The
flockfile(),
ftrylockfile(), and
funlockfile() functions provide applications with explicit control of locking of stdio stream objects. They can be used by a thread to execute a sequence of I/O operations as a unit, without interference from another thread.
Locks on stdio streams are recursive, and a lock count is maintained. stdio streams are created unlocked, with a lock count of zero. After successful acquisition of the lock, its count is incremented to one, indicating locked state of the stdio stream. Each subsequent relock operation performed by the owner thread increments the lock count by one, and each subsequent unlock operation performed by the owner thread decrements the lock count by one, allowing matching lock and unlock operations to be nested. After its lock count is decremented to zero, the stdio stream returns to unlocked state, and ownership of the stdio stream is relinquished.
The
flockfile() function acquires the ownership of
file for the calling thread. If
file is already owned by another thread, the calling thread is suspended until the acquisition is possible (i.e.,
file is relinquished again and the calling thread is scheduled to acquire it).
The
ftrylockfile() function acquires the ownership of
file for the calling thread only if
file is available.
The
funlockfile() function relinquishes the ownership of
file previously granted to the calling thread. Only the current owner of
file may
funlockfile() it.