Forcing GCC to compile .cpp file as C

Question

I have an externally provided .cpp file. It is a mixture of C compatible code and a bit of C++ as well. The C++ code is just a wrapper around the C to take advantage of C++ features.

It uses #ifdef __cplusplus macros to protect the C++ code, which is great. Unfortunately, if I try to compile using GCC, it treats it as C++ because of the file ending. I'm aware of the differences between gcc and g++ - I don't want to compile as C++.

Is there any way I can force GCC to treat this file as a C file? I've tried using e.g. --std=c99, but this correctly produces the error that C99 isn't valid for C++.

Renaming the file to .c works, but I'd like to avoid this if possible because it's externally provided and it'd be nice for it to remain as a pristine copy.

Sometimes when I see SO questions. I just want to scream a big RTFM!! man gcc. For this king of question it actually takes less time than writing the question. — log0
– log0, Commented Nov 18, 2010 at 15:21
I had the same problem because I wanted to compile pure C code within a .cpp file...obviously I'm quite a beginner concerning C. RTFM! is not such a good advice then. gcc man pages are 11.000 lines and it's kind of hard to find a solution in there if you don't know what to look for. — mort
– mort, Commented Nov 4, 2011 at 8:05
I disagree, reading documentation is never a loss of time, and the documentation of GCC is IMHO well written — Basile Starynkevitch
– Basile Starynkevitch, Commented Apr 26, 2021 at 14:26

Peter Mortensen · Accepted Answer · 2021-04-26 14:29:44Z

78

The -x option for gcc lets you specify the language of all input files following it:

$ gcc -x c your-file-name.cpp

If you only want to special-case that one file, you can use -x none to shut off the special treatment:

$ gcc -x c your-filename.cpp -x none other-file-name.cpp

(your-filename.cpp will be compiled as C, while other-file-name.cpp will use the extension and compile as C++)

edited Apr 26, 2021 at 14:29

Peter Mortensen

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answered Nov 18, 2010 at 15:13

Michael Mrozek

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1 Comment

ralight Over a year ago

How embarrassing, I must have paged right past it! Thanks very much, the same to everybody else who answered.

Rob Kennedy · Accepted Answer · 2010-11-18 16:02:08Z

18

To compile foo.cpp as a C file, you can create a new file named foo.c and put the following as its entire contents:

#include "foo.cpp"

Now compile foo.c instead of foo.cpp.

We've used this to go the other way (compile a .c file as C++) in order to start using C++ features in some files while preserving their decade-long CVS history. Also, we build using each platform's native compiler, not just GCC, so we didn't have to find the -x equivalent command for a half-dozen compilers, and then make our build system apply that command only to certain files.

answered Nov 18, 2010 at 16:02

Rob Kennedy

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1 Comment

Mischo5500 Over a year ago

Works as charm and quickest mentioned solution possible I think.

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