Programming

           struct mq_attr {
               long mq_flags;       /* Flags: 0 or O_NONBLOCK */
               long mq_maxmsg;      /* Max. # of messages on queue */
               long mq_msgsize;     /* Max. message size (bytes) */
               long mq_curmsgs;     /* # of messages currently in queue */
           };

$ cat mq.c
#include <fcntl.h>           /* For O_* constants */
#include <sys/stat.h>        /* For mode constants */
#include <mqueue.h>

#include <stdio.h>
#include <string.h>

mqd_t mqopen2(const char * pName,
                    unsigned long Flags,
                    long  maxMsg,
                    long  msgSz);

int main(void)
{
        mqd_t m=mqopen2(NULL, 0, 16, 1024);

        printf("mq is %d\n", m);

        mq_close(m);
}

mqd_t mqopen2(const char * pName,
                    unsigned long Flags,
                    long  maxMsg,
                    long  msgSz)
{
   int perms = 0600;          /* permissions */
   int oflags= O_RDWR| O_CREAT;
   int rd=0, wr=0;            /* -r and -w options */
   mqd_t mqd;                 /* returned msg queue descriptor */
   int c;
   char *path="/iothread8";                /* ->first non-option argument */
   struct mq_attr buf;        /* buffer for stat info */

   memset(&buf, 0, sizeof(buf)); // WILL NOT WORK without this!

   buf.mq_msgsize = msgSz;
   buf.mq_maxmsg = maxMsg;
   buf.mq_flags = Flags;

   /* while */
   //if (optind < argc)
   //   path = argv[optind]; /* first non-option argument */
   //else
   //   { printf("Queue pathname required\n"); return -1; }
   mqd = mq_open(path,oflags,perms,&buf);
   if (-1 != mqd)
   {
    printf("Reached inside the loop\n");

      if ( ! mq_getattr(mqd,&buf) )
      {
         printf("flags: 0x%lx  maxmsg: %ld  msgsize: %ld  curmsgs: %ld\n",
         buf.mq_flags, buf.mq_maxmsg, buf.mq_msgsize, buf.mq_curmsgs);
      }
      else
         perror("mq_getattr()");
   }
   else
      perror("mq_open()");

   return mqd;
}
$ gcc mq.c -o mq -lrt
$ ./mq
Reached inside the loop
flags: 0x0  maxmsg: 10  msgsize: 8192  curmsgs: 0
mq is 3
$