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FLOCKFILE(3) NetBSD Library Functions Manual FLOCKFILE(3)
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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 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 decre-
ments 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
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 relin-
quished again and the calling thread is scheduled to acquire it).
The ftrylockfile() function acquires the ownership of file for the call-
ing 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
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),
The flockfile(), ftrylockfile() and funlockfile() functions conform to
IEEE Std 1003.1-2001 (``POSIX.1'').
The flockfile() function first appeared in FreeBSD 2.0.
The design of these interfaces does not allow for addressing the problem
of priority inversion.
NetBSD 9.3 October 15, 2011 NetBSD 9.3