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.\"
.\" Copyright (c) 1996 Brian Somers <brian@awfulhak.demon.co.uk>
.\"
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.\" $Id$
.\" "
.Dd March 30, 1997
.Os
.Dt uucplock 3
.Sh NAME
.Nm uu_lock ,
.Nm uu_unlock
.Nd acquire and release control of a serial device
.Sh SYNOPSIS
.Fd #include <libutil.h>
.Ft int
.Fn uu_lock "char *ttyname"
.Ft int
.Fn uu_unlock "char *ttyname"
.Pp
Link with
.Va -lutil
on the
.Xr cc 1
command line.
.Sh DESCRIPTION
The
.Fn uu_lock
function attempts to create a lock file called
.Dq /var/spool/lock/LCK..
with a suffix given by the passed
.Fa ttyname .
If the file already exists, it is expected to contain the process
id of the locking program.
.Pp
If the file does not already exist, or the owning process given by
the process id found in the lock file is no longer running,
.Fn uu_lock
will write its own process id into the file and return success.
.Pp
.Fn uu_unlock
removes the lockfile created by
.Fn uu_lock
for the given
.Fa ttyname .
Care should be taken that
.Fn uu_lock
was successful before calling
.Fn uu_unlock .
.Sh RETURN VALUES
Both
.Fn uu_lock
and
.Fn uu_unlock
return 0 on success and -1 on failure.
.Sh ERRORS
On failure,
.Fn uu_lock
will log any unexpected errors using
.Xr syslog 2 .
If the lock already exists and contains the process id of a running
process,
.Fn uu_lock
will silently fail. The value of
.Dv errno
can not be used to determine the cause of failure.
.Pp
.Fn uu_unlock
will set the global variable
.Dv errno
to reflect the reason that the lock file could not be removed.
Refer to the description of
.Xr unlink 2
for further details.
.Sh BUGS
Locking is not atomic. Should a race condition occur, it's entirely
possible that both processes obtain the lock.
.Pp
It is possible that a stale lock is not recognised as such if a new
processes is assigned the same processes id as the program that left
the stale lock.
.Pp
The calling process must have write permissions to the
.Dq /var/spool/lock
directory. There is no mechanism in place to ensure that the
permissions of this directory are the same as those of the
serial devices that might be locked.
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