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.

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