Question

I have different processes concurrently accessing a named pipe in Linux and I want to make this access mutually exclusive.

I know is possible to achieve that using a mutex placed in a shared memory area, but being this a sort of homework assignment I have some restrictions.

Thus, what I thought about is to use locking primitives on files to achieve mutual exclusion; I made some try but I can’t make it work.

This is what i tried:

flock(lock_file, LOCK_EX)

// critic section

flock(lock_file, LOCK_UN)

Different projects will use different file descriptors but referring to the same file.
Is it possible to achieve something like that? Can you provide some example.

Answer 1

Your example is as good as you’re going to get using flock (2) (which is after all, merely an “advisory” lock (which is to say not a lock at all, really)). The man page for it on my Mac OS X system has a couple of possibly important provisos:

Locks are on files, not file descriptors. That is, file descriptors duplicated through dup(2) or fork(2) do not result in multiple instances of a
lock, but rather multiple references to a single lock. If a process holding a lock on a file forks and the child explicitly unlocks the file, the
parent will lose its lock

and

Processes blocked awaiting a lock may be awakened by signals.

both of which suggest ways it could fail.

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// would have been a comment, but I wanted to quote the man page at some length