Detecting whether a Message Queue with a given qid exists?

Detecting whether a Message Queue with a given qid exists?

[Rajat Jain]

Hi,

This is the reagrding the message queue mecahnism of sys V IPC.

Is there a way to find out that whether a message queue with a
particular qid exists?

Thanks,

Rajat

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

[JOSEPH ALPHONSE]

Hi all,
	send the command "ipcs -q "
	
	u will get a list of queues	
	grep from the list to get the required id

Thanks 
Joseph

-----Original Message-----
From: linux-c-programming-owner@vger.kernel.org
[mailto:linux-c-programming-owner@vger.kernel.org]On Behalf Of Rajat Jain
Sent: Monday, October 31, 2005 12:41 PM
To: linux-c-programming@vger.kernel.org
Subject: Detecting whether a Message Queue with a given qid exists?


Hi,

This is the reagrding the message queue mecahnism of sys V IPC.

Is there a way to find out that whether a message queue with a
particular qid exists?

Thanks,

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

[Steve Graegert]

On 10/31/05, Rajat Jain <rajat.noida.india@gmail.com> wrote:
> Hi,
>
> This is the reagrding the message queue mecahnism of sys V IPC.
>
> Is there a way to find out that whether a message queue with a
> particular qid exists?

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.



	\Steve

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

[Rajat Jain]

> 
> 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

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

[Steve Graegert]

On 11/1/05, Rajat Jain <rajat.noida.india@gmail.com> wrote:
> >
> > 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 :-(

Try msgctl with the IPC_STAT command:

	#include <sys/msg.h>
	int msgctl(int msqid, int cmd, struct msqid_ds *buf);

If the queue ID is not valid this call fails with EINVAL and buf is not filled.

	\Steve

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

[Rajat Jain]

>
> Try msgctl with the IPC_STAT command:
>
>        #include <sys/msg.h>
>        int msgctl(int msqid, int cmd, struct msqid_ds *buf);
>
> If the queue ID is not valid this call fails with EINVAL and buf is not filled.
>
>        \Steve
>

Thanks a lot. I think it would solve my problem

:-)