171 lines
4.6 KiB
Groff
171 lines
4.6 KiB
Groff
.\"
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.\" Copyright (c) 1996 Brian Somers <brian@awfulhak.demon.co.uk>
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.\"
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $Id: uucplock.3,v 1.6 1997/05/11 08:50:33 davidn Exp $
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.\" "
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.Dd March 30, 1997
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.Os
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.Dt uucplock 3
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.Sh NAME
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.Nm uu_lock ,
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.Nm uu_unlock ,
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.Nm uu_lockerr
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.Nd acquire and release control of a serial device
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.Sh SYNOPSIS
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.Fd #include <sys/types.h>
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.Fd #include <libutil.h>
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.Ft int
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.Fn uu_lock "const char *ttyname"
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.Ft int
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.Fn uu_unlock "const char *ttyname"
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.Ft const char *
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.Fn uu_lockerr "int uu_lockresult"
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.Pp
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Link with
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.Va -lutil
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on the
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.Xr cc 1
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command line.
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.Sh DESCRIPTION
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The
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.Fn uu_lock
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function attempts to create a lock file called
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.Pa /var/spool/lock/LCK..
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with a suffix given by the passed
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.Fa ttyname .
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If the file already exists, it is expected to contain the process
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id of the locking program.
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.Pp
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If the file does not already exist, or the owning process given by
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the process id found in the lock file is no longer running,
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.Fn uu_lock
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will write its own process id into the file and return success.
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.Pp
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.Fn uu_unlock
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removes the lockfile created by
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.Fn uu_lock
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for the given
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.Fa ttyname .
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Care should be taken that
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.Fn uu_lock
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was successful before calling
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.Fn uu_unlock .
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.Pp
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.Fn uu_lockerr
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returns an error string representing the error
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.Fa uu_lockresult ,
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as returned from
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.Fn uu_lock .
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.Sh RETURN VALUES
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.Fn uu_unlock
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returns 0 on success and -1 on failure.
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.Pp
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.Fn uu_lock
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may return any of the following values:
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.Pp
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.Dv UU_LOCK_INUSE:
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The lock is in use by another process.
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.Pp
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.Dv UU_LOCK_OK:
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The lock was successfully created.
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.Pp
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.Dv UU_LOCK_OPEN_ERR:
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The lock file could not be opened via
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.Xr open 2 .
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.Pp
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.Dv UU_LOCK_READ_ERR:
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The lock file could not be read via
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.Xr read 2 .
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.Pp
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.Dv UU_LOCK_SEEK_ERR:
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The lock file was
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.Dq stale ,
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but the call to
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.Xr lseek 2
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necessary to write the current process id failed.
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.Pp
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.Dv UU_LOCK_WRITE_ERR:
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The current process id could not be written to the lock file via a call to
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.Xr write 2 .
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.Pp
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If a value of
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.Dv UU_LOCK_OK
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is passed to
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.Fn uu_lockerr ,
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an empty string is returned.
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Otherwise, a string specifying
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the reason for failure is returned.
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.Fn uu_lockerr
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uses the current value of
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.Va errno
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to determine the exact error. Care should be made not to allow
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.Va errno
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to be changed between calls to
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.Fn uu_lock
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and
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.Fn uu_lockerr .
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.Sh ERRORS
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If
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.Fn uu_lock
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returns one of the four error values above, the global value
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.Va errno
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can be used to determine the cause. Refer to the respective manual pages
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for further details.
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.Pp
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.Fn uu_unlock
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will set the global variable
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.Va errno
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to reflect the reason that the lock file could not be removed.
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Refer to the description of
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.Xr unlink 2
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for further details.
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.Sh SEE ALSO
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.Xr open 2 ,
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.Xr read 2 ,
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.Xr lseek 2 ,
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.Xr write 2
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.Sh BUGS
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Locking is not atomic. Should a race condition occur, it's
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possible that the
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.Dq losing
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process fails to identify the
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.Dq winning
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process. If this happens,
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.Fn uu_lock
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returns
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.Dv UU_LOCK_READ_ERR
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and errno is set to
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.Er EINVAL .
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.Pp
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It is possible that a stale lock is not recognised as such if a new
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processes is assigned the same processes id as the program that left
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the stale lock.
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.Pp
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The calling process must have write permissions to the
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.Pa /var/spool/lock
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directory. There is no mechanism in place to ensure that the
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permissions of this directory are the same as those of the
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serial devices that might be locked.
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