This is exactly the situation where flock makes sense. But I think you need LOCK_EX. EX stands for "exclusive" while SH stands for "shared". If you had lots of data to read before the write it might make sense to first use LOCK_SH (prevents others to get LOCK_EX, but doesn't prevent the LOCK_SH of other readers). But since you do a quick test+optional write, LOCK_SH probably wastes more time than it saves on simultaneous reads.

So my advice: Just do your operation between a LOCK_EX and a LOCK_UN.

Linux and most other unixes work with flock, but you should avoid nfs-mounted partitions. It is probably best to include a test in your code if you anticipate the program to use different OSes

UPDATE: the seek in your example code seeks to the end of the file. Not a good idea if you want to read it or rewrite it, only good for appending.