path: root/lib/libutil/pidfile.3

.\" Copyright (c) 2005 Pawel Jakub Dawidek <pjd@FreeBSD.org> .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .Dd August 2, 2025 .Dt PIDFILE 3 .Os .Sh NAME .Nm pidfile_open , .Nm pidfile_write , .Nm pidfile_close , .Nm pidfile_remove , .Nm pidfile_fileno , .Nm pidfile_signal .Nd "library for PID files handling" .Sh LIBRARY .Lb libutil .Sh SYNOPSIS .In libutil.h .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" .Ft int .Fn pidfile_signal "const char *path" "int sig" "pid_t *pidptr" .Sh DESCRIPTION The .Nm pidfile family of functions allows daemons to handle PID files. It uses .Xr flopen 3 to lock a pidfile and detect already running daemons. .Pp 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 .Dv NULL and file can not be locked, the function will use it to store a PID of an already running daemon or .Li -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 .Dv NULL , .Pa /var/run/ Ns Ao Va progname Ac Ns Pa .pid file will be used. The .Fn pidfile_open function sets the O_CLOEXEC close-on-exec flag when opening the pidfile. .Pp 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. .Pp The .Fn pidfile_close function closes a pidfile. It should be used after daemon .Fn fork Ns s to start a child process. .Pp The .Fn pidfile_remove function closes and removes a pidfile. .Pp The .Fn pidfile_fileno function returns the file descriptor for the open pidfile. .Pp The .Fn pidfile_signal function looks for the pidfile specified by .Va path , and if it exists and is locked, sends the signal specified by .Va sig to the PID it contains. If .Va pidptr is not .Dv NULL , the PID that was found in the pidfile is stored in the location it points to. Note that calling .Fn pidfile_signal with .Va sig set to zero is an effective way to verify the existence of a pidfile and of the process that owns it. .Sh RETURN VALUES The .Fn pidfile_open function returns a valid pointer to a .Vt pidfh structure on success, or .Dv NULL if an error occurs. If an error occurs, .Va errno will be set. .Pp .Rv -std pidfile_write pidfile_close pidfile_remove .Pp The .Fn pidfile_fileno function returns the low-level file descriptor. It returns .Li -1 and sets .Va errno if a NULL .Vt pidfh is specified, or if the pidfile is no longer open. .Pp The .Fn pidfile_signal function returns 0 if it successfully signaled a process, and an appropriate .Va errno value otherwise. .Sh EXAMPLES The following example shows in which order these functions should be used. Note that it is safe to pass .Dv NULL to .Fn pidfile_write , .Fn pidfile_remove , .Fn pidfile_close , and .Fn pidfile_fileno functions. .Bd -literal 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 EDOOFUS. */ } 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); .Ed .Sh ERRORS The .Fn pidfile_open function will fail if: .Bl -tag -width Er .It 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 .Dv NULL the function will use it to store a PID of an already running daemon or .Li -1 in case daemon did not write its PID yet. .It Bq Er ENAMETOOLONG Specified pidfile's name is too long. .It Bq Er EINVAL Some process already holds the lock on the given pidfile, but PID read from there is invalid. .El .Pp The .Fn pidfile_open function may also fail and set .Va errno for any errors specified for the .Xr fstat 2 , .Xr open 2 , and .Xr read 2 calls. .Pp The .Fn pidfile_write function will fail if: .Bl -tag -width Er .It Bq Er EDOOFUS Improper function use. Probably called before .Fn pidfile_open . .El .Pp The .Fn pidfile_write function may also fail and set .Va errno for any errors specified for the .Xr fstat 2 , .Xr ftruncate 2 , and .Xr write 2 calls. .Pp The .Fn pidfile_close function may fail and set .Va errno for any errors specified for the .Xr close 2 and .Xr fstat 2 calls. .Pp The .Fn pidfile_remove function will fail if: .Bl -tag -width Er .It Bq Er EDOOFUS Improper function use. Probably called not from the process which made .Fn pidfile_write . .El .Pp The .Fn pidfile_remove function may also fail and set .Va errno for any errors specified for the .Xr close 2 , .Xr fstat 2 , .Xr write 2 , and .Xr unlink 2 system calls and the .Xr flopen 3 library function. .Pp The .Fn pidfile_fileno function will fail if: .Bl -tag -width Er .It Bq Er EDOOFUS Improper function use. Probably called not from the process which used .Fn pidfile_open . .El .Pp The .Fn pidfile_signal function will fail if: .Bl -tag -width Er .It Bq Er ENOENT The pidfile does not exist, or exists but is not locked. .It Bq Er EDOM The pidfile contains a negative number. .El .Pp The .Fn pidfile_signal function may also fail and return any of the .Va errno values specified for the .Fn pidfile_read function and the .Xr kill 2 system call. .Sh SEE ALSO .Xr kill 2 , .Xr open 2 , .Xr daemon 3 , .Xr flopen 3 .Sh HISTORY The functions .Fn pidfile_open , .Fn pidfile_write , .Fn pidfile_close , and .Fn pidfile_remove first appeared in .Fx 5.5 . The .Fn pidfile_fileno function was added in .Fx 9.1 . The .Fn pidfile_signal function was added in .Fx 14.0 . .Sh AUTHORS .An -nosplit The .Nm pidfile functionality is based on ideas from .An John-Mark Gurney Aq Mt jmg@FreeBSD.org . .Pp The code and manual page was written by .An Pawel Jakub Dawidek Aq Mt pjd@FreeBSD.org .