本文共 5422 字，大约阅读时间需要 18 分钟。
消息队列可以认为是一个消息链表，System V 消息队列使用消息队列标识符标识。具有足够特权的任何进程都可以往一个队列放置一个消息，具有足够特权的任何进程都可以从一个给定队列读出一个消息。在某个进程往一个队列写入消息之前，并不需要另外某个进程在该队列上等待消息的到达。System V 消息队列是随内核持续的，只有在内核重起或者显示删除一个消息队列时，该消息队列才会真正被删除。对于系统中没个消息队列，内核维护一个msqid_ds的信息结构：
struct 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 */
};
下面这个程序是创建一个消息队列，然后网消息队列中放置一个含有1个字节的消息，发出msgctl的IPC_STAT命令，使用system函数执行ipcs命令，最后使用IPC_RMID命令删除该队列。(书本例子)
#include#include #include #include #include #include #define MSG_R 0400 /* read permission */#define MSG_W 0200 /* write permission */#define SVMSG_MODE (MSG_R | MSG_W | MSG_R >>3 | MSG_R >>6)typedef unsigned long ulong_t;int main(){ int msqid; struct msqid_ds info; struct msgbuf buf; if((msqid = msgget(IPC_PRIVATE,SVMSG_MODE|IPC_CREAT)) == -1) { perror("msgget() error"); exit(-1); } buf.mtype = 1; buf.mtext[0] = 1; if(msgsnd(msqid,&buf,1,0) == -1) { perror("msgsnd() error"); exit(-1); } if(msgctl(msqid,IPC_STAT,&info) == -1) { perror("msgctl() error"); exit(-1); } printf("read-write: %03o,cbytes = %lu,qnum =%lu,qbytes = %lu\n", info.msg_perm.mode & 0777,(ulong_t) info.msg_cbytes, (ulong_t) info.msg_qnum,(ulong_t) info.msg_qbytes); system("ipcs -q"); if(msgctl(msqid,IPC_RMID,NULL) == -1) { perror("msgctl() error"); exit(-1); } exit(0); return 0;}
这个是对System V的一些函数的测试，下面看一个客户服务器的例子：
先服务器创建两个System V的消息队列，然后发送消息给服务器，服务器接受到消息后开始给客户反馈消息。
客服端：
#include服务器端：#include #include #include #include #include #include #define MQ_KEY1 1234L //定义两个KEY#define MQ_KEY2 2345L#define MAXMESGDATA 1024#define MAXMSG sizeof(long)*2+1024void client(int ,int);int main(int argc,char *argv[]){ int readid,writeid; //打开消息队列 //MO_KEY2用于客户从服务器读取 if((readid = msgget(MQ_KEY2,0)) == -1) { perror("msgget() error"); exit(-1); }//MO_KEY用于客户向服务器写 if((writeid = msgget(MQ_KEY1,0))==-1) { perror("msgget() error"); exit(-1); } client(readid,writeid); msgctl(readid,IPC_RMID,NULL); //删除消息队列 msgctl(writeid,IPC_RMID,NULL); exit(0);}void client(int readid ,int writeid){ ssize_t n,len; struct msgbuf *buff,*ptr; char content[MAXMESGDATA]; long type; printf("Enter the msg_type: "); scanf("%ld",&type); printf("Enter the msg_data: "); scanf("%s",content); len = strlen(content); ptr =(msgbuf *) calloc(sizeof(long)*2+len,sizeof(char)); ptr->mtype = type; strcpy(ptr->mtext,content); printf("Sent to server.\n"); if(msgsnd(writeid,ptr,len+sizeof(long),0) == -1) { perror("msgsnd() error"); exit(-1); } printf("Sent to server successfully.\n"); free(ptr); printf("*********************************\n"); buff = (msgbuf*)malloc(MAXMSG ); printf("waiting for server......\n"); //以阻塞形式读取 if((n=msgrcv(readid,buff,MAXMSG,0,0)) == -1) { perror("msgrcv() error"); exit(-1); } printf("Received message from server is:\nmsg_type = %ld\nmsg_data = %s\n", buff->mtype,buff->mtext); free(buff);}
#include消息复用： 消息队列中的消息结构可以由我们自由定义，具备较强的灵活性。通过消息结构可以共享一个队列，进行消息复用。通常定义一个类似如下的消息结构：#include #include #include #include #include #include #define MQ_KEY1 1234L //定义两个KEY#define MQ_KEY2 2345L#define MAXMESGDATA 1024#define MAXMSG sizeof(long)*2+1024#define MSG_R 0400 /* read permission */#define MSG_W 0200 /* write permission */#define SVMSG_MODE (MSG_R | MSG_W | MSG_R >>3 | MSG_R >>6)void server(int ,int);int main(int argc,char *argv[]){ int readid,writeid; if((readid = msgget(MQ_KEY1,SVMSG_MODE | IPC_CREAT)) == -1) { perror("msgget() error"); exit(-1); } if((writeid = msgget(MQ_KEY2,SVMSG_MODE | IPC_CREAT)) == -1) { perror("msgget() error"); exit(-1); } server(readid,writeid); exit(0);}void server(int readid,int writeid){ ssize_t n,len; struct msgbuf *buff,*ptr; char content[MAXMESGDATA]; long type; buff =(msgbuf*) malloc(MAXMSG ); printf("waiting for client......\n"); if((n=msgrcv(readid,buff,MAXMSG,0,0)) == -1) { perror("msgrcv() error"); exit(-1); } printf("Received message from client is:\nmsg_type = %ld\nmsg_data = %s\n", buff->mtype,buff->mtext); free(buff); printf("*********************************\n"); printf("Enter the msg_type: "); scanf("%ld",&type); printf("Enter the msg_data: "); scanf("%s",content); len = strlen(content); ptr =(msgbuf*) calloc(sizeof(long)*2+len,sizeof(char)); ptr->mtype = type; strcpy(ptr->mtext,content); printf("Sent to client.\n"); if(msgsnd(writeid,ptr,len,0) == -1) { perror("msgsnd() error"); exit(-1); } printf("Sent to client successfully.\n"); free(buff);}
#define MSGMAXDAT 1024struct mymsg{ long msg_len; //消息长度 long msg_type; //消息类型 long msg_data[MSGMAXDATA]; //消息内容}; 消息结构相关联的类型字段（msg_type）提供了两个特性：
（1）标识消息，使得多个进程在单个队列上复用消息。
（2）用作优先级字段，允许接收者以不同于先进先出的某个顺序读出各个消息。
转载地址：http://wwifi.baihongyu.com/