Next: Invoking G++, Previous: Option Summary, Up: Invoking GCC
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:
You can specify the input language explicitly with the -x option:
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
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.
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
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.
Input files that don't require preprocessing are ignored.
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
./-_. This is useful for shell scripts to capture the driver-generated command lines.
These are the supported qualifiers:
Thus for example to display all the undocumented target-specific switches supported by the compiler, use:
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 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:
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
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 ...’.
funcdeclarations which may be a useful way to start writing a Go interface to code written in some other language.
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.