Remove code between #if 0 and #endif when exporting a C file to a new one

Question

I want to remove all comments in a toy.c file. From Remove comments from C/C++ code I see that I could use

gcc -E -fpreprocessed -P -dD toy.c

But some of my code (say deprecated functions that I don't want to compile) are wrapped up between #if 0 and endif, as if they were commented out.

• One one hand, the above command does not remove this type of "comment" because its removal is only possible during macro expansion, which -fpreprocessed prevents;
• On the other hand, I have other macros I don't want to expand, so dropping -fpreprocessed is a bad idea.

I see a dilemma here. Is there a way out of this situation? Thanks.

---

The following toy example "toy.c" is sufficient to illustrate the problem.

#define foo 3  /* this is a macro */

// a toy function
int main (void) {
  return foo;
  }

// this is deprecated
#if 0
int main (void) {
  printf("%d\n", foo);
  return 0;
  }
#endif

gcc -E -fpreprocessed -P -dD toy.c gives

#define foo 3
int main (void) {
  return foo;
  }
#if 0
int main (void) {
  printf("%d\n", foo);
  return 0;
  }
#endif

while gcc -E -P toy.c gives

int main (void) {
  return 3;
  }

Answer 1

There's a pair of programs, sunifdef ("Son of unifdef", which is available from unifdef) and coan, that can be used to do what you want. The question Is there a C pre-processor which eliminates #ifdef blocks based on values defined/undefined? has answers which discuss these programs.

For example, given "xyz37.c":

#define foo 3  /* this is a macro */

// a toy function
int main (void) {
  return foo;
  }

// this is deprecated
#if 0
int main (void) {
  printf("%d\n", foo);
  }
#endif

Using sunifdef

sunifdef -DDEFINED -ned < xyz37.c

gives

#define foo 3  /* this is a macro */

// a toy function
int main (void) {
  return foo;
  }

// this is deprecated

and given this file "xyz23.c":

#if 0
This is deleted
#else
This is not deleted
#endif

#if 0
Deleted
#endif

#if defined(XYZ)
XYZ is defined
#else
XYZ is not defined
#endif

#if 1
This is persistent
#else
This is inconsistent
#endif

The program

sunifdef -DDEFINE -ned < xyz23.c

gives

This is not deleted

#if defined(XYZ)
XYZ is defined
#else
XYZ is not defined
#endif

This is persistent

This is, I think, what you're after. The -DDEFINED options seems to be necessary; choose any name that you do not use in your code. You could use -UNEVER_DEFINE_THIS instead, if you prefer. The -ned option evaluates the constant terms and eliminates the relevant code. Without it, the constant terms like 0 and 1 are not eliminated.

I've used sunifdef happily for a number of years (encroaching on a decade). I've not yet found it to make a mistake, and I've used it to clean up some revoltingly abstruse sets of 'ifdeffery'. The program coan is a development of sunifdef with even more capabilities.

Answer 2

The preprocessor doesn't make exceptions. You cannot use it here to do that.

A simple state machine using python can work. It even handles nesting (well, maybe not all cases are covered like nested #if 0 but you can compare the source before & after and manually validate). Also commented code isn't supported (but it seems that you have it covered)

the input (slightly more complex than yours for the demo):

#define foo 3
int main (void) {
  return foo;
  }
#if 0
int main (void) {
  #ifdef DDD
  printf("%d\n", foo);
  #endif
  }
#endif

void other_function()
{}

now the code, using regexes to detect #if & #endif.

import re
rif0 = re.compile("\s*#if\s+0")
rif = re.compile("\s*#(if|ifn?def)")
endif = re.compile("\s*#endif")

if_nesting = 0
if0_nesting = 0
suppress = False

with open("input.c") as fin, open("output.c","w") as fout:
    for l in fin:
        if rif.match(l):
            if_nesting += 1
            if rif0.match(l):
                suppress = True
                if0_nesting = if_nesting
        elif endif.match(l):
            if if0_nesting == if_nesting:
                suppress = False
            if_nesting -= 1
            continue  # don't write the #endif

        if not suppress:
            fout.write(l))

the output file contains:

#define foo 3
int main (void) {
  return foo;
  }

void other_function()
{}

so the nesting worked and the #if 0 part was successfully removed. Not something that sed "/#if 0/,/#endif/d can achieve.

Answer 3

Thanks for the other two answers.

I am now aware of unifdef and sunifdef. I am happy to know the existence of these tools, and that I am not the only one who wants to do this kind of code cleaning.

I have also written a rm_if0_endif.c (attached below) for removing an #if 0 ... #endif block which is sufficient for me. Its philosophy is based on text processing. It scans an input C script, locating #if 0 and the correct enclosing endif, so that this block can be omitted during char-to-char copying.

The text processing approach is limited, as it is designed for #if 0 ... #endif case only, but is all I need for now. A C program is not the only way for this kind of text processing. Jean-François Fabre's answer demonstrates how to do it in Python. I can also do something similar in R, using readLines, startsWith and writeLines. I chose to do it in C as I am not yet an expert in C so this task drives me to learn. Here is a demo of my rm_if0_endif.c. Note that the program can concatenate several C files and add header for each file.

original input file input.c

#define foo 3  /* this is a macro */

// a toy function
int test1 (void) {
  return foo;
  }

#if 0

#undef foo
#define foo 4

#ifdef bar
  #warning "??"
#endif

// this is deprecated
int main (void) {
  printf("%d\n", foo);
  return 0;
  }

#endif

// another toy
int test2 (void) {
  return foo;
  }

gcc pre-processing output "gcc_output.c" (taken as input for my program)

gcc -E -fpreprocessed -P -dD input.c > gcc_output.c

#define foo 3
int test1 (void) {
  return foo;
  }
#if 0
#undef foo
#define foo 4
#ifdef bar
  #warning "??"
#endif
int main (void) {
  printf("%d\n", foo);
  return 0;
  }
#endif
int test2 (void) {
  return foo;
  }

final output final_output.c from my program

rm_if0_endif.c has a utility function pattern_matching and a workhorse function rm_if0_endif:

void rm_if0_endif (char *InputFile,
                   char *OutputFile, char *WriteMode, char *OutputHeader);

The attached file below has a main function, doing

rm_if0_endif("gcc_output.c",
             "final_output.c", "w", "// this is a demo of 'rm_if0_endif.c'\n");

It produces:

// this is a demo of 'rm_if0_endif.c'
#define foo 3
int test1 (void) {
  return foo;
  }

int test2 (void) {
  return foo;
  }

---

Appendix: rm_if0_endif.c

#include <stdio.h>
int pattern_matching (FILE *fp, const char *pattern, int length_pattern) {
  int flag = 1;
  int i, c;
  for (i = 0; i < length_pattern; i++) {
    c = fgetc(fp);
    if (c != pattern[i]) {
      flag = 0; break;
      }
    }
  return flag;
  }
void rm_if0_endif (char *InputFile,
                   char *OutputFile, char *WriteMode, char *OutputHeader) {
  FILE *fp_r = fopen(InputFile, "r");
  FILE *fp_w = fopen(OutputFile, WriteMode);
  fpos_t pos;
  if (fp_r == NULL) perror("error when opening input file!");
  fputs(OutputHeader, fp_w);
  int c, i, a1, a2;
  int if_0_flag, if_flag, endif_flag, EOF_flag;
  const char *if_0 = "if 0";
  const char *endif = "endif";
  EOF_flag = 0;
  while (EOF_flag == 0) {
    do {
      c = fgetc(fp_r);
      while ((c != '#') && (c != EOF)) {
        fputc(c, fp_w);
        c = fgetc(fp_r);
        }
      if (c == EOF) {
        EOF_flag = 1; break;
        }
      fgetpos(fp_r, &pos);
      if_0_flag = pattern_matching(fp_r, if_0, 4);
      fsetpos(fp_r, &pos);
      if (if_0_flag == 0) fputc('#', fp_w);
      } while (if_0_flag == 0);
    if (EOF_flag == 1) break;
    a1 = 1; a2 = 0;
    do {
      c = fgetc(fp_r);
      while (c != '#') c = fgetc(fp_r);
      fgetpos(fp_r, &pos);
      if_flag = pattern_matching(fp_r, if_0, 2);
      fsetpos(fp_r, &pos);
      if (if_flag == 1) a1++;
      fgetpos(fp_r, &pos);
      endif_flag = pattern_matching(fp_r, endif, 5);
      fsetpos(fp_r, &pos);
      if (endif_flag == 1) a2++;
      } while (a1 != a2);
    for (i = 0; i < 5; i++) c = fgetc(fp_r);
    if (c == EOF) {
      EOF_flag == 1;
      }
    }
  fclose(fp_r);
  fclose(fp_w);
  }
int main (void) {
  rm_if0_endif("gcc_output.c",
               "final_output.c", "w", "// this is a demo of 'rm_if0_endif.c'\n");
  return 0;
  }