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

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$Id: uucplock.3,v 1.6 1997/05/11 08:50:33 davidn 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"

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

a /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 .

p .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.

p .Fn uu_lock may return any of the following values:

p .Dv UU_LOCK_INUSE: The lock is in use by another process.

p .Dv UU_LOCK_OK: The lock was successfully created.

p .Dv UU_LOCK_OPEN_ERR: The lock file could not be opened via .Xr open 2 .

p .Dv UU_LOCK_READ_ERR: The lock file could not be read via .Xr read 2 .

p .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.

p .Dv UU_LOCK_WRITE_ERR: The current process id could not be written to the lock file via a call to .Xr write 2 .

p 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.

p .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 .

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

a /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.