pmsg/mq_notify_thread.c

+/* mq_notify_thread.c
+
+   Demonstrate message notification via threads on a POSIX message queue.
+*/
 #include <pthread.h>
 #include <mqueue.h>
 #include <signal.h>
 #include <fcntl.h>              /* For definition of O_NONBLOCK */
 #include "tlpi_hdr.h"
 
+/* 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_thread.c for an example of how to deal with that case. */
+
 static void notifySetup(mqd_t *mqdp);
 
 static void                     /* Thread notification function */
 threadFunc(union sigval sv)
 {
     ssize_t numRead;
     mqd_t *mqdp;
     void *buffer;
     struct mq_attr attr;
 
     mqdp = sv.sival_ptr;
 
     /* Determine mq_msgsize for message queue, and allocate an input buffer
        of that size */
 
     if (mq_getattr(*mqdp, &attr) == -1)
         errExit("mq_getattr");
 
     buffer = malloc(attr.mq_msgsize);
     if (buffer == NULL)
         errExit("malloc");
 
+    /* Reregister for message notification */
+
     notifySetup(mqdp);
 
     while ((numRead = mq_receive(*mqdp, buffer, attr.mq_msgsize, NULL)) >= 0)
         printf("Read %ld bytes\n", (long) numRead);
 
     if (errno != EAGAIN)                        /* Unexpected error */
         errExit("mq_receive");
 
     free(buffer);
 }
 
 static void
 notifySetup(mqd_t *mqdp)
 {
     struct sigevent sev;
 
     sev.sigev_notify = SIGEV_THREAD;            /* Notify via thread */
     sev.sigev_notify_function = threadFunc;
     sev.sigev_notify_attributes = NULL;
             /* Could be pointer to pthread_attr_t structure */
     sev.sigev_value.sival_ptr = mqdp;           /* Argument to threadFunc() */
 
     if (mq_notify(*mqdp, &sev) == -1)
         errExit("mq_notify");
 }
 
 int
 main(int argc, char *argv[])
 {
     mqd_t mqd;
 
     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");
 
     notifySetup(&mqd);
     pause();                    /* Wait for notifications via thread function */
 }