Thread safe multi-file writing

Question

I have a daemon that accepts socket connections and reads or writes a dynamic set of files, depending on the nature of the connection. Because my daemon is multithreaded, the possibility exists that the same file may be written to by more than one thread. Because my list of files is dynamic and not fixed, I'm not sure how to keep one thread from bumping into the other. For performance reasons, I want threads to be writing to different files at the same time, just not the same file at the same time.

Other questions have suggested using mutexes, but I'm not entirely clear how a mutex would help in this scenario - the list of files being dynamic and only known to the thread.

Would it be appropriate to use file locking in this case? If so, how would one implement file locking in a thread-safe way?

Answer 1

flock will work OK. It doesn't lock file descriptors, it locks the actual file.

A file that has been exclusively flock'ed can't be exclusively locked again by another process or thread. That would defeat the entire purpose of locks.

One note is that these locks are advisory. A process that doesn't use flock can happily overwrite the file, even if another process has exclusive-flock'ed it.