### 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 that must be preprocessed.
file`.i`
  C source code that should not be preprocessed.
file`.ii`
  C++ source code that 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 that 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 that should not be preprocessed.
file`.h`
  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).
file`.cc`
file`.cp`
file`.cxx`
file`.cpp`
file`.CPP`
file`.c++`
file`.C`
  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.
file`.mm`
file`.M`
  Objective-C++ source code that must be preprocessed.
file`.mii`
  Objective-C++ source code that 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 or Ada spec.
file`.f`
file`.for`
file`.ftn`
  Fixed form Fortran source code that should not be preprocessed.
file`.F`
file`.FOR`
file`.fpp`
file`.FPP`
file`.FTN`
  Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).
file`.f90`
file`.f95`
file`.f03`
file`.f08`
  Free form Fortran source code that should not be preprocessed.
file`.F90`
file`.F95`
file`.F03`
file`.F08`
  Free form Fortran source code that must be preprocessed (with the traditional preprocessor).
file`.go`
  Go source code.
file`.ads`
  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 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 that must be preprocessed.
other
  An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.

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

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

                c  c-header  cpp-output
                c++  c++-header  c++-cpp-output
                objective-c  objective-c-header  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
                go
                java

`-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 exits 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 instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends 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 that don't require preprocessing are ignored.

`-o` file
  Place output in file file. This applies 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 arguments are quoted unless they contain only alphanumeric characters or `./-_`. 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.
`--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` is also 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 are also 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 all specified classes and qualifiers. These are the supported classes:
  ‘`optimizers`’
    Display all of the optimization options supported by the compiler.
  ‘`warnings`’
    Display all of the options controlling warning messages produced by the compiler.
  ‘`target`’
    Display target-specific options. Unlike the `--target-help` option however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended `--help=` syntax.
  ‘`params`’
    Display the values recognized by the `--param` option.
  language
    Display the options supported for language, where language is the name of one of the languages supported in this version of GCC.
  ‘`common`’
    Display the options that are common to all languages.

  These are the supported qualifiers:

  ‘`undocumented`’
    Display only those options that are undocumented.
  ‘`joined`’
    Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as: ‘`--help=target`’.
  ‘`separate`’
    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, use:

                --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) that have a description, use:

                --help=warnings,^joined,^undocumented

  The argument to `--help=` should not consist solely of inverted qualifiers.

  Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use:

                --help=target,optimizers

  The `--help=` option can be repeated on the command line. Each successive use displays 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

`-no-canonical-prefixes`
  Do not expand any symbolic links, resolve references to ‘`/../`’ or ‘`/./`’, or make the path absolute when generating a relative prefix.
`--version`
  Display the version number and copyrights of the invoked GCC.
`-wrapper`
  Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.

                gcc -c t.c -wrapper gdb,--args

  This invokes all subprograms of `gcc` under ‘`gdb --args`’, thus the invocation of `cc1` is ‘`gdb --args cc1 ...`’.

`-fplugin=`name`.so`
  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 `-fplugin-arg-name-key=value` below). Each plugin should define the callback functions specified in the Plugins API.
`-fplugin-arg-`name`-`key`=`value
  Define an argument called key with a value of value for the plugin called name.
`-fdump-ada-spec`[`-slim`]
  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.
`-fada-spec-parent=`unit
  In conjunction with `-fdump-ada-spec[-slim]` above, generate Ada specs as child units of parent unit.
`-fdump-go-spec=`file
  For input files in any language, generate corresponding Go declarations in file. This generates Go `const`, `type`, `var`, and `func` declarations which may be a useful way to start writing a Go interface to code written in some other language.
`@`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.