pmsg/mq_notify_sig.c

+/* mq_notify_sig.c
+
+   Usage: mq_notify_sig mq-name
+
+   Demonstrate message notification via signals (catching the signals with
+   a signal handler) on a POSIX message queue.
+*/
 #include <signal.h>
 #include <mqueue.h>
 #include <fcntl.h>              /* For definition of O_NONBLOCK */
 #include "tlpi_hdr.h"
 
 #define NOTIFY_SIG SIGUSR1
 
 static void
 handler(int sig)
 {
     /* Just interrupt sigsuspend() */
 }
 
+/* This program does not handle the case where a message already exists on
+   the queue by the time the first attempt is made to register for message
+   notification. In that case, the program would never receive a notification.
+   See mq_notify_via_signal.c for an example of how to deal with that case. */
+
 int
 main(int argc, char *argv[])
 {
     struct sigevent sev;
     mqd_t mqd;
     struct mq_attr attr;
     void *buffer;
     ssize_t numRead;
     sigset_t blockMask, emptyMask;
     struct sigaction sa;
 
     if (argc != 2 || strcmp(argv[1], "--help") == 0)
         usageErr("%s mq-name\n", argv[0]);
 
     mqd = mq_open(argv[1], O_RDONLY | O_NONBLOCK);
     if (mqd == (mqd_t) -1)
         errExit("mq_open");
 
+    /* Determine mq_msgsize for message queue, and allocate an input buffer
+       of that size */
+
     if (mq_getattr(mqd, &attr) == -1)
         errExit("mq_getattr");
 
     buffer = malloc(attr.mq_msgsize);
     if (buffer == NULL)
         errExit("malloc");
 
+    /* Block the notification signal and establish a handler for it */
+
     sigemptyset(&blockMask);
     sigaddset(&blockMask, NOTIFY_SIG);
     if (sigprocmask(SIG_BLOCK, &blockMask, NULL) == -1)
         errExit("sigprocmask");
 
     sigemptyset(&sa.sa_mask);
     sa.sa_flags = 0;
     sa.sa_handler = handler;
     if (sigaction(NOTIFY_SIG, &sa, NULL) == -1)
         errExit("sigaction");
 
+    /* Register for message notification via a signal */
+
     sev.sigev_notify = SIGEV_SIGNAL;
     sev.sigev_signo = NOTIFY_SIG;
     if (mq_notify(mqd, &sev) == -1)
         errExit("mq_notify");
 
     sigemptyset(&emptyMask);
 
     for (;;) {
         sigsuspend(&emptyMask);         /* Wait for notification signal */
 
+        /* Reregister for message notification */
+
         if (mq_notify(mqd, &sev) == -1)
             errExit("mq_notify");
 
         while ((numRead = mq_receive(mqd, buffer, attr.mq_msgsize, NULL)) >= 0)
             printf("Read %ld bytes\n", (long) numRead);
 
         if (errno != EAGAIN)            /* Unexpected error */
             errExit("mq_receive");
     }
 }