GCC preprocessor

Question

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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?

Exact duplicate (looks like same user, earlier today): [gcc preprocessor](http://stackoverflow.com/questions/3916979/gcc-preprocessor) — Paul R, Oct 12 '10 at 17:43
@Paul Is indeed the same user; the original version of this question before Michael cleaned it up was word-for-word identical, including the *Richard Luo* sig. — user229044, Oct 12 '10 at 17:57

score 43 · Answer 1 · answered Oct 12 '10 at 17:25

43

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.

answered Oct 12 '10 at 17:25

Benjamin Bannier

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thanks for answer. maybe my question is not clear enough. but honk answered my question. – richard Oct 12 '10 at 17:54
What do the `# 1` lines mean? – 425nesp Apr 13 '22 at 20:35

score 5 · Answer 2 · answered Oct 12 '10 at 17:30

5

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

answered Oct 12 '10 at 17:30

Christoffer

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score 2 · Answer 3 · edited May 23 '17 at 10:31

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.

score -1 · Answer 4 · answered Oct 12 '10 at 18:41

-1

You probably can use:

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

With switch the #define you know will be undefined.

answered Oct 12 '10 at 18:41

Nicolas Viennot

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GCC preprocessor

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