relative_prefix.c

#include <string.h>
#include <stdio.h>
#include <sys/param.h>
#include <stdlib.h>

char *relative_prefix(char *argv0, char *prefix) {
  char *file_name, *result;
  size_t size;

  if (argv0 == NULL) {
    return realpath(".", prefix);
  }

  size = strlen(argv0) + 7;
  file_name = malloc(size);

  if (file_name == NULL) {
    return NULL;
  }

  snprintf(file_name, size, "%s/../..", argv0);
  result = realpath(file_name, prefix);

  free(file_name);
  return result;
}

int main(int argc, char **argv) {
  char prefix[PATH_MAX];
  fprintf(stderr, "libpath = %s\n", relative_prefix(argv[0], prefix));
  return 0;
}