Linux下获取消息队列的信息

在程序中想要获取消息队列长度可使用消息队列的属性这个数据结构：

需要#include <sys/msg.h>

/* 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;    //当前消息队列中的字节数
    ushort msg_qnum;     //当前消息队列中的消息个数
    ushort msg_qbytes;      /* max number of bytes on queue */
    ushort msg_lspid;       /* pid of last msgsnd */
    ushort msg_lrpid;       /* last receive pid */
};

简单程序示例：

struct msqid_ds msg_info;
if (msgctl(msgQid,IPC_STAT,&msg_info) == ERROR)
{
    return ERROR;
}
else
{
    return msg_info.msg_qnum;
}

数据结构各个参数详解如下：

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.