Possible Duplicate:
Running the GCC preprocessor
Is there a GCC option to make the GCC preprocessor generate C source code but filter out irrelevant source code?
For example, a C file has #define switch to define for many different platforms. I'm only intersted in one platform, so I want the C preprocessor to filter out unrelated code. Does GCC support this?
Use gcc -E to only run the preprocessor part, e.g., given a file, in.c,
#if 0
0;
#endif
#if 1
1;
#endif
running
gcc -E in.c -o in.i
yields a file, in.i:
# 1 "in.cpp"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "in.cpp"
1;
I.e., the parts behind the #if 0 got removed. If you would have #include'd files, they would have been pasted too though, so I am not sure how much help this is.
It sounds like you actually want unifdef, not the GCC preprocessor.
Yes - almost certainly your compiler provides certain default definitions in the environment that you can use to turn code on and off for different systems. __GNUC__ is a good one for GCC. For example:
#ifdef __GNUC__
#define SOME_VALUE 12
#else
#define SOME_VALUE 14
#endif
If you compile that block with GCC, SOME_VALUE will be 12, and if you compile with MSVC, for example, SOME_VALUE will be 14. A list of platform specific definitions is available at this question.
You probably can use:
gcc -CC -P -Uswitch -undef -nostdinc -fdirectives-only -dDI -E
With switch the #define you know will be undefined.
