Message Queue(消息隊列) Back

• 消息隊列就是一個消息的鏈表.
• 消息具有特定的格式以及特定的優先級.
• 消息隊列是隨內核持續的.
• 與管道相比:
  • 消息隊列提供有格式字节流, 從而減少開發人員的工作量
  • 消息具有類型, 可分優先級
  • 消息隊列的生命週期更長

0. Header files

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>

1. Create a Message Queue

method

int msgget(key_t key, int msgflag)

parameters

• key: 鍵值, 由ftok()獲取
• msgflag: 消息標誌
  • IPC_CREATE: 如果內核中不存在則創建
  • IPD_EXCEL: 若和IPC_CREATE一起使用, 若內核中存在則返回失敗

return value

• 0: 消息隊列描述字

• -1: failure

2. Receive messages

• receive method will be blocked when there is no data to read until one of three situations has happened:
  • there is data
  • the message queue of msgid was killed
  • received a signal of interruption, return error (errno = EINTR)

method

int msgrcv(int msqid, struct msgbuf* msgp, int msgsz, long msgtyp, int msgflg)

parameters

• msqid: 消息隊列描述字
• msgp: 接受信息的數字結構
• msgsz: 消息長度(指定msgp中mtex成員的長度)
• msgtyp: 請求讀取的消息類型
• msgflg: 讀消息的標誌
  • IPC_NOWAIT: 若無滿足條件的消息, 調用立即返回(errno = ENOMSG)
  • IPC_EXCEPT: 返回隊列中第一個類型不為msgtyp的消息
  • IPC_NOERROR: 若滿足條件的消息內容大於所請求的msgsz, 則把消息截斷, 截斷部分將丟失

return value

• 0: 讀出消息的實際字節數

• -1: failure

3. Send messages

• send method will be blocked when there is no data to read until one of three situations has happened:
  • there is sufficient space for sending messages
  • the message queue of msgid was killed
  • received a signal of interruption, return error (errno = EINTR)

method

int msgsnd(int msqid, struct msgbur* msgp, int msgsz, int msgflg)

parameters

• msqid: 消息隊列描述字
• msgp: 發送信息的數字結構
• msgsz: 消息長度(指定msgp中mtex成員的長度)
• msgflg: 讀消息的標誌
  • IPC_NOWAIT: 若消息隊列沒有足夠空間容納發送消息時, 調用立即返回(errno = ENOMSG)

return value

• 0: success
• -1: failure

4. Control a message queue

method

int msgctl(int msqid, int cmd, struct msqid_ds* buf)

/* msqid_ds */
struct msqid_ds
{
    struct ipc_perm msg_perm; 
    struct msg *msg_first; /* first message on queue,unused */
    struct msg *msg_last; /* last message in queue,unused */
    __kernel_time_t msg_stime; /* last msgsnd time */ 
    __kernel_time_t msg_rtime; /* last msgrcv time */ 
    __kernel_time_t msg_ctime; /* last change time */ 
    unsigned long msg_lcbytes; /* Reuse junk fields for 32 bit */ 
    unsigned long msg_lqbytes; /* ditto */ 
    unsigned short msg_cbytes; /* current number of bytes on queue */
    unsigned short msg_qnum; /* number of messages in queue */
    unsigned short msg_qbytes; /* max number of bytes on queue */
    __kernel_ipc_pid_t msg_lspid; /* pid of last msgsnd */
    __kernel_ipc_pid_t msg_lrpid; /* last receive pid */
}

/* ipc_perm */
struct ipc_perm 
{ 
    key_t key; 
    ushort uid; /* owner euid and egid */ 
    ushort gid; 
    ushort cuid; /* creator euid and egid */ 
    ushort cgid; 
    ushort mode; /* access */ 
    ushort seq; /* slot usage sequence number */ 
};

parameters

• msqid: 消息隊列描述字
• cmd: 命令標誌
  • IPC_STAT: 獲取消息隊列信息, 返回的信息存儲在buf指向的msqid_ds結構體中
  • IPC_SET: 設置消息隊列屬性, 要設計的屬性存儲在buf指向的msqid_ds結構體中
    • msg_perm.uid
    • msg_perm.gid
    • msg_perm.mode
    • msg_qbytes
    • 影響msg_ctime成員
  • IPC_RMID: 刪除msqid標識的消息隊列

return value

• 0: success
• -1: failure