Append-only writes are actually amazing, having several processes writing into the same file and have their writes interleaved instead of destroying each other is almost impossible to re-create in the user space.
And I still don't understand why processes "modifying their own envp or argv" are met with such revulsion in this comment thread except from the "I dislike that on ideological grounds" reason. Now, the ability to modify envp and/or argv of other processes while those are running, yes, that's a horrible idea. But modifying your own internal process state?
Oh, and fcntl file locks are horrible for the historical reasons: basically, when POSIX (or its predecessor?) were trying to decide on a portable interface, the representative of one of the vendors cobbled together this API and its implementation in a week or two, and then showed to the meeting with it. To his surprise, instead of arguing everyone else basically said "eh, looks fine", and that was it, we now have broken "why on earth does close()/fork()/exec() interact with locks like that" behaviour.
