1. The "key" field of the Shared memory segments is usually 0x00000000
. This indicates the IPC_PRIVATE
key specified during creation of the shared memory segment. The manual of shmget() contains more details.
2. AFAIK, this cannot be done. If any msg is "de-queued" from the msgQ, then the intended receiver will not see it.
3. The 2 entries in the list of message queues indicates that there are currently 2 active message queues on the system identified by their corresponding unique keys.
Creating additional msgQ : ipcmk -Q
Deleting an existing msgQ : ipcrm -Q <unique-key>
4. The used-bytes
and messages
fields set to 0 indicate that currently no transfers have occurred using that particular msgQ.
5. Currently one way to do this to obtain the number of msgs currently queued-up in the msgQ programmatically as shown in the following C snippet. Next this can be compared with the size of the msgQ as demonstrated in this answer.
int ret = msgctl(msqid, IPC_STAT, &buf);
uint msg = (uint)(buf.msg_qnum);
printf("msgs in Q = %u\n", msg);
6. There exists a limit on the total memory used by all the msgQs on the system combined together. This can be obtained by ulimit -q
. The amount of bytes used in a msgQ is listed under the used-bytes
column in the output of ipcs -Q
. The total number of msgQs is limited only by the amount of memory available to create a new msgQ from the msgQ memory pool limit seen above.
Also checkout the latter part of this answer for a few sample operations on POSIX message queues.