Reading from message queue (non-blocking if empty)

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

score 4 · Accepted Answer · answered Dec 20 '12 at 14:42

4

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].

answered Dec 20 '12 at 14:42

NPE

score 4 · Answer 2 · answered Dec 20 '12 at 14:43

4

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

answered Dec 20 '12 at 14:43

Alex F

score 0 · Answer 3 · edited Aug 13 '14 at 11:47

0

You can check if queue is empty using the

ipcs

command in linux terminal.

It will show you the queues that you have created.

edited Aug 13 '14 at 11:47

Felix Frank

answered Aug 13 '14 at 11:31

paras

Thanks for the feedback, but please note that the OP is asking about `C` programming, so that shelling out to an external utility is likely not an option. – Felix Frank Aug 13 '14 at 11:45

3 Answers3