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:
- C source code that must be preprocessed.
- C source code that should not be preprocessed.
- C++ source code that 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 that 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 that should not be preprocessed.
- C, C++, Objective-C or Objective-C++ header file to be turned into a
precompiled header (default), or C, C++ header file to be turned into an
Ada spec (via the -fdump-ada-spec switch).
- C++ source code that 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 that must be preprocessed.
- Objective-C++ source code that should not be preprocessed.
- C++ header file to be turned into a precompiled header or Ada spec.
- Fixed form Fortran source code that should not be preprocessed.
- Fixed form Fortran source code that must be preprocessed (with the traditional
- Free form Fortran source code that should not be preprocessed.
- Free form Fortran source code that must be preprocessed (with the
- Go source code.
- Ada source code file that 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 that must be preprocessed.
- 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:
- Specify explicitly the language for the following input files
(rather than letting the compiler choose a default based on the file
name suffix). This option applies to all following input files until
the next -x option. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
f77 f77-cpp-input f95 f95-cpp-input
- 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
- 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 that 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
- 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 arguments
are quoted unless they contain only alphanumeric characters or
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
- 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 that
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 that are undocumented.
- Display options taking an argument that appears after an equal
sign in the same continuous piece of text, such as:
- Display options taking 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) that have a description, use:
The argument to --help= should not consist solely of inverted
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:
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
- Do not expand any symbolic links, resolve references to `/../'
or `/./', or make the path absolute when generating a relative
- Display the version number and copyrights of the invoked GCC.
- Invoke all subcommands under a wrapper program. The name of the
wrapper program and its parameters are passed as a comma separated
gcc -c t.c -wrapper gdb,--args
This will invoke all subprograms of gcc under
`gdb --args', thus the invocation of cc1 will be
`gdb --args cc1 ...'.
- Load the plugin code in file name.so, assumed to be a
shared object to be dlopen'd by the compiler. The base name of
the shared object file is used to identify the plugin for the
purposes of argument parsing (See
Each plugin should define the callback functions specified in the
- Define an argument called key with a value of value
for the plugin called name.
- For C and C++ source and include files, generate corresponding Ada
specs. See Generating Ada Bindings for C and C++ headers, which provides detailed documentation on this feature.
- For input files in any language, generate corresponding Go
declarations in file. This generates Go
func declarations which may be a
useful way to start writing a Go interface to code written in some
- 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.