read_callback     681 contrib/libtests/pngunknown.c          png_set_read_user_chunk_fn(d->png_ptr, d, read_callback);
read_callback    3322 contrib/tools/pngfix.c                read_callback(png_ptr, buffer, count);
read_callback    3597 contrib/tools/pngfix.c       png_set_read_fn(png_ptr, control, read_callback);