How do I lock files using fopen()?

I would strongly disagree with the claim that fopen is prefered over open. It’s impossible to use fopen safely when writing a file in a directory that’s writable by other users due to symlink vulnerabilities/race conditions, since there is no O_EXCL option. If you need to use stdio on POSIX systems, it’s best to use open and fdopen rather than calling fopen directly.

Now, as for locking it depends on what you want to do. POSIX does not have mandatory locking like Windows, but if you just want to ensure you’re working with a new file and not clobbering an existing file or following a symlink, use the O_EXCL and O_NOFOLLOW options, as appropriate. If you want to do cooperative locking beyond the initial open, use fcntl locks.

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