Make -P or --pidfile keep programs from running if already running.

controller/controller.8.in

@@ -117,6 +117,16 @@ the PID of the running process.  If \fIpidfile\fR is not specified, or
 if it does not begin with \fB/\fR, then it is created in
 \fB@rundir@\fR.
 
+.TP
+\fB-f\fR, \fB--force\fR
+By default, when \fB-P\fR or \fB--pidfile\fR is specified and the
+specified pidfile already exists and is locked by a running process,
+\fBcontroller\fR refuses to start.  Specify \fB-f\fR or \fB--force\fR
+to cause it to instead overwrite the pidfile.
+
+When \fB-P\fR or \fB--pidfile\fR is not specified, this option has no
+effect.
+
 .TP
 \fB-D\fR, \fB--detach\fR
 Causes \fBcontroller\fR to detach itself from the foreground session and

controller/controller.c

@@ -133,6 +133,7 @@ main(int argc, char *argv[])
         fatal(0, "no active or passive switch connections");
     }
 
+    die_if_already_running();
     daemonize();
 
     while (n_switches > 0 || n_listeners > 0) {
@@ -231,6 +232,7 @@ parse_options(int argc, char *argv[])
     static struct option long_options[] = {
         {"detach",      no_argument, 0, 'D'},
         {"pidfile",     optional_argument, 0, 'P'},
+        {"force",       no_argument, 0, 'f'},
         {"hub",         no_argument, 0, 'H'},
         {"noflow",      no_argument, 0, 'n'},
         {"max-idle",    required_argument, 0, OPT_MAX_IDLE},
@@ -260,6 +262,10 @@ parse_options(int argc, char *argv[])
             set_pidfile(optarg);
             break;
 
+        case 'f':
+            ignore_existing_pidfile();
+            break;
+
         case 'H':
             learn_macs = false;
             break;
@@ -314,6 +320,7 @@ usage(void)
     printf("\nOther options:\n"
            "  -D, --detach            run in background as daemon\n"
            "  -P, --pidfile[=FILE]    create pidfile (default: %s/controller.pid)\n"
+           "  -f, --force             with -P, start even if already running\n"
            "  -H, --hub               act as hub instead of learning switch\n"
            "  -n, --noflow            pass traffic, but don't add flows\n"
            "  --max-idle=SECS         max idle time for new flows\n"