how to get back existing msg queue id

vdx · 04-05-2012, 02:17 AM

I am using msgget() syscall to get new msg queue. I have used the IPC_CREAT & IPC_EXCL flags in that. like

Quote:

message_queue = msgget(ftok("/tmp", 100), (0666 | IPC_CREAT | IPC_EXCL));

Now, when my prog exists unexpectedly, the msg queue remains and I am failed to re-create the msg queue. So, my question is "How can I get back the existing msg queues's ID ?"

and by the way, where does msg queue stores its id ?

Nominal Animal · 04-05-2012, 03:22 AM

How about

Code:

message_queue = msgget(ftok("/tmp", 100), 0666);

to get a handle back to the message queue?

The message queue details are stored in internal kernel structures. You can use msgctl(index,MSG_STAT,&info) to find information on all existing message queues. You need to scan with index starting from zero, increasing by one, until the call returns -1 with errno==EINVAL (for no more message queues). The key used for the msgget() call (ftok("/tmp",100)) is in info.msg_perm.__key.

You can use the ipcs -q command to list all message queues, and ipcrm -Q key or ipcrm -q id to delete a message queue.

This is all documented in the man 2 msgget and man 2 msgctl man pages.

Hope this helps,

vdx · 04-05-2012, 03:31 AM

thnx for the reply, Nominal Animal. It was very helpful

sundialsvcs · 04-05-2012, 02:30 PM

man ipcrm for information on how to remove queues. They are intended to be persistent objects for rendezvous purposes.