I'm not really a *nix programmer, but I slowly memorize libraries and functions and prototypes over time. If I cant remember it, I look up. Since I dont like looking it up, I tend to try to memorize it for next time
Its pretty obvious that the more you use something the more you'll remember how to use it. I dont think too many programmers would go out of their way to memorize a library they have never used, if thats your question.