readpng_cleanup()

With that, readpng_get_image() returns control to our main program, which closes the input file and promptly calls another readpng routine to clean up all allocated memory (except for the image data itself, of course):

void readpng_cleanup(int free_image_data)
{
    if (free_image_data && image_data) {
        free(image_data);
        image_data = NULL;
    }

    if (png_ptr && info_ptr) {
        png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
        png_ptr = NULL;
        info_ptr = NULL;
    }
}

That is, the main program calls readpng_cleanup() with a zero (FALSE) argument here so that image_data is not freed. If it had waited to clean up until after the user requested the program to end, it would have passed a nonzero (TRUE) argument instead. Setting png_ptr and info_ptr to NULL is unnecessary here, since png_destroy_read_struct() does that for us; but we do it anyway, since it's a habit that tends to save on debugging time in the long run.