pmsg/mq_notify_sig.c

This is pmsg/mq_notify_sig.c (Listing 52-6, page 1080), an example from the book, The Linux Programming Interface.

The source code file is copyright 2024, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

  Cover of The Linux Programming Interface
+/* 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");
     }
 }

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