Actually use zmc_heartbeat_time. Set it on every capture and use it in ShmValid to determine if zmc has gone away.

src/zm_monitor.cpp

@@ -2505,6 +2505,7 @@ int Monitor::Capture() {
   packet->timestamp = new struct timeval;
   packet->image_index = image_count;
   gettimeofday(packet->timestamp, nullptr);
+  shared_data->zmc_heartbeat_time = packet->timestamp->tv_sec;
 
   Image* capture_image = image_buffer[index].image;
   int captureResult = 0;

src/zm_monitor.h

@@ -410,7 +410,9 @@ public:
   bool disconnect();
 
   inline int ShmValid() const {
-    return shared_data && shared_data->valid;
+    struct timeval now;
+    gettimeofday(&now, nullptr);
+    return shared_data && shared_data->valid && ((now.tv_sec - shared_data->zmc_heartbeat_time) > config.watch_max_delay);
   }