Understanding concurrent file writes from multiple processes

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Atomicity of writes less than PIPE_BUF applies only to pipes and FIFOs. For file writes, POSIX says:

This volume of POSIX.1-2008 does not specify behavior of concurrent
writes to a file from multiple processes. Applications should use some
form of concurrency control.

…which means that you’re on your own – different UNIX-likes will give different guarantees.

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