A demonstration of how to read and set tags using xattr APIs.

FileTagger.m

- (NSSet *)tagsForFileAtPath:(NSString *)path {
	const char *attrName = [@"com.apple.metadata:_kMDItemUserTags" UTF8String];
	const char *filePath = [path fileSystemRepresentation];
	ssize_t bufferLength = getxattr(filePath, attrName, NULL, 0, 0, 0);
	void *buffer = malloc(bufferLength);

	// now actually get the attribute string
	getxattr(filePath, attrName, buffer, 255, 0, 0);

	// convert to NSString
	NSData *tagsData = [NSData dataWithBytes:buffer length:(NSUInteger)bufferLength];
	free(buffer);
	NSArray *tags = [NSPropertyListSerialization propertyListWithData:tagsData options:NSPropertyListImmutable format:NULL error:nil];
	return [NSSet setWithArray:tags];
}

- (void)setTags:(NSSet *)newTags forFileAtPath:(NSString *)path{
	const char *attrName = [@"com.apple.metadata:_kMDItemUserTags" UTF8String];
	const char *filePath = [path fileSystemRepresentation];
	NSData *tagData = [NSPropertyListSerialization dataWithPropertyList:[newTags allObjects] format:NSPropertyListBinaryFormat_v1_0 options:0 error:nil];
	setxattr(filePath, attrName, [tagData bytes], [tagData length], 0, 0);
}