Linux进程学习之：消息队列

以你之姓@ 2023-06-13 11:23 15阅读 0赞

消息队列是一个链式队列

创建消息队列需要用到msgget函数，函数声明在“sys/msg.h”头文件里  
函数声明如下：

int msgget(key_t key, int msgflg);

msgget　函数有两个参数，第一个是key，可以传入宏 IPC\_PRIVATE或者有ftok函数生成的key值，如果传入的是IPC\_PRIVATE宏，消息队列将用于具有亲缘关系的进程间通信，如果是ftok函数生成的key值，则可以用于非亲缘关系的进程间通信，第二个参数是生成的消息队列的读写权限，一般使用0777

下面贴一段生成消息队列的代码

最后的system(“ipcs -q”)的作用是将系统中所有的消息的队列信息打印出来，包括刚才创建的消息队列

删除消息队列用的msgctl函数，同样声明在“sys/msg.h”头文件中  
声明如下：

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

第一个参数是告诉函数要操作的msg对象的id，第二个参数是告诉函数要怎么做，因为我们要删除创建的消息队列，所以传入的IPC\_RMID宏，第三个参数直接传入空就可以

贴一个创建之后又删除消息队列的问题

#include <iostream>
    #include <sys/msg.h>
    #include <stdlib.h>
    
    using namespace std;
    
    int main() {
        
    	int msgid = msgget(IPC_PRIVATE, 0777);
    	if ( msgid == -1) {
        
    		cout << "Create msgid failed ..." << endl;
    		return -1;
    	}
    	else
    		cout << "Create msgid successfully, msgid = " << msgid <<endl;
    	system("ipcs -q");
    	// do something ...
    	//delete msg
    	int ret = msgctl(msgid, IPC_RMID, NULL);
    	if (ret == -1) {
        
    		cout << "delete msg failed ..." << endl;
    		return -2;
    	}
    	else
    		cout << "delete msg successfully ..." << endl;
    	system("ipcs -q");
    	return 0;
    }

接下来说消息队列的消息发送和接受，消息发送用的msgsnd函数，接受用的是msgrcv函数，两个函数声明分别如下：

int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
    
    ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg);

1、msgsnd函数第一个参数是消息队列的id，第二个参数是一个指向用户自定义结构体的指针，不过这个结构体必须具有如下格式，第一个是参数类型，必须大于０，第二个是一个字符数组，大小可以自己定，不过也必须大于０：

struct msgbuf {
        
    	 long mtype;       /* message type, must be > 0 */
         char mtext[1];    /* message data */
    };

msgsnd函数的第三个参数是阻塞标志，０表示阻塞，还有其他的各种标志，可以自己去看

２、msgrcv函数和msgsnd函数的区别是只有第四个参数不一样，第四个参数是意思是当前队列消息的类型，需要明确才可以，如果输入的消息类型有误，程序就会被阻塞

接下来贴一个消息队列发送和接受消息的代码：

#include <iostream>
    #include <sys/msg.h>
    #include <stdlib.h>
    #include <string.h>
    
    using namespace std;
    
    struct msgBuff {
        
    	long type;
    	char voltage[124];
    };
    
    int main() {
        
    	int msgid = msgget(IPC_PRIVATE, 0777);
    	if ( msgid == -1) {
        
    		cout << "Create msgid failed ..." << endl;
    		return -1;
    	}
    	else
    		cout << "Create msgid successfully, msgid = " << msgid <<endl;
    	system("ipcs -q");
    
    	msgBuff sendBuf, rcvBuf;
    	sendBuf.type = 100;
    	cout << "please input message: ";
    	cin >> sendBuf.voltage;
    	msgsnd(msgid, (void *)&sendBuf, sizeof(sendBuf.voltage), 0);
    	memset(rcvBuf.voltage, 0, sizeof(rcvBuf.voltage));
    	int ret = msgrcv(msgid, (void *)&rcvBuf, sizeof(rcvBuf.voltage), 100, 0);
    	if (ret == -1){
        
    		cout << "read rcvBuf failed ..." << endl;
    	}
    	else {
        
    		cout << "read rcvBuf successfully, rcvBuf = " << rcvBuf.voltage << endl;
    	}
    	//delete msg
    	ret = msgctl(msgid, IPC_RMID, NULL);
    	if (ret == -1) {
        
    		cout << "delete msg failed ..." << endl;
    		return -2;
    	}
    	else
    		cout << "delete msg successfully ..." << endl;
    	system("ipcs -q");
    	return 0;
    }

运行以上代码，可以实现先往消息队列写入信息，接着从消息队列读取信息

接下来贴一个两个无亲缘关系的进程间通过消息队列通信的代码：

首先是写进程代码：

#include <iostream>
    #include <sys/msg.h>
    #include <stdlib.h>
    #include <string.h>
    #include <sys/ipc.h>
    
    using namespace std;
    
    struct msgBuff {
        
    	long type;
    	char voltage[124];
    };
    
    int main() {
        
    	int key = ftok("./a", 1);
    	if (key < 0) {
        
    		cout << "Create key failed" << endl;
    		return -1;
    	}
    	else
    		cout << "Create key successfully, key = " << key << endl;
    	int msgid = msgget(key, IPC_CREAT | 0777);
    	if ( msgid == -1) {
        
    		cout << "Create msgid failed ..." << endl;
    		return -1;
    	}
    	else
    		cout << "Create msgid successfully, msgid = " << msgid <<endl;
    	system("ipcs -q");
    
    	msgBuff sendBuf;
    	sendBuf.type = 100;
    	while(1) {
        
    		cout << "please input message: ";
    		cin >> sendBuf.voltage;
    		msgsnd(msgid, (void *)&sendBuf, sizeof(sendBuf.voltage), 0);
    	}
    	int ret = msgctl(msgid, IPC_RMID, NULL);
    	if (ret == -1) {
        
    		cout << "delete msg failed ..." << endl;
    		return -2;
    	}
    	else
    		cout << "delete msg successfully ..." << endl;
    	system("ipcs -q");
    	return 0;
    }

然后是读进程的代码

#include <iostream>
    #include <sys/msg.h>
    #include <stdlib.h>
    #include <string.h>
    #include <sys/ipc.h>
    
    using namespace std;
    
    struct msgBuff {
        
    	long type;
    	char voltage[124];
    };
    
    int main() {
        
    	int key = ftok("./a", 1);
    	if (key < 0) {
        
    		cout << "Create key failed" << endl;
    		return -1;
    	}
    	else
    		cout << "Create key successfully, key = " << key << endl;
    	int msgid = msgget(key, IPC_CREAT | 0777);
    	if ( msgid == -1) {
        
    		cout << "Create msgid failed ..." << endl;
    		return -1;
    	}
    	else
    		cout << "Create msgid successfully, msgid = " << msgid <<endl;
    	system("ipcs -q");
    
    	msgBuff rcvBuf;
    	while(1) {
        
    		memset(rcvBuf.voltage, 0, sizeof(rcvBuf.voltage));
    		msgrcv(msgid, (void *)&rcvBuf, sizeof(rcvBuf.voltage), 100, 0);
    		cout << "read buff:" << rcvBuf.voltage << endl;
    	}
    	int ret = msgctl(msgid, IPC_RMID, NULL);
    	if (ret == -1) {
        
    		cout << "delete msg failed ..." << endl;
    		return -2;
    	}
    	else
    		cout << "delete msg successfully ..." << endl;
    	system("ipcs -q");
    	return 0;
    }

运行以上代码：可以实现在写进程写入消息，在读进程读取信息的功能

接下里贴一个可以在server进程和client进程双向通信的代码：

首先是server代码：

#include <iostream>
    #include <sys/msg.h>
    #include <stdlib.h>
    #include <string.h>
    #include <sys/ipc.h>
    #include <unistd.h>
    
    using namespace std;
    
    struct msgBuff {
        
    	long type;
    	char voltage[124];
    };
    
    int main() {
        
    	int key = ftok("./a", 1);
    	if (key < 0) {
        
    		cout << "Create key failed" << endl;
    		return -1;
    	}
    	else
    		cout << "Create key successfully, key = " << key << endl;
    	int msgid = msgget(key, IPC_CREAT | 0777);
    	if ( msgid == -1) {
        
    		cout << "Create msgid failed ..." << endl;
    		return -1;
    	}
    	else
    		cout << "Create msgid successfully, msgid = " << msgid <<endl;
    	system("ipcs -q");
    
    	int pid = fork();
    	if (pid > 0) {
        
    		msgBuff sendBuf;
    		sendBuf.type = 100;
    		while(1) {
        
    			cout << "please input message: ";
    			cin >> sendBuf.voltage;
    			msgsnd(msgid, (void *)&sendBuf, sizeof(sendBuf.voltage), 0);
    		}
    	}
    	else if (pid == 0){
        
    		msgBuff rcvBuf;
    		while(1) {
        
    			memset(rcvBuf.voltage, 0, sizeof(rcvBuf.voltage));
    			msgrcv(msgid, (void *)&rcvBuf, sizeof(rcvBuf.voltage), 200, 0);
    			cout << "server recive: " << rcvBuf.voltage << endl;
    		}
    	}
    	int ret = msgctl(msgid, IPC_RMID, NULL);
    	if (ret == -1) {
        
    		cout << "delete msg failed ..." << endl;
    		return -2;
    	}
    	else
    		cout << "delete msg successfully ..." << endl;
    	system("ipcs -q");
    	return 0;
    }

然后是client代码：

#include <iostream>
    #include <sys/msg.h>
    #include <stdlib.h>
    #include <string.h>
    #include <sys/ipc.h>
    #include <unistd.h>
    
    using namespace std;
    
    struct msgBuff {
        
    	long type;
    	char voltage[124];
    };
    
    int main() {
        
    	int key = ftok("./a", 1);
    	if (key < 0) {
        
    		cout << "Create key failed" << endl;
    		return -1;
    	}
    	else
    		cout << "Create key successfully, key = " << key << endl;
    	int msgid = msgget(key, IPC_CREAT | 0777);
    	if ( msgid == -1) {
        
    		cout << "Create msgid failed ..." << endl;
    		return -1;
    	}
    	else
    		cout << "Create msgid successfully, msgid = " << msgid <<endl;
    	system("ipcs -q");
    
    	int pid = fork();
    	if (pid > 0) {
        
    		msgBuff sendBuf;
    		sendBuf.type = 200;
    		while(1) {
        
    			cout << "please input message: ";
    			cin >> sendBuf.voltage;
    			msgsnd(msgid, (void *)&sendBuf, sizeof(sendBuf.voltage), 0);
    		}
    	}
    	else if (pid == 0){
        
    		msgBuff rcvBuf;
    		while(1) {
        
    			memset(rcvBuf.voltage, 0, sizeof(rcvBuf.voltage));
    			msgrcv(msgid, (void *)&rcvBuf, sizeof(rcvBuf.voltage), 100, 0);
    			cout << "server recive: " << rcvBuf.voltage << endl;
    		}
    	}
    	int ret = msgctl(msgid, IPC_RMID, NULL);
    	if (ret == -1) {
        
    		cout << "delete msg failed ..." << endl;
    		return -2;
    	}
    	else
    		cout << "delete msg successfully ..." << endl;
    	system("ipcs -q");
    	return 0;
    }

运行以上代码，可以实现两个非亲缘关系进程间的双向通信，其中server进程写入消息的类型是100，读取消息的类型是200，而client进程读取消息的类型是100，写入消息的类型是200，这样双方就可以正常通信了。