pmsg/pmsg_create.c

+/* pmsg_create.c
+
+   Create a POSIX message queue.
+
+   Usage as shown in usageError().
+
+   Linux supports POSIX message queues since kernel 2.6.6.
+*/
 #include <mqueue.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 #include "tlpi_hdr.h"
 
 static void
 usageError(const char *progName)
 {
     fprintf(stderr, "Usage: %s [-cx] [-m maxmsg] [-s msgsize] mq-name "
             "[octal-perms]\n", progName);
     fprintf(stderr, "    -c          Create queue (O_CREAT)\n");
     fprintf(stderr, "    -m maxmsg   Set maximum # of messages\n");
     fprintf(stderr, "    -s msgsize  Set maximum message size\n");
     fprintf(stderr, "    -x          Create exclusively (O_EXCL)\n");
     exit(EXIT_FAILURE);
 }
 
 int
 main(int argc, char *argv[])
 {
     int flags, opt;
     mode_t perms;
     mqd_t mqd;
     struct mq_attr attr, *attrp;
 
+    /* If 'attrp' is NULL, mq_open() uses default attributes. If an
+       option specifying a message queue attribute is supplied on the
+       command line, we save the attribute in 'attr' and set 'attrp'
+       pointing to 'attr'. We assign some (arbitrary) default values
+       to the fields of 'attr' in case the user specifies the value
+       for one of the queue attributes, but not the other. */
+
     attrp = NULL;
     attr.mq_maxmsg = 10;
     attr.mq_msgsize = 2048;
     flags = O_RDWR;
 
     /* Parse command-line options */
 
     while ((opt = getopt(argc, argv, "cm:s:x")) != -1) {
         switch (opt) {
         case 'c':
             flags |= O_CREAT;
             break;
 
         case 'm':
             attr.mq_maxmsg = atoi(optarg);
             attrp = &attr;
             break;
 
         case 's':
             attr.mq_msgsize = atoi(optarg);
             attrp = &attr;
             break;
 
         case 'x':
             flags |= O_EXCL;
             break;
 
         default:
             usageError(argv[0]);
         }
     }
 
     if (optind >= argc)
         usageError(argv[0]);
 
     perms = (argc <= optind + 1) ? (S_IRUSR | S_IWUSR) :
                 getInt(argv[optind + 1], GN_BASE_8, "octal-perms");
 
     mqd = mq_open(argv[optind], flags, perms, attrp);
     if (mqd == (mqd_t) -1)
         errExit("mq_open");
 
     exit(EXIT_SUCCESS);
 }