Thread: ipc message-passing hangs

hi,

wrote 2 applications, one for sender and one for receiver. To send a msg, the sender application is started manually at commandline and 1 msg is sent and received. everthing works great. But when i penetrate the commandline and try to send as fast as i can with
the keyboard, the receiver everytime hangs up and relieves some msgs in the message
queue. (as i can see with ipcs). Is there any problem with synchronization?

I use basically:

msgget(2407, (IPC_CREAT | 0755))
msgsnd(msgID, &dataMsg, MSGSIZE, 0)) ....

and

msgget(2407, 0755);
msgrcv(msgID, &dataMsg, MSGSIZE, msgTyp, IPC_NOWAIT)) ....

Greetz

you'r right...

Code:

IN HEADERFILE---------------

#define MSGSIZE 20

struct myMsg {
	  long mtype;
	  char mtext[MSGSIZE];
	} dataMsg;


SENDER -------------

void mysend(long type, int pid)
{
	int msgID;
	char msg[MSGSIZE];

	dataMsg.mtype = type;
        sprintf(msg, "%d", pid);
	strncpy(dataMsg.mtext, msg, MSGSIZE);

	msgID = msgget(2407, (IPC_CREAT | 0755));
    	if (msgID >= 0) {
        	
        if (-1==msgsnd(msgID, &dataMsg, MSGSIZE, 0)) {
                perror("msgsnd"); 
        } else {
        	printf("PID gesendet: %s \n", dataMsg.mtext);
        }
    	} else {
        	perror("msgget");
    	}
} 

int main(int argc, char **argv) {

    int pid;
    
    mysend(3333,pid);

    return 0;
}

RECEIVER---------------

struct myMsg my_msgrcv(long type) {
	int msgID;
	struct myMsg dataMsg;
	long msgTyp = 0;

	msgTyp = type;

	msgID = msgget(2407, 0755);
	if (msgID >= 0) {
		if (-1==msgrcv(msgID, &dataMsg, MSGSIZE, msgTyp, IPC_NOWAIT)) {
		    perror("msgrcv"); 
		} else {
		     printf("received: %s \n", dataMsg.mtext);
		}
	    } else {
			perror("msgget");
	    }
	return dataMsg;
}

int main() {

    while(1) {
           mymsgrcv(3333);
    }
}

The original code is to long, but i only shorten in main. It finally always stucks in the line with the msgrcv cmd.

Its an piece of code, that looks similar whereever i searched. I only experimented with
the flags to have a non-blocking behaviour and a receiver that runs in endless loop.

.... and please do not regard to the typing error in the RECEIVER main

1)everything is compiled well and works, my first thought was that it is relied on the flags
maybe i forgot something.

2) i checked the also the /proc/sys/kernel/msgm* informations, but they are default-valued.

3) when i kill the hanging process, and start the receiver again, the last message that is not
taken from queue is received. could it be that there are some queue entries that are empty for some reasons?

> Well the first big problem is that MSGSIZE isn't the same as sizeof(dataMsg)

havent seen that, very good, thanks i will work it out

> What output do you get?

None, i have some printf() in the while loop and, sorry i forgot: a sleep(1) at the end to see
the progression of the loop. i can use kbd and type in the cmdline-window. But it simply stops every
output. Shouldnt it normally go into the if-clause to print perror("msgrcv"); ?