GCC preprocessor

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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?

Answer 1

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.

Answer 2

Use gcc -E to only run the preprocessor part, e.g. give 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.

Answer 3

It sounds like you actually want unifdef, not the GCC preprocessor.

Answer 4

You probably can use:

gcc -CC -P -Uswitch -undef -nostdinc -fdirectives-only -dDI -E 

With switch the #define you know will be undefined.