Copyright (c) 1996 Brian Somers <brian@awfulhak.demon.co.uk>

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$Id$
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.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"

p 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.

p 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.

p .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.

p .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.

p 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.

p 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.