if (!fread (buf, 24, 1, input))
g_error("error reading header");
if (!fwrite(buf, 24, 1, output))
g_error("error writing header");
if (!fread (buf, 24, 1, input))
g_error("error reading header");
if (!fwrite(buf, 24, 1, output))
g_error("error writing header");