Re: Detecting whether a Message Queue with a given qid exists?

[Rajat Jain <rajat.noida.india@gmail.com>]

> 
> msgget returns a valid queue identifier.  To see if the queue still
> exists when you try to place a message into the queue via msgsend,
> check for EINVAL, which is returned if the queue identifier is not
> valid (any more):
> 
> int result, msqid;
> 
> struct m {
>        long type;
>        char text[64];
> } msg;
> 
> m.type = 1;
> strcpy(m.text, "Hi, I am a message");
> 
> if ((queue_id = msgget(KEY, 0666)) == -1) {
>        /* handle error */
> }
> ... /* do some stuff here */
> result = msgsnd(queue_id, (void *)&m, sizeof(m.text), IPC_NOWAIT);
> ... /* check error here */
> 
> Hope this helps.
> 

Hi,

Thanks for the help. I understood this. But the above method uses msgsnd. So if the Queue actually exists, then a message will ACTUALLY be inserted. Given a qid, I want to check if a queue with that ID exists, without actually inserting a message. Is there a way or the only way out is try to insert a message, and if successful, remove that message :-(

The background og this is that I am using message queues to communicate between 2 independent programs (say A & B). Program "A" creates a message queue, writes a message in the queue and tells the qid to program B via a file. Program "B" reads the queue ID, and gets the message from the Message queue. After one message has been successfully sent, "A" wants to send another message, but before that just wants to check if the message queue it had created earlier already exists. If it does, it would insert messages in the old queue, otherwise would create a new queue and tell the new qid to "b" via the same file.

TIA,

Thanks & Best Regards,

Rajat