These functions provide explicit application-level locking of stdio streams. They can be used to avoid output from multiple threads being interspersed, input being dispersed among multiple readers, and to avoid the overhead of locking the stream for each operation.
The flockfile function acquires an exclusive lock on the specified stream. If another thread has already locked the stream, flockfile will block until the lock is released.
The ftrylockfile function is a non-blocking version of flockfile; if the lock cannot be acquired immediately, ftrylockfile returns non-zero instead of blocking.
The funlockfile function releases the lock on a stream acquired by an earlier call to flockfile or ftrylockfile.
These functions behave as if there is a lock count associated with each stream. Each time flockfile is called on the stream, the count is incremented, and each time funlockfile is called on the stream, the count is decremented. The lock is only actually released when the count reaches zero.
The flockfile and funlockfile functions return no value.
The ftrylockfile function returns zero if the stream was successfully locked, non-zero otherwise.