# extracts APP1 data from jpeg file.
# usage: perl jpeggetapp1.pl < foo.jpeg > foo.app1

sub skip {
	local($len) = @_;
	local($junk);
	if (read(STDIN, $junk, $len) != $len) {
		print STDERR "error skipping $len\n";
		exit 1;
	}
}

sub dump {
	local($len) = @_;
	local($body);
	if (read(STDIN, $body, $len) != $len) {
		print STDERR "error skipping $len\n";
		exit 1;
	}
	syswrite(STDOUT, $body, $len);
}

if (read(STDIN, $magic, 2) != 2) {
	print STDERR "not jpeg file\n";
	exit 1;
}
if ($magic ne "\377\330") {	# ff d8
	print STDERR "not jpeg file\n";
	exit 1;
}

while (1) {
	if (read(STDIN, $magic, 2) != 2) {
		print STDERR "error reading section header\n";
		exit 1;
	}
	if (substr($magic, 0, 1) ne "\377") {	# ff
		print STDERR "error reading section header\n";
		exit 1;
	}
	if (substr($magic, 1, 1) eq "\331") {
		print STDERR "APP1 not found\n";
		exit 1;
	} elsif (substr($magic, 1, 1) eq "\341") {	# e1, APP1
		if (read(STDIN, $len, 2) != 2) {
			print STDERR "error reading section len\n";
			exit 1;
		}
		$len = unpack('n', $len);
		&dump($len - 2);
		exit 0;
	} else {
		print STDERR 'section is ' . unpack('xC', $magic) . "\n";
		if (read(STDIN, $len, 2) != 2) {
			print STDERR "error reading section len\n";
			exit 1;
		}
		$len = unpack('n', $len);
		&skip($len - 2);
	}
}