Kernel msqid_ds structure

Each of the three types of IPC objects has an internal data structure which is maintained by the kernel. For message queues, this is the msqid_ds structure. The kernel creates, stores, and maintains an instance of this structure for every message queue created on the system. It is defined in linux/msg.h as follows:

/* one msqid structure for each queue on the system */
struct msqid_ds {
    struct ipc_perm msg_perm;
    struct msg *msg_first;  /* first message on queue */
    struct msg *msg_last;   /* last message in queue */
    time_t msg_stime;       /* last msgsnd time */
    time_t msg_rtime;       /* last msgrcv time */
    time_t msg_ctime;       /* last change time */
    struct wait_queue *wwait;
    struct wait_queue *rwait;
    ushort msg_cbytes;    
    ushort msg_qnum;     
    ushort msg_qbytes;      /* max number of bytes on queue */
    ushort msg_lspid;       /* pid of last msgsnd */
    ushort msg_lrpid;       /* last receive pid */
};

While you will rarely have to concern yourself with most of the members of this structure, a brief description of each is in order to complete our tour:

msg_perm

An instance of the ipc_perm structure, which is defined for us in linux/ipc.h. This holds the permission information for the message queue, including the access permissions, and information about the creator of the queue (uid, etc).

msg_first

Link to the first message in the queue (the head of the list).

msg_last

Link to the last message in the queue (the tail of the list).

msg_stime

Timestamp (time_t) of the last message that was sent to the queue.

msg_rtime

Timestamp of the last message retrieved from the queue.

msg_ctime

Timestamp of the last ``change'' made to the queue (more on this later).

wwait

and

rwait

Pointers into the kernel's wait queue. They are used when an operation on a message queue deems the process go into a sleep state (i.e. queue is full and the process is waiting for an opening).

msg_cbytes

Total number of bytes residing on the queue (sum of the sizes of all messages).

msg_qnum

Number of messages currently in the queue.

msg_qbytes

Maximum number of bytes on the queue.

msg_lspid

The PID of the process who sent the last message.

msg_lrpid

The PID of the process who retrieved the last message.