File locking seems good enough to me. I wasn't consiciously aware of this short-coming but when you think about it, it makes total sense: the syscalls for file

File locking seems good enough to me. I wasn't consiciously aware of this short-coming but when you think about it, it makes total sense: the syscalls for file locks (at least on Linux and macOS) are related to the file descriptors. So when you close the file, the lock is gone. Why would you in any case hold 2 file descriptors open of the same file in the same process, especially when you use locking? For a "multi-process application" (this sounds so antiquated) probably one thread should hold the fd and exclusively write on it.