Listening Message Queues

[ycinar]

I am implementing an IPC mechanism using message queues (Sys V).

I can receive the messages using msgrcv() function. I am wondering how can I listen on the new messages. I would like to implement a mechanism that would notify me when there is a new message in the queue for me and I will call the msgrcv() function to retrieve the message.

Thanks much!

[ycinar]

I think I found my answer... probably msgflg argument passed in to the function defines the behavior here:

int msgrcv ( int msqid, struct msgbuf *msgp, int msgsz, long mtype, int msgflg );

I was expecting a call back mechanism like receiving an event etc but seems like this the mechanism here.

One can still confirm

[ycinar]

OK, what I dont understand is that if I pass 0 as the msgflg, the msgrcv will block until a message arrives in the queue. In some cases, I might want to wake the msgrcv function and resume the execution a defined manner. How can I do that? Looks like you cannot pass a timeout parameter or such to wake after certain amount of time.

I found this on a website,

"The calling thread receives a signal that is to be caught; in this case a message is not received and the calling thread resumes execution in the manner prescribed in sigaction()."

Not sure what it means, though.

I cannot paste the link it since I am a newbie...

[white_hound]

It looks like you could benefit from 'pthread' functionality.
Create a message receiver thread that would always listen to incoming messages.
Each time a message arrives, call a callback function.
Meanwhile, your _main_ thread goes on, so you actually don't need no timeout functionality.
Note that in any case you can signal the receiver thread to break the msgrcv().

[abarclay]

I agree with white_hound - using threads will make the solution much cleaner, however, if you want to implement a "timeout", you can do so using signals.

First, you'll need to write a function to execute when the time expires (the handler):

void timehandler(int dummy)
{
time_t currtime;
char *timeAsString;

time(&currtime);
timeAsString=ctime(&currtime);
write(1,timeAsString,TIMESTRINGLEN);
alarm(10);
}


Now, you need to install the handler:
sigaction(SIGALRM,(struct sigaction *)NULL, &action);
action.sa_handler=timehandler;
sigaction(SIGALRM,&action,(struct sigaction *)NULL);

Set the initial timeout:
alarm(10);

Now, call your msgrcv....
You need to check "errno" after the msgrcv. If it's set to EINTR, then your timeout expired...