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====== What Is It? ======
''lockf'' is a piece of software that comes with FreeBSD (as well as other BSDs, I would assume).  It's a simple C program that allows you to create a lock file with an exclusive lock and start a program running.  [[http://fuse4bsd.creo.hu/localcgi/man-cgi.cgi?lockf|Here is the man page]].  It's a simple program with a simple implementation that is very useful.  This was written by **John D. Polstra**, copyright 1997, and I have taken the source and modified it so that it will work on linux systems.

====== Why Use It? ======
This is very helpful when running a cron job that might run long enough that it would "overlap" with the next cron run.  This will cause the overlapping instance to exit, alternatively after ''-t <seconds>'' timeout.

====== Download ======
Download the source package: {{programming:c:lockf_linux.tar.gz}}

====== Install ======
After downloading the above tar.gz, run the following as root in the directory you downloaded it to:
<code>
# tar -xzf lockf_linux.tar.gz
# cd lockf
# make && make install
</code>
The above will install the binary in ''/usr/bin'' and the man page in ''/usr/share/man/man1''.  If you want something different, just modify the paths in the ''Makefile''.

====== Usage ======
<code>
$ lockf --help
usage: lockf [-ks] [-t seconds] file command [arguments]
</code>
A typical call would be:
<code>
$ lockf -t 30 /tmp/mybinary.lock mybinary --arg=blah --arg2=blah2
</code>
This would create a lockfile, ''/tmp/mybinary.lock'', that would cause any more ''lockf'' calls with that file to fail.  The ''-t 30'' means that if lockf cannot obtain a lock after 30 seconds, it will exit.