PIDFILE

Section: Misc. Reference Manual Pages (3bsd)
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NAME

pidfile_open pidfile_write pidfile_close pidfile_remove pidfile_fileno - library for PID files handling

LIBRARY

Lb libbsd

SYNOPSIS

In libutil.h (See libbsd(7) for include usage.) Ft struct pidfh * Fn pidfile_open const char *path mode_t mode pid_t *pidptr Ft int Fn pidfile_write struct pidfh *pfh Ft int Fn pidfile_close struct pidfh *pfh Ft int Fn pidfile_remove struct pidfh *pfh Ft int Fn pidfile_fileno struct pidfh *pfh

DESCRIPTION

The pidfile family of functions allows daemons to handle PID files. It uses flopen(3bsd) to lock a pidfile and detect already running daemons.

The Fn pidfile_open function opens (or creates) a file specified by the Fa path argument and locks it. If Fa pidptr argument is not NULL and file can not be locked, the function will use it to store a PID of an already running daemon or -1 in case daemon did not write its PID yet. The function does not write process' PID into the file here, so it can be used before Fn fork Ns ing and exit with a proper error message when needed. If the Fa path argument is NULL /var/run/ Ao progname Ac .pid file will be used. The Fn pidfile_open function sets the O_CLOEXEC close-on-exec flag when opening the pidfile.

The Fn pidfile_write function writes process' PID into a previously opened file. The file is truncated before write, so calling the Fn pidfile_write function multiple times is supported.

The Fn pidfile_close function closes a pidfile. It should be used after daemon Fn fork Ns s to start a child process.

The Fn pidfile_remove function closes and removes a pidfile.

The Fn pidfile_fileno function returns the file descriptor for the open pidfile.

RETURN VALUES

The Fn pidfile_open function returns a valid pointer to a Vt pidfh structure on success, or NULL if an error occurs. If an error occurs, errno will be set.

Rv -std pidfile_write pidfile_close pidfile_remove

The Fn pidfile_fileno function returns the low-level file descriptor. It returns -1 and sets errno if a NULL Vt pidfh is specified, or if the pidfile is no longer open.

EXAMPLES

The following example shows in which order these functions should be used. Note that it is safe to pass NULL to Fn pidfile_write , Fn pidfile_remove , Fn pidfile_close and Fn pidfile_fileno 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");
        /*
         * Even though pfh is NULL we can continue, as the other pidfile_*
         * function can handle such situation by doing nothing except setting
         * errno to EINVAL.
         */
}

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 Fn pidfile_open function will fail if:

Bq Er EEXIST: Some process already holds the lock on the given pidfile, meaning that a daemon is already running. If Fa pidptr argument is not NULL the function will use it to store a PID of an already running daemon or -1 in case daemon did not write its PID yet.
Bq Er ENAMETOOLONG: Specified pidfile's name is too long.
Bq Er EINVAL: Some process already holds the lock on the given pidfile, but PID read from there is invalid.

The Fn pidfile_open function may also fail and set errno for any errors specified for the fstat(2), open(2), and read(2) calls.

The Fn pidfile_write function will fail if:

Bq Er EINVAL: Improper function use. Probably called before Fn pidfile_open .

The Fn pidfile_write function may also fail and set errno for any errors specified for the fstat(2), ftruncate(2), and write(2) calls.

The Fn pidfile_close function may fail and set errno for any errors specified for the close(2) and fstat(2) calls.

The Fn pidfile_remove function will fail if:

Bq Er EINVAL: Improper function use. Probably called not from the process which made Fn pidfile_write .

The Fn 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(3bsd) library function.

The Fn pidfile_fileno function will fail if:

Bq Er EINVAL: Improper function use. Probably called not from the process which used Fn pidfile_open .

AUTHORS

An -nosplit The pidfile functionality is based on ideas from An John-Mark Gurney Aq Mt jmg@FreeBSD.org .

The code and manual page was written by An Pawel Jakub Dawidek Aq Mt pjd@FreeBSD.org .