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.
- C source code which must be preprocessed.
- C source code which should not be preprocessed.
- C++ source code which should not be preprocessed.
- Objective-C source code. Note that you must link with the libobjc
library to make an Objective-C program work.
- Objective-C source code which should not be preprocessed.
- 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.
- Objective-C++ source code which should not be preprocessed.
- C, C++, Objective-C or Objective-C++ header file to be turned into a
- 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.
- Objective-C++ source code which must be preprocessed.
- Objective-C++ source code which should not be preprocessed.
- C++ header file to be turned into a precompiled header.
- Fixed form Fortran source code which should not be preprocessed.
- Fixed form Fortran source code which must be preprocessed (with the traditional
- Free form Fortran source code which should not be preprocessed.
- Free form Fortran source code which must be preprocessed (with the
- 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
- Ada source code file containing a library unit body (a subprogram or
package body). Such files are also called bodies.
- Assembler code.
- Assembler code which must be preprocessed.
- An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.
- 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 f77 f77-cpp-input f95 f95-cpp-input java
- 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).
- 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.
- 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.
- 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
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.
- 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
Input files which don't require preprocessing are ignored.
- 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.
- 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.
- 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
- 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
(one for each source file), but only one (combined) .o or
- 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.
- 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.
- Print (on the standard output) a description of the command line
options understood by the compiler that fit into all specified classes
and qualifiers. These are the supported classes:
- This will display all of the optimization options supported by the
- This will display all of the options controlling warning messages
produced by the compiler.
- 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.
- This will display the values recognized by the --param
- This will display the options supported for language, where
language is the name of one of the languages supported in this
version of GCC.
- This will display the options that are common to all languages.
These are the supported qualifiers:
- Display only those options which are undocumented.
- Display options which take an argument that appears after an equal
sign in the same continuous piece of text, such as:
- 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:
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:
The argument to --help= should not consist solely of inverted qualifiers.
Combining several classes is possible, 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 classes is target. So for example to display all the target-specific optimization options the following can be used:
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:
-Q -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
- Display the version number and copyrights of the invoked GCC.
- Invoke all subcommands under a wrapper program. It takes a single
comma separated list as an argument, which will be used to invoke
gcc -c t.c -wrapper gdb,--args
This will invoke all subprograms of gcc under "gdb –args", thus cc1 invocation will be "gdb –args cc1 ...".
- 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.