POSIX4 messages queues "mq_open: No such file or directory"

Question

I'm trying to use POSIX4 messages queues. So I'm using mq_open to create the queue, and for all the options I give to it an struct mq_attr, that I fill.

He can't find the queue, while I put the O_CREATE flag.

Here is my code (no-indented lines are debug code):

...
/***
 * Queues' names
 */
#define GUI_QUEUE "/guiQ"
...
  struct mq_attr attrAct;       /* Queue parameters */
  /***
   * Message queue to send action
   */
  attrAct.mq_maxmsg=1;
  attrAct.mq_msgsize=sizeof(gui_action);
  attrAct.mq_flags=0;
  attrAct.mq_curmsgs=0;

printf("serveur first sizeof(gui_action) : %lu\tmsgsize : %lu\n", sizeof(gui_action), attrAct.mq_msgsize);
  if ((guiQue=mq_open(GUI_QUEUE, O_CREAT | O_NONBLOCK | O_WRONLY
      , S_IWUSR | S_IRUSR , &attrAct))!=0) {
    perror("mq_open");
    exit(EXIT_FAILURE);
  }
if (mq_getattr(guiQue, &attrAct)!=0) {
perror("mq_getattr");
}
else {
printf("serveur second sizeof(gui_action) : %lu\tmsgsize : %lu\n", sizeof(gui_action), attrAct.mq_msgsize);
}
struct mq_attr new;
new=attrAct;
new.mq_msgsize=sizeof(gui_action);
printf("serveur third sizeof(gui_action) : %lu\tmsgsize : %lu\n", sizeof(gui_action), new.mq_msgsize);
if (mq_setattr(guiQue, &new, &attrAct)!=0) perror("mq_setattr");
if (mq_getattr(guiQue, &attrAct)!=0) {
perror("mq_getattr");
}
else {
printf("serveur fourth sizeof(gui_action) : %lu\tmsgsize : %lu\n", sizeof(gui_action), attrAct.mq_msgsize);
}
...

And here is the output:

serveur first sizeof(gui_action) : 16   msgsize : 16
mq_open: No such file or directory

What am I doing wrong?

Answer 1

mq_open returns (mqd_t) -1 on failure and a message queue descriptor on success.

You are mistaking the successful return of mq_open (which, in practice, is an integer >= 0) for failure, and perror is reporting some previous system call's errno.