IPC message queue. msgrcv system call. System V. how to get out of loop

Question

I have a message queue from which I am getting messages in a loop. The problem is that I don't know how to exit the loop. msgrcv returns type size_t so it keeps going. What value should I compare the return value so I can exit the loop?

while(msgrcv(msqid, &msgreceived, sizeof(msgreceived), BUFFER_CHANGED, 0) != -1){
    printf("%d %d %d %d ",msgreceived.value0,msgreceived.value1,msgreceived.value2,msgreceived.value3);

Answer 1

Because your last parameter does not include IPC_NOWAIT, msgrcv blocks while waiting for future messages.

If there is no message to be read immediately, msgrcv(..., ... | IPC_NOWAIT) will return (size_t)-1 and set errno = ENOMSG.