IPCS message passing related queries

Question

I am dealing with Message Passing IPCS method. I do have few question regarding this:

1. KEY field in ipcs -q shows me 0x00000000 what does this means ?
2. Can i see what messsage is passes using msqid ?
3. If two entries are present (for a particular user) after executing command ipcs -q. Does this means that two messages were passed by this particular user ?
4. If used-bytes and message fields are set as 0 what does this mean?
5. Is there away to see if message queue is full or not?
6. How many queues can we have for one particular user?

I tried goggling, but was not able to find answer to these questions. Please help

Answer 1

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.