#include <stdio.h>
// debugging macros so we can pin down message origin at a glance
#define WHERESTR  "[file %s, line %d]: "
#define WHEREARG  __FILE__, __LINE__
#define DEBUGPRINT2(...)       fprintf(stderr, __VA_ARGS__)
#define DEBUGPRINT(_fmt, ...)  DEBUGPRINT2(WHERESTR _fmt, WHEREARG, __VA_ARGS__)
//...

int main(main) {
    int x = 9;
  printf("Real code....\n");
  DEBUGPRINT("I am a debug string...., x=%d\n", x);
  printf("Done\n");
return(0);
}

[root@oldmars bin]#gcc debug.c -o debug
[root@oldmars bin]#./debug
Real code....
[file debug.c, line 13]: I am a debug string...., x=9
Done