一. 消息队列的通信原理

消息队列放在链表当中，链表中每一个消息放在结构体当中，
（链表中每一项都是一个结构体）。

AB使用同一个队列进行通讯

消息队列按照消息的类型进行读取

二、在Linux中使用消息队列

（1）创建或打开消息队列函数

int msgget(key_t, key, int msgflg);

（2）添加消息函数

int msgsnd(int msgid, const void *msg_ptr, size_t msg_sz, int msgflg);

如果调用成功，消息数据的一分副本将被放到消息队列中，并返回0，失败时返回-1.

（3）从一个消息队列中获取消息：

int msgrcv(int msgid, void *msg_ptr, size_t msg_st, long int msgtype, int msgflg);

调用成功时，该函数返回放到接收缓存区中的字节数，消息被复制到由msg_ptr指向的用户分配的缓存区中，然后删除消息队列中的对应消息。失败时返回-1.

（4）消息队列控制函数：

int msgctl(int msgid, int command, struct msgid_ds *buf);

command是将要采取的动作，它可以取3个值，

IPC_STAT：把msgid_ds结构中的数据设置为消息队列的当前关联值，即用消息队列的当前关联值覆盖msgid_ds的值。

IPC_SET：如果进程有足够的权限，就把消息列队的当前关联值设置为msgid_ds结构中给出的值

IPC_RMID：删除消息队列
buf是指向msgid_ds结构的指针，它指向消息队列模式和访问权限的结构。msgid_ds结构至少包括以下成员：
struct msgid_ds { uid_t shm_perm.uid; uid_t shm_perm.gid; mode_t shm_perm.mode; };
成功时返回0，失败时返回-1.

1.发送消息：

//文件msgsend.c
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include<stdio.h>
#include<string.h>

//       int msgget(key_t key, int msgflg);
//       int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

 //      size_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,int msgflg);

struct msgbuf{
        long mtype;
        char mtext[128];
};

int main()
{
        struct msgbuf sendBuf = {888,"this is message from quen"};
        struct msgbuf readBuf;
        int msgId = msgget(0x1235,IPC_CREAT|0777);
        if(msgId == -1){
              printf("get que failuer\n");
        }


        msgsnd(msgId,&sendBuf,strlen(sendBuf.mtext),0);
        printf("send over\n");

        msgrcv(msgId,&readBuf,sizeof(readBuf.mtext),988,0);
        printf("return from get:%s\n",readBuf.mtext);

        return 0;
}

接收消息：

//文件msgGet.c
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include<stdio.h>
#include<string.h>

//       int msgget(key_t key, int msgflg);
//       int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

 //      size_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,int msgflg);
struct msgbuf{

        long mtype;
        char mtext[128];
};

int main()
{
        struct msgbuf readBuf;
        int msgId = msgget(0x1235,IPC_CREAT|0777);
        if(msgId == -1){
                printf("get que failuer\n");
        }
        msgrcv(msgId,&readBuf,sizeof(readBuf.mtext),888,0);
        printf("read from que:%s\n",readBuf.mtext);

        struct msgbuf sendBuf = {988,"thank you for reach"};
        msgsnd(msgId,&sendBuf,strlen(sendBuf.mtext),0);

        return 0;
}

 2.发送消息：

//文件msgSend2.c
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include<stdio.h>
#include<string.h>

//       int msgget(key_t key, int msgflg);
//       int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

 //      size_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,int msgflg);
struct msgbuf{

        long mtype;
        char mtext[128];
};

int main()
{
        struct msgbuf sendBuf = {888,"this is message from quen"};
        struct msgbuf readBuf;

        key_t key;
        key = ftok(".",'z');
        printf("key=%x\n",key);

        int msgId = msgget(key,IPC_CREAT|0777);
        if(msgId == -1){
                printf("get que failuer\n");
        }


        msgsnd(msgId,&sendBuf,strlen(sendBuf.mtext),0);
        printf("send over\n");

        msgrcv(msgId,&readBuf,sizeof(readBuf.mtext),988,0);
        printf("return from get:%s\n",readBuf.mtext);
        msgctl(msgId,&sendBuf,strlen(sendBuf.mtext),0);

        return 0;
}

 接收消息：

//文件msgGet2.c
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include<stdio.h>
#include<string.h>

//       int msgget(key_t key, int msgflg);
//       int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

 //      size_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,int msgflg);
struct msgbuf{

        long mtype;
        char mtext[128];
};

int main()
{
        struct msgbuf readBuf;

        key_t key;
        key = ftok(".",'z');
        printf("key=%x\n",key);
        int msgId = msgget(key,IPC_CREAT|0777);
        if(msgId == -1){
                printf("get que failuer\n");
        }

        msgrcv(msgId,&readBuf,sizeof(readBuf.mtext),888,0);
        printf("read from que:%s\n",readBuf.mtext);

        struct msgbuf sendBuf = {988,"thank you for reach"};
        msgsnd(msgId,&sendBuf,strlen(sendBuf.mtext),0);
        msgctl(msgId,&sendBuf,strlen(sendBuf.mtext),0);

        return 0;
}

运行结果：

CLC@Embed_Learn:~$ ./send
key=7a055bb6
send over
return from get:thank you for reach

CLC@Embed_Learn:~$ ./get
key=7a055bb6
read from que:this is message from quen