gistfile1.txt

/* This function is called when there is a warning, but the library thinks
 * it can continue anyway.  Replacement functions don't have to do anything
 * here if you don't want to.  In the default configuration, png_ptr is
 * not used, but it is passed in case it may be useful.
 */
static void PNGCBAPI
pngtest_warning(png_structp png_ptr, png_const_charp message)
{
   PNG_CONST char *name = "UNKNOWN (ERROR!)";
   char *test;
   test = png_get_error_ptr(png_ptr);

   if (test == NULL)
     fprintf(STDERR, "%s: libpng warning: %s\n", name, message);

   else
     fprintf(STDERR, "%s: libpng warning: %s\n", test, message);
}