freebsd-dev/lib/libutil/uucplock.3

.\" 
.\" 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: uucplock.3,v 1.5 1997/04/02 05:52:22 mpp Exp $
.\" "
.Dd March 30, 1997
.Os
.Dt uucplock 3
.Sh NAME
.Nm uu_lock , 
.Nm uu_unlock ,
.Nm uu_lockerr
.Nd acquire and release control of a serial device
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <libutil.h>
.Ft int
.Fn uu_lock "char *ttyname"
.Ft int
.Fn uu_unlock "char *ttyname"
.Ft char *
.Fn uu_lockerr "int uu_lockresult"
.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
.Pa /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 .
.Pp
.Fn uu_lockerr
returns an error string representing the error
.Fa uu_lockresult ,
as returned from
.Fn uu_lock .
.Sh RETURN VALUES
.Fn uu_unlock
returns 0 on success and -1 on failure.
.Pp
.Fn uu_lock
may return any of the following values:
.Pp
.Dv UU_LOCK_INUSE:
The lock is in use by another process.
.Pp
.Dv UU_LOCK_OK:
The lock was successfully created.
.Pp
.Dv UU_LOCK_OPEN_ERR:
The lock file could not be opened via
.Xr open 2 .
.Pp
.Dv UU_LOCK_READ_ERR:
The lock file could not be read via
.Xr read 2 .
.Pp
.Dv UU_LOCK_SEEK_ERR:
The lock file was
.Dq stale ,
but the call to
.Xr lseek 2
necessary to write the current process id failed.
.Pp
.Dv UU_LOCK_WRITE_ERR:
The current process id could not be written to the lock file via a call to
.Xr write 2 .
.Pp
If a value of
.Dv UU_LOCK_OK
is passed to
.Fn uu_lockerr ,
an empty string is returned.
Otherwise, a string specifying
the reason for failure is returned.
.Fn uu_lockerr
uses the current value of
.Va errno
to determine the exact error.  Care should be made not to allow
.Va errno
to be changed between calls to
.Fn uu_lock
and
.Fn uu_lockerr .
.Sh ERRORS
If
.Fn uu_lock
returns one of the four error values above, the global value
.Va errno
can be used to determine the cause.  Refer to the respective manual pages
for further details.
.Pp
.Fn uu_unlock
will set the global variable
.Va 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 SEE ALSO
.Xr open 2 ,
.Xr read 2 ,
.Xr lseek 2 ,
.Xr write 2
.Sh BUGS
Locking is not atomic.  Should a race condition occur, it's
possible that the
.Dq losing
process fails to identify the
.Dq winning
process.  If this happens,
.Fn uu_lock
returns
.Dv UU_LOCK_READ_ERR
and errno is set to
.Er EINVAL .
.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
.Pa /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.