Remove comments from C/C++ code

Run the following command on your source file:

gcc -fpreprocessed -dD -E test.c

Thanks to KennyTM for finding the right flags. Here’s the result for completeness:

test.c:

#define foo barfoo foo foo#ifdef foo#undef foo#define foo baz#endiffoo foo/* comments? comments. */// c++ style comments

gcc -fpreprocessed -dD -E test.c:

#define foo barfoo foo foo#ifdef foo#undef foo#define foo baz#endiffoo foo

It depends on how perverse your comments are. I have a program scc to strip C and C++ comments. I also have a test file for it, and I tried GCC (4.2.1 on MacOS X) with the options in the currently selected answer - and GCC doesn't seem to do a perfect job on some of the horribly butchered comments in the test case.

NB: This isn't a real-life problem - people don't write such ghastly code.

Consider the (subset - 36 of 135 lines total) of the test case:

/\*\Regularcomment*\/The regular C comment number 1 has finished./\\/ This is not a C++/C99 comment!This is followed by C++/C99 comment number 3./\\\/ But this is a C++/C99 comment!The C++/C99 comment number 3 has finished./\\* This is not a C or C++ comment!This is followed by regular C comment number 2./\*/ This is a regular C comment *\but this is just a routine continuation *\and that was not the end either - but this is *\\/The regular C comment number 2 has finished.This is followed by regular C comment number 3./\\\\* C comment */

On my Mac, the output from GCC (gcc -fpreprocessed -dD -E subset.c) is:

/\*\Regularcomment*\/The regular C comment number 1 has finished./\\/ This is not a C++/C99 comment!This is followed by C++/C99 comment number 3./\\\/ But this is a C++/C99 comment!The C++/C99 comment number 3 has finished./\\* This is not a C or C++ comment!This is followed by regular C comment number 2./\*/ This is a regular C comment *\but this is just a routine continuation *\and that was not the end either - but this is *\\/The regular C comment number 2 has finished.This is followed by regular C comment number 3./\\\\* C comment */

The output from 'scc' is:

The regular C comment number 1 has finished./\\/ This is not a C++/C99 comment!This is followed by C++/C99 comment number 3./\\\/ But this is a C++/C99 comment!The C++/C99 comment number 3 has finished./\\* This is not a C or C++ comment!This is followed by regular C comment number 2.The regular C comment number 2 has finished.This is followed by regular C comment number 3.

The output from 'scc -C' (which recognizes double-slash comments) is:

The regular C comment number 1 has finished./\\/ This is not a C++/C99 comment!This is followed by C++/C99 comment number 3.The C++/C99 comment number 3 has finished./\\* This is not a C or C++ comment!This is followed by regular C comment number 2.The regular C comment number 2 has finished.This is followed by regular C comment number 3.

Source for SCC now available on GitHub

The current version of SCC is 6.60 (dated 2016-06-12), though the Git versions were created on 2017-01-18 (in the US/Pacific time zone). The code is available from GitHub at https://github.com/jleffler/scc-snapshots. You can also find snapshots of the previous releases (4.03, 4.04, 5.05) and two pre-releases (6.16, 6.50) — these are all tagged release/x.yz.

The code is still primarily developed under RCS. I'm still working out how I want to use sub-modules or a similar mechanism to handle common library files like stderr.c and stderr.h (which can also be found in https://github.com/jleffler/soq).

SCC version 6.60 attempts to understand C++11, C++14 and C++17 constructs such as binary constants, numeric punctuation, raw strings, and hexadecimal floats. It defaults to C11 mode operation. (Note that the meaning of the -C flag — mentioned above — flipped between version 4.0x described in the main body of the answer and version 6.60 which is currently the latest release.)

gcc -fpreprocessed -dD -E did not work for me but this program does it:

#include <stdio.h>static void process(FILE *f){ int c; while ( (c=getc(f)) != EOF ) {  if (c=='\'' || c=='"')            /* literal */  {   int q=c;   do   {    putchar(c);    if (c=='\\') putchar(getc(f));    c=getc(f);   } while (c!=q);   putchar(c);  }  else if (c=='/')              /* opening comment ? */  {   c=getc(f);   if (c!='*')                  /* no, recover */   {    putchar('/');    ungetc(c,f);   }   else   {    int p;    putchar(' ');               /* replace comment with space */    do    {     p=c;     c=getc(f);    } while (c!='/' || p!='*');   }  }  else  {   putchar(c);  } }}int main(int argc, char *argv[]){ process(stdin); return 0;}