System V 消息队列—复用消息

　　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：

#define MSGMAXDAT     1024
struct 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。

例子1：每个应用一个队列，可以在多个客户和单个服务器之间复用消息。使用一个消息队列进行通信，由消息类型标识消息是从客户到服务器，还是服务器到客户。通信模型如下：

按照通信模型编写程序如下：

公共头文件svmsg.h

#ifndef  SVMSG_H
#define  SVMSG_H
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <errno.h>

#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)
#define MQ_KEY  1234L
#define MSGMAX  1024
//消息结构
struct mymesg
{
    long mesg_len;
    long mesg_type;
    char mesg_data[MSGMAX];
};
#endif

客户端程序sysv_client.c

#include "svmsg.h"
void client(int ,int);

int main(int argc,char *argv[])
{
    int     msqid;
    if((msqid = msgget(MQ_KEY,0)) == -1)
    {
        perror("megget()");
        exit(-1);
    }
    client(msqid,msqid);
    exit(0);
}

void client(int readfd,int writefd)
{
    size_t len;
    ssize_t n;
    char *ptr;
    struct mymesg mesg;
    printf("Send request to server.\n");
    //set pid to message
    snprintf(mesg.mesg_data,MSGMAX,"%ld",(long)getpid());
    len = strlen(mesg.mesg_data);
    mesg.mesg_data[len] = ' '; //blank
    ptr = mesg.mesg_data+len+1;
    printf("Enter filename: ");
    fgets(ptr,MSGMAX-len,stdin);
    len = strlen(mesg.mesg_data);
    if(mesg.mesg_data[len-1] == '\n')
        len--;
    mesg.mesg_len = len;
    mesg.mesg_type = 1;
    printf("mesg_data is :%s len=%ld\n",mesg.mesg_data, mesg.mesg_len);
    if(msgsnd(writefd,&(mesg.mesg_type),mesg.mesg_len,0) == -1)
    {
        perror("msgsnd() error");
        exit(-1);
    }
    //read from IPC,write to standard output
    mesg.mesg_type = getpid();
    while( (n = msgrcv(readfd,&(mesg.mesg_type),MSGMAX,mesg.mesg_type,0))>0)
    {
        write(STDOUT_FILENO,mesg.mesg_data,n);
        putchar('\n');
    }
    if(n == 0 )
    {
        printf("Read file from server is completed.\n");
    }
    if(n == -1)
    {
        perror("msgrcv() error");
        exit(-1);
    }
}

服务器程序sysv_server.c

#include "svmsg.h"
void server(int ,int);
int main(int argc,char *argv[])
{
    int     msqid;
    if((msqid = msgget(MQ_KEY,SVMSG_MODE | IPC_CREAT)) == -1)
    {
        perror("megget()");
        exit(-1);
    }
    server(msqid,msqid);
    exit(0);
}

void server(int readfd,int writefd)
{
    FILE            *fp;
    char            *ptr;
    pid_t           pid;
    ssize_t         n;
    ssize_t         len;
    struct mymesg   mesg;
    printf("Waiting for client......\n");
    for(; ;)
    {
        mesg.mesg_type = 1;
        if((n = msgrcv(readfd,&(mesg.mesg_type),MSGMAX,mesg.mesg_type,0)) == 0)
        {
            printf("pathname missing.\n");
            continue;
        }
        mesg.mesg_data[n] = '\0';
        printf("Received message from client is: %s\n",mesg.mesg_data);
        if ((ptr = strchr(mesg.mesg_data,' ')) == NULL)
        {
            printf("bogus request: %s\n",mesg.mesg_data);
            continue;
        }
        *ptr++ = 0;
        pid = atoi(mesg.mesg_data);
        mesg.mesg_type = pid;
        //open fiel and read data
        if((fp = fopen(ptr,"r")) == NULL)
        {
            printf("open file failed.sent msg to client\n");
            snprintf(mesg.mesg_data+n,sizeof(mesg.mesg_data)-n,": can't open,%s\n",strerror(errno));
            mesg.mesg_len = strlen(ptr);
            memmove(mesg.mesg_data,ptr,mesg.mesg_len);
            if(msgsnd(writefd,&(mesg.mesg_type),mesg.mesg_len,0) == -1)
            {
                perror("msgsnd() error");
                exit(-1);
            }
        }
        else
        {
            printf("open file successed.sent file to client\n");
            while(fgets(mesg.mesg_data,MSGMAX,fp) != NULL)
            {
                mesg.mesg_len = strlen(mesg.mesg_data);
                if(msgsnd(writefd,&(mesg.mesg_type),mesg.mesg_len,0) == -1)
                {
                    perror("msgsnd() error");
                    exit(-1);
                }
            }
            fclose(fp);
        }
        printf("send compelted.\n");
        mesg.mesg_len = 0;
        if(msgsnd(writefd,&(mesg.mesg_type),mesg.mesg_len,0) == -1)
        {
            perror("msgsnd() error");
            exit(-1);
        }
    }
}

程序测试结果如下所示：

 例子2：每个客户一个队列，将例子1改成所有用户用一个共同的消息队列向服务器发送消息，给每个客户分配一个消息队列，使得服务器对每个客户进行应答。通信模型如下：

以并发服务器模型编写这个程序，服务器给每个客户fork一个子进程进行处理。程序如下：

公共头文件svmsg.h和svmsg.c：

//svmsg.h file
#ifndef  SVMSG_H
#define  SVMSG_H
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <errno.h>

#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)
#define MQ_KEY  1234L
#define MSGMAX  1024
//message structure
struct mymesg
{
    long mesg_len;
    long mesg_type;
    char mesg_data[MSGMAX];
};

ssize_t mesg_send(int id,struct mymesg *mptr);
ssize_t mesg_recv(int id,struct mymesg *mptr);

void    Mesg_send(int id,struct mymesg *mptr);
ssize_t Mesg_recv(int id,struct mymesg *mptr);
#endif

//svmsg.c file
#include "svmsg.h"

ssize_t mesg_send(int id,struct mymesg *mptr)
{
    return (msgsnd(id,&(mptr->mesg_type),mptr->mesg_len,0));
}

ssize_t mesg_recv(int id,struct mymesg *mptr)
{
    ssize_t n;
    n = msgrcv(id,&(mptr->mesg_type),MSGMAX,mptr->mesg_type,0);
    mptr->mesg_len = n;
    return n;
}

void Mesg_send(int id,struct mymesg *mptr)
{
    if(mesg_send(id,mptr) == -1)
    {
        perror("mesg_send() error");
        exit(-1);
    }
}
ssize_t Mesg_recv(int id,struct mymesg *mptr)
{
    ssize_t n;
    do
    {
        n = mesg_recv(id,mptr);
    }while(n==-1 && errno == EINTR);
    if(n == -1)
    {
        perror("mesg_recv() error");
        exit(-1);
    }
    return n;
}

客户端程序如下：

#include "svmsg.h"

void client(int ,int);

int main(int argc,char *argv[])
{
    int    readid,writeid;
    if((writeid = msgget(MQ_KEY,0)) == -1)
    {
        perror("megget()");
        exit(-1);
    }
    if((readid = msgget(IPC_PRIVATE,SVMSG_MODE | IPC_CREAT)) == -1)
    {
        perror("megget()");
        exit(-1);
    }
    client(readid,writeid);
    msgctl(readid,IPC_RMID,NULL);
    exit(0);
}

void client(int readid,int writeid)
{
    size_t len;
    ssize_t n;
    char *ptr;
    struct mymesg mesg;
    printf("Send request to server.\n");
    //set pid to message
    snprintf(mesg.mesg_data,MSGMAX,"%d",readid);
    len = strlen(mesg.mesg_data);
    mesg.mesg_data[len] = ' '; //blank
    ptr = mesg.mesg_data+len+1;
    printf("Enter filename: ");
    fgets(ptr,MSGMAX-len,stdin);
    len = strlen(mesg.mesg_data);
    if(mesg.mesg_data[len-1] == '\n')
        len--;
    mesg.mesg_len = len;
    mesg.mesg_type = 1;
    printf("mesg_data is :%s\n",mesg.mesg_data);
    Mesg_send(writeid,&mesg);
    printf("Send messge to server successed.\n");
    //read from IPC,write to standard output
    while( (n = Mesg_recv(readid,&mesg))>0)
    {
        write(STDOUT_FILENO,mesg.mesg_data,n);
        putchar('\n');
    }
    if(n == 0 )
    {
        printf("Read file from server is completed.\n");
    }
}

服务器程序如下：

#include "svmsg.h"

void server(int ,int);
void sig_child(int signo);

int main(int argc,char *argv[])
{
    int     msqid;
    if((msqid = msgget(MQ_KEY,SVMSG_MODE | IPC_CREAT)) == -1)
    {
        perror("megget()");
        exit(-1);
    }
    server(msqid,msqid);
    exit(0);
}

void server(int readid,int writeid)
{
    FILE            *fp;
    char            *ptr;
    pid_t           pid;
    ssize_t         n;
    ssize_t         len;
    struct mymesg   mesg;
    signal(SIGCHLD,sig_child);
    printf("Waiting for client......\n");
    for(; ;)
    {
        mesg.mesg_type = 1;
        if((n = Mesg_recv(readid,&mesg)) == 0)
        {
            printf("pathname missing.\n");
            continue;
        }
        mesg.mesg_data[n] = '\0';
        printf("Received message from client is: %s\n",mesg.mesg_data);
        if ((ptr = strchr(mesg.mesg_data,' ')) == NULL)
        {
            printf("bogus request: %s\n",mesg.mesg_data);
            continue;
        }
        *ptr++ = 0;
        writeid = atoi(mesg.mesg_data);
        if(fork() == 0)
        {
            //open fiel and read data
            if((fp = fopen(ptr,"r")) == NULL)
            {
                printf("open file failed.sent msg to client\n");
                snprintf(mesg.mesg_data+n,sizeof(mesg.mesg_data)-n,": can't open,%s\n",strerror(errno));
                mesg.mesg_len = strlen(ptr);
                memmove(mesg.mesg_data,ptr,mesg.mesg_len);
                Mesg_send(writeid,&mesg);
            }
            else
            {
                printf("open file successed.sent file to client\n");
                while(fgets(mesg.mesg_data,MSGMAX,fp) != NULL)
                {
                    mesg.mesg_len = strlen(mesg.mesg_data);
                    Mesg_send(writeid,&mesg);
                }
                fclose(fp);
            }
            printf("send compelted.\n");
            mesg.mesg_len = 0;
            Mesg_send(writeid,&mesg);
        }
    }
}

void sig_child(int signo)
{
    pid_t   pid;
    int     stat;
    while ((pid = waitpid(-1,&stat,WNOHANG)) > 0);
    return ;
}

程序测试结果如下：