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
1 #ifndef SVMSG_H
2 #define SVMSG_H
3 #include <stdio.h>
4 #include <stdlib.h>
5 #include <string.h>
6 #include <unistd.h>
7 #include <sys/types.h>
8 #include <sys/ipc.h>
9 #include <sys/msg.h>
10 #include <errno.h>
11
12 #define MSG_R 0400 /* read permission */
13 #define MSG_W 0200 /* write permission */
14 #define SVMSG_MODE (MSG_R | MSG_W | MSG_R >>3 | MSG_R >>6)
15 #define MQ_KEY 1234L
16 #define MSGMAX 1024
17 //消息结构
18 struct mymesg
19 {
20 long mesg_len;
21 long mesg_type;
22 char mesg_data[MSGMAX];
23 };
24 #endif
客户端程序sysv_client.c
1 #include "svmsg.h"
2 void client(int ,int);
3
4 int main(int argc,char *argv[])
5 {
6 int msqid;
7 if((msqid = msgget(MQ_KEY,0)) == -1)
8 {
9 perror("megget()");
10 exit(-1);
11 }
12 client(msqid,msqid);
13 exit(0);
14 }
15
16 void client(int readfd,int writefd)
17 {
18 size_t len;
19 ssize_t n;
20 char *ptr;
21 struct mymesg mesg;
22 printf("Send request to server.
");
23 //set pid to message
24 snprintf(mesg.mesg_data,MSGMAX,"%ld",(long)getpid());
25 len = strlen(mesg.mesg_data);
26 mesg.mesg_data[len] = ' '; //blank
27 ptr = mesg.mesg_data+len+1;
28 printf("Enter filename: ");
29 fgets(ptr,MSGMAX-len,stdin);
30 len = strlen(mesg.mesg_data);
31 if(mesg.mesg_data[len-1] == '
')
32 len--;
33 mesg.mesg_len = len;
34 mesg.mesg_type = 1;
35 printf("mesg_data is :%s len=%ld
",mesg.mesg_data, mesg.mesg_len);
36 if(msgsnd(writefd,&(mesg.mesg_type),mesg.mesg_len,0) == -1)
37 {
38 perror("msgsnd() error");
39 exit(-1);
40 }
41 //read from IPC,write to standard output
42 mesg.mesg_type = getpid();
43 while( (n = msgrcv(readfd,&(mesg.mesg_type),MSGMAX,mesg.mesg_type,0))>0)
44 {
45 write(STDOUT_FILENO,mesg.mesg_data,n);
46 putchar('
');
47 }
48 if(n == 0 )
49 {
50 printf("Read file from server is completed.
");
51 }
52 if(n == -1)
53 {
54 perror("msgrcv() error");
55 exit(-1);
56 }
57 }
服务器程序sysv_server.c
1 #include "svmsg.h"
2 void server(int ,int);
3 int main(int argc,char *argv[])
4 {
5 int msqid;
6 if((msqid = msgget(MQ_KEY,SVMSG_MODE | IPC_CREAT)) == -1)
7 {
8 perror("megget()");
9 exit(-1);
10 }
11 server(msqid,msqid);
12 exit(0);
13 }
14
15 void server(int readfd,int writefd)
16 {
17 FILE *fp;
18 char *ptr;
19 pid_t pid;
20 ssize_t n;
21 ssize_t len;
22 struct mymesg mesg;
23 printf("Waiting for client......
");
24 for(; ;)
25 {
26 mesg.mesg_type = 1;
27 if((n = msgrcv(readfd,&(mesg.mesg_type),MSGMAX,mesg.mesg_type,0)) == 0)
28 {
29 printf("pathname missing.
");
30 continue;
31 }
32 mesg.mesg_data[n] = ' ';
33 printf("Received message from client is: %s
",mesg.mesg_data);
34 if ((ptr = strchr(mesg.mesg_data,' ')) == NULL)
35 {
36 printf("bogus request: %s
",mesg.mesg_data);
37 continue;
38 }
39 *ptr++ = 0;
40 pid = atoi(mesg.mesg_data);
41 mesg.mesg_type = pid;
42 //open fiel and read data
43 if((fp = fopen(ptr,"r")) == NULL)
44 {
45 printf("open file failed.sent msg to client
");
46 snprintf(mesg.mesg_data+n,sizeof(mesg.mesg_data)-n,": can't open,%s
",strerror(errno));
47 mesg.mesg_len = strlen(ptr);
48 memmove(mesg.mesg_data,ptr,mesg.mesg_len);
49 if(msgsnd(writefd,&(mesg.mesg_type),mesg.mesg_len,0) == -1)
50 {
51 perror("msgsnd() error");
52 exit(-1);
53 }
54 }
55 else
56 {
57 printf("open file successed.sent file to client
");
58 while(fgets(mesg.mesg_data,MSGMAX,fp) != NULL)
59 {
60 mesg.mesg_len = strlen(mesg.mesg_data);
61 if(msgsnd(writefd,&(mesg.mesg_type),mesg.mesg_len,0) == -1)
62 {
63 perror("msgsnd() error");
64 exit(-1);
65 }
66 }
67 fclose(fp);
68 }
69 printf("send compelted.
");
70 mesg.mesg_len = 0;
71 if(msgsnd(writefd,&(mesg.mesg_type),mesg.mesg_len,0) == -1)
72 {
73 perror("msgsnd() error");
74 exit(-1);
75 }
76 }
77 }
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：
1 //svmsg.h file
2 #ifndef SVMSG_H
3 #define SVMSG_H
4 #include <stdio.h>
5 #include <stdlib.h>
6 #include <string.h>
7 #include <unistd.h>
8 #include <signal.h>
9 #include <sys/types.h>
10 #include <sys/ipc.h>
11 #include <sys/msg.h>
12 #include <errno.h>
13
14 #define MSG_R 0400 /* read permission */
15 #define MSG_W 0200 /* write permission */
16 #define SVMSG_MODE (MSG_R | MSG_W | MSG_R >>3 | MSG_R >>6)
17 #define MQ_KEY 1234L
18 #define MSGMAX 1024
19 //message structure
20 struct mymesg
21 {
22 long mesg_len;
23 long mesg_type;
24 char mesg_data[MSGMAX];
25 };
26
27 ssize_t mesg_send(int id,struct mymesg *mptr);
28 ssize_t mesg_recv(int id,struct mymesg *mptr);
29
30 void Mesg_send(int id,struct mymesg *mptr);
31 ssize_t Mesg_recv(int id,struct mymesg *mptr);
32 #endif
1 //svmsg.c file
2 #include "svmsg.h"
3
4 ssize_t mesg_send(int id,struct mymesg *mptr)
5 {
6 return (msgsnd(id,&(mptr->mesg_type),mptr->mesg_len,0));
7 }
8
9 ssize_t mesg_recv(int id,struct mymesg *mptr)
10 {
11 ssize_t n;
12 n = msgrcv(id,&(mptr->mesg_type),MSGMAX,mptr->mesg_type,0);
13 mptr->mesg_len = n;
14 return n;
15 }
16
17 void Mesg_send(int id,struct mymesg *mptr)
18 {
19 if(mesg_send(id,mptr) == -1)
20 {
21 perror("mesg_send() error");
22 exit(-1);
23 }
24 }
25 ssize_t Mesg_recv(int id,struct mymesg *mptr)
26 {
27 ssize_t n;
28 do
29 {
30 n = mesg_recv(id,mptr);
31 }while(n==-1 && errno == EINTR);
32 if(n == -1)
33 {
34 perror("mesg_recv() error");
35 exit(-1);
36 }
37 return n;
38 }
客户端程序如下：
1 #include "svmsg.h"
2
3 void client(int ,int);
4
5 int main(int argc,char *argv[])
6 {
7 int readid,writeid;
8 if((writeid = msgget(MQ_KEY,0)) == -1)
9 {
10 perror("megget()");
11 exit(-1);
12 }
13 if((readid = msgget(IPC_PRIVATE,SVMSG_MODE | IPC_CREAT)) == -1)
14 {
15 perror("megget()");
16 exit(-1);
17 }
18 client(readid,writeid);
19 msgctl(readid,IPC_RMID,NULL);
20 exit(0);
21 }
22
23 void client(int readid,int writeid)
24 {
25 size_t len;
26 ssize_t n;
27 char *ptr;
28 struct mymesg mesg;
29 printf("Send request to server.
");
30 //set pid to message
31 snprintf(mesg.mesg_data,MSGMAX,"%d",readid);
32 len = strlen(mesg.mesg_data);
33 mesg.mesg_data[len] = ' '; //blank
34 ptr = mesg.mesg_data+len+1;
35 printf("Enter filename: ");
36 fgets(ptr,MSGMAX-len,stdin);
37 len = strlen(mesg.mesg_data);
38 if(mesg.mesg_data[len-1] == '
')
39 len--;
40 mesg.mesg_len = len;
41 mesg.mesg_type = 1;
42 printf("mesg_data is :%s
",mesg.mesg_data);
43 Mesg_send(writeid,&mesg);
44 printf("Send messge to server successed.
");
45 //read from IPC,write to standard output
46 while( (n = Mesg_recv(readid,&mesg))>0)
47 {
48 write(STDOUT_FILENO,mesg.mesg_data,n);
49 putchar('
');
50 }
51 if(n == 0 )
52 {
53 printf("Read file from server is completed.
");
54 }
55 }
服务器程序如下：
1 #include "svmsg.h"
2
3 void server(int ,int);
4 void sig_child(int signo);
5
6 int main(int argc,char *argv[])
7 {
8 int msqid;
9 if((msqid = msgget(MQ_KEY,SVMSG_MODE | IPC_CREAT)) == -1)
10 {
11 perror("megget()");
12 exit(-1);
13 }
14 server(msqid,msqid);
15 exit(0);
16 }
17
18 void server(int readid,int writeid)
19 {
20 FILE *fp;
21 char *ptr;
22 pid_t pid;
23 ssize_t n;
24 ssize_t len;
25 struct mymesg mesg;
26 signal(SIGCHLD,sig_child);
27 printf("Waiting for client......
");
28 for(; ;)
29 {
30 mesg.mesg_type = 1;
31 if((n = Mesg_recv(readid,&mesg)) == 0)
32 {
33 printf("pathname missing.
");
34 continue;
35 }
36 mesg.mesg_data[n] = ' ';
37 printf("Received message from client is: %s
",mesg.mesg_data);
38 if ((ptr = strchr(mesg.mesg_data,' ')) == NULL)
39 {
40 printf("bogus request: %s
",mesg.mesg_data);
41 continue;
42 }
43 *ptr++ = 0;
44 writeid = atoi(mesg.mesg_data);
45 if(fork() == 0)
46 {
47 //open fiel and read data
48 if((fp = fopen(ptr,"r")) == NULL)
49 {
50 printf("open file failed.sent msg to client
");
51 snprintf(mesg.mesg_data+n,sizeof(mesg.mesg_data)-n,": can't open,%s
",strerror(errno));
52 mesg.mesg_len = strlen(ptr);
53 memmove(mesg.mesg_data,ptr,mesg.mesg_len);
54 Mesg_send(writeid,&mesg);
55 }
56 else
57 {
58 printf("open file successed.sent file to client
");
59 while(fgets(mesg.mesg_data,MSGMAX,fp) != NULL)
60 {
61 mesg.mesg_len = strlen(mesg.mesg_data);
62 Mesg_send(writeid,&mesg);
63 }
64 fclose(fp);
65 }
66 printf("send compelted.
");
67 mesg.mesg_len = 0;
68 Mesg_send(writeid,&mesg);
69 }
70 }
71 }
72
73 void sig_child(int signo)
74 {
75 pid_t pid;
76 int stat;
77 while ((pid = waitpid(-1,&stat,WNOHANG)) > 0);
78 return ;
79 }
程序测试结果如下：