libc/stdio/flockfile.3

.\"	$NetBSD: flockfile.3,v 1.6 2011/10/15 21:43:19 wiz Exp $
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.\" by Klaus Klein.
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.Dd October 15, 2011
.Dt FLOCKFILE 3
.Os
.Sh NAME
.Nm flockfile ,
.Nm ftrylockfile ,
.Nm funlockfile
.Nd stdio stream locking functions
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft void
.Fn flockfile "FILE *file"
.Ft int
.Fn ftrylockfile "FILE *file"
.Ft void
.Fn funlockfile "FILE *file"
.Sh DESCRIPTION
The
.Fn flockfile ,
.Fn ftrylockfile ,
and
.Fn 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.
.Pp
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.
.Pp
The
.Fn flockfile
function acquires the ownership of
.Fa file
for the calling thread.
If
.Fa file
is already owned by another thread, the calling thread is suspended
until the acquisition is possible (i.e.,
.Fa file
is relinquished again and the calling thread is scheduled to acquire it).
.Pp
The
.Fn ftrylockfile
function acquires the ownership of
.Fa file
for the calling thread only if
.Fa file
is available.
.Pp
The
.Fn funlockfile
function relinquishes the ownership of
.Fa file
previously granted to the calling thread.
Only the current owner of
.Fa file
may
.Fn funlockfile
it.
.Sh RETURN VALUES
If successful, the
.Fn ftrylockfile
function returns 0.
Otherwise, it returns non-zero to indicate that the lock cannot be acquired.
.Sh SEE ALSO
.Xr flock 2 ,
.Xr getc_unlocked 3 ,
.Xr getchar_unlocked 3 ,
.Xr lockf 3 ,
.Xr putc_unlocked 3 ,
.Xr putchar_unlocked 3
.Sh STANDARDS
The
.Fn flockfile ,
.Fn ftrylockfile
and
.Fn funlockfile
functions conform to
.St -p1003.1-2001 .
.Sh HISTORY
The
.Fn flockfile
function first appeared in
.Fx 2.0 .
.Sh BUGS
The design of these interfaces does not allow for addressing the
problem of priority inversion.