freebsd-skq/lib/libutil/uucplock.3

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.\" Copyright (c) 1996 Brian Somers <brian@awfulhak.demon.co.uk>
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.\" $Id: uucplock.3,v 1.7 1997/05/12 10:36:14 brian 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 "const char *ttyname"
.Ft int
.Fn uu_unlock "const char *ttyname"
.Ft const 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_CREAT_ERR:
Can't create temporary lock file via
.Xr creat 2 .
.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
.Dv UU_LOCK_LINK_ERR:
Can't link temporary lock file via
.Xr link 2 .
.Pp
.Dv UU_LOCK_TRY_ERR:
Locking attempts are failed after 5 tries.
.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.