1 GNU CC Command Options

When you invoke GNU CC, 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.

The gcc program accepts options and file names as operands. Many options have multiletter 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.

1.1 Option Summary

Here is a summary of all the options, grouped by type. Explanations are in the following sections.

Overall Options

See section Options Controlling the Kind of Output.

-c  -S  -E  -o file  -pipe  -v  -x language

Language Options

See section Options Controlling Dialect.

-ansi  -fcond-mismatch  -fno-asm  -fno-builtin
-fsigned-bitfields  -fsigned-char 
-funsigned-bitfields  -funsigned-char  -fwritable-strings
-traditional  -traditional-cpp  -trigraphs
-fall-virtual  -fdollars-in-identifiers  -fenum-int-equiv
-fno-strict-prototype  -fthis-is-variable

Warning Options

See section Options to Request or Suppress Warnings.

-fsyntax-only  -pedantic  -pedantic-errors
-w  -W  -Wall  -Waggregate-return 
-Wcast-align  -Wcast-qual  -Wcomment  -Wconversion  -Werror
-Wformat  -Wid-clash-len  -Wenum-clash  -Wimplicit  -Wimport
-Winline -Wmissing-prototypes  -Wnested-externs  -Wparentheses
-Wpointer-arith  -Wredundant-decls  -Wreturn-type  -Wshadow
-Wstrict-prototypes  -Wswitch  -Wtraditional  -Wtrigraphs
-Wuninitialized  -Wunused  -Wwrite-strings  -Wchar-subscripts

Debugging Options

See section Options for Debugging Your Program or GCC.

-a  -dletters  -fpretend-float 
-g  -glevel -ggdb  -gdwarf -gdwarf+
-gstabs  -gstabs+  -gcoff -gxcoff -gxcoff+
-p  -pg  -save-temps

Optimization Options

See section Options that Control Optimization.

-fcaller-saves  -fcse-follow-jumps  -fcse-skip-blocks
-fdelayed-branch   -fexpensive-optimizations  -ffast-math 
-ffloat-store  -fforce-addr  -fforce-mem
-finline-functions  -fkeep-inline-functions  -fno-defer-pop
-fno-function-cse  -fno-inline  -fno-peephole  -fomit-frame-pointer
-frerun-cse-after-loop  -fschedule-insns  -fschedule-insns2
-fstrength-reduce  -fthread-jumps
-funroll-all-loops  -funroll-loops 
-felide-constructors  -fmemoize-lookups  -fno-default-inline
-O  -O2

Preprocessor Options

See section Options Controlling the Preprocessor.

-Aassertion  -C  -dD  -dM  -dN
-Dmacro[=defn]  -E  -H
-include file  -imacros file
-M  -MD  -MM  -MMD  -nostdinc  -P  -trigraphs  -Umacro

Assembler Option

See section Passing Options to the Assembler.

-Wa,option

Linker Options

See section Options for Linking.

object-file-name
-llibrary  -nostdlib  
-static  -shared  -symbolic  
-Xlinker option
-u symbol

Directory Options

See section Options for Directory Search.

-Bprefix  -Idir  -I-  -Ldir

Target Options

See section Target Machine and Compiler Version.

-b machine  -V version

Machine Dependent Options

See section Hardware Models and Configurations.

M680x0 Options
-m68000 -m68020 -m68020-40 -m68030 -m68040 -m68881 -mbitfield
-mc68000 -mc68020 -mfpa -mnobitfield -mrtd -mshort -msoft-float

VAX Options
-mg -mgnu -munix

SPARC Options
-mforce-align  -mno-epilogue

Convex Options
-margcount -mc1 -mc2 -mnoargcount

AMD29K Options
-m29000 -m29050 -mbw -mdw -mkernel-registers -mlarge 
-mnbw -mnodw -msmall -mstack-check -muser-registers

M88K Options
-m88000 -m88100 -m88110 -mbig-pic -mcheck-zero-division
-mhandle-large-shift -midentify-revision
-mno-check-zero-division -mno-ocs-debug-info
-mno-ocs-frame-position -mno-optimize-arg-area -mno-underscores
-mocs-debug-info -mocs-frame-position -moptimize-arg-area
-mshort-data-num -msvr3 -msvr4 -mtrap-large-shift
-muse-div-instruction -mversion-03.00 -mwarn-passed-structs

RS/6000 Options
-mfp-in-toc -mno-fop-in-toc

RT Options
-mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs
-mfull-fp-blocks -mhc-struct-return -min-line-mul
-mminimum-fp-blocks -mnohc-struct-return

MIPS Options
-mcpu=cpu type -mips2 -mips3 -mint64 -mlong64 -mlonglong128
-mmips-as -mgas -mrnames -mno-rnames -mgpopt -mno-gpopt -mstats
-mno-stats -mmemcpy -mno-memcpy -mno-mips-tfile -mmips-tfile
-msoft-float -mhard-float -mabicalls -mno-abicalls -mhalf-pic
-mno-half-pic -G num -nocpp

i386 Options
-m486 -mno-486 -msoft-float -msvr3-shlib -mieee-fp
-mno-fp-ret-in-387

HPPA Options
-mno-bss
-mpa-risc-1-0
-mpa-risc-1-1
-mkernel
-mshared-libs
-mno-shared-libs
-mlong-calls

Intel 960 Options
-mcpu type
-mnumerics -msoft-float
-mcode-align -mno-code-align
-mleaf-procedures -mno-leaf-procedures
-mtail-call -mno-tail-call
-mcomplex-addr -mno-complex-addr
-mclean-linkage -mno-clean-linkage
-mic-compat -mic2.0-compat -mic3.0-compat
-masm-compat -mintel-asm
-mstrict-align -mno-strict-align
-mold-align -mno-old-align

DEC Alpha Options
-mfp-regs -mno-fp-regs -mno-soft-float -msoft-float 

System V Options
-G  -Qy  -Qn  -YP,paths  -Ym,dir

Code Generation Options

See section Options for Code Generation Conventions.

-fcall-saved-reg  -fcall-used-reg  -ffixed-reg
-finhibit-size-directive  -fnonnull-objects  -fno-common
-fno-ident  -fno-gnu-linker  -fpcc-struct-return  -fpic  -fPIC
-fshared-data  -fshort-enums  -fshort-double  -fvolatile
-fverbose-asm

1.2 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.h

C header file (not to be compiled or linked).

file.cc

file.cxx

file.C

C++ source code which must be preprocessed.

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 choosing a default based on the file name suffix). This option applies to all following input files until the next ‘-x’ option. Possible values of language are ‘c’, ‘objective-c’, ‘c-header’, ‘c++’, ‘cpp-output’, ‘assembler’, and ‘assembler-with-cpp’.

-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).

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.

-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.

1.3 Options Controlling Dialect

The following options control the dialect of C or C++ that the compiler accepts:

-ansi

Support all ANSI standard C programs.

This turns off certain features of GNU C that are incompatible with ANSI C, such as the asm, inline 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 ANSI trigraph feature, and disallows ‘$’ as part of identifiers.

The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite ‘-ansi’. You would not want to use them in an ANSI C program, of course, but it 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-ANSI programs to be rejected gratuitously. For that, ‘-pedantic’ is required in addition to ‘-ansi’. See section 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 ANSI standard doesn’t call for; this is to avoid interfering with any programs that might use these names for other things.

The functions alloca, abort, exit, and _exit are not builtin functions when ‘-ansi’ is used.

-fall-virtual

Treat certain member functions as virtual, implicitly (C++ only). This applies to all member functions declared in the same class with a “method-call” operator method (except for constructor functions and new or delete member operators). In effect, all of these methods become “implicitly virtual.”

This does not mean that all calls to these methods will be made through the internal table of virtual functions. There are some circumstances under which it is obvious that a call to a given virtual function can be made directly, and in these cases the calls still go direct.

The effect of making all methods of a class with a declared ‘operator->()()’ implicitly virtual using ‘-fall-virtual’ extends also to all non-constructor methods of any class derived from such a class.

-fdollars-in-identifiers

Permit the use of ‘$’ in identifiers (C++ only). You can also use ‘-fno-dollars-in-identifiers’ to explicitly prohibit use of ‘$’. (GNU C++ allows ‘$’ by default on some target systems but not others.)

-fenum-int-equiv

Permit implicit conversion of int to enumeration types (C++ only). Normally GNU C++ allows conversion of enum to int, but not the other way around.

-fno-asm

Do not recognize asm, inline or typeof as a keyword. These words may then be used as identifiers. You can use __asm__, __inline__ and __typeof__ instead. ‘-ansi’ implies ‘-fno-asm’.

-fno-builtin

Don’t recognize built-in functions that do not begin with two leading underscores. Currently, the functions affected include _exit, abort, abs, alloca, cos, exit, fabs, labs, memcmp, memcpy, sin, sqrt, strcmp, strcpy, and strlen.

The ‘-ansi’ option prevents alloca and _exit from being builtin functions.

-fno-strict-prototype

Treat a function declaration with no arguments, such as ‘int foo ();’, as C would treat it—as saying nothing about the number of arguments or their types (C++ only). Normally, such a declaration in C++ means that the function foo takes no arguments.

-fthis-is-variable

Permit assignment to this (C++ only). The incorporation of user-defined free store management into C++ has made assignment to ‘this’ an anachronism. Therefore, by default it is invalid to assign to this within a class member function. However, for backwards compatibility, you can make it valid with ‘-fthis-is-variable’.

-trigraphs

Support ANSI C trigraphs. You don’t want to know about this brain-damage. The ‘-ansi’ option implies ‘-trigraphs’.

-traditional

Attempt to support some aspects of traditional C compilers. Specifically:

• All extern declarations take effect globally even if they are written inside of a function definition. This includes implicit declarations of functions.
• The keywords typeof, inline, signed, const and volatile are not recognized. (You can still use the alternative keywords such as __typeof__, __inline__, and so on.)
• Comparisons between pointers and integers are always allowed.
• Integer types unsigned short and unsigned char promote to unsigned int.
• Out-of-range floating point literals are not an error.
• String “constants” are not necessarily constant; they are stored in writable space, and identical looking constants are allocated separately. (This is the same as the effect of ‘-fwritable-strings’.)
• All automatic variables not declared register are preserved by longjmp. Ordinarily, GNU C follows ANSI C: automatic variables not declared volatile may be clobbered.
• In the preprocessor, comments convert to nothing at all, rather than to a space. This allows traditional token concatenation.
• In the preprocessor, macro arguments are recognized within string constants in a macro definition (and their values are stringified, though without additional quote marks, when they appear in such a context). The preprocessor always considers a string constant to end at a newline.
• The predefined macro __STDC__ is not defined when you use ‘-traditional’, but __GNUC__ is (since the GNU extensions which __GNUC__ indicates are not affected by ‘-traditional’). If you need to write header files that work differently depending on whether ‘-traditional’ is in use, by testing both of these predefined macros you can distinguish four situations: GNU C, traditional GNU C, other ANSI C compilers, and other old C compilers.

You may wish to use ‘-fno-builtin’ as well as ‘-traditional’ if your program uses names that are normally GNU C builtin functions for other purposes of its own.

-traditional-cpp

Attempt to support some aspects of traditional C preprocessors. This includes the last three items in the table immediately above, but none of the other effects of ‘-traditional’.

-fcond-mismatch

Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void.

-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, ‘-fno-signed-char’ is equivalent to ‘-funsigned-char’.

-fsigned-bitfields

-funsigned-bitfields

-fno-signed-bitfields

-fno-unsigned-bitfields

These options control whether a bitfield is signed or unsigned, when the declaration does not use either signed or unsigned. By default, such a bitfield is signed, because this is consistent: the basic integer types such as int are signed types.

However, when ‘-traditional’ is used, bitfields are all unsigned no matter what.

-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. ‘-traditional’ also has this effect.

Writing into string constants is a very bad idea; “constants” should be constant.

1.4 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.

These options control the amount and kinds of warnings produced by GNU CC:

-fsyntax-only

Check the code for syntax errors, but don’t emit any output.

-w

Inhibit all warning messages.

-Wno-import

Inhibit warning messages about the use of ‘#import’.

-pedantic

Issue all the warnings demanded by strict ANSI standard C; reject all programs that use forbidden extensions.

Valid ANSI standard C programs should compile properly with or without this option (though a rare few will require ‘-ansi’). However, without this option, certain GNU extensions and traditional 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. @xref{Alternate Keywords}.

This option is not intended to be useful; it exists only to satisfy pedants who would otherwise claim that GNU CC fails to support the ANSI standard.

Some users try to use ‘-pedantic’ to check programs for strict ANSI C conformance. They soon find that it does not do quite what they want: it finds some non-ANSI practices, but not all—only those for which ANSI C requires a diagnostic.

A feature to report any failure to conform to ANSI C might be useful in some instances, but would require considerable additional work and would be quite different from ‘-pedantic’. We recommend, rather, that users take advantage of the extensions of GNU C and disregard the limitations of other compilers. Aside from certain supercomputers and obsolete small machines, there is less and less reason ever to use any other C compiler other than for bootstrapping GNU CC.

-pedantic-errors

Like ‘-pedantic’, except that errors are produced rather than warnings.

-W

Print extra warning messages for these events:

• A nonvolatile 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.

• 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 contains no side effects.
• 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.
• An aggregate has a partly bracketed initializer. For example, the following code would evoke such a warning, because braces are missing around the initializer for x.h:

  struct s { int f, g; };
  struct t { struct s h; int i; };
  struct t x = { 1, 2, 3 };
  

-Wenum-clash

Warn about conversion between different enumeration types (C++ only).

-Wimplicit

Warn whenever a function or parameter is implicitly declared.

-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.

-Wunused

Warn whenever a local variable is unused aside from its declaration, whenever a function is declared static but never defined, and whenever a statement computes a result that is explicitly not used.

If you want to prevent a warning for a particular variable, you can use this macro:

#define USE(var) \
  static void * use_##var = (&use_##var, (void *) &var)

USE (string);

-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.

-Wcomment

Warn whenever a comment-start sequence ‘/*’ appears in a comment.

-Wtrigraphs

Warn if any trigraphs are encountered (assuming they are enabled).

-Wformat

Check calls to printf and scanf, etc., to make sure that the arguments supplied have types appropriate to the format string specified.

-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.

-Wuninitialized

An automatic variable is used without first being initialized.

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 GNU CC 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 GNU CC 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.

Some spurious warnings can be avoided if you declare as volatile all the functions you use that never return. @xref{Function Attributes}.

-Wparentheses

Warn if parentheses are omitted in certain contexts.

-Wall

All of the above ‘-W’ options combined. These are all the options which pertain to usage that we recommend avoiding and that we believe is easy to avoid, even in conjunction with macros.

The remaining ‘-W…’ options are not implied by ‘-Wall’ because they warn about constructions that we consider reasonable to use, on occasion, in clean programs.

-Wtraditional

Warn about certain constructs that behave differently in traditional and ANSI C.

• Macro arguments occurring within string constants in the macro body. These would substitute the argument in traditional C, but are part of the constant in ANSI C.
• 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.

-Wshadow

Warn whenever a local variable shadows another local variable.

-Wid-clash-len

Warn whenever two distinct identifiers match in the first len characters. This may help you prepare a program that will compile with certain obsolete, brain-damaged compilers.

-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.

-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

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. 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.

-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

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

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.

-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

Warn if an extern declaration is encountered within an function.

-Winline

Warn if a function can not be inlined, and either it was declared as inline, or else the ‘-finline-functions’ option was given.

-Werror

Make all warnings into errors.

1.5 Options for Debugging Your Program or GNU CC

GNU CC has various special options that are used for debugging either your program or GCC:

-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+’, or ‘-gdwarf’ (see below).

Unlike most other C compilers, GNU CC 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 GNU CC is generated with the capability for more than one debugging format.

-ggdb

Produce debugging information in the native format (if that is 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). This is the format used by SDB on most System V systems prior to System V Release 4.

-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.

-gdwarf

Produce debugging information in DWARF format (if that is supported). This is the format used by SDB on most System V Release 4 systems.

-gdwarf+

Produce debugging information in DWARF 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.

-glevel

-ggdblevel

-gstabslevel

-gcofflevel

-gxcofflevel

-gdwarflevel

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’.

-p

Generate extra code to write profile information suitable for the analysis program prof.

-pg

Generate extra code to write profile information suitable for the analysis program gprof.

-a

Generate extra code to write profile information for basic blocks, which will record the number of times each basic block is executed. This data could be analyzed by a program like tcov. Note, however, that the format of the data is not what tcov expects. Eventually GNU gprof should be extended to process this data.

-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 word to the source file name (e.g. ‘foo.c.rtl’ or ‘foo.c.jump’). Here are the possible letters for use in letters, and their meanings:

‘M’

Dump all macro definitions, at the end of preprocessing, and write no output.

‘N’

Dump all macro names, at the end of preprocessing.

‘D’

Dump all macro definitions, at the end of preprocessing, in addition to normal output.

‘y’

Dump debugging information during parsing, to standard error.

‘r’

Dump after RTL generation, to ‘file.rtl’.

‘x’

Just generate RTL for a function instead of compiling it. Usually used with ‘r’.

‘j’

Dump after first jump optimization, to ‘file.jump’.

‘s’

Dump after CSE (including the jump optimization that sometimes follows CSE), to ‘file.cse’.

‘L’

Dump after loop optimization, to ‘file.loop’.

‘t’

Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to ‘file.cse2’.

‘f’

Dump after flow analysis, to ‘file.flow’.

‘c’

Dump after instruction combination, to ‘file.combine’.

‘S’

Dump after the first instruction scheduling pass, to ‘file.sched’.

‘l’

Dump after local register allocation, to
‘file.lreg’.

‘g’

Dump after global register allocation, to
‘file.greg’.

‘R’

Dump after the second instruction scheduling pass, to ‘file.sched2’.

‘J’

Dump after last jump optimization, to ‘file.jump2’.

‘d’

Dump after delayed branch scheduling, to ‘file.dbr’.

‘k’

Dump after conversion from registers to stack, to ‘file.stack’.

‘a’

Produce all the dumps listed above.

‘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.

-fpretend-float

When running a cross-compiler, pretend that the target machine uses the same floating point format as the host machine. This causes incorrect output of the actual floating constants, but the actual instruction sequence will probably be the same as GNU CC would make when running on the target machine.

-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’.

1.6 Options That Control Optimization

These options control various sorts of optimizations:

-O

-O1

Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.

Without ‘-O’, 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.

Without ‘-O’, only variables declared register are allocated in registers. The resulting compiled code is a little worse than produced by PCC without ‘-O’.

With ‘-O’, the compiler tries to reduce code size and execution time.

When ‘-O’ is specified, ‘-fthread-jumps’ and ‘-fdelayed-branch’ are turned on. On some machines other flags may also be turned on.

-O2

Optimize even more. Nearly all supported optimizations that do not involve a space-speed tradeoff are performed. As compared to ‘-O’, this option increases both compilation time and the performance of the generated code.

‘-O2’ turns on all ‘-fflag’ options that enable more optimization, except for ‘-funroll-loops’, ‘-funroll-all-loops’ and ‘-fomit-frame-pointer’.

-O0

Do not optimize.

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 which is not the default. You can figure out the other form by either removing ‘no-’ or adding it.

-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. 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.

-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.

-fforce-mem

Force memory operands to be copied into registers before doing arithmetic on them. This may produce better code by making all memory references potential common subexpressions. When they are not common subexpressions, instruction combination should eliminate the separate register-load. I am interested in hearing about the difference this makes.

-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. I am interested in hearing about the difference this makes.

-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, such as the Vax, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn’t exist. The machine-description macro FRAME_POINTER_REQUIRED controls whether a target machine supports this flag. See Register Usage in Using and Porting GCC.

-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.

-fkeep-inline-functions

Even if all calls to a given function are integrated, and the function is declared static, nevertheless output a separate run-time callable version of the function.

-fno-default-inline

Don’t make member functions inline by default merely because they are defined inside the class scope (C++ only).

-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.

-ffast-math

This option allows GCC to violate some ANSI or IEEE rules/specifications in the interest of optimizing code for speed. For example, it allows the compiler to assume arguments to the sqrt function are non-negative numbers.

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 ANSI rules/specifications for math functions.

-felide-constructors

Elide constructors when this seems plausible (C++ only). With this option, GNU C++ initializes y directly from the call to foo without going through a temporary in the following code:

A foo ();
A y = foo ();

Without this option, GNU C++ first initializes y by calling the appropriate constructor for type A; then assigns the result of foo to a temporary; and, finally, replaces the initial value of y with the temporary.

The default behavior (‘-fno-elide-constructors’) is specified by the draft ANSI C++ standard. If your program’s constructors have side effects, ‘-felide-constructors’ can change your program’s behavior, since some constructor calls may be omitted.

-fmemoize-lookups

-fsave-memoized

Use heuristics to compile faster (C++ only). These heuristics are not enabled by default, since they are only effective for certain input files. Other input files compile more slowly.

The first time the compiler must build a call to a member function (or reference to a data member), it must (1) determine whether the class implements member functions of that name; (2) resolve which member function to call (which involves figuring out what sorts of type conversions need to be made); and (3) check the visibility of the member function to the caller. All of this adds up to slower compilation. Normally, the second time a call is made to that member function (or reference to that data member), it must go through the same lengthy process again. This means that code like this

cout << "This " << p << " has " << n << " legs.\n";

makes six passes through all three steps. By using a software cache, a “hit” significantly reduces this cost. Unfortunately, using the cache introduces another layer of mechanisms which must be implemented, and so incurs its own overhead. ‘-fmemoize-lookups’ enables the software cache.

Because access privileges (visibility) to members and member functions may differ from one function context to the next, G++ may need to flush the cache. With the ‘-fmemoize-lookups’ flag, the cache is flushed after every function that is compiled. The ‘-fsave-memoized’ flag enables the same software cache, but when the compiler determines that the context of the last function compiled would yield the same access privileges of the next function to compile, it preserves the cache. This is most helpful when defining many member functions for the same class: with the exception of member functions which are friends of other classes, each member function has exactly the same access privileges as every other, and the cache need not be flushed.

The following options control specific optimizations. The ‘-O2’ option turns on all of these optimizations except ‘-funroll-loops’ and ‘-funroll-all-loops’. The ‘-O’ option usually turns on the ‘-fthread-jumps’ and ‘-fdelayed-branch’ options, but specific machines may change the default optimizations.

You can use the following flags in the rare cases when “fine-tuning” of optimizations to be performed is desired.

-fstrength-reduce

Perform the optimizations of loop strength reduction and elimination of iteration variables.

-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.

-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.

-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.

-frerun-cse-after-loop

Re-run common subexpression elimination after loop optimizations has been performed.

-fexpensive-optimizations

Perform a number of minor optimizations that are relatively expensive.

-fdelayed-branch

If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions.

-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.

-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.

-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 enabled by default on certain machines, usually those which have no call-preserved registers to use instead.

-funroll-loops

Perform the optimization of loop unrolling. This is only done for loops whose number of iterations can be determined at compile time or run time. ‘-funroll-loop’ implies ‘-fstrength-reduce’ and ‘-frerun-cse-after-loop’.

-funroll-all-loops

Perform the optimization of loop unrolling. This is done for all loops and usually makes programs run more slowly. ‘-funroll-all-loops’ implies ‘-fstrength-reduce’ and ‘-frerun-cse-after-loop’.

-fno-peephole

Disable any machine-specific peephole optimizations.

1.7 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.

-include file

Process file as input before processing the regular input file. In effect, the contents of file are compiled first. Any ‘-D’ and ‘-U’ options on the command line are always processed before ‘-include file’, regardless of the order in which they are written. All the ‘-include’ and ‘-imacros’ options are processed in the order in which they are written.

-imacros file

Process file as input, discarding the resulting output, before processing the regular input file. Because the output generated from file is discarded, the only effect of ‘-imacros file’ is to make the macros defined in file available for use in the main input.

Any ‘-D’ and ‘-U’ options on the command line are always processed before ‘-imacros file’, regardless of the order in which they are written. All the ‘-include’ and ‘-imacros’ options are processed in the order in which they are written.

-nostdinc

Do not search the standard system directories for header files. Only the directories you have specified with ‘-I’ options (and the current directory, if appropriate) are searched. See section Options for Directory Search, for information on ‘-I’.

By using both ‘-nostdinc’ and ‘-I-’, you can limit the include-file search path to only those directories you specify explicitly.

-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 ‘libg++’.)

-undef

Do not predefine any nonstandard macros. (Including architecture flags).

-E

Run only the C preprocessor. Preprocess all the C source files specified and output the results to standard output or to the specified output file.

-C

Tell the preprocessor not to discard comments. Used with the ‘-E’ option.

-P

Tell the preprocessor not to generate ‘#line’ commands. Used with the ‘-E’ option.

-M

Tell the preprocessor to output a rule suitable for make describing the dependencies of each object file. For each source file, the preprocessor outputs one make-rule whose target is the object file name for that source file and whose dependencies are all the files ‘#include’d in it. This rule may be a single line or may be continued with ‘\’-newline if it is long. The list of rules is printed on standard output instead of the preprocessed C program.

‘-M’ implies ‘-E’.

Another way to specify output of a make rule is by setting the environment variable DEPENDENCIES_OUTPUT (see section Environment Variables Affecting GNU CC).

-MM

Like ‘-M’ but the output mentions only the user header files included with ‘#include "file"’. System header files included with ‘#include <file>’ are omitted.

-MD

Like ‘-M’ but the dependency information is written to files with names made by replacing ‘.o’ with ‘.d’ at the end of the output file names. This is in addition to compiling the input files as specified—‘-MD’ does not inhibit ordinary compilation the way ‘-M’ does.

The Mach utility ‘md’ can be used to merge the ‘.d’ files into a single dependency file suitable for using with the ‘make’ command.

-MMD

Like ‘-MD’ except mention only user header files, not system header files.

-H

Print the name of each header file used, in addition to other normal activities.

-Aquestion(answer)

Assert the answer answer for question, in case it is tested with a preprocessor conditional such as ‘#if #question(answer)’. ‘-A-’ disables the standard assertions that normally describe the target machine.

-Dmacro

Define macro macro with the string ‘1’ as its definition.

-Dmacro=defn

Define macro macro as defn. All instances of ‘-D’ on the command line are processed before any ‘-U’ options.

-Umacro

Undefine macro macro. ‘-U’ options are evaluated after all ‘-D’ options, but before any ‘-include’ and ‘-imacros’ options.

-dM

Tell the preprocessor to output only a list of the macro definitions that are in effect at the end of preprocessing. Used with the ‘-E’ option.

-dD

Tell the preprocessing to pass all macro definitions into the output, in their proper sequence in the rest of the output.

-dN

Like ‘-dD’ except that the macro arguments and contents are omitted. Only ‘#define name’ is included in the output.

-trigraphs

Support ANSI C trigraphs. You don’t want to know about this brain-damage. The ‘-ansi’ option also has this effect.

1.8 Passing 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.

1.9 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. See section Options Controlling the Kind of Output.

-llibrary

Search the library named library when linking.

It makes a difference where in the command you write this option; the linker searches 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

This special case of the ‘-l’ option is what you need to do when you link an Objective C program.

-nostdlib

Don’t use the standard system libraries and startup files when linking. Only the files you specify will be passed to the linker.

-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. Only a few systems support this option.

-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 GNU CC 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 "-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.

1.10 Options for Directory Search

These options specify directories to search for header files, for libraries and for parts of the compiler:

-Idir

Append directory dir to the list of directories searched for include files.

-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 and data files of the compiler itself.

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/’ (see section 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.

‘-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.

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. See section Environment Variables Affecting GNU CC.

1.11 Specifying Target Machine and Compiler Version

By default, GNU CC compiles code for the same type of machine that you are using. However, it can also be installed as a cross-compiler, to compile for some other type of machine. In fact, several different configurations of GNU CC, for different target machines, can be installed side by side. Then you specify which one to use with the ‘-b’ option.

In addition, older and newer versions of GNU CC can be installed side by side. One of them (probably the newest) will be the default, but you may sometimes wish to use another.

-b machine

The argument machine specifies the target machine for compilation. This is useful when you have installed GNU CC as a cross-compiler.

The value to use for machine is the same as was specified as the machine type when configuring GNU CC as a cross-compiler. For example, if a cross-compiler was configured with ‘configure i386v’, meaning to compile for an 80386 running System V, then you would specify ‘-b i386v’ to run that cross compiler.

When you do not specify ‘-b’, it normally means to compile for the same type of machine that you are using.

-V version

The argument version specifies which version of GNU CC to run. This is useful when multiple versions are installed. For example, version might be ‘2.0’, meaning to run GNU CC version 2.0.

The default version, when you do not specify ‘-V’, is controlled by the way GNU CC is installed. Normally, it will be a version that is recommended for general use.

The ‘-b’ and ‘-V’ options actually work by controlling part of the file name used for the executable files and libraries used for compilation. A given version of GNU CC, for a given target machine, is normally kept in the directory ‘/usr/local/lib/gcc-lib/machine/version’.

It follows that sites can customize the effect of ‘-b’ or ‘-V’ either by changing the names of these directories or adding alternate names (or symbolic links). Thus, if ‘/usr/local/lib/gcc-lib/80386’ is a link to ‘/usr/local/lib/gcc-lib/i386v’, then ‘-b 80386’ becomes an alias for ‘-b i386v’.

In one respect, the ‘-b’ or ‘-V’ do not completely change to a different compiler: the top-level driver program gcc that you originally invoked continues to run and invoke the other executables (preprocessor, compiler per se, assembler and linker) that do the real work. However, since no real work is done in the driver program, it usually does not matter that the driver program in use is not the one for the specified target and version.

The only way that the driver program depends on the target machine is in the parsing and handling of special machine-specific options. However, this is controlled by a file which is found, along with the other executables, in the directory for the specified version and target machine. As a result, a single installed driver program adapts to any specified target machine and compiler version.

The driver program executable does control one significant thing, however: the default version and target machine. Therefore, you can install different instances of the driver program, compiled for different targets or versions, under different names.

For example, if the driver for version 2.0 is installed as ogcc and that for version 2.1 is installed as gcc, then the command gcc will use version 2.1 by default, while ogcc will use 2.0 by default. However, you can choose either version with either command with the ‘-V’ option.

1.12 Specifying Hardware Models and Configurations

Earlier we discussed the standard option ‘-b’ which chooses among different installed compilers for completely different target machines, such as Vax vs. 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.

1.12.1 M680x0 Options

These are the ‘-m’ options defined for the 68000 series. The default values for these options depends on which style of 68000 was selected when the compiler was configured; the defaults for the most common choices are given below.

-m68000

-mc68000

Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems.

-m68020

-mc68020

Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems.

-m68881

Generate output containing 68881 instructions for floating point. This is the default for most 68020 systems unless ‘-nfp’ was specified when the compiler was configured.

-m68030

Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems.

-m68040

Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems.

-m68020-40

Generate output for a 68040, without using any of the new instructions. This results in code which can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040.

-mfpa

Generate output containing Sun FPA instructions for floating point.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GNU CC. Normally the facilities of the machine’s usual C compiler are used, but this can’t be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

-mshort

Consider type int to be 16 bits wide, like short int.

-mnobitfield

Do not use the bit-field instructions. ‘-m68000’ implies ‘-mnobitfield’.

-mbitfield

Do use the bit-field instructions. ‘-m68020’ implies ‘-mbitfield’. This is the default if you use the unmodified sources configured for a 68020.

-mrtd

Use a different function-calling convention, in which functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.

This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.

Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code will be generated for calls to those functions.

In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)

The rtd instruction is supported by the 68010 and 68020 processors, but not by the 68000.

1.12.2 VAX Options

These ‘-m’ options are defined for the Vax:

-munix

Do not output certain jump instructions (aobleq and so on) that the Unix assembler for the Vax cannot handle across long ranges.

-mgnu

Do output those jump instructions, on the assumption that you will assemble with the GNU assembler.

-mg

Output code for g-format floating point numbers instead of d-format.

1.12.3 SPARC Options

These ‘-m’ switches are supported on the Sparc:

-mforce-align

Make sure all objects of type double are 8-byte aligned in memory and use double-word instructions to reference them.

-mno-epilogue

Generate separate return instructions for return statements. This has both advantages and disadvantages; I don’t recall what they are.

1.12.4 Convex Options

These ‘-m’ options are defined for the Convex:

-mc1

Generate output for a C1. This is the default when the compiler is configured for a C1.

-mc2

Generate output for a C2. This is the default when the compiler is configured for a C2.

-margcount

Generate code which puts an argument count in the word preceding each argument list. Some nonportable Convex and Vax programs need this word. (Debuggers don’t, except for functions with variable-length argument lists; this info is in the symbol table.)

-mnoargcount

Omit the argument count word. This is the default if you use the unmodified sources.

1.12.5 AMD29K Options

These ‘-m’ options are defined for the AMD Am29000:

-mdw

Generate code that assumes the DW bit is set, i.e., that byte and halfword operations are directly supported by the hardware. This is the default.

-mnodw

Generate code that assumes the DW bit is not set.

-mbw

Generate code that assumes the system supports byte and halfword write operations. This is the default.

-mnbw

Generate code that assumes the systems does not support byte and halfword write operations. ‘-mnbw’ implies ‘-mnodw’.

-msmall

Use a small memory model that assumes that all function addresses are either within a single 256 KB segment or at an absolute address of less than 256K. This allows the call instruction to be used instead of a const, consth, calli sequence.

-mlarge

Do not assume that the call instruction can be used; this is the default.

-m29050

Generate code for the Am29050.

-m29000

Generate code for the Am29000. This is the default.

-mkernel-registers

Generate references to registers gr64-gr95 instead of gr96-gr127. This option can be used when compiling kernel code that wants a set of global registers disjoint from that used by user-mode code.

Note that when this option is used, register names in ‘-f’ flags must use the normal, user-mode, names.

-muser-registers

Use the normal set of global registers, gr96-gr127. This is the default.

-mstack-check

Insert a call to __msp_check after each stack adjustment. This is often used for kernel code.

1.12.6 M88K Options

These ‘-m’ options are defined for Motorola 88K architectures:

-m88000

Generate code that works well on both the m88100 and the m88110.

-m88100

Generate code that works best for the m88100, but that also runs on the m88110.

-m88110

Generate code that works best for the m88110, and may not run on the m88100.

-midentify-revision

Include an ident directive in the assembler output recording the source file name, compiler name and version, timestamp, and compilation flags used.

-mno-underscores

In assembler output, emit symbol names without adding an underscore character at the beginning of each name. The default is to use an underscore as prefix on each name.

-mocs-debug-info

-mno-ocs-debug-info

Include (or omit) additional debugging information (about registers used in each stack frame) as specified in the 88open Object Compatibility Standard, “OCS”. This extra information allows debugging of code that has had the frame pointer eliminated. The default for DG/UX, SVr4, and Delta 88 SVr3.2 is to include this information; other 88k configurations omit this information by default.

-mocs-frame-position

When emitting COFF debugging information for automatic variables and parameters stored on the stack, use the offset from the canonical frame address, which is the stack pointer (register 31) on entry to the function. The DG/UX, SVr4, Delta88 SVr3.2, and BCS configurations use ‘-mocs-frame-position’; other 88k configurations have the default ‘-mno-ocs-frame-position’.

-mno-ocs-frame-position

When emitting COFF debugging information for automatic variables and parameters stored on the stack, use the offset from the frame pointer register (register 30). When this option is in effect, the frame pointer is not eliminated when debugging information is selected by the -g switch.

-moptimize-arg-area

-mno-optimize-arg-area

Control how to store function arguments in stack frames. ‘-moptimize-arg-area’ saves space, but conflicts with the 88open specifications. ‘-mno-optimize-arg-area’ conforms to the 88open standards. By default GNU CC does not optimize the argument area.

-mshort-data-num

Generate smaller data references by making them relative to r0, which allows loading a value using a single instruction (rather than the usual two). You control which data references are affected by specifying num with this option. For example, if you specify ‘-mshort-data-512’, then the data references affected are those involving displacements of less than 512 bytes. ‘-mshort-data-num’ is not effective for num greater than 64K.

-mserialize-volatile

-mno-serialize-volatile

Do, or don’t, generate code to guarantee sequential consistency of volatile memory references.

GNU CC always guarantees consistency by default, for the preferred processor submodel. How this is done depends on the submodel.

The m88100 processor does not reorder memory references and so always provides sequential consistency. If you use ‘-m88100’, GNU CC does not generate any special instructions for sequential consistency.

The order of memory references made by the m88110 processor does not always match the order of the instructions requesting those references. In particular, a load instruction may execute before a preceding store instruction. Such reordering violates sequential consistency of volatile memory references, when there are multiple processors. When you use ‘-m88000’ or ‘-m88110’, GNU CC generates special instructions when appropriate, to force execution in the proper order.

The extra code generated to guarantee consistency may affect the performance of your application. If you know that you can safely forgo this guarantee, you may use ‘-mno-serialize-volatile’.

If you use ‘-m88100’ but require sequential consistency when running on the m88110 processor, you should use ‘-mserialize-volatile’.

-msvr4

-msvr3

Turn on (‘-msvr4’) or off (‘-msvr3’) compiler extensions related to System V release 4 (SVr4). This controls the following:

1. Which variant of the assembler syntax to emit (which you can select independently using ‘-mversion-03.00’).
2. ‘-msvr4’ makes the C preprocessor recognize ‘#pragma weak’ that is used on System V release 4.
3. ‘-msvr4’ makes GNU CC issue additional declaration directives used in SVr4.

‘-msvr3’ is the default for all m88K configurations except the SVr4 configuration.

-mversion-03.00

In the DG/UX configuration, there are two flavors of SVr4. This option modifies ‘-msvr4’ to select whether the hybrid-COFF or real-ELF flavor is used. All other configurations ignore this option.

-mno-check-zero-division

-mcheck-zero-division

Early models of the 88K architecture had problems with division by zero; in particular, many of them didn’t trap. Use these options to avoid including (or to include explicitly) additional code to detect division by zero and signal an exception. All GNU CC configurations for the 88K use ‘-mcheck-zero-division’ by default.

-muse-div-instruction

Do not emit code to check both the divisor and dividend when doing signed integer division to see if either is negative, and adjust the signs so the divide is done using non-negative numbers. Instead, rely on the operating system to calculate the correct value when the div instruction traps. This results in different behavior when the most negative number is divided by -1, but is useful when most or all signed integer divisions are done with positive numbers.

-mtrap-large-shift

-mhandle-large-shift

Include code to detect bit-shifts of more than 31 bits; respectively, trap such shifts or emit code to handle them properly. By default GNU CC makes no special provision for large bit shifts.

-mwarn-passed-structs

Warn when a function passes a struct as an argument or result. Structure-passing conventions have changed during the evolution of the C language, and are often the source of portability problems. By default, GNU CC issues no such warning.

1.12.7 IBM RS/6000 Options

Only one pair of ‘-m’ options is defined for the IBM RS/6000:

-mfp-in-toc

-mno-fp-in-toc

Control whether or not floating-point constants go in the Table of Contents (TOC), a table of all global variable and function addresses. By default GNU CC puts floating-point constants there; if the TOC overflows, ‘-mno-fp-in-toc’ will reduce the size of the TOC, which may avoid the overflow.

1.12.8 IBM RT Options

These ‘-m’ options are defined for the IBM RT PC:

-min-line-mul

Use an in-line code sequence for integer multiplies. This is the default.

-mcall-lib-mul

Call lmul$$ for integer multiples.

-mfull-fp-blocks

Generate full-size floating point data blocks, including the minimum amount of scratch space recommended by IBM. This is the default.

-mminimum-fp-blocks

Do not include extra scratch space in floating point data blocks. This results in smaller code, but slower execution, since scratch space must be allocated dynamically.

-mfp-arg-in-fpregs

Use a calling sequence incompatible with the IBM calling convention in which floating point arguments are passed in floating point registers. Note that varargs.h and stdargs.h will not work with floating point operands if this option is specified.

-mfp-arg-in-gregs

Use the normal calling convention for floating point arguments. This is the default.

-mhc-struct-return

Return structures of more than one word in memory, rather than in a register. This provides compatibility with the MetaWare HighC (hc) compiler. Use ‘-fpcc-struct-return’ for compatibility with the Portable C Compiler (pcc).

-mnohc-struct-return

Return some structures of more than one word in registers, when convenient. This is the default. For compatibility with the IBM-supplied compilers, use either ‘-fpcc-struct-return’ or ‘-mhc-struct-return’.

1.12.9 MIPS Options

These ‘-m’ options are defined for the MIPS family of computers:

-mcpu=cpu type

Assume the defaults for the machine type cpu type when scheduling instructions. The default cpu type is ‘default’, which picks the longest cycles times for any of the machines, in order that the code run at reasonable rates on all MIPS cpu’s. Other choices for cpu type are ‘r2000’, ‘r3000’, ‘r4000’, and ‘r6000’. While picking a specific cpu type will schedule things appropriately for that particular chip, the compiler will not generate any code that does not meet level 1 of the MIPS ISA (instruction set architecture) without the ‘-mips2’ or ‘-mips3’ switches being used.

-mips2

Issue instructions from level 2 of the MIPS ISA (branch likely, square root instructions). The ‘-mcpu=r4000’ or ‘-mcpu=r6000’ switch must be used in conjunction with ‘-mips2’.

-mips3

Issue instructions from level 3 of the MIPS ISA (64 bit instructions). You must use the ‘-mcpu=r4000’ switch along with ‘-mips3’.

-mint64

-mlong64

-mlonglong128

These options don’t work at present.

-mmips-as

Generate code for the MIPS assembler, and invoke ‘mips-tfile’ to add normal debug information. This is the default for all platforms except for the OSF/1 reference platform, using the OSF/rose object format. If the either of the ‘-gstabs’ or ‘-gstabs+’ switches are used, the ‘mips-tfile’ program will encapsulate the stabs within MIPS ECOFF.

-mgas

Generate code for the GNU assembler. This is the default on the OSF/1 reference platform, using the OSF/rose object format.

-mrnames

-mno-rnames

The ‘-mrnames’ switch says to output code using the MIPS software names for the registers, instead of the hardware names (ie, a0 instead of $4). The GNU assembler does not support the ‘-mrnames’ switch, and the MIPS assembler will be instructed to run the MIPS C preprocessor over the source file. The ‘-mno-rnames’ switch is default.

-mgpopt

-mno-gpopt

The ‘-mgpopt’ switch says to write all of the data declarations before the instructions in the text section, this allows the MIPS assembler to generate one word memory references instead of using two words for short global or static data items. This is on by default if optimization is selected.

-mstats

-mno-stats

For each non-inline function processed, the ‘-mstats’ switch causes the compiler to emit one line to the standard error file to print statistics about the program (number of registers saved, stack size, etc.).

-mmemcpy

-mno-memcpy

The ‘-mmemcpy’ switch makes all block moves call the appropriate string function (‘memcpy’ or ‘bcopy’) instead of possibly generating inline code.

-mmips-tfile

-mno-mips-tfile

The ‘-mno-mips-tfile’ switch causes the compiler not postprocess the object file with the ‘mips-tfile’ program, after the MIPS assembler has generated it to add debug support. If ‘mips-tfile’ is not run, then no local variables will be available to the debugger. In addition, ‘stage2’ and ‘stage3’ objects will have the temporary file names passed to the assembler embedded in the object file, which means the objects will not compare the same. The ‘-mno-mips-tfile’ switch should only be used when there are bugs in the ‘mips-tfile’ program that prevents compilation.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GNU CC. Normally the facilities of the machine’s usual C compiler are used, but this can’t be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

-mhard-float

Generate output containing floating point instructions. This is the default if you use the unmodified sources.

-mfp64

Assume that the FR bit in the status word is on, and that there are 32 64-bit floating point registers, instead of 32 32-bit floating point registers. You must also specify the ‘-mcpu=r4000’ and ‘-mips3’ switches.

-mfp32

Assume that there are 32 32-bit floating point registers. This is the default.

-mabicalls

-mno-abicalls

Emit the ‘.abicalls’, ‘.cpload’, and ‘.cprestore’ pseudo operations that some System V.4 ports use for position independent code.

-mhalf-pic

-mno-half-pic

Put pointers to extern references into the data section and load them up, rather than put the references in the text section. These options do not work at present.

-G num

Put global and static items less than or equal to num bytes into the small data or bss sections instead of the normal data or bss section. This allows the assembler to emit one word memory reference instructions based on the global pointer (gp or $28), instead of the normal two words used. By default, num is 8 when the MIPS assembler is used, and 0 when the GNU assembler is used. The ‘-G num’ switch is also passed to the assembler and linker. All modules should be compiled with the same ‘-G num’ value.

-nocpp

Tell the MIPS assembler to not run it’s preprocessor over user assembler files (with a ‘.s’ suffix) when assembling them.

1.12.10 Intel 386 Options

These ‘-m’ options are defined for the i386 family of computers:

-m486

-mno-486

Control whether or not code is optimized for a 486 instead of an 386. Code generated for an 486 will run on a 386 and vice versa.

-msoft-float

Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GNU CC. Normally the facilities of the machine’s usual C compiler are used, but this can’t be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

On machines where a function returns floating point results in the 80387 register stack, some floating point opcodes may be emitted even if ‘-msoft-float’ is used.

-mno-fp-ret-in-387

Don’t use the FPU registers for return values of functions.

The usual calling convention has functions return values of types float and double in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU.

The option ‘-mno-fp-ret-in-387’ causes such values to be returned in ordinary CPU registers instead.

1.12.11 HPPA Options

This ‘-m’ option is defined for the HPPA family of computers:

-mno-bss

Disable the use of the BSS section. This may be necessary with older versions of pa-gas. It is highly recommended that you pick up a new version of pa-gas from jaguar.cs.utah.edu.

-mpa-risc-1-0

Generate code for a PA 1.0 processor.

-mpa-risc-1-1

Generate code for a PA 1.1 processor.

-mkernel

Generate code which is suitable for use in kernels. Specifically, avoid add instructions in which one of the arguments is the DP register; generate addil instructions instead. This avoids a rather serious bug in the HP-UX linker.

-mshared-libs

Generate code that can be linked against HP-UX shared libraries. This option is not fully function yet, and is not on by default for any PA target.

-mno-shared-libs

Don’t generate code that will be linked against shared libraries. This is the default for all PA targets.

-mlong-calls

Generate code which allows calls to functions greater than 256K away from the caller when the caller and callee are in the same source file. Do not turn this option on unless code refuses to link with "branch out of range errors" from the linker.

1.12.12 Intel 960 Options

These ‘-m’ options are defined for the Intel 960 implementations:

-mcpu type

Assume the defaults for the machine type cpu type for some of the other options, including instruction scheduling, floating point support, and addressing modes. The choices for cpu type are ‘ka’, ‘kb’, ‘mc’, ‘ca’, ‘cf’, ‘sa’, and ‘sb’. The default is ‘kb’.

-mnumerics

-msoft-float

The ‘-mnumerics’ option indicates that the processor does support floating-point instructions. The ‘-msoft-float’ option indicates that floating-point support should not be assumed.

-mleaf-procedures

-mno-leaf-procedures

Do (or do not) attempt to alter leaf procedures to be callable with the bal instruction as well as call. This will result in more efficient code for explicit calls when the bal instruction can be substituted by the assembler or linker, but less efficient code in other cases, such as calls via function pointers, or using a linker that doesn’t support this optimization.

-mtail-call

-mno-tail-call

Do (or do not) make additional attempts (beyond those of the machine-independent portions of the compiler) to optimize tail-recursive calls into branches. You may not want to do this because the detection of cases where this is not valid is not totally complete. The default is ‘-mno-tail-call’.

-mcomplex-addr

-mno-complex-addr

Assume (or do not assume) that the use of a complex addressing mode is a win on this implementation of the i960. Complex addressing modes may not be worthwhile on the K-series, but they definitely are on the C-series. The default is currently ‘-mcomplex-addr’ for all processors except the CB and CC.

-mcode-align

-mno-code-align

Align code to 8-byte boundaries for faster fetching (or don’t bother). Currently turned on by default for C-series implementations only.

-mic-compat

-mic2.0-compat

-mic3.0-compat

Enable compatibility with iC960 v2.0 or v3.0.

-masm-compat

-mintel-asm

Enable compatibility with the iC960 assembler.

-mstrict-align

-mno-strict-align

Do not permit (do permit) unaligned accesses.

-mold-align

Enable structure-alignment compatibility with Intel’s gcc release version 1.3 (based on gcc 1.37). Currently this is buggy in that ‘#pragma align 1’ is always assumed as well, and cannot be turned off.

1.12.13 DEC Alpha Options

These ‘-m’ options are defined for the DEC Alpha implementations:

-mno-soft-float

-msoft-float

Use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is specified, functions in ‘libgcc1.c’ will be used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines will issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them.

Note that Alpha implementations without floating-point operations are required to have floating-point registers.

-mfp-reg

-mno-fp-regs

Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the floating-point register set is not used, floating point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option.

A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.

1.12.14 Options for System V

These additional options are available on System V Release 4 for compatibility with other compilers on those systems:

-Qy

Identify the versions of each tool used by the compiler, in a .ident assembler directive in the output.

-Qn

Refrain from adding .ident directives to the output file (this is the default).

-YP,dirs

Search the directories dirs, and no others, for libraries specified with ‘-l’.

-Ym,dir

Look in the directory dir to find the M4 preprocessor. The assembler uses this option.

1.13 Options for Code Generation Conventions

These machine-independent options control the interface conventions used in code generation.

Most of them 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 which is not the default. You can figure out the other form by either removing ‘no-’ or adding it.

-fpcc-struct-return

Use the same convention for returning struct and union values that is used by the usual C compiler on your system. This convention is less efficient for small structures, and on many machines it fails to be reentrant; but it has the advantage of allowing intercallability between GNU CC-compiled code and PCC-compiled code.

-fshort-enums

Allocate to an enum type only as many bytes as it needs for the declared range of possible values. Specifically, the enum type will be equivalent to the smallest integer type which has enough room.

-fshort-double

Use the same size for double as for float.

-fshared-data

Requests that the data and non-const variables of this compilation be shared data rather than private data. The distinction makes sense only on certain operating systems, where shared data is shared between processes running the same program, while private data exists in one copy per process.

-fno-common

Allocate even uninitialized global variables in the bss section of the object file, rather than generating them as common blocks. This has the effect that if the same variable is declared (without extern) in two different compilations, you will get an error when you link them. The only reason this might be useful is if you wish to verify that the program will work on other systems which always work this way.

-fno-ident

Ignore the ‘#ident’ directive.

-fno-gnu-linker

Don’t output global initializations such as C++ constructors and destructors in the form used by the GNU linker (on systems where the GNU linker is the standard method of handling them). Use this option when you want to use a “collect” program and a non-GNU linker.

-finhibit-size-directive

Don’t output a .size assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling ‘crtstuff.c’; you should not need to use it for anything else.

-fnonnull-objects

Assume that objects reached through references are not null (C++ only).

Normally, GNU C++ makes conservative assumptions about objects reached through references. For example, the compiler must check that a is not null in code like the following:

obj &a = g ();
a.f (2);

Checking that references of this sort have non-null values requires extra code, however, and it is unnecessary for many programs. You can use ‘-fnonnull-objects’ to omit the checks for null, if your program doesn’t require checking.

-fverbose-asm

Put extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself).

-fvolatile

Consider all memory references through pointers to be volatile.

-fpic

If supported for the target machine, generate position-independent code (PIC) suitable for use in a shared library. All addresses will be accessed through a global offset table (GOT). If the GOT size for the linked executable exceeds a machine-specific maximum size, you will get an error message from the linker indicating that ‘-fpic’ does not work; recompile with ‘-fPIC’ instead. (These maximums are 16k on the m88k, 8k on the Sparc, and 32k on the m68k and RS/6000. The 386 has no such limit.)

Position-independent code requires special support, and therefore works only on certain machines. For the 386, GNU CC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.

The GNU assembler does not fully support PIC. Currently, you must use some other assembler in order for PIC to work. We would welcome volunteers to upgrade GAS to handle this; the first part of the job is to figure out what the assembler must do differently.

-fPIC

If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on the m68k, m88k and the Sparc.

Position-independent code requires special support, and therefore works only on certain machines.

-ffixed-reg

Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role).

reg must be the name of a register. The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file.

This flag does not have a negative form, because it specifies a three-way choice.

-fcall-used-reg

Treat the register named reg as an allocatable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way will not save and restore the register reg.

Use of this flag for a register that has a fixed pervasive role in the machine’s execution model, such as the stack pointer or frame pointer, will produce disastrous results.

This flag does not have a negative form, because it specifies a three-way choice.

-fcall-saved-reg

Treat the register named reg as an allocatable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way will save and restore the register reg if they use it.

Use of this flag for a register that has a fixed pervasive role in the machine’s execution model, such as the stack pointer or frame pointer, will produce disastrous results.

A different sort of disaster will result from the use of this flag for a register in which function values may be returned.

This flag does not have a negative form, because it specifies a three-way choice.

1.14 Environment Variables Affecting GNU CC

This section describes several environment variables that affect how GNU CC operates. They work by specifying directories or prefixes to use when searching for various kinds of files.

Note that you can also specify places to search using options such as ‘-B’, ‘-I’ and ‘-L’ (see section Options for Directory Search). These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GNU CC.

TMPDIR

If TMPDIR is set, it specifies the directory to use for temporary files. GNU CC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper.

GCC_EXEC_PREFIX

If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish.

If GNU CC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.

Other prefixes specified with ‘-B’ take precedence over this prefix.

This prefix is also used for finding files such as ‘crt0.o’ that are used for linking.

In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with ‘/usr/local/lib/gcc-lib’ (more precisely, with the value of GCC_INCLUDE_DIR), GNU CC tries replacing that beginning with the specified prefix to produce an alternate directory name. Thus, with ‘-Bfoo/’, GNU CC will search ‘foo/bar’ where it would normally search ‘/usr/local/lib/bar’. These alternate directories are searched first; the standard directories come next.

COMPILER_PATH

The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GNU CC tries the directories thus specified when searching for subprograms, if it can’t find the subprograms using GCC_EXEC_PREFIX.

LIBRARY_PATH

The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. GNU CC tries the directories thus specified when searching for special linker files, if it can’t find them using GCC_EXEC_PREFIX. Linking using GNU CC also uses these directories when searching for ordinary libraries for the ‘-l’ option (but directories specified with ‘-L’ come first).

C_INCLUDE_PATH

CPLUS_INCLUDE_PATH

OBJC_INCLUDE_PATH

These environment variables pertain to particular languages. Each variable’s value is a colon-separated list of directories, much like PATH. When GNU CC searches for header files, it tries the directories listed in the variable for the language you are using, after the directories specified with ‘-I’ but before the standard header file directories.

DEPENDENCIES_OUTPUT

If this variable is set, its value specifies how to output dependencies for Make based on the header files processed by the compiler. This output looks much like the output from the ‘-M’ option (see section Options Controlling the Preprocessor), but it goes to a separate file, and is in addition to the usual results of compilation.

The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form ‘file target’, in which case the rules are written to file file using target as the target name.

1.15 Running Protoize

The program protoize is an optional part of GNU C. You can use it to add prototypes to a program, thus converting the program to ANSI C in one respect. The companion program unprotoize does the reverse: it removes argument types from any prototypes that are found.

When you run these programs, you must specify a set of source files as command line arguments. The conversion programs start out by compiling these files to see what functions they define. The information gathered about a file foo is saved in a file named ‘foo.X’.

After scanning comes actual conversion. The specified files are all eligible to be converted; any files they include (whether sources or just headers) are eligible as well.

But not all the eligible files are converted. By default, protoize and unprotoize convert only source and header files in the current directory. You can specify additional directories whose files should be converted with the ‘-d directory’ option. You can also specify particular files to exclude with the ‘-x file’ option. A file is converted if it is eligible, its directory name matches one of the specified directory names, and its name within the directory has not been excluded.

Basic conversion with protoize consists of rewriting most function definitions and function declarations to specify the types of the arguments. The only ones not rewritten are those for varargs functions.

protoize optionally inserts prototype declarations at the beginning of the source file, to make them available for any calls that precede the function’s definition. Or it can insert prototype declarations with block scope in the blocks where undeclared functions are called.

Basic conversion with unprotoize consists of rewriting most function declarations to remove any argument types, and rewriting function definitions to the old-style pre-ANSI form.

Both conversion programs print a warning for any function declaration or definition that they can’t convert. You can suppress these warnings with ‘-q’.

The output from protoize or unprotoize replaces the original source file. The original file is renamed to a name ending with ‘.save’. If the ‘.save’ file already exists, then the source file is simply discarded.

protoize and unprotoize both depend on GNU CC itself to scan the program and collect information about the functions it uses. So neither of these programs will work until GNU CC is installed.

Here is a table of the options you can use with protoize and unprotoize. Each option works with both programs unless otherwise stated.

‘-B directory’

Look for the file ‘SYSCALLS.c.X’ in directory, instead of the usual directory (normally ‘/usr/local/lib’). This file contains prototype information about standard system functions. This option applies only to protoize.

‘-c compilation-options’

Use compilation-options as the options when running gcc to produce the ‘.X’ files. The special option ‘-aux-info’ is always passed in addition, to tell gcc to write a ‘.X’ file.

Note that the compilation options must be given as a single argument to protoize or unprotoize. If you want to specify several gcc options, you must quote the entire set of compilation options to make them a single word in the shell.

There are certain gcc arguments that you cannot use, because they would produce the wrong kind of output. These include ‘-g’, ‘-O’, ‘-c’, ‘-S’, and ‘-o’ If you include these in the compilation-options, they are ignored.

‘-C’

Rename files to end in ‘.C’ instead of ‘.c’. This is convenient if you are converting a C program to C++. This option applies only to protoize.

‘-g’

Add explicit global declarations. This means inserting explicit declarations at the beginning of each source file for each function that is called in the file and was not declared. These declarations precede the first function definition that contains a call to an undeclared function. This option applies only to protoize.

‘-i string’

Indent old-style parameter declarations with the string string. This option applies only to protoize.

unprotoize converts prototyped function definitions to old-style function definitions, where the arguments are declared between the argument list and the initial ‘{’. By default, unprotoize uses five spaces as the indentation. If you want to indent with just one space instead, use ‘-i " "’.

‘-k’

Keep the ‘.X’ files. Normally, they are deleted after conversion is finished.

‘-l’

Add explicit local declarations. protoize with ‘-l’ inserts a prototype declaration for each function in each block which calls the function without any declaration. This option applies only to protoize.

‘-n’

Make no real changes. This mode just prints information about the conversions that would have been done without ‘-n’.

‘-N’

Make no ‘.save’ files. The original files are simply deleted. Use this option with caution.

‘-p program’

Use the program program as the compiler. Normally, the name ‘gcc’ is used.

‘-q’

Work quietly. Most warnings are suppressed.

‘-v’

Print the version number, just like ‘-v’ for gcc.

If you need special compiler options to compile one of your program’s source files, then you should generate that file’s ‘.X’ file specially, by running gcc on that source file with the appropriate options and the option ‘-aux-info’. Then run protoize on the entire set of files. protoize will use the existing ‘.X’ file because it is newer than the source file. For example:

gcc -Dfoo=bar file1.c -aux-info
protoize *.c

You need to include the special files along with the rest in the protoize command, even though their ‘.X’ files already exist, because otherwise they won’t get converted.

@xref{Protoize Caveats}, for more information on how to use protoize successfully.

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