Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation, assembly, and linking; always in that order. The first three stages apply to an individual source file, and end by producing an object file; linking combines all the object files (those newly compiled, and those specified as input) into an executable file.
For any given input file, the filename suffix determines what kind of compilation is done, as the following descriptions help to clarify.
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 library libobjc.a
to make an Objective-C program work.
file.h
C header file
(not to be compiled or linked).
file.s
Assembler code.
file.S
Assembler code
which must be preprocessed.
other
An object
file to be fed straight into linking. Any filename 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 filename suffix). This option applies to all following
input files until the next -x
option.
Possible input for language include the following optional values.
c
objective-c c++
c-header cpp-output c++-cpp-output
assembler assembler-with-cpp
-x none
Turn off any specification of a language, so that subsequent files are
handled according to their filename suffixes (as they are if -x
has not been used at all).
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 filename 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 filename for a source file is made by replacing
the suffix .c,
.i,
etc., with .s.
Input files that dont 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 dont 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. Since only one output file can be specified, it does not make sense
to use -o
when compiling more than one input file, unless you are producing an executable
file as output. 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,
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.
-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.