Implement synchronous signal handling

This is to demonstrate how SIGINT, etc. can be handled properly even if the
application thread is blocked on zmq_recv().

1 files changed, 49 insertions, 24 deletions

diff --git a/zmq-camera.c b/zmq-camera.c
index 717eade..fb46899 100644
--- a/zmq-camera.c
+++ b/zmq-camera.c
@@ -64,16 +64,9 @@ void *sender_thread (void *arg)
     unicap_data_buffer_t src_buffer;
     unicap_data_buffer_t dest_buffer;
     unicap_data_buffer_t *returned_buffer;
-    sigset_t block_set;
 
     sender_args = (struct sender_args_t *)arg;
 
-    /* Block SIGALRM in the sender thread since it's used for timers in the
-       main thread */
-    sigemptyset (&block_set);
-    sigaddset (&block_set, SIGALRM);
-    pthread_sigmask (SIG_BLOCK, &block_set, NULL);
-
     /* Create a ZMQ_PUB socket for sending video data and connect it to both
        an inproc endpoint for the local loopback view and bind to the endpoint
        specified by the user. */
@@ -218,9 +211,47 @@ void *sender_thread (void *arg)
 }
 
 /*
- *  Print usage and exit.
+ *  Signal handling thread
+ */
+
+/* Global variable used to signal if an alarm has expired */
+volatile int alarm_expired = 0;
+
+void *signal_thread (void *arg)
+{
+    sigset_t wait_set;
+    int rc, signo;
+
+    sigemptyset (&wait_set);
+    sigaddset (&wait_set, SIGTERM);
+    sigaddset (&wait_set, SIGINT);
+    sigaddset (&wait_set, SIGQUIT);
+    sigaddset (&wait_set, SIGALRM);
+
+    while (1) {
+        rc = sigwait (&wait_set, &signo);
+        assert (rc == 0);
+
+        switch (signo) {
+        case SIGTERM:
+        case SIGINT:
+        case SIGQUIT:
+            fprintf (stderr, "Exiting.\n");
+            exit (1);
+        case SIGALRM:
+            alarm_expired = 1;
+            break;
+        default:
+            ; /* Ignored */
+        }
+    }
+}
+
+/*
+ *  Main thread
  */
 
+/* Print usage and exit. */
 void err_usage(char *error)
 {
     const char usage[] = \
@@ -247,17 +278,7 @@ void err_usage(char *error)
     exit (1);
 }
 
-/*
- *  Main thread
- */
-
-volatile int alarm_expired = 0;
-
-void alarm_handler (int signo)
-{
-    alarm_expired = 1;
-}
-
+/* Main program. */
 int main (int argc, char *argv [])
 {
     void *ctx, *s;
@@ -269,7 +290,7 @@ int main (int argc, char *argv [])
     uint32_t image_width, image_height;
     int sdl_initialised = 0;
     int quit = 0;
-    struct sigaction alarm_action;
+    sigset_t block_set;
     struct itimerval alarm_timer;
     uint64_t received_fps = 0;
     uint64_t received_bps = 0;
@@ -304,6 +325,14 @@ int main (int argc, char *argv [])
         err_usage ("Please specify either the -r or -s option");
     endpoint = argv [optind];
 
+    /* Block signal handling in all application and 0MQ threads and
+       start a separate thread to handle all signals synchronously */
+    sigfillset (&block_set);
+    pthread_sigmask (SIG_SETMASK, &block_set, NULL);
+    pthread_t signal;
+    rc = pthread_create (&signal, NULL, signal_thread, NULL);
+    assert (rc == 0);
+
     /* Initialise 0MQ infrastructure for 2 application threads and 
        a single I/O thread */
     ctx = zmq_init (2, 1, 0);
@@ -345,10 +374,6 @@ int main (int argc, char *argv [])
     
     /* Set up an interval timer to fire SIGALRM once a second so that
        we can update bandwidth and fps statistics */
-    alarm_action.sa_flags = 0;
-    alarm_action.sa_handler = alarm_handler;
-    rc = sigaction (SIGALRM, &alarm_action, NULL);
-    assert (rc == 0);
     alarm_timer.it_interval.tv_sec = 1;
     alarm_timer.it_interval.tv_usec = 0;
     alarm_timer.it_value.tv_sec = 1;