases both compilation time and the performance of the generated code.
-O2 turns on all optimization flags specified by -O. It also turns on the following optimization
flags:
-fforce-mem
-foptimize-sibling-calls
-fstrength-reduce
-fcse-follow-jumps -fcse-skip-blocks
-frerun-cse-after-loop -frerun-loop-opt
-fgcse -fgcse-lm -fgcse-sm
-fdelete-null-pointer-checks
-fexpensive-optimizations
48 Chapter 4. GCC Command Options
-fregmove
-fschedule-insns -fschedule-insns2
-fsched-interblock -fsched-spec
-fcaller-saves
-fpeephole2
-freorder-blocks -freorder-functions
-fstrict-aliasing
-falign-functions -falign-jumps
-falign-loops -falign-labels
Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.
-O3
Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the
-finline-functions, -funit-at-a-time and -frename-registers options.
-O0
Do not optimize. This is the default.
-Os
Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It
also performs further optimizations designed to reduce code size.
-Os disables the following optimization flags:
-falign-functions -falign-jumps -falign-loops
-falign-labels -freorder-blocks -fprefetch-loop-arrays
If you use multiple -O options, with or without level numbers, the last such option is the one that
is effective.
Options of the form -fflag specify machine-independent flags. Most flags have both positive and
negative forms; the negative form of -ffoo would be -fno-foo. In the table below, only one of the
forms is listed--the one you typically will use. You can figure out the other form by either removing
no- or adding it.
The following options control specific optimizations. They are either activated by -O options or are
related to ones that are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired.
-fno-default-inline
Do not make member functions inline by default merely because they are defined inside the
class scope (C++ only). Otherwise, when you specify -O, member functions defined inside class
scope are compiled inline by default; i.e., you don’t need to add inline in front of the member
function name.
-fno-defer-pop
Always pop the arguments to each function call as soon as that function returns. For machines
which must pop arguments after a function call, the compiler normally lets arguments accumulate
on the stack for several function calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os.
-fforce-mem
Force memory operands to be copied into registers before doing arithmetic on them. This produces better code by making all memory references potential common subexpressions. When
they are not common subexpressions, instruction combination should eliminate the separate
register-load.
Chapter 4. GCC Command Options 49
Enabled at levels -O2, -O3, -Os.
-fforce-addr
Force memory address constants to be copied into registers before doing arithmetic on them.
This may produce better code just as -fforce-mem may.
-fomit-frame-pointer
Don’t keep the frame pointer in a register for functions that don’t need one. This avoids the
instructions to save, set up and restore frame pointers; it also makes an extra register available in
many functions. It also makes debugging impossible on some machines.
On some machines, this flag has no effect, because the standard calling sequence automatically
handles the frame pointer and nothing is saved by pretending it does not exist. The machinedescription macro FRAME_POINTER_REQUIRED controls whether a target machine supports this
flag. .
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-fno-inline
Don’t pay attention to the inline keyword. Normally this option is used to keep the compiler
from expanding any functions inline. Note that if you are not optimizing, no functions can be
expanded inline.
-finline-functions
Integrate all simple functions into their callers. The compiler heuristically decides which functions are simple enough to be worth integrating in this way.
If all calls to a given function are integrated, and the function is declared static, then the
function is normally not output as assembler code in its own right.
Enabled at level -O3.
-finline-limit=n
By default, gcc limits the size of functions that can be inlined. This flag allows the control of
this limit for functions that are explicitly marked as inline (i.e., marked with the inline keyword
or defined within the class definition in c++). n is the size of functions that can be inlined in
number of pseudo instructions (not counting parameter handling). The default value of n is 600.
Increasing this value can result in more inlined code at the cost of compilation time and memory
consumption. Decreasing usually makes the compilation faster and less code will be inlined
(which presumably means slower programs). This option is particularly useful for programs that
use inlining heavily such as those based on recursive templates with C++.
Inlining is actually controlled by a number of parameters, which may be specified individually
by using -param name=value. The -finline-limit=n option sets some of these parameters
as follows:
max-inline-insns
is set to n.
max-inline-insns-single
is set to n/2.
50 Chapter 4. GCC Command Options
max-inline-insns-auto
is set to n/2.
min-inline-insns
is set to 130 or n/4, whichever is smaller.
max-inline-insns-rtl
is set to n.
Using -finline-limit=600 thus results in the default settings for these parameters. See below
for a documentation of the individual parameters controlling inlining.
Note: pseudo instruction represents, in this particular context, an abstract measurement of function’s size. In no way, it represents a count of assembly instructions and as such its exact meaning
might change from one release to an another.
-fkeep-inline-functions
Even if all calls to a given function are integrated, and the function is declared static, nev-
ertheless output a separate run-time callable version of the function. This switch does not affect
extern inline functions.
-fkeep-static-consts
Emit variables declared static const when optimization isn’t turned on, even if the variables
aren’t referenced.
GCC enables this option by default. If you want to force the compiler to check if the
variable was referenced, regardless of whether or not optimization is turned on, use the
-fno-keep-static-consts option.
-fmerge-constants
Attempt to merge identical constants (string constants and floating point constants) across compilation units.
This option is the default for optimized compilation if the assembler and linker support it. Use
-fno-merge-constants to inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-constants this considers for example, even constant initialized arrays or initialized constant variables with integral
or floating point types. Languages like C or C++ require each non-automatic variable to have
distinct location, so using this option will result in non-conforming behavior.
-fnew-ra
Use a graph coloring register allocator. Currently this option is meant for testing, so we are
interested to hear about miscompilations with -fnew-ra.
-fno-branch-count-reg
Do not use "decrement and branch" instructions on a count register, but instead generate a
sequence of instructions that decrement a register, compare it against zero, then branch based
upon the result. This option is only meaningful on architectures that support such instructions,
which include x86, PowerPC, IA-64 and S/390.
Chapter 4. GCC Command Options 51
The default is -fbranch-count-reg, enabled when -fstrength-reduce is enabled.
-fno-function-cse
Do not put function addresses in registers; make each instruction that calls a constant function
contain the function’s address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler output
may be confused by the optimizations performed when this option is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables that are initialized to zero
into BSS. This can save space in the resulting code.
This option turns off this behavior because some programs explicitly rely on variables going to
the data section. E.g., so that the resulting executable can find the beginning of that section and/or
make assumptions based on that.
The default is -fzero-initialized-in-bss.
-fstrength-reduce
Perform the optimizations of loop strength reduction and elimination of iteration variables.
Enabled at levels -O2, -O3, -Os.
-fthread-jumps
Perform optimizations where we check to see if a jump branches to a location where another
comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the
condition is known to be true or false.
Enabled at levels -O, -O2, -O3, -Os.
-fcse-follow-jumps
In common subexpression elimination, scan through jump instructions when the target of the
jump is not reached by any other path. For example, when CSE encounters an if statement with
an else clause, CSE will follow the jump when the condition tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow jumps which conditionally
skip over blocks. When CSE encounters a simple if statement with no else clause,
-fcse-skip-blocks causes CSE to follow the jump around the body of the if.
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations has been performed.
Enabled at levels -O2, -O3, -Os.
-frerun-loop-opt
Run the loop optimizer twice.
Enabled at levels -O2, -O3, -Os.
52 Chapter 4. GCC Command Options
-fgcse
Perform a global common subexpression elimination pass. This pass also performs global con-
stant and copy propagation.
Note: When compiling a program using computed gotos, a GCC extension, you may get better
runtime performance if you disable the global common subexpression elimination pass by adding
-fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression elimination will attempt to move
loads which are only killed by stores into themselves. This allows a loop containing a load/store
sequence to be changed to a load outside the loop, and a copy/store within the loop.
Enabled by default when gcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, A store motion pass is run after global common subexpression
elimination. This pass will attempt to move stores out of loops. When used in conjunction with
-fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and
a store after the loop.
Enabled by default when gcse is enabled.
-floop-optimize
Perform loop optimizations: move constant expressions out of loops, simplify exit test conditions
and optionally do strength-reduction and loop unrolling as well.
Enabled at levels -O, -O2, -O3, -Os.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies equivalent code and save
code size. The resulting code may or may not perform better than without cross-jumping.
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion
Attempt to transform conditional jumps into branch-less equivalents. This include use of conditional moves, min, max, set flags and abs instructions, and some tricks doable by standard
arithmetics. The use of conditional execution on chips where it is available is controlled by
if-conversion2.
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion2
Use conditional execution (where available) to transform conditional jumps into branch-less
equivalents.
Enabled at levels -O, -O2, -O3, -Os.
-fdelete-null-pointer-checks
Use global dataflow analysis to identify and eliminate useless checks for null pointers. The
compiler assumes that dereferencing a null pointer would have halted the program. If a pointer
is checked after it has already been dereferenced, it cannot be null.
Chapter 4. GCC Command Options 53
In some environments, this assumption is not true, and programs can safely dereference null
pointers. Use -fno-delete-null-pointer-checks to disable this optimization for programs
which depend on that behavior.
Enabled at levels -O2, -O3, -Os.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
-foptimize-register-move
-fregmove
Attempt to reassign register numbers in move instructions and as operands of other simple
instructions in order to maximize the amount of register tying. This is especially helpful on
machines with two-operand instructions.
Note -fregmove and -foptimize-register-move are the same optimization.
Enabled at levels -O2, -O3, -Os.
-fdelayed-branch
If supported for the target machine, attempt to reorder instructions to exploit instruction slots
available after delayed branch instructions.
Enabled at levels -O, -O2, -O3, -Os.
-fschedule-insns
If supported for the target machine, attempt to reorder instructions to eliminate execution stalls
due to required data being unavailable. This helps machines that have slow floating point or
memory load instructions by allowing other instructions to be issued until the result of the load
or floating point instruction is required.
Enabled at levels -O2, -O3, -Os.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of instruction scheduling after
register allocation has been done. This is especially useful on machines with a relatively small
number of registers and where memory load instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Don’t schedule instructions across basic blocks. This is normally enabled by default when
scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fno-sched-spec
Don’t allow speculative motion of non-load instructions. This is normally enabled by default
when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-spec-load
Allow speculative motion of some load instructions. This only makes sense when scheduling
before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only makes sense when scheduling
before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
54 Chapter 4. GCC Command Options
-fsched2-use-superblocks
When schedulilng after register allocation, do use superblock scheduling algorithm. Superblock
scheduling allows motion across basic block boundaries resulting on faster schedules. This option
is experimental, as not all machine descriptions used by GCC model the CPU closely enough to
avoid unreliable results from the algorithm.
This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2
or at -O2 or higher.
-fsched2-use-traces
Use -fsched2-use-superblocks algorithm when scheduling after register allocation and
additionally perform code duplication in order to increase the size of superblocks using tracer
pass. See -ftracer for details on trace formation.
This mode should produce faster but significantly longer programs. Also without
-fbranch-probabilities the traces constructed may not match the reality and hurt the
performance. This only makes sense when scheduling after register allocation, i.e. with
-fschedule-insns2 or at -O2 or higher.
-fcaller-saves
Enable values to be allocated in registers that will be clobbered by function calls, by emitting
extra instructions to save and restore the registers around such calls. Such allocation is done only
when it seems to result in better code than would otherwise be produced.
This option is always enabled by default on certain machines, usually those which have no callpreserved registers to use instead.
Enabled at levels -O2, -O3, -Os.
-fmove-all-movables
Forces all invariant computations in loops to be moved outside the loop.
-freduce-all-givs
Forces all general-induction variables in loops to be strength-reduced.
Note: When compiling programs written in Fortran, -fmove-all-movables and
-freduce-all-givs are enabled by default when you use the optimizer.
These options may generate better or worse code; results are highly dependent on the structure
of loops within the source code.
These two options are intended to be removed someday, once they have helped determine the
efficacy of various approaches to improving loop optimizations.
Please let us (mailto:gcc@@gcc.gnu.org and mailto:fortran@@gnu.org) know how use of these
options affects the performance of your production code. We’re very interested in code that runs
slower when these options are enabled.
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The difference between -fno-peephole
and -fno-peephole2 is in how they are implemented in the compiler; some targets use one,
some use the other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
Chapter 4. GCC Command Options 55
-fno-guess-branch-probability
Do not guess branch probabilities using a randomized model.
Sometimes gcc will opt to use a randomized model to guess branch probabilities, when none
are available from either profiling feedback (-fprofile-arcs) or __builtin_expect. This
means that different runs of the compiler on the same program may produce different object
code.
In a hard real-time system, people don’t want different runs of the compiler to produce code that
has different behavior; minimizing non-determinism is of paramount import. This switch allows
users to reduce non-determinism, possibly at the expense of inferior optimization.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce number of taken branches and
improve code locality.
Enabled at levels -O2, -O3, -Os.
-freorder-functions
Reorder basic blocks in the compiled function in order to reduce number of taken branches and
improve code locality. This is implemented by using special subsections text.hot for most
frequently executed functions and text.unlikely for unlikely executed functions. Reordering
is done by the linker so object file format must support named sections and linker must place
them in a reasonable way.
Also profile feedback must be available in to make this option effective. See -fprofile-arcs
for details.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allows the compiler to assume the strictest aliasing rules applicable to the language being compiled. For C (and C++), this activates optimizations based on the type of expressions. In particular, an object of one type is assumed never to reside at the same address as an object of a different
type, unless the types are almost the same. For example, an unsigned int can alias an int,
but not a void* or a double. A character type may alias any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one most recently written to
(called "type-punning") is common. Even with -fstrict-aliasing, type-punning is allowed,
provided the memory is accessed through the union type. So, the code above will work as expected. However, this code might not:
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
56 Chapter 4. GCC Command Options
return *ip;
}
Every language that wishes to perform language-specific alias analysis should define a function
that computes, given an tree node, an alias set for the node. Nodes in different alias sets are not
allowed to alias. For an example, see the C front-end function c_get_alias_set.
Enabled at levels -O2, -O3, -Os.
-falign-functions
-falign-functions=n
Align the start of functions to the next power-of-two greater than n, skipping up to n bytes.
For instance, -falign-functions=32 aligns functions to the next 32-byte boundary, but
-falign-functions=24 would align to the next 32-byte boundary only if this can be done by
skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions
will not be aligned.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
If n is not specified, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-labels
-falign-labels=n
Align all branch targets to a power-of-two boundary, skipping up to n bytes like
-falign-functions. This option can easily make code slower, because it must insert dummy
operations for when the branch target is reached in the usual flow of the code.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their
values are used instead.
If n is not specified, use a machine-dependent default which is very likely to be 1, meaning no
alignment.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
Align loops to a power-of-two boundary, skipping up to n bytes like -falign-functions. The
hope is that the loop will be executed many times, which will make up for any execution of the
dummy operations.
If n is not specified, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
Align branch targets to a power-of-two boundary, for branch targets where the targets can only be
reached by jumping, skipping up to n bytes like -falign-functions. In this case, no dummy
operations need be executed.
If n is not specified, use a machine-dependent default.
Enabled at levels -O2, -O3.
Chapter 4. GCC Command Options 57
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of registers left over after
register allocation. This optimization will most benefit processors with lots of registers. It can,
however, make debugging impossible, since variables will no longer stay in a "home register".
Enabled at levels -O3.
-fno-cprop-registers
After register allocation and post-register allocation instruction splitting, we perform a copy-
propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.
Disabled at levels -O, -O2, -O3, -Os.
The following options control compiler behavior regarding floating point arithmetic. These options
trade off between speed and correctness. All must be specifically enabled.
-ffloat-store
Do not store floating point variables in registers, and inhibit other options that might change
whether a floating point value is taken from a register or memory.
This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have. Similarly for
the x86 architecture. For most programs, the excess precision does only good, but a few programs
rely on the precise definition of IEEE floating point. Use -ffloat-store for such programs,
after modifying them to store all pertinent intermediate computations into variables.
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations, -fno-trapping-math,
-ffinite-math-only and -fno-signaling-nans.
This option causes the preprocessor macro __FAST_MATH__ to be defined.
This option should never be turned on by any -O option since it can result in incorrect output
for programs which depend on an exact implementation of IEEE or ISO rules/specifications for
math functions.
-fno-math-errno
Do not set ERRNO after calling math functions that are executed with a single instruction, e.g.,
sqrt. A program that relies on IEEE exceptions for math error handling may want to use this flag
for speed while maintaining IEEE arithmetic compatibility.
This option should never be turned on by any -O option since it can result in incorrect output
for programs which depend on an exact implementation of IEEE or ISO rules/specifications for
math functions.
The default is -fmath-errno.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume that arguments and results
are valid and (b) may violate IEEE or ANSI standards. When used at link-time, it may include
libraries or startup files that change the default FPU control word or other similar optimizations.
This option should never be turned on by any -O option since it can result in incorrect output
for programs which depend on an exact implementation of IEEE or ISO rules/specifications for
math functions.
The default is -fno-unsafe-math-optimizations.
58 Chapter 4. GCC Command Options
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that arguments and results are not
NaNs or +-Infs.
This option should never be turned on by any -O option since it can result in incorrect output for
programs which depend on an exact implementation of IEEE or ISO rules/specifications.
The default is -fno-finite-math-only.
-fno-trapping-math
Compile code assuming that floating-point operations cannot generate user-visible traps. These
traps include division by zero, overflow, underflow, inexact result and invalid operation. This
option implies -fno-signaling-nans. Setting this option may allow faster code if one relies
on "non-stop" IEEE arithmetic, for example.
This option should never be turned on by any -O option since it can result in incorrect output
for programs which depend on an exact implementation of IEEE or ISO rules/specifications for
math functions.
The default is -ftrapping-math.
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-visible traps during
floating-point operations. Setting this option disables optimizations that may change the number
of exceptions visible with signaling NaNs. This option implies -ftrapping-math.
This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all GCC optimizations
that affect signaling NaN behavior.
-fsingle-precision-constant
Treat floating point constant as single precision constant instead of implicitly converting it to
double precision constant.
The following options control optimizations that may improve performance, but are not enabled by
any -O options. This section includes experimental options that may produce broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs (refer to Section 4.9
Options for Debugging Your Program or GCC), you can compile it a second time using
-fbranch-probabilities, to improve optimizations based on the number of times each
branch was taken. When the program compiled with -fprofile-arcs exits it saves arc
execution counts to a file called sourcename.da for each source file The information in this
data file is very dependent on the structure of the generated code, so you must use the same
source code and the same optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and
CALL_INSN. These can be used to improve optimization. Currently, they are only used in one
place: in reorg.c, instead of guessing which path a branch is mostly to take, the REG_BR_PROB
values are used to exactly determine which path is taken more often.
-fnew-ra
Use a graph coloring register allocator. Currently this option is meant for testing, so we are
interested to hear about miscompilations with -fnew-ra.
Chapter 4. GCC Command Options 59
-ftracer
Perform tail duplication to enlarge superblock size. This transformation simplifies the control
flow of the function allowing other optimizations to do better job.
-funit-at-a-time
Parse the whole compilation unit before starting to produce code. This allows some extra opti-
mizations to take place but consumes more memory.
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the
loop. -funroll-loops implies -frerun-cse-after-loop. It also turns on complete loop
peeling (i.e. complete removal of loops with small constant number of iterations). This option
makes code larger, and may or may not make it run faster.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This
usually makes programs run more slowly. -funroll-all-loops implies the same options as
-funroll-loops.
-fpeel-loops
Peels the loops for that there is enough information that they do not roll much (from profile
feedback). It also turns on complete loop peeling (i.e. complete removal of loops with small
constant number of iterations).
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates of the loop on
both branches (modified according to result of the condition).
-fold-unroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to
the loop, using the old loop unroller whose loop recognition is based on notes from frontend.
-fold-unroll-loops implies both -fstrength-reduce and -frerun-cse-after-loop.
This option makes code larger, and may or may not make it run faster.
-fold-unroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This
is done using the old loop unroller whose loop recognition is based on notes from frontend.
This usually makes programs run more slowly. -fold-unroll-all-loops implies the same
options as -fold-unroll-loops.
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates of the loop on
both branches (modified according to result of the condition).
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates of the loop on
both branches (modified according to result of the condition).
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to improve the
performance of loops that access large arrays.
60 Chapter 4. GCC Command Options
Disabled at level -Os.
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output file if the target supports
arbitrary sections. The name of the function or the name of the data item determines the section’s
name in the output file.
Use these options on systems where the linker can perform optimizations to improve locality of
reference in the instruction space. Most systems using the ELF object format and SPARC processors running Solaris 2 have linkers with such optimizations. AIX may have these optimizations
in the future.
Only use these options when there are significant benefits from doing so. When you specify these
options, the assembler and linker will create larger object and executable files and will also be
slower. You will not be able to use gprof on all systems if you specify this option and you may
have problems with debugging if you specify both this option and -g.
-fssa
Perform optimizations in static single assignment form. Each function’s flow graph is translated
into SSA form, optimizations are performed, and the flow graph is translated back from SSA
form. Users should not specify this option, since it is not yet ready for production use.
-fssa-ccp
Perform Sparse Conditional Constant Propagation in SSA form. Requires -fssa. Like -fssa,
this is an experimental feature.
-fssa-dce
Perform aggressive dead-code elimination in SSA form. Requires -fssa. Like -fssa, this is an
experimental feature.
-param name=value
In some places, GCC uses various constants to control the amount of optimization that is done.
For example, GCC will not inline functions that contain more that a certain number of instructions. You can control some of these constants on the command-line using the -param option.
In each case, the value is an integer. The allowable choices for name are given in the following
table:
max-crossjump-edges
The maximum number of incoming edges to consider for crossjumping. The algorithm used
by -fcrossjumping is O(N^2) in the number of edges incoming to each block. Increasing
values mean more aggressive optimization, making the compile time increase with probably
small improvement in executable size.
max-delay-slot-insn-search
The maximum number of instructions to consider when looking for an instruction to fill a
delay slot. If more than this arbitrary number of instructions is searched, the time savings
from filling the delay slot will be minimal so stop searching. Increasing values mean more
aggressive optimization, making the compile time increase with probably small improvement in executable run time.
Chapter 4. GCC Command Options 61
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of instructions to consider when
searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compile time. This parameter
should be removed when the delay slot code is rewritten to maintain the control-flow graph.
max-gcse-memory
The approximate maximum amount of memory that will be allocated in order to perform
the global common subexpression elimination optimization. If more memory than specified
is required, the optimization will not be done.
max-gcse-passes
The maximum number of passes of GCSE to run.
max-pending-list-length
The maximum number of pending dependencies scheduling will allow before flushing the
current state and starting over. Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources.
max-inline-insns-single
Several parameters control the tree inliner used in gcc. This number sets the maximum
number of instructions (counted in gcc’s internal representation) in a single function that
the tree inliner will consider for inlining. This only affects functions declared inline and
methods implemented in a class declaration (C++). The default value is 300.
max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot of functions that would
otherwise not be considered for inlining by the compiler will be investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied. The default value is 300.
max-inline-insns
The tree inliner does decrease the allowable size for single functions to be inlined after we
already inlined the number of instructions given here by repeated inlining. This number
should be a factor of two or more larger than the single function limit. Higher numbers
result in better runtime performance, but incur higher compile-time resource (CPU time,
memory) requirements and result in larger binaries. Very high values are not advisable, as
too large binaries may adversely affect runtime performance. The default value is 600.
max-inline-slope
After exceeding the maximum number of inlined instructions by repeated inlining, a linear
function is used to decrease the allowable size for single functions. The slope of that function
is the negative reciprocal of the number specified here. The default value is 32.
min-inline-insns
The repeated inlining is throttled more and more by the linear function after exceeding the
limit. To avoid too much throttling, a minimum for this function is specified here to allow
repeated inlining for very small functions even when a lot of repeated inlining already has
been done. The default value is 130.
62 Chapter 4. GCC Command Options
max-inline-insns-rtl
For languages that use the RTL inliner (this happens at a later stage than tree inlining), you
can set the maximum allowable size (counted in RTL instructions) for the RTL inliner with
this parameter. The default value is 600.
max-unrolled-insns
The maximum number of instructions that a loop should have if that loop is unrolled, and if
the loop is unrolled, it determines how many times the loop code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by probabilities of their execution that a loop
should have if that loop is unrolled, and if the loop is unrolled, it determines how many
times the loop code is unrolled.
max-unroll-times
The maximum number of unrollings of a single loop.
max-peeled-insns
The maximum number of instructions that a loop should have if that loop is peeled, and if
the loop is peeled, it determines how many times the loop code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.
max-completely-peel-times
The maximum number of iterations of a loop to be suitable for complete peeling.
max-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-level
The maximum number of branches unswitched in a single loop.
hot-bb-count-fraction
Select fraction of the maximal count of repetitions of basic block in program given basic
block needs to have to be considered hot.
hot-bb-frequency-fraction
Select fraction of the maximal frequency of executions of basic block in function given
basic block needs to have to be considered hot
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given percentage of executed
instructions is covered. This limits unnecessary code size expansion.
The tracer-dynamic-coverage-feedback is used only when profile feedback is available. The real profiles (as opposed to statically estimated ones) are much less balanced
allowing the threshold to be larger value.
Chapter 4. GCC Command Options 63
tracer-max-code-growth
Stop tail duplication once code growth has reached given percentage. This is rather hokey
argument, as most of the duplicates will be eliminated later in cross jumping, so it may be
set to much higher values than is the desired code growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge is less than this threshold (in
percent).
tracer-min-branch-ratio
tracer-min-branch-ratio-feedback
Stop forward growth if the best edge do have probability lower than this threshold.
Similarly to tracer-dynamic-coverage two values are present, one for compilation for
profile feedback and one for compilation without. The value for compilation with profile
feedback needs to be more conservative (higher) in order to make tracer effective.
max-cse-path-length
Maximum number of basic blocks on path that cse considers.
ggc-min-expand
GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum percentage by which the garbage collector’s heap should be allowed to
expand between collections. Tuning this may improve compilation speed; it has no effect
on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM=
1GB. If getrlimit is available, the notion of "RAM" is the smallest of actual RAM,
RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. If GCC is not able to calculate RAM
on a particular platform, the lower bound of 30% is used. Setting this parameter and
ggc-min-heapsize to zero causes a full collection to occur at every opportunity. This is
extremely slow, but can be useful for debugging.
ggc-min-heapsize
Minimum size of the garbage collector’s heap before it begins bothering to collect garbage.
The first collection occurs after the heap expands by ggc-min-expand% beyond
ggc-min-heapsize. Again, tuning this may improve compilation speed, and has no
effect on code generation.
The default is RAM/8, with a lower bound of 4096 (four megabytes) and an upper bound of
131072 (128 megabytes). If getrlimit is available, the notion of "RAM" is the smallest
of actual RAM, RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. If GCC is not able to
calculate RAM on a particular platform, the lower bound is used. Setting this parameter very
large effectively disables garbage collection. Setting this parameter and ggc-min-expand
to zero causes a full collection to occur at every opportunity.
reorder-blocks-duplicate
reorder-blocks-duplicate-feedback
Used by basic block reordering pass to decide whether to use unconditional branch or duplicate the code on it is destination. Code is duplicated when it is estimated size is smaller
than this value multiplied by the estimated size of unconditional jump in the hot spots of the
program.
The reorder-block-duplicate-feedback is used only when profile feedback is available and may be set to higher values than reorder-block-duplicate since information
about the hot spots is more accurate.
64 Chapter 4. GCC Command Options
4.11. Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source file before actual compilation.
If you use the -E option, nothing is done except preprocessing. Some of these options make sense only
together with -E because they cause the preprocessor output to be unsuitable for actual compilation.
You can use -Wp,option to bypass the compiler driver and pass option
directly through to the preprocessor. If option contains commas, it is split
into multiple options at the commas. However, many options are modified,
translated or interpreted by the compiler driver before being passed to the
preprocessor, and -Wp forcibly bypasses this phase. The preprocessor’s
direct interface is undocumented and subject to change, so whenever
possible you should avoid using -Wp and let the driver handle the options
instead.
-Xpreprocessor option
Pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options which GCC does not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xpreprocessor twice,
once for the option and once for the argument.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
Predefine name as a macro, with definition definition. There are no restrictions on the contents of definition, but if you are invoking the preprocessor from a shell or shell-like program
you may need to use the shell’s quoting syntax to protect characters such as spaces that have a
meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write its argument list with
surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most
shells, so you will need to quote the option. With sh and csh, -D’name(args...)=definition’
works.
-D and -U options are processed in the order they are given on the command line. All -imacros
file and -include file options are processed after all -D and -U options.
-U name
Cancel any previous definition of name, either built in or provided with a -D option.
-undef
Do not predefine any system-specific or GCC-specific macros. The standard predefined macros
remain defined.
-I dir
Add the directory dir to the list of directories to be searched for header files. Directories named
by -I are searched before the standard system include directories. If the directory dir is a standard system include directory, the option is ignored to ensure that the default search order for
system directories and the special treatment of system headers are not defeated .
Chapter 4. GCC Command Options 65
-o file
Write output to file. This is the same as specifying file as the second non-option argument
to cpp. gcc has a different interpretation of a second non-option argument, so you must use -o
to specify the output file.
-Wall
Turns on all optional warnings which are desirable for normal code. At present this is
-Wcomment, -Wtrigraphs, -Wmultichar and a warning about integer promotion causing a
change of sign in #if expressions. Note that many of the preprocessor’s warnings are on by
default and have no options to control them.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a
backslash-newline appears in a // comment. (Both forms have the same effect.)
-Wtrigraphs
Most trigraphs in comments cannot affect the meaning of the program. However, a trigraph that
would form an escaped newline (??/ at the end of a line) can, by changing where the comment
begins or ends. Therefore, only trigraphs that would form escaped newlines produce warnings
inside a comment.
This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs
are enabled. To get trigraph conversion without warnings, but get the other -Wall warnings, use
-trigraphs -Wall -Wno-trigraphs.
-Wtraditional
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about
ISO C constructs that have no traditional C equivalent, and problematic constructs which should
be avoided.
-Wimport
Warn the first time #import is used.
-Wundef
Warn whenever an identifier which is not a macro is encountered in an #if directive, outside of
defined. Such identifiers are replaced with zero.
-Wunused-macros
Warn about macros defined in the main file that are unused. A macro is used if it is expanded or
tested for existence at least once. The preprocessor will also warn if the macro has not been used
at the time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros defined in include files are
not warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will
report it as unused. To avoid the warning in such a case, you might improve the scope of the
macro’s definition by, for example, moving it into the first skipped block. Alternatively, you
could provide a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif
66 Chapter 4. GCC Command Options
-Wendif-labels
Warn whenever an #else or an #endif are followed by text. This usually happens in code of
the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third FOO should be in comments, but often are not in older programs. This
warning is on by default.
-Werror
Make all warnings into hard errors. Source code which triggers warnings will be rejected.
-Wsystem-headers
Issue warnings for code in system headers. These are normally unhelpful in finding bugs in your
own code, therefore suppressed. If you are responsible for the system library, you may want to
see them.
-w
Suppress all warnings, including those which GNU CPP issues by default.
-pedantic
Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by default,
since they trigger frequently on harmless code.
-pedantic-errors
Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors. This in-
cludes mandatory diagnostics that GCC issues without -pedantic but treats as warnings.
-M
Instead of outputting the result of preprocessing, output a rule suitable for make describing the
dependencies of the main source file. The preprocessor outputs one make rule containing the
object file name for that source file, a colon, and the names of all the included files, including
those coming from -include or -imacros command line options.
Unless specified explicitly (with -MT or -MQ), the object file name consists of the basename of
the source file with any suffix replaced with object file suffix. If there are many included files
then the rule is split into several lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor’s debug output, such as -dM. To avoid mixing such
debug output with the dependency rules you should explicitly specify the dependency output file
with -MF, or use an environment variable like DEPENDENCIES_OUTPUT (refer to Section 4.19
Environment Variables Affecting GCC). Debug output will still be sent to the regular output
stream as normal.
Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.
-MM
Like -M but do not mention header files that are found in system header directories, nor header
files that are included, directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an #include directive does
not in itself determine whether that header will appear in -MM dependency output. This is a slight
change in semantics from GCC versions 3.0 and earlier.
Chapter 4. GCC Command Options 67
-MF file
When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is
given the preprocessor sends the rules to the same place it would have sent preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default dependency output
file.
-MG
In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error.
The dependency filename is taken directly from the #include directive without prepending any
path. -MG also suppresses preprocessed output, as a missing header file renders this useless.
This feature is used in automatic updating of makefiles.
-MP
This option instructs CPP to add a phony target for each dependency other than the main file,
causing each to depend on nothing. These dummy rules work around errors make gives if you
remove header files without updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency generation. By default CPP takes the name
of the main input file, including any path, deletes any file suffix such as .c, and appends the
platform’s usual object suffix. The result is the target.
An -MT option will set the target to be exactly the string you specify. If you want multiple targets,
you can specify them as a single argument to -MT, or use multiple -MT options.
For example, -MT ’$(objpfx)foo.o’ might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to Make. -MQ ’$(objpfx)foo.o’
gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with -MQ.
-MD
-MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file
based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of
.d, otherwise it take the basename of the input file and applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency
output file, but if used without -E, each -o is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate a dependency output file as a side-effect of
the compilation process.
-MMD
Like -MD except mention only user header files, not system -header files.
68 Chapter 4. GCC Command Options
-fpch-deps
When using precompiled headers (refer to Section 4.20 Using Precompiled Headers), this flag
will cause the dependency-output flags to also list the files from the precompiled header’s dependencies. If not specified only the precompiled header would be listed and not the files that were
used to create it because those files are not consulted when a precompiled header is used.
-x c
-x c++
-x objective-c
-x assembler-with-cpp
Specify the source language: C, C++, Objective-C, or assembly. This has nothing to do with
standards conformance or extensions; it merely selects which base syntax to expect. If you give
none of these options, cpp will deduce the language from the extension of the source file: .c,
.cc, .m, or .S. Some other common extensions for C++ and assembly are also recognized. If
cpp does not recognize the extension, it will treat the file as C; this is the most generic mode.
Note: Previous versions of cpp accepted a -lang option which selected both the language and
the standards conformance level. This option has been removed, because it conflicts with the -l
option.
-std=standard
-ansi
Specify the standard to which the code should conform. Currently CPP knows about C and C++
standards; others may be added in the future.
standard may be one of:
iso9899:1990
c89
The ISO C standard from 1990. c89 is the customary shorthand for this version of the
standard.
The -ansi option is equivalent to -std=c89.
iso9899:199409
The 1990 C standard, as amended in 1994.
iso9899:1999
c99
iso9899:199x
c9x
The revised ISO C standard, published in December 1999. Before publication, this was
known as C9X.
gnu89
The 1990 C standard plus GNU extensions. This is the default.
gnu99
gnu9x
The 1999 C standard plus GNU extensions.
c++98
The 1998 ISO C++ standard plus amendments.
Chapter 4. GCC Command Options 69
gnu++98
The same as -std=c++98 plus GNU extensions. This is the default for C++ code.
-I-
Split the include path. Any directories specified with -I options before -I- are searched only
for headers requested with #include "file"; they are not searched for #include
file
.
If additional directories are specified with -I options after the -I-, those directories are searched
for all #include directives.
In addition, -I- inhibits the use of the directory of the current file directory as the first search
directory for #include "file".
-nostdinc
Do not search the standard system directories for header files. Only the directories you have
specified with -I options (and the directory of the current file, if appropriate) are searched.
-nostdinc++
Do not search for header files in the C++-specific standard directories, but do still search the
other standard directories. (This option is used when building the C++ library.)
-include file
Process file as if #include "file" appeared as the first line of the primary source file.
However, the first directory searched for file is the preprocessor’s working directory instead of
the directory containing the main source file. If not found there, it is searched for in the remainder
of the #include "..." search chain as normal.
If multiple -include options are given, the files are included in the order they appear on the
command line.
-imacros file
Exactly like -include, except that any output produced by scanning file is thrown away.
Macros it defines remain defined. This allows you to acquire all the macros from a header without
also processing its declarations.
All files specified by -imacros are processed before all files specified by -include.
-idirafter dir
Search dir for header files, but do it after all directories specified with -I and the standard
system directories have been exhausted. dir is treated as a system include directory.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a
directory, you should include the final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. -iwithprefixbefore puts it in the same place -I would;
-iwithprefix puts it where -idirafter would.
Use of these options is discouraged.
70 Chapter 4. GCC Command Options
-isystem dir
Search dir for header files, after all directories specified by -I but before the standard system
directories. Mark it as a system directory, so that it gets the same special treatment as is applied
to the standard system directories.
-fdollars-in-identifiers
Accept $ in identifiers.
-fpreprocessed
Indicate to the preprocessor that the input file has already been preprocessed. This suppresses
things like macro expansion, trigraph conversion, escaped newline splicing, and processing of
most directives. The preprocessor still recognizes and removes comments, so that you can pass a
file preprocessed with -C to the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These
are the extensions that GCC uses for preprocessed files created by -save-temps.
-ftabstop=width
Set the distance between tab stops. This helps the preprocessor report correct column numbers
in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than
100, the option is ignored. The default is 8.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary if diagnostics are being
scanned by a program that does not understand the column numbers, such as dejagnu.
-A predicate=answer
Make an assertion with the predicate predicate and answer answer. This form is preferred
to the older form -A predicate(answer), which is still supported, because it does not use shell
special characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer answer.
-dCHARS
CHARS is a sequence of one or more of the following characters, and must not be preceded by a
space. Other characters are interpreted by the compiler proper, or reserved for future versions of
GCC, and so are silently ignored. If you specify characters whose behavior conflicts, the result
is undefined.
M
Instead of the normal output, generate a list of #define directives for all the macros defined
during the execution of the preprocessor, including predefined macros. This gives you a way
of finding out what is predefined in your version of the preprocessor. Assuming you have
no file foo.h, the command
touch foo.h; cpp -dM foo.h
will show all the predefined macros.
Chapter 4. GCC Command Options 71
D
Like M except in two respects: it does not include the predefined macros, and it outputs
both the #define directives and the result of preprocessing. Both kinds of output go to the
standard output file.
N
Like D, but emit only the macro names, not their expansions.
I
Output #include directives in addition to the result of preprocessing.
-P
Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when
running the preprocessor on something that is not C code, and will be sent to a program which
might be confused by the linemarkers.
-C
Do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.
You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would
be a directive line have the effect of turning that line into an ordinary source line, since the first
token on the line is no longer a #.
-CC
Do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is
expanded.
In addition to the side-effects of the -C option, the -CC option causes all C++-style comments
inside a macro to be converted to C-style comments. This is to prevent later use of that macro
from inadvertently commenting out the remainder of the source line.
The -CC option is generally used to support lint comments.
-traditional-cpp
Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors.
-trigraphs
Process trigraph sequences. These are three-character sequences, all starting with ??, that are
defined by ISO C to stand for single characters. For example, ??/ stands for \, so ’??/n’ is a
character constant for a newline. By default, GCC ignores trigraphs, but in standard-conforming
modes it converts them. See the -std and -ansi options.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??
????= ??/ ??’ ??! ??-
Replacement: [ ] { } # \ ^ | ~
-remap
Enable special code to work around file systems which only permit very short file names, such
as MS-DOS.
72 Chapter 4. GCC Command Options
-help
-target-help
Print text describing all the command line options instead of preprocessing anything.
-v
Verbose mode. Print out GNU CPP’s version number at the beginning of execution, and report
the final form of the include path.
-H
Print the name of each header file used, in addition to other normal activities. Each name is
indented to show how deep in the #include stack it is. Precompiled header files are also printed,
even if they are found to be invalid; an invalid precompiled header file is printed with ...x and
a valid one with ...! .
-version
-version
Print out GNU CPP’s version number. With one dash, proceed to preprocess as normal. With
two dashes, exit immediately.
4.12. Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option contains commas, it is split into multiple
options at the commas.
-Xassembler option
Pass option as an option to the assembler. You can use this to supply system-specific assembler
options which GCC does not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xassembler twice, once
for the option and once for the argument.
4.13. Options for Linking
These options come into play when the compiler links object files into an executable output file. They
are meaningless if the compiler is not doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is considered to name an object file or
library. (Object files are distinguished from libraries by the linker according to the file contents.)
If linking is done, these object files are used as input to the linker.
-c
-S
-E
If any of these options is used, then the linker is not run, and object file names should not be
used as arguments. Refer to Section 4.2 Options Controlling the Kind of Output.
Chapter 4. GCC Command Options 73
-llibrary
-l library
Search the library named library when linking. (The second alternative with the library as a
separate argument is only for POSIX compliance and is not recommended.)
It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches
library z after file foo.o but before bar.o. If bar.o refers to functions in z, those functions
may not be loaded.
The linker searches a standard list of directories for the library, which is actually a file named
liblibrary.a. The linker then uses this file as if it had been specified precisely by name.
The directories searched include several standard system directories plus any that you specify
with -L.
Normally the files found this way are library files--archive files whose members are object files.
The linker handles an archive file by scanning through it for members which define symbols that
have so far been referenced but not defined. But if the file that is found is an ordinary object file,
it is linked in the usual fashion. The only difference between using an -l option and specifying
a file name is that -l surrounds library with lib and .a and searches several directories.
-lobjc
You need this special case of the -l option in order to link an Objective-C program.
-nostartfiles
Do not use the standard system startup files when linking. The standard system libraries are used
normally, unless -nostdlib or -nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the libraries you specify will be
passed to the linker. The standard startup files are used normally, unless -nostartfiles is
used. The compiler may generate calls to bcopy and bzero for BSD environments. These entries
are usually resolved by entries in libc. These entry points should be supplied through some other
mechanism when this option is specified.
-nostdlib
Do not use the standard system startup files or libraries when linking. No startup files and only
the libraries you specify will be passed to the linker. The compiler may generate calls to bcopy
and bzero for BSD environments. These entries are usually resolved by entries in libc. These
entry points should be supplied through some other mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a
library of internal subroutines that GCC uses to overcome shortcomings of particular machines,
or special needs for some languages. (, for more discussion of libgcc.a.) In most cases, you
need libgcc.a even when you want to avoid other standard libraries. In other words, when you
specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines. (For example,
__main, used to ensure C++ constructors will be called; .)
-pie
Produce a position independent executable on targets which support it. For predictable results,
you must also specify the same set of options that were used to generate code (-fpie, -fPIE,
or model suboptions) when you specify this option.
74 Chapter 4. GCC Command Options
-s
Remove all symbol table and relocation information from the executable.
-static
On systems that support dynamic linking, this prevents linking with the shared libraries. On
other systems, this option has no effect.
-shared
Produce a shared object which can then be linked with other objects to form an executable. Not
all systems support this option. For predictable results, you must also specify the same set of
options that were used to generate code (-fpic, -fPIC, or model suboptions) when you specify
this option.
1
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these options force the use of either the
shared or static version respectively. If no shared version of libgcc was built when the compiler
was configured, these options have no effect.
There are several situations in which an application should use the shared libgcc instead of
the static version. The most common of these is when the application wishes to throw and catch
exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared libgcc.
Therefore, the G++ and GCJ drivers automatically add -shared-libgcc whenever you build a
shared library or a main executable, because C++ and Java programs typically use exceptions, so
this is the right thing to do.
If, instead, you use the GCC driver to create shared libraries, you may find that they will not
always be linked with the shared libgcc. If GCC finds, at its configuration time, that you have
a GNU linker that does not support option -eh-frame-hdr, it will link the shared version of
libgcc into shared libraries by default. Otherwise, it will take advantage of the linker and opti-
mize away the linking with the shared version of libgcc, linking with the static version of libgcc
by default. This allows exceptions to propagate through such shared libraries, without incurring
relocation costs at library load time.
However, if a library or main executable is supposed to throw or catch exceptions, you must link
it using the G++ or GCJ driver, as appropriate for the languages used in the program, or using
the option -shared-libgcc, such that it is linked with the shared libgcc.
-symbolic
Bind references to global symbols when building a shared object. Warn about any unresolved
references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only
a few systems support this option.
-Xlinker option
Pass option as an option to the linker. You can use this to supply system-specific linker options
which GCC does not know how to recognize.
If you want to pass an option that takes an argument, you must use -Xlinker twice, once for the
option and once for the argument. For example, to pass -assert definitions, you must
write -Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker
1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On
multi-libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply
the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous.
Chapter 4. GCC Command Options 75
"-assert definitions", because this passes the entire string as a single argument, which
is not what the linker expects.
-Wl,option
Pass option as an option to the linker. If option contains commas, it is split into multiple
options at the commas.
-u symbol
Pretend the symbol symbol is undefined, to force linking of library modules to define it. You
can use -u multiple times with different symbols to force loading of additional library modules.
4.14. Options for Directory Search
These options specify directories to search for header files, for libraries and for parts of the compiler:
-Idir
Add the directory dir to the head of the list of directories to be searched for header files. This
can be used to override a system header file, substituting your own version, since these directories
are searched before the system header file directories. However, you should not use this option
to add directories that contain vendor-supplied system header files (use -isystem for that). If
you use more than one -I option, the directories are scanned in left-to-right order; the standard
system directories come after.
If a standard system include directory, or a directory specified with -isystem, is also specified
with -I, the -I option will be ignored. The directory will still be searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC’s procedure
to fix buggy system headers and the ordering for the include_next directive are not inadvertently
changed. If you really need to change the search order for system directories, use the -nostdinc
and/or -isystem options.
-I-
Any directories you specify with -I options before the -I- option are searched only for the case
of #include "file"; they are not searched for #include
file
.
If additional directories are specified with -I options after the -I-, these directories are searched
for all #include directives. (Ordinarily all -I directories are used this way.)
In addition, the -I- option inhibits the use of the current directory (where the current input file
came from) as the first search directory for #include "file". There is no way to override this
effect of -I-. With -I. you can specify searching the directory which was current when the
compiler was invoked. That is not exactly the same as what the preprocessor does by default, but
it is often satisfactory.
-I- does not inhibit the use of the standard system directories for header files. Thus, -I- and
-nostdinc are independent.
-Ldir
Add directory dir to the list of directories to be searched for -l.
-Bprefix
This option specifies where to find the executables, libraries, include files, and data files of the
compiler itself.
76 Chapter 4. GCC Command Options
The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries
prefix as a prefix for each program it tries to run, both with and without machine/version/
(refer to Section 4.16 Specifying Target Machine and Compiler Version).
For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that
name is not found, or if -B was not specified, the driver tries two standard prefixes, which are
/usr/lib/gcc/ and /usr/local/lib/gcc-lib/. If neither of those results in a file name
that is found, the unmodified program name is searched for using the directories specified in
your PATH environment variable.
The compiler will check to see if the path provided by the -B refers to a directory, and if necessary
it will add a directory separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to libraries in the linker, because
the compiler translates these options into -L options for the linker. They also apply to includes
files in the preprocessor, because the compiler translates these options into -isystem options
for the preprocessor. In this case, the compiler appends include to the prefix.
The run-time support file libgcc.a can also be searched for using the -B prefix, if needed. If it
is not found there, the two standard prefixes above are tried, and that is all. The file is left out of
the link if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use the environment variable
GCC_EXEC_PREFIX. Refer to Section 4.19 Environment Variables Affecting GCC.
As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the
range 0 to 9, then it will be replaced by [dir/]include. This is to help with boot-strapping the
compiler.
-specs=file
Process file after the compiler reads in the standard specs file, in order to override the defaults
that the gcc driver program uses when determining what switches to pass to cc1, cc1plus,
as, ld, etc. More than one -specs=file can be specified on the command line, and they are
processed in order, from left to right.
4.15. Specifying subprocesses and the switches to pass to them
gcc is a driver program. It performs its job by invoking a sequence of other programs to do the work
of compiling, assembling and linking. GCC interprets its command-line parameters and uses these to
deduce which programs it should invoke, and which command-line options it ought to place on their
command lines. This behavior is controlled by spec strings. In most cases there is one spec string for
each program that GCC can invoke, but a few programs have multiple spec strings to control their
behavior. The spec strings built into GCC can be overridden by using the -specs= command-line
switch to specify a spec file.
Spec files are plaintext files that are used to construct spec strings. They consist of a sequence of
directives separated by blank lines. The type of directive is determined by the first non-whitespace
character on the line and it can be one of the following:
%command
Issues a command to the spec file processor. The commands that can appear here are:
%include
file
Search for file and insert its text at the current point in the specs file.
Chapter 4. GCC Command Options 77
%include_noerr
file
Just like %include, but do not generate an error message if the include file cannot be found.
%rename old_name new_name
Rename the spec string old_name to new_name.
*[spec_name]:
This tells the compiler to create, override or delete the named spec string. All lines after this
directive up to the next directive or blank line are considered to be the text for the spec string. If
this results in an empty string then the spec will be deleted. (Or, if the spec did not exist, then
nothing will happened.) Otherwise, if the spec does not currently exist a new spec will be created.
If the spec does exist then its contents will be overridden by the text of this directive, unless the
first character of that text is the + character, in which case the text will be appended to the spec.
[suffix]:
Creates a new [suffix] spec pair. All lines after this directive and up to the next directive or
blank line are considered to make up the spec string for the indicated suffix. When the compiler
encounters an input file with the named suffix, it will processes the spec string in order to work
out how to compile that file. For example:
.ZZ:
z-compile -input %i
This says that any input file whose name ends in .ZZ should be passed to the program
z-compile, which should be invoked with the command-line switch -input and with the
result of performing the %i substitution. (See below.)
As an alternative to providing a spec string, the text that follows a suffix directive can be one of
the following:
@language
This says that the suffix is an alias for a known language. This is similar to using the -x
command-line switch to GCC to specify a language explicitly. For example:
.ZZ:
@c++
Says that .ZZ files are, in fact, C++ source files.
#name
This causes an error messages saying:
name compiler not installed on this system.
GCC already has an extensive list of suffixes built into it. This directive will add an entry to the
end of the list of suffixes, but since the list is searched from the end backwards, it is effectively
possible to override earlier entries using this technique.
GCC has the following spec strings built into it. Spec files can override these strings or create their
own. Note that individual targets can also add their own spec strings to this list.
asm Options to pass to the assembler
asm_final Options to pass to the assembler post-processor
cpp Options to pass to the C preprocessor
cc1 Options to pass to the C compiler
cc1plus Options to pass to the C++ compiler
endfile Object files to include at the end of the link
link Options to pass to the linker
lib Libraries to include on the command line to the linker
78 Chapter 4. GCC Command Options
libgcc Decides which GCC support library to pass to the linker
linker Sets the name of the linker
predefines Defines to be passed to the C preprocessor
signed_char Defines to pass to CPP to say whether char is signed
by default
startfile Object files to include at the start of the link
Here is a small example of a spec file:
%rename lib old_lib
*lib:
--start-group -lgcc -lc -leval1 --end-group %(old_lib)
This example renames the spec called lib to old_lib and then overrides the previous definition of
lib with a new one. The new definition adds in some extra command-line options before including
the text of the old definition.
Spec strings are a list of command-line options to be passed to their corresponding program. In addition, the spec strings can contain %-prefixed sequences to substitute variable text or to conditionally
insert text into the command line. Using these constructs it is possible to generate quite complex
command lines.
Here is a table of all defined %-sequences for spec strings. Note that spaces are not generated automatically around the results of expanding these sequences. Therefore you can concatenate them together
or combine them with constant text in a single argument.
%%
Substitute one % into the program name or argument.
%i
Substitute the name of the input file being processed.
%b
Substitute the basename of the input file being processed. This is the substring up to (and not
including) the last period and not including the directory.
%B
This is the same as %b, but include the file suffix (text after the last period).
%d
Marks the argument containing or following the %d as a temporary file name, so that that file will
be deleted if GCC exits successfully. Unlike %g, this contributes no text to the argument.
%gsuffix
Substitute a file name that has suffix suffix and is chosen once per compilation, and mark
the argument in the same way as %d. To reduce exposure to denial-of-service attacks, the
file name is now chosen in a way that is hard to predict even when previously chosen file
names are known. For example, %g.s ... %g.o ... %g.s might turn into ccUVUUAU.s
ccXYAXZ12.o ccUVUUAU.s. suffix matches the regexp [.A-Za-z]* or the special string
%O, which is treated exactly as if %O had been preprocessed. Previously, %g was simply
substituted with a file name chosen once per compilation, without regard to any appended suffix
(which was therefore treated just like ordinary text), making such attacks more likely to succeed.
Chapter 4. GCC Command Options 79
%usuffix
Like %g, but generates a new temporary file name even if %usuffix was already seen.
%Usuffix
Substitutes the last file name generated with %usuffix, generating a new one if there is no such
last file name. In the absence of any %usuffix, this is just like %gsuffix, except they don’t share
the same suffix space, so %g.s ... %U.s ... %g.s ... %U.s would involve the generation
of two distinct file names, one for each %g.s and another for each %U.s. Previously, %U was
simply substituted with a file name chosen for the previous %u, without regard to any appended
suffix.
%jsuffix
Substitutes the name of the HOST_BIT_BUCKET, if any, and if it is writable, and if save-temps
is off; otherwise, substitute the name of a temporary file, just like %u. This temporary file is not
meant for communication between processes, but rather as a junk disposal mechanism.
%|suffix
%msuffix
Like %g, except if -pipe is in effect. In that case %| substitutes a single dash and %m substitutes
nothing at all. These are the two most common ways to instruct a program that it should read
from standard input or write to standard output. If you need something more elaborate you can
use an %{pipe:X} construct: see for example f/lang-specs.h.
%.SUFFIX
Substitutes .SUFFIX for the suffixes of a matched switch’s args when it is subsequently output
with %*. SUFFIX is terminated by the next space or %.
%w
Marks the argument containing or following the %w as the designated output file of this compilation. This puts the argument into the sequence of arguments that %o will substitute later.
%o
Substitutes the names of all the output files, with spaces automatically placed around them. You
should write spaces around the %o as well or the results are undefined. %o is for use in the specs
for running the linker. Input files whose names have no recognized suffix are not compiled at all,
but they are included among the output files, so they will be linked.
%O
Substitutes the suffix for object files. Note that this is handled specially when it immediately follows %g, %u, or %U, because of the need for those to form complete file names. The handling
is such that %O is treated exactly as if it had already been substituted, except that %g, %u, and
%U do not currently support additional suffix characters following %O as they would following,
for example, .o.
%p
Substitutes the standard macro predefinitions for the current target machine. Use this when running cpp.
%P
Like %p, but puts __ before and after the name of each predefined macro, except for macros that
start with __ or with _L, where L is an uppercase letter. This is for ISO C.
80 Chapter 4. GCC Command Options
%I
Substitute any of -iprefix (made from GCC_EXEC_PREFIX), -isysroot (made from
TARGET_SYSTEM_ROOT), and -isystem (made from COMPILER_PATH and -B options) as
necessary.
%s
Current argument is the name of a library or startup file of some sort. Search for that file in a
standard list of directories and substitute the full name found.
%estr
Print str as an error message. str is terminated by a newline. Use this when inconsistent options
are detected.
%(name)
Substitute the contents of spec string name at this point.
%[name]
Like %(...) but put __ around -D arguments.
%x{option}
Accumulate an option for %X.
%X
Output the accumulated linker options specified by -Wl or a %x spec string.
%Y
Output the accumulated assembler options specified by -Wa.
%Z
Output the accumulated preprocessor options specified by -Wp.
%a
Process the asm spec. This is used to compute the switches to be passed to the assembler.
%A
Process the asm_final spec. This is a spec string for passing switches to an assembler postprocessor, if such a program is needed.
%l
Process the link spec. This is the spec for computing the command line passed to the linker.
Typically it will make use of the %L %G %S %D and %E sequences.
%D
Dump out a -L option for each directory that GCC believes might contain startup files. If the
target supports multilibs then the current multilib directory will be prepended to each of these
paths.
%M
Output the multilib directory with directory separators replaced with _. If multilib directories are
not set, or the multilib directory is . then this option emits nothing.
Chapter 4. GCC Command Options 81
%L
Process the lib spec. This is a spec string for deciding which libraries should be included on the
command line to the linker.
%G
Process the libgcc spec. This is a spec string for deciding which GCC support library should
be included on the command line to the linker.
%S
Process the startfile spec. This is a spec for deciding which object files should be the first
ones passed to the linker. Typically this might be a file named crt0.o.
%E
Process the endfile spec. This is a spec string that specifies the last object files that will be
passed to the linker.
%C
Process the cpp spec. This is used to construct the arguments to be passed to the C preprocessor.
%c
Process the signed_char spec. This is intended to be used to tell cpp whether a char is signed.
It typically has the definition:
%{funsigned-char:-D__CHAR_UNSIGNED__}
%1
Process the cc1 spec. This is used to construct the options to be passed to the actual C compiler
(cc1).
%2
Process the cc1plus spec. This is used to construct the options to be passed to the actual C++
compiler (cc1plus).
%*
Substitute the variable part of a matched option. See below. Note that each comma in the substituted string is replaced by a single space.
%
S
Remove all occurrences of -S from the command line. Note--this command is position dependent. % commands in the spec string before this one will see -S, % commands in the spec string
after this one will not.
%:function(args)
Call the named function function, passing it args. args is first processed as a nested spec
string, then split into an argument vector in the usual fashion. The function returns a string which
is processed as if it had appeared literally as part of the current spec.
The following built-in spec functions are provided:
if-exists
The if-exists spec function takes one argument, an absolute pathname to a file. If the file
exists, if-exists returns the pathname. Here is a small example of its usage:
*startfile:
82 Chapter 4. GCC Command Options
crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s
if-exists-else
The if-exists-else spec function is similar to the if-exists spec function, except
that it takes two arguments. The first argument is an absolute pathname to a file. If the file
exists, if-exists-else returns the pathname. If it does not exist, it returns the second
argument. This way, if-exists-else can be used to select one file or another, based on
the existence of the first. Here is a small example of its usage:
*startfile:
crt0%O%s %:if-exists(crti%O%s) \
%:if-exists-else(crtbeginT%O%s crtbegin%O%s)
%{S}
Substitutes the -S switch, if that switch was given to GCC. If that switch was not specified, this
substitutes nothing. Note that the leading dash is omitted when specifying this option, and it is
automatically inserted if the substitution is performed. Thus the spec string %{foo} would match
the command-line option -foo and would output the command line option -foo.
%W{S}
Like %{S} but mark last argument supplied within as a file to be deleted on failure.
%{S*}
Substitutes all the switches specified to GCC whose names start with -S, but which also take an
argument. This is used for switches like -o, -D, -I, etc. GCC considers -o foo as being one
switch whose names starts with o. %{o*} would substitute this text, including the space. Thus
two arguments would be generated.
%{S*&T*}
Like %{S*}, but preserve order of S and T options (the order of S and T in the spec is not
significant). There can be any number of ampersand-separated variables; for each the wild card
is optional. Useful for CPP as %{D*&U*&A*}.
%{S:X}
Substitutes X, if the -S switch was given to GCC.
%{!S:X}
Substitutes X, if the -S switch was not given to GCC.
%{S*:X}
Substitutes X if one or more switches whose names start with -S are specified to GCC. Normally
X is substituted only once, no matter how many such switches appeared. However, if %* appears
somewhere in X, then X will be substituted once for each matching switch, with the %* replaced
by the part of that switch that matched the *.
%{.S:X}
Substitutes X, if processing a file with suffix S.
%{!.S:X}
Substitutes X, if not processing a file with suffix S.
Chapter 4. GCC Command Options 83
%{S|P:X}
Substitutes X if either -S or -P was given to GCC. This may be combined with !, ., and *
sequences as well, although they have a stronger binding than the |. If %* appears in X, all of the
alternatives must be starred, and only the first matching alternative is substituted.
For example, a spec string like this:
%{.c:-foo} %{!.c:-bar} %{.c|d:-baz} %{!.c|d:-boggle}
will output the following command-line options from the following input command-line options:
fred.c -foo -baz
jim.d -bar -boggle
-d fred.c -foo -baz -boggle
-d jim.d -bar -baz -boggle
%{S:X; T:Y; :D}
If S was given to GCC, substitues X; else if T was given to GCC, substitues Y; else substitutes D.
There can be as many clauses as you need. This may be combined with ., !, |, and * as needed.
The conditional text X in a %{S:X} or similar construct may contain other nested % constructs or
spaces, or even newlines. They are processed as usual, as described above. Trailing white space in X
is ignored. White space may also appear anywhere on the left side of the colon in these constructs,
except between . or * and the corresponding word.
The -O, -f, -m, and -W switches are handled specifically in these constructs. If another value of -O or
the negated form of a -f, -m, or -W switch is found later in the command line, the earlier switch value
is ignored, except with {S*} where S is just one letter, which passes all matching options.
The character | at the beginning of the predicate text is used to indicate that a command should be
piped to the following command, but only if -pipe is specified.
It is built into GCC which switches take arguments and which do not. (You might think it would be
useful to generalize this to allow each compiler’s spec to say which switches take arguments. But this
cannot be done in a consistent fashion. GCC cannot even decide which input files have been specified
without knowing which switches take arguments, and it must know which input files to compile in
order to tell which compilers to run).
GCC also knows implicitly that arguments starting in -l are to be treated as compiler output files, and
passed to the linker in their proper position among the other output files.
4.16. Specifying Target Machine and Compiler Version
The usual way to run GCC is to run the executable called gcc, or
machine-gcc when cross-
compiling, or
machine-gcc-version
to run a version other than the one that was installed
last. Sometimes this is inconvenient, so GCC provides options that will switch to another crosscompiler or version.
-b machine
The argument machine specifies the target machine for compilation.
The value to use for machine is the same as was specified as the machine type when configuring
GCC as a cross-compiler.
-V version
The argument version specifies which version of GCC to run. This is useful when multiple
versions are installed. For example, version might be 2.0, meaning to run GCC version 2.0.
The -V and -b options work by running the
machine-gcc-version
executable, so there’s
no real reason to use them if you can just run that directly.
84 Chapter 4. GCC Command Options
4.17. Hardware Models and Configurations
Earlier we discussed the standard option -b which chooses among different installed compilers for
completely different target machines, such as 68000 vs. 80386.
In addition, each of these target machine types can have its own special options, starting with -m,
to choose among various hardware models or configurations--for example, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can compile for any model or
configuration, according to the options specified.
Some configurations of the compiler also support additional special options, usually for compatibility
with other compilers on the same platform.
These options are defined by the macro TARGET_SWITCHES in the machine description. The default
for the options is also defined by that macro, which enables you to change the defaults.
4.17.1. SPARC Options
These -m switches are supported on the SPARC:
-mno-app-regs, -mapp-regs
Specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC
SVR4 ABI reserves for applications. This is the default.
To be fully SVR4 ABI compliant at the cost of some performance loss, specify -mno-app-regs.
You should compile libraries and system software with this option.
-mfpu, -mhard-float
Generate output containing floating point instructions. This is the default.
-mno-fpu, -msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are
not available for all SPARC targets. Normally the facilities of the machine’s usual C compiler
are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets
sparc-*-aout and sparclite-*-* do provide software floating point support.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if
you compile all of a program with this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this to work.
-mhard-quad-float
Generate output containing quad-word (long double) floating point instructions.
-msoft-quad-float
Generate output containing library calls for quad-word (long double) floating point instructions.
The functions called are those specified in the SPARC ABI. This is the default.
As of this writing, there are no sparc implementations that have hardware support for the quadword floating point instructions. They all invoke a trap handler for one of these instructions, and
then the trap handler emulates the effect of the instruction. Because of the trap handler overhead,
this is much slower than calling the ABI library routines. Thus the -msoft-quad-float option
is the default.
Chapter 4. GCC Command Options 85
-mno-flat, -mflat
With -mflat, the compiler does not generate save/restore instructions and will use a "flat" or
single register window calling convention. This model uses %i7 as the frame pointer and is
compatible with the normal register window model. Code from either may be intermixed. The
local registers and the input registers (0-5) are still treated as "call saved" registers and will be
saved on the stack as necessary.
With -mno-flat (the default), the compiler emits save/restore instructions (except for leaf functions) and is the normal mode of operation.
-mno-unaligned-doubles, -munaligned-doubles
Assume that doubles have 8 byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8 byte alignment only if they
are contained in another type, or if they have an absolute address. Otherwise, it assumes they
have 4 byte alignment. Specifying this option avoids some rare compatibility problems with
code generated by other compilers. It is not the default because it results in a performance loss,
especially for floating point code.
-mno-faster-structs, -mfaster-structs
With -mfaster-structs, the compiler assumes that structures should have 8 byte alignment.
This enables the use of pairs of ldd and std instructions for copies in structure assignment, in
place of twice as many ld and st pairs. However, the use of this changed alignment directly
violates the SPARC ABI. Thus, it is intended only for use on targets where the developer acknowledges that their resulting code will not be directly in line with the rules of the ABI.
-mv8, -msparclite
These two options select variations on the SPARC architecture.
By default (unless specifically configured for the Fujitsu SPARClite), GCC generates code for
the v7 variant of the SPARC architecture.
-mv8 will give you SPARC v8 code. The only difference from v7 code is that the compiler emits
the integer multiply and integer divide instructions which exist in SPARC v8 but not in SPARC
v7.
-msparclite will give you SPARClite code. This adds the integer multiply, integer divide step
and scan (ffs) instructions which exist in SPARClite but not in SPARC v7.
These options are deprecated and will be deleted in a future GCC release. They have been replaced with -mcpu=xxx.
-mcypress, -msupersparc
These two options select the processor for which the code is optimized.
With -mcypress (the default), the compiler optimizes code for the Cypress CY7C602 chip,
as used in the SPARCStation/SPARCServer 3xx series. This is also appropriate for the older
SPARCStation 1, 2, IPX etc.
With -msupersparc the compiler optimizes code for the SuperSPARC cpu, as used in the
SPARCStation 10, 1000 and 2000 series. This flag also enables use of the full SPARC v8 instruction set.
These options are deprecated and will be deleted in a future GCC release. They have been replaced with -mcpu=xxx.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters for machine
type cpu_type. Supported values for cpu_type are v7, cypress, v8, supersparc,
86 Chapter 4. GCC Command Options
sparclite, hypersparc, sparclite86x, f930, f934, sparclet, tsc701, v9,
ultrasparc, and ultrasparc3.
Default instruction scheduling parameters are used for values that select an architecture and not
an implementation. These are v7, v8, sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported implementations.
v7: cypress
v8: supersparc, hypersparc
sparclite: f930, f934, sparclite86x
sparclet: tsc701
v9: ultrasparc, ultrasparc3
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option -mcpu=cpu_type would.
The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful
values are those that select a particular cpu implementation. Those are cypress, supersparc,
hypersparc, f930, f934, sparclite86x, tsc701, ultrasparc, and ultrasparc3.
These -m switches are supported in addition to the above on the SPARCLET processor.
-mlittle-endian
Generate code for a processor running in little-endian mode.
-mlive-g0
Treat register %g0 as a normal register. GCC will continue to clobber it as necessary but will not
assume it always reads as 0.
-mbroken-saverestore
Generate code that does not use non-trivial forms of the save and restore instructions. Early
versions of the SPARCLET processor do not correctly handle save and restore instructions
used with arguments. They correctly handle them used without arguments. A save instruction
used without arguments increments the current window pointer but does not allocate a new stack
frame. It is assumed that the window overflow trap handler will properly handle this case as will
interrupt handlers.
These -m switches are supported in addition to the above on SPARC V9 processors in 64-bit environments.
-mlittle-endian
Generate code for a processor running in little-endian mode.
-m32, -m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and
pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
-mcmodel=medlow
Generate code for the Medium/Low code model: the program must be linked in the low 32 bits
of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked.
Chapter 4. GCC Command Options 87
-mcmodel=medmid
Generate code for the Medium/Middle code model: the program must be linked in the low 44
bits of the address space, the text segment must be less than 2G bytes, and data segment must be
within 2G of the text segment. Pointers are 64 bits.
-mcmodel=medany
Generate code for the Medium/Anywhere code model: the program may be linked anywhere in
the address space, the text segment must be less than 2G bytes, and data segment must be within
2G of the text segment. Pointers are 64 bits.
-mcmodel=embmedany
Generate code for the Medium/Anywhere code model for embedded systems: assume a 32-bit
text and a 32-bit data segment, both starting anywhere (determined at link time). Register %g4
points to the base of the data segment. Pointers are still 64 bits. Programs are statically linked,
PIC is not supported.
-mstack-bias, -mno-stack-bias
With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset
by -2047 which must be added back when making stack frame references. Otherwise, assume no
such offset is present.
4.17.2. IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
GCC supports two related instruction set architectures for the RS/6000 and PowerPC. The
POWER instruction set are those instructions supported by the rios chip set used in the original
RS/6000 systems and the PowerPC instruction set is the architecture of the Motorola MPC5xx,
MPC6xx, MPC8xx microprocessors, and the IBM 4xx microprocessors.
Neither architecture is a subset of the other. However there is a large common subset of instructions supported by both. An MQ register is included in processors supporting the POWER
architecture.
You use these options to specify which instructions are available on the processor you are
using. The default value of these options is determined when configuring GCC. Specifying
the -mcpu=cpu_type overrides the specification of these options. We recommend you use the
-mcpu=cpu_type option rather than the options listed above.
The -mpower option allows GCC to generate instructions that are found only in the POWER
architecture and to use the MQ register. Specifying -mpower2 implies -power and also allows
88 Chapter 4. GCC Command Options
GCC to generate instructions that are present in the POWER2 architecture but not the original
POWER architecture.
The -mpowerpc option allows GCC to generate instructions that are found only in the 32-bit subset of the PowerPC architecture. Specifying -mpowerpc-gpopt implies -mpowerpc and also
allows GCC to use the optional PowerPC architecture instructions in the General Purpose group,
including floating-point square root. Specifying -mpowerpc-gfxopt implies -mpowerpc and
also allows GCC to use the optional PowerPC architecture instructions in the Graphics group,
including floating-point select.
The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are
found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities.
GCC defaults to -mno-powerpc64.
If you specify both -mno-power and -mno-powerpc, GCC will use only the instructions in
the common subset of both architectures plus some special AIX common-mode calls, and will
not use the MQ register. Specifying both -mpower and -mpowerpc permits GCC to use any
instruction from either architecture and to allow use of the MQ register; specify this for the
Motorola MPC601.
-mnew-mnemonics
-mold-mnemonics
Select which mnemonics to use in the generated assembler code. With -mnew-mnemonics,
GCC uses the assembler mnemonics defined for the PowerPC architecture. With
-mold-mnemonics it uses the assembler mnemonics defined for the POWER architecture.
Instructions defined in only one architecture have only one mnemonic; GCC uses that
mnemonic irrespective of which of these options is specified.
GCC defaults to the mnemonics appropriate for the architecture in use. Specifying
-mcpu=cpu_type sometimes overrides the value of these option. Unless you are
building a cross-compiler, you should normally not specify either -mnew-mnemonics or
-mold-mnemonics, but should instead accept the default.
-mcpu=cpu_type
Set architecture type, register usage, choice of mnemonics, and instruction scheduling parame-
ters for machine type cpu_type. Supported values for cpu_type are rios, rios1, rsc, rios2,
rs64a, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, power, power2,
powerpc, 403, 505, 801, 821, 823, and 860 and common.
-mcpu=common selects a completely generic processor. Code generated under this option will
run on any POWER or PowerPC processor. GCC will use only the instructions in the common
subset of both architectures, and will not use the MQ register. GCC assumes a generic processor
model for scheduling purposes.
-mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64 specify generic
POWER, POWER2, pure 32-bit PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture
machine types, with an appropriate, generic processor model assumed for scheduling purposes.
The other options specify a specific processor. Code generated under those options will run best
on that processor, and may not run at all on others.
The -mcpu options automatically enable or disable other -m options as follows:
common
-mno-power, -mno-powerpc
Chapter 4. GCC Command Options 89
power
power2
rios1
rios2
rsc
-mpower, -mno-powerpc, -mno-new-mnemonics
powerpc
rs64a
602
603
603e
604
620
630
740
7400
7450
750
505
-mno-power, -mpowerpc, -mnew-mnemonics
601
-mpower, -mpowerpc, -mnew-mnemonics
403
821
860
-mno-power, -mpowerpc, -mnew-mnemonics, -msoft-float
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture type, register usage, or choice of mnemonics, as -mcpu=cpu_type would. The same
values for cpu_type are used for -mtune as for -mcpu. If both are specified, the code generated
will use the architecture, registers, and mnemonics set by -mcpu, but the scheduling parameters
set by -mtune.
-maltivec
-mno-altivec
These switches enable or disable the use of built-in functions that allow access to the AltiVec
instruction set. You may also need to set -mabi=altivec to adjust the current ABI with AltiVec
ABI enhancements.
-mabi=spe
Extend the current ABI with SPE ABI extensions. This does not change the default ABI, instead
it adds the SPE ABI extensions to the current ABI.
-mabi=no-spe
Disable Booke SPE ABI extensions for the current ABI.
90 Chapter 4. GCC Command Options
-misel=yes/no
-misel
This switch enables or disables the generation of ISEL instructions.
-mspe=yes/no
-mspe
This switch enables or disables the generation of SPE simd instructions.
-mfloat-gprs=yes/no
-mfloat-gprs
This switch enables or disables the generation of floating point operations on the general purpose
registers for architectures that support it. This option is currently only available on the MPC8540.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created for every executable file.
The -mfull-toc option is selected by default. In that case, GCC will allocate at least one TOC
entry for each unique non-automatic variable reference in your program. GCC will also place
floating-point constants in the TOC. However, only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have overflowed the available TOC
space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and
-mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point
constants in the TOC and -mno-sum-in-toc forces GCC to generate code to calculate the sum
of an address and a constant at run-time instead of putting that sum into the TOC. You may
specify one or both of these options. Each causes GCC to produce very slightly slower and
larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead. This option causes GCC to make only one TOC entry for every
file. When you specify this option, GCC will produce code that is slower and larger but which
uses extremely little TOC space. You may wish to use this option only on files that contain less
frequently executed code.
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit long type, and the
infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64 and
-mpowerpc, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC
defaults to -maix32.
-mxl-call
-mno-xl-call
On AIX, pass floating-point arguments to prototyped functions beyond the register save area
(RSA) on the stack in addition to argument FPRs. The AIX calling convention was extended but
not initially documented to handle an obscure K&R C case of calling a function that takes the
address of its arguments with fewer arguments than declared. AIX XL compilers access floating
point arguments which do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and
rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by AIX XL compilers without optimization.
Chapter 4. GCC Command Options 91
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message
passing with special startup code to enable the application to run. The system must have PE
installed in the standard location (/usr/lpp/ppe.poe/), or the specs file must be overridden
with the -specs= option to specify the appropriate directory location. The Parallel Environment
does not support threads, so the -mpe option and the -pthread option are incompatible.
-malign-natural
-malign-power
On AIX and 64-bit PowerPC Linux, the option -malign-natural overrides the ABI-defined
alignment of larger types, such as floating-point doubles, on their natural size-based boundary.
The option -malign-power instructs GCC to follow the ABI-specified alignment rules. GCC
defaults to the standard alignment defined in the ABI.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register set. Software floating point
emulation is provided if you use the -msoft-float option, and pass the option to GCC when
linking.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word instructions and the store multiple
word instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use -mmultiple on little endian PowerPC systems, since
those instructions do not work when the processor is in little endian mode. The exceptions are
PPC740 and PPC750 which permit the instructions usage in little endian mode.
-mstring
-mno-string
Generate code that uses (does not use) the load string instructions and the store string word
instructions to save multiple registers and do small block moves. These instructions are generated
by default on POWER systems, and not generated on PowerPC systems. Do not use -mstring
on little endian PowerPC systems, since those instructions do not work when the processor is in
little endian mode. The exceptions are PPC740 and PPC750 which permit the instructions usage
in little endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store instructions that update the base register
to the address of the calculated memory location. These instructions are generated by default.
If you use -mno-update, there is a small window between the time that the stack pointer is
updated and the address of the previous frame is stored, which means code that walks the stack
frame across interrupts or signals may get corrupted data.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating point multiply and accumulate instructions.
These instructions are generated by default if hardware floating is used.
92 Chapter 4. GCC Command Options
-mno-bit-align
-mbit-align
On embedded PowerPC systems do not (do) force structures and unions that contain bit-fields
to be aligned to the base type of the bit-field.
For example, by default a structure containing nothing but 8 unsigned bit-fields of length 1
would be aligned to a 4 byte boundary and have a size of 4 bytes. By using -mno-bit-align,
the structure would be aligned to a 1 byte boundary and be one byte in size.
-mno-strict-align
-mstrict-align
On embedded PowerPC systems do not (do) assume that unaligned memory references will be
handled by the system.
-mrelocatable
-mno-relocatable
On embedded PowerPC systems generate code that allows (does not allow) the program to be
relocated to a different address at runtime. If you use -mrelocatable on any module, all objects
linked together must be compiled with -mrelocatable or -mrelocatable-lib.
-mrelocatable-lib
-mno-relocatable-lib
On embedded PowerPC systems generate code that allows (does not allow) the program to be
relocated to a different address at runtime. Modules compiled with -mrelocatable-lib can
be linked with either modules compiled without -mrelocatable and -mrelocatable-lib or
with modules compiled with the -mrelocatable options.
-mno-toc
-mtoc
On embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global
area pointing to the addresses used in the program.
-mlittle
-mlittle-endian
On embedded PowerPC systems compile code for the processor in little endian mode. The
-mlittle-endian option is the same as -mlittle.
-mbig
-mbig-endian
On embedded PowerPC systems compile code for the processor in big endian mode. The
-mbig-endian option is the same as -mbig.
-mcall-sysv
On embedded PowerPC systems compile code using calling conventions that adheres to the
March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement.
This is the default unless you configured GCC using powerpc-*-eabiaix.
-mcall-sysv-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
Chapter 4. GCC Command Options 93
-mcall-solaris
On embedded PowerPC systems compile code for the Solaris operating system.
-mcall-linux
On embedded PowerPC systems compile code for the Linux-based GNU system.
-mcall-gnu
On embedded PowerPC systems compile code for the Hurd-based GNU system.
-mcall-netbsd
On embedded PowerPC systems compile code for the NetBSD operating system.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).
-mabi=altivec
Extend the current ABI with AltiVec ABI extensions. This does not change the default ABI,
instead it adds the AltiVec ABI extensions to the current ABI.
-mabi=no-altivec
Disable AltiVec ABI extensions for the current ABI.
-mprototype
-mno-prototype
On embedded PowerPC systems assume that all calls to variable argument functions are properly
prototyped. Otherwise, the compiler must insert an instruction before every non prototyped call
to set or clear bit 6 of the condition code register (CR) to indicate whether floating point values
were passed in the floating point registers in case the function takes a variable arguments. With
-mprototype, only calls to prototyped variable argument functions will set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is called sim-crt0.o
and that the standard C libraries are libsim.a and libc.a. This is the default for
powerpc-*-eabisim. configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is called crt0.o and the
standard C libraries are libmvme.a and libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is called crt0.o and the
standard C libraries are libads.a and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is called crt0.o and the
standard C libraries are libyk.a and libc.a.
94 Chapter 4. GCC Command Options
-mvxworks
On embedded PowerPC systems, specify that you are compiling for a VxWorks system.
-mwindiss
Specify that you are compiling for the WindISS simulation environment.
-memb
On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that
eabi extended relocations are used.
-meabi
-mno-eabi
On embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary Interface (eabi) which is a set of modifications to the System V.4 specifications. Selecting -meabi
means that the stack is aligned to an 8 byte boundary, a function __eabi is called to from main
to set up the eabi environment, and the -msdata option can use both r2 and r13 to point to
two separate small data areas. Selecting -mno-eabi means that the stack is aligned to a 16 byte
boundary, do not call an initialization function from main, and the -msdata option will only use
r13 to point to a single small data area. The -meabi option is on by default if you configured
GCC using one of the powerpc*-*-eabi* options.
-msdata=eabi
On embedded PowerPC systems, put small initialized const global and static data in the
.sdata2 section, which is pointed to by register r2. Put small initialized non-const global
and static data in the .sdata section, which is pointed to by register r13. Put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdata section. The
-msdata=eabi option is incompatible with the -mrelocatable option. The -msdata=eabi
option also sets the -memb option.
-msdata=sysv
On embedded PowerPC systems, put small global and static data in the .sdata section, which
is pointed to by register r13. Put small uninitialized global and static data in the .sbss section,
which is adjacent to the .sdata section. The -msdata=sysv option is incompatible with the
-mrelocatable option.
-msdata=default
-msdata
On embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi,
otherwise compile code the same as -msdata=sysv.
-msdata-data
On embedded PowerPC systems, put small global and static data in the .sdata section. Put
small uninitialized global and static data in the .sbss section. Do not use register r13 to address
small data however. This is the default behavior unless other -msdata options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static data in the .data section,
and all uninitialized data in the .bss section.
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