Problem with msgget() - High Level Programming

Ashwini · #1 09-13-2006

Hi,

I am having problem with msgget() function.

Here is the problem that I am having on Unix :

I have two processes sender and receiver. Sender generates queue (msgget()) with some key e.g. 938, for output.

Receiver reads from the same queue. i.e. receiver also tries to get queue id calling msgget() function and using same key, 938 that sender used.

But the qid that sender is receiving from msgget() function is 16777219 and receiver is getting different qid. Ideally it should get the same qid i.e. 16777219.

I tried to remove this queue using

ipcrm –q <qid>

And I tried to bring up sender with key as 338. But still it is building qid as such huge number and receiver is getting different qid.

Now I am able to read from this queue.

But still not sure whether this qid is correct or not

Please let me know how do I resove this.

Thanks,
Ashwini

Corona688 · #2 09-14-2006

While not extremely experienced in this subject, it strikes me that if the id's were guaranteed to be the identical, they wouldn't need keys to find them.

blowtorch · #3 09-14-2006

test1.c:

Code:

#include<stdio.h>
#include<sys/msg.h>

int main() {
        int msqid;

        msqid=msgget(938,666|IPC_CREAT);
        fprintf(stdout,"msqid from program 1: %d\n",msqid);
        fprintf(stdout,"now sleeping for 30 seconds\n");
        sleep(30);
        msgctl(msqid,IPC_RMID,NULL);
}

test2.c:

Code:

#include<stdio.h>
#include<sys/msg.h>

int main() {
        int msqid;

        msqid=msgget(938,666|IPC_CREAT);
        fprintf(stdout,"msqid from program 2: %d\n",msqid);
        msgctl(msqid,IPC_RMID,NULL);
}

I used the above bits of test code to check if at any point the message queue id changed. It didn't
In fact, the man page of msgget makes it quite clear:

Quote:

The msgget() argument returns the message queue identifier
associated with key.

A message queue identifier and associated message queue and
data structure (see intro(3)) are created for key if one of
the following are true:

o key is IPC_PRIVATE.

o key does not already have a message queue identifier
associated with it, and (msgflg&IPC_CREAT) is true.

Remember that msgget will always return a queue id that is associated with the key. This means that if there is already a queueid that is associated with the key, thats what you'll get. A new queue id is only created under the circumstances specified (key is IPC_PRIVATE or queueid does not already exist).

So if you have a program that created a message queue with a certain key, then another program that tries to create a message queue with the same key will *always* get the existing message queue id ,unless one of the programs removes the queue from the system.

In your case, if you have two programs, is one of the programs removing the message queue before the second one can access it? If this happens, then the queue id associated with your key will no longer exist and you will get a new queue id.

And don't worry about the size of the queue id. It makes no difference to your program.

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