FLOCKFILE(3) BSD Library Functions Manual FLOCKFILE(3)
flockfile, ftrylockfile, funlockfile -- stdio stream locking functions
Standard C Library (libc, -lc)
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 exe-
cute 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 decre-
mented 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
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.
If successful, the ftrylockfile() function returns 0. Otherwise, it returns non-zero to
indicate that the lock cannot be acquired.
flock(2), getc_unlocked(3), getchar_unlocked(3), lockf(3), putc_unlocked(3),
The flockfile(), ftrylockfile() and funlockfile() functions conform to IEEE Std 1003.1-2001
The flockfile() function first appeared in FreeBSD 2.0.
The design of these interfaces does not allow for addressing the problem of priority inver-
BSD October 15, 2011 BSD