Return Error Strings

Good Code:

int perform_backup(char* from_file, char* to_file, char** error_msg)
{
   int success = 0;  /* assume failure. */
   static char error_buffer[1024]; /* buffer for returned */
                                   /* error messages.     */

   if (error_msg)    /* put buffer's address in */
      *error_msg = error_buffer;   /* error_msg parameter. */

   FILE* f_from = fopen(from_file, "r");
   if (f_from) {
      FILE* f_to = fopen(to_file, "w");
      if (f_to) {
         success = copy_file(f_from, f_to, error_msg);
         fclose(f_from);
         fclose(f_to);
      }
      else
         sprintf(error_buffer,
                 "Cannot open file '%s' for writing - %s",
                 to_file, strerror(errno));
   }
   else
      sprintf(error_buffer, "Cannot open file '%s' - %s",
              from_file, strerror(errno));
   return success;
}