Overall Options - Using the GNU Compiler Collection (GCC)- Weblessons

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3.2 Options Controlling the Kind of Output

Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. GCC is capable of preprocessing and compiling several files either into several assembler input files, or into one assembler input file; then each assembler input file produces an object file, and 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 libobjc library to make an Objective-C program work.
file .mi: Objective-C source code which should not be preprocessed.
file .mm
file .M: Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++ program work. Note that .M refers to a literal capital M.
file .mii: Objective-C++ source code which should not be preprocessed.
file .h: C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header.
file .cc
file .cp
file .cxx
file .cpp
file .CPP
file .c++
file .C: C++ source code which must be preprocessed. Note that in .cxx, the last two letters must both be literally x. Likewise, .C refers to a literal capital C.
file .mm
file .M: Objective-C++ source code which must be preprocessed.
file .mii: Objective-C++ source code which should not be preprocessed.
file .hh
file .H
file .hp
file .hxx
file .hpp
file .HPP
file .h++
file .tcc: C++ header file to be turned into a precompiled header.
file .f
file .for
file .FOR: Fixed form Fortran source code which should not be preprocessed.
file .F
file .fpp
file .FPP: Fixed form Fortran source code which must be preprocessed (with the traditional preprocessor).
file .f90
file .f95: Free form Fortran source code which should not be preprocessed.
file .F90
file .F95: Free form Fortran source code which must be preprocessed (with the traditional preprocessor).
file .ads: Ada source code file which contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs.
file .adb: Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.
file .s: Assembler code.
file .S
file .sx: Assembler code which must be preprocessed.
other: An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.

You can specify the input language explicitly with the -x option:

-x language

Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix). This option applies to all following input files until the next -x option. Possible values for language are:

          c  c-header  c-cpp-output
          c++  c++-header  c++-cpp-output
          objective-c  objective-c-header  objective-c-cpp-output
          objective-c++ objective-c++-header objective-c++-cpp-output
          assembler  assembler-with-cpp
          ada
          f95  f95-cpp-input
          java
          treelang

-x none

Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if -x has not been used at all).

-pass-exit-codes

Normally the gcc program will exit with the code of 1 if any phase of the compiler returns a non-success return code. If you specify -pass-exit-codes, the gcc program will instead return with numerically highest error produced by any phase that returned an error indication. The C, C++, and Fortran frontends return 4, if an internal compiler error is encountered.

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.

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, a precompiled header file in source . suffix .gch, and all preprocessed C source on standard output.

-v

Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.

-###

Like -v except the commands are not executed and all command arguments are quoted. This is useful for shell scripts to capture the driver-generated command lines.

-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.

-combine

If you are compiling multiple source files, this option tells the driver to pass all the source files to the compiler at once (for those languages for which the compiler can handle this). This will allow intermodule analysis (IMA) to be performed by the compiler. Currently the only language for which this is supported is C. If you pass source files for multiple languages to the driver, using this option, the driver will invoke the compiler(s) that support IMA once each, passing each compiler all the source files appropriate for it. For those languages that do not support IMA this option will be ignored, and the compiler will be invoked once for each source file in that language. If you use this option in conjunction with -save-temps, the compiler will generate multiple pre-processed files (one for each source file), but only one (combined) .o or .s file.

--help

Print (on the standard output) a description of the command line options understood by gcc. If the -v option is also specified then --help will also be passed on to the various processes invoked by gcc, so that they can display the command line options they accept. If the -Wextra option has also been specified (prior to the --help option), then command line options which have no documentation associated with them will also be displayed.

--target-help

Print (on the standard output) a description of target-specific command line options for each tool. For some targets extra target-specific information may also be printed.

--help= class [ , qualifier ]

Print (on the standard output) a description of the command line options understood by the compiler that fit into a specific class. The class can be one of optimizers, warnings, target, params, or language:

optimizers: This will display all of the optimization options supported by the compiler.
warnings: This will display all of the options controlling warning messages produced by the compiler.
target: This will display target-specific options. Unlike the --target-help option however, target-specific options of the linker and assembler will not be displayed. This is because those tools do not currently support the extended --help= syntax.
params: This will display the values recognized by the --param option.
language: This will display the options supported for language, where language is the name of one of the languages supported in this version of GCC.
common: This will display the options that are common to all languages.

It is possible to further refine the output of the --help= option by adding a comma separated list of qualifiers after the class. These can be any from the following list:

undocumented: Display only those options which are undocumented.
joined: Display options which take an argument that appears after an equal sign in the same continuous piece of text, such as: --help=target.
separate: Display options which take an argument that appears as a separate word following the original option, such as: -o output-file.

Thus for example to display all the undocumented target-specific switches supported by the compiler the following can be used:

          --help=target,undocumented

The sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument), which have a description the following can be used:

          --help=warnings,^joined,^undocumented

A class can also be used as a qualifier, although this usually restricts the output by so much that there is nothing to display. One case where it does work however is when one of the subjects is target. So for example to display all the target-specific optimization options the following can be used:

          --help=target,optimizers

The --help= option can be repeated on the command line. Each successive use will display its requested class of options, skipping those that have already been displayed.

If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by --help= is changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the --help= option is used).

Here is a truncated example from the ARM port of gcc:

            % gcc -Q -mabi=2 --help=target -c
            The following options are target specific:
            -mabi=                                2
            -mabort-on-noreturn                   [disabled]
            -mapcs                                [disabled]

The output is sensitive to the effects of previous command line options, so for example it is possible to find out which optimizations are enabled at -O2 by using:

          -O2 --help=optimizers

Alternatively you can discover which binary optimizations are enabled by -O3 by using:

          gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
          gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
          diff /tmp/O2-opts /tmp/O3-opts | grep enabled

--version

Display the version number and copyrights of the invoked GCC.

@ file

Read command-line options from file. The options read are inserted in place of the original @file option. If file does not exist, or cannot be read, then the option will be treated literally, and not removed.

Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively.