NAME
pidfile_open, pidfile_write, pidfile_close, pidfile_remove - library for
PID files handling
LIBRARY
Utility functions from BSD systems (libbsd, -lbsd)
SYNOPSIS
#include <libutil.h>
struct pidfh *
pidfile_open(const char *path, mode_t mode, pid_t *pidptr);
int
pidfile_write(struct pidfh *pfh);
int
pidfile_close(struct pidfh *pfh);
int
pidfile_remove(struct pidfh *pfh);
DESCRIPTION
The pidfile family of functions allows daemons to handle PID files. It
uses flopen(3) to lock a pidfile and detect already running daemons.
The pidfile_open() function opens (or creates) a file specified by the
path argument and locks it. If a file can not be locked, a PID of an
already running daemon is returned in the pidptr argument (if it is not
NULL). The function does not write process’ PID into the file here, so
it can be used before fork()ing and exit with a proper error message when
needed. If the path argument is NULL, /var/run/〈progname〉.pid file will
be used.
The pidfile_write() function writes process’ PID into a previously opened
file.
The pidfile_close() function closes a pidfile. It should be used after
daemon fork()s to start a child process.
The pidfile_remove() function closes and removes a pidfile.
RETURN VALUES
The pidfile_open() function returns a valid pointer to a pidfh structure
on success, or NULL if an error occurs. If an error occurs, errno will
be set.
The pidfile_write(), pidfile_close(), and pidfile_remove() functions
return the value 0 if successful; otherwise the value -1 is returned and
the global variable errno is set to indicate the error.
EXAMPLES
The following example shows in which order these functions should be
used. Note that it is safe to pass NULL to pidfile_write(),
pidfile_remove() and pidfile_close() functions.
struct pidfh *pfh;
pid_t otherpid, childpid;
pfh = pidfile_open("/var/run/daemon.pid", 0600, &otherpid);
if (pfh == NULL) {
if (errno == EEXIST) {
errx(EXIT_FAILURE, "Daemon already running, pid: %jd.",
(intmax_t)otherpid);
}
/* If we cannot create pidfile from other reasons, only warn. */
warn("Cannot open or create pidfile");
}
if (daemon(0, 0) == -1) {
warn("Cannot daemonize");
pidfile_remove(pfh);
exit(EXIT_FAILURE);
}
pidfile_write(pfh);
for (;;) {
/* Do work. */
childpid = fork();
switch (childpid) {
case -1:
syslog(LOG_ERR, "Cannot fork(): %s.", strerror(errno));
break;
case 0:
pidfile_close(pfh);
/* Do child work. */
break;
default:
syslog(LOG_INFO, "Child %jd started.", (intmax_t)childpid);
break;
}
}
pidfile_remove(pfh);
exit(EXIT_SUCCESS);
ERRORS
The pidfile_open() function will fail if:
[EEXIST] Some process already holds the lock on the given
pidfile, meaning that a daemon is already running.
[ENAMETOOLONG] Specified pidfile’s name is too long.
[EINVAL] Some process already holds the lock on the given
pidfile, but PID read from there is invalid.
[EAGAIN] Some process already holds the lock on the given
pidfile, but the file is truncated. Most likely, the
existing daemon is writing new PID into the file.
The pidfile_open() function may also fail and set errno for any errors
specified for the fstat(2), open(2), and read(2) calls.
The pidfile_write() function will fail if:
[EINVAL] Improper function use. Probably called before
pidfile_open().
The pidfile_write() function may also fail and set errno for any errors
specified for the fstat(2), ftruncate(2), and write(2) calls.
The pidfile_close() function may fail and set errno for any errors
specified for the close(2) and fstat(2) calls.
The pidfile_remove() function will fail if:
[EINVAL] Improper function use. Probably called not from the
process which made pidfile_write().
The pidfile_remove() function may also fail and set errno for any errors
specified for the close(2), fstat(2), write(2), and unlink(2) system
calls and the flopen(3) library function.
SEE ALSO
open(2), daemon(3), flopen(3)
AUTHORS
The pidfile functionality is based on ideas from John-Mark Gurney
〈jmg@FreeBSD.org〉.
The code and manual page was written by Pawel Jakub Dawidek
〈pjd@FreeBSD.org〉.