PIDFILE(3)

NAME
pidfile - write a daemon pid file

LIBRARY
libutil

SYNOPSIS
#include <util.h>
int
pidfile(const char *path);
pid_t
pidfile_lock(const char *path);
pid_t
pidfile_read(const char *path);
int
pidfile_clean(void);

DESCRIPTION
pidfile() and pidfile_lock() create and lock a file containing the process ID of the calling program. The pid file can be used as a quick reference if the process needs to be sent a signal. The pid file is truncated and removed automatically when the program exits, unless the program receives a fatal signal.

If path is NULL or a plain basename (a name containing no directory components), the pid file is created in the /var/run directory. The file name has the form /var/run/basename.pid. The basename part is either the value of path if it was not NULL, or the program name as returned by getprogname(3) otherwise.

If path is an absolute or relative path (i.e. it contains the ‘/’ character), the pid file is created in the provided location.

If called with a new path, pidfile() and pidfile_lock() will remove the old pid file.

The pid file is truncated, so these functions can be called multiple times and allow a child process to take over the lock.

pidfile_read() will read the last pid file created, or specified by path, and return the process ID it contains.

pidfile_clean() will ftruncate(2), close(2), and unlink(2) the last opening pid file if, and only if, the current process wrote it. This function should be called if the program needs to call _exit(2) (such as from a signal handler) and needs to clean up the pid file.

RETURN VALUES
pidfile() and pidfile_clean() returns 0 on success and -1 on failure.

pidfile_lock() returns 0 on success. Otherwise, the process ID who owns the lock is returned and if that cannot be derived then -1 is returned.

pidfile_read() returns the process ID if known, otherwise -1.

ERRORS
The pidfile() and pidfile_lock() functions will fail if:

[EEXIST]
    Some process already holds the lock on the given pid file, meaning that a daemon is already running.

[ENAMETOOLONG]
    Specified pidfile's name is too long.

SEE ALSO
flock(2), atexit(3)

HISTORY
The pidfile() function call appeared in NetBSD 1.5. Support for creating pid files in any arbitrary path was added in NetBSD 6.0.

The pidfile_lock(), pidfile_read(), and pidfile_clean() function calls appeared in NetBSD 8.

CAVEATS
pidfile() and pidfile_lock() use atexit(3) to ensure the pid file is cleaned at program exit. However, programs that use the _exit(2) function (for example, in signal handlers) will not trigger this behaviour and should call pidfile_clean(). Like-wise, if the program creates a pid file before fork(2)ing a child to take over, it should use the _exit(2) function instead of returning or using the exit(3) function to ensure the pid file is not cleaned.