xref: /freebsd/lib/libutil/uucplock.3 (revision 568b59b9df9cbf2d11dda28cefc9fc105d6ff8c6)

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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"
.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.