write a daemon pid file
writes a file containing the process ID of the program to the
/var/run directory. The file name has the form
/var/run/basename.pid. If the
basename argument is NULL,
pidfile will determine the program name and use that
The pid file can be used as a quick reference if the process needs to be sent a signal. When the program exits, the pid file will be removed automatically, unless the program receives a fatal signal.
pidfile() returns 0 on success and -1 on
pidfile function call appeared in
pidfile() is called multiple times with
different basename, only the last pidfile will be
removed upon exit.
atexit() to ensure the pidfile is unlinked at
program exit. However, programs that use the
function (for example, in signal handlers) will not trigger this