从消息队列中读取（如果为空则不阻塞）

Question

I'm writing to message queue

if (msgsnd(q, &msg, sizeof(message), slaves_list[to]) == -1)

and reading

if (msgrcv(q, &msg, sizeof(message), id, 0) == -1)

but what if this queue is empty? How to check that? If there is nothing I want execute next instruction in the loop

Answer 1

Use IPC_NOWAIT . From the documentation :

If (msgflg & IPC_NOWAIT) is non-zero, the calling thread will return immediately with a return value of -1 and errno set to [ENOMSG] .

Answer 2

Use IPC_NOWAIT for msgflg parameter: http://man7.org/linux/man-pages/man2/msgsnd.2.html

IPC_NOWAIT Return immediately if no message of the requested type is in the queue. The system call fails with errno set to ENOMSG.

Answer 3

You can check if queue is empty using the

ipcs

command in linux terminal.

It will show you the queues that you have created.