Question

I have the following macro function in vanilla C:

#define GLOG(format_string, ...) { \
  const char *file = strrchr(__FILE__, '/'); \
  char format[256] = "%s:%s!%d\t"; \
  strncat(format, format_string, 248); \
  strcat(format, "\n"); \
  printf(format, __FUNCTION__, file ? file : __FILE__, __LINE__, ##__VA_ARGS__); \
}

which lets me print a debug message containing the current function, file and line number, e.g.

GLOG("count=%d", count);

might print

do_count:counter.c!123  count=456

1. How can I modify the function to print all local variables if caller omits format_string? e.g.

   GLOG();
   

   might print

   do_count:counter.c!123  count=456, message="Hello world", array=[7, 8] structure={ptr=0xACE0FBA5E, coord={x=9, y=0}}
   

2. If that’s not possible, how can I modify it to print just the current function, file and line number? e.g.

   do_count:counter.c!123
   

   As is, this returns an error:

   error: expected expression before ‘,’ token

   as the strncat line is simply

   strncat(format, , 248);
   

Answer 1

First, inspecting all the local variables at runtime by the process itself seems impossible because C doesn’t have any means for reflection.

Second, you would be much better off if you wrote the logging macro like that:

#include <stdio.h>

#define STRINGIFY(x) #x
#define TOSTRING(x) STRINGIFY(x)

#define GLOGF(fmt, ...) \
  printf("%s:%s " fmt "\n", __func__, __FILE__ "!" TOSTRING(__LINE__), ##__VA_ARGS__)

int main (void) {
  /* main:test.c!xx count=5 */
  GLOGF("count=%d", 5);
  /* main:test.c!xx */
  GLOGF();
  return 0;
}

It is simpler and doesn’t incur any additional runtime overhead since the string is concatenated at compile-time.

Also note that I have used __func__ instead of __FUNCTION__, because the latter is non-standard.