Modify '-F' option to work nicely with pidfile(3) - a pidfile given as

an argument has to be locked.

usr.bin/pkill/pkill.1

@@ -91,6 +91,12 @@ The following options are available:
 Restrict matches to a process whose PID is stored in the
 .Ar pidfile
 file.
+The
+.Ar pidfile
+file must be locked with the
+.Xr flock 2
+syscall or created with
+.Xr pidfile 3 .
 .It Fl G Ar gid
 Restrict matches to processes with a real group ID in the comma-separated
 list
@@ -233,8 +239,10 @@ An internal error occurred.
 .Xr kill 1 ,
 .Xr killall 1 ,
 .Xr ps 1 ,
+.Xr flock 2 ,
 .Xr kill 2 ,
 .Xr sigaction 2 ,
+.Xr pidfile 3 ,
 .Xr re_format 7
 .\" Xr signal 7
 .Sh HISTORY

usr.bin/pkill/pkill.c

@@ -641,6 +641,20 @@ takepid(const char *pidfile)
 	if (fh == NULL)
 		err(STATUS_ERROR, "can't open pid file `%s'", pidfile);
 
+	/*
+	 * If we can lock pidfile, this means that daemon is not running,
+	 * so better don't kill the process from the pidfile.
+	 */
+	if (flock(fileno(fh), LOCK_EX | LOCK_NB) == 0) {
+		(void)fclose(fh);
+		errx(STATUS_ERROR, "file '%s' can be locked", pidfile);
+	} else {
+		if (errno != EWOULDBLOCK) {
+			errx(STATUS_ERROR, "error while locking file '%s'",
+			    pidfile);
+		}
+	}
+
 	if (fgets(line, sizeof(line), fh) == NULL) {
 		if (feof(fh)) {
 			(void)fclose(fh);