System V Message Queues in Debian 6.x/Knoppix 6.x

A message queue is a System V IPC object that allows different or unrelated processes to exchange messages. Any process can create a message queue, modify it and leave it for other processes to modify later on. Any process can immediately remove a message queue regardless of whether another process is using it. Message queues exist until they are removed or system shutdown.

The data strucuture of the message queue object is:

struct msqid_ds

{

  struct ipc_perm msg_perm;

  msgqnum_t msg_qnum;    /* no of messages on queue */

  msglen_t msg_qbytes;   /* bytes max on a queue */

  pid_t msg_lspid;       /* PID of last msgsnd(2) call */

  pid_t msg_lrpid;       /* PID of last msgrcv(2) call */

  time_t msg_stime;      /* last msgsnd(2) time */

  time_t msg_rtime;      /* last msgrcv(2) time */

  time_t msg_ctime;      /* last change time */

};

Both commands list all the message queue objects in use on the system:

cat /proc/sysvipc/msg;

ipcs -q; ipcs -q -t; ipcs -q -p; ipcs -q -c; ipcs -q -l; ipcs -q -u;

Create message queue objects on the system:

ipcmk -Q -p <permission bits>;

Remove message queue objects from the system:

ipcrm -Q <msg key>;

ipcrm -q <msg id>;

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