使用msgsnd和msgrcv的IPC

Question

为了学习，我编写了2个IPC程序，一个服务器和一个使用消息发送的客户端。由于某些原因，服务器似乎没有接收到所发送的数据。你能给我一些提示，说明我如何着手调试这样的问题吗？

#include <sys/ipc.h>
#include <sys/types.h>
#include <sys/msg.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define CENTRAL_MAILBOX 1200
#define CLIENT_MAILBOX 1201

struct {
long priority;
int value;
int pid;
} msgp, cmbox;

int main(int argc, char **argv) {

int uid = 0; // Process id of the server(this process) set to 0
int length = -1; // Length of the structure we are going to send over
int result = -1; // Result of the msgrcv operation
int status = -1;

if ( argc != 2 ) {
    fprintf(stderr, "Usage: ./server <temperature>\n");
    exit(-1);
}

printf("[+] Starting server\n");

printf("[+] Creating the mailbox\n");
int server_msgid = msgget(CENTRAL_MAILBOX, 0600 | IPC_CREAT);

printf("[+] Creating a mailbox for the client process\n");
int client_msgid = msgget(CLIENT_MAILBOX, 0600 | IPC_CREAT);

printf("[+] Initializing data to be sent across\n");
msgp.priority = 1;
msgp.value = 31337;
msgp.pid = uid;

length = ( sizeof(msgp) > sizeof(long) ? sizeof(msgp)-sizeof(long) : sizeof(long)-sizeof(msgp) );
printf("[+] Calculating the size of the message we are about to send across=%d\n", length);

// msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg);
result = msgrcv(server_msgid, &cmbox, length, 1, 0);
printf("Result = %d\n", result);
printf("[+] Received message pid=%d, value=%d, priority=%ld\n", cmbox.pid, cmbox.value, cmbox.priority);

printf("[+] Sending message\n");
// int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
result = msgsnd(client_msgid, &msgp, length, 0);

printf("[+] Shutting down server\n");
status = msgctl(server_msgid, IPC_RMID, 0);

if ( status != 0 ) {
fprintf(stderr, "[*] ERROR: closing mailbox failed\n");
}

}

我的当事人:-

#include <sys/ipc.h>
#include <sys/types.h>
#include <sys/msg.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define CENTRAL_MAILBOX 1200
#define CLIENT_MAILBOX 1201

struct {
long priority;
int value;
int pid;
} msgp, cmbox;

int main(int argc, char **argv) {

int temp = atoi(argv[1]);
int uid = getpid(); //7171;
int length, result, status;

if ( argc != 2 ) {
// TODO find actual process id
fprintf(stderr, "Usage: ./client <temperature>\n");
exit(-1);
}

printf("[+] Creating server mailbox\n");
int server_msgid = msgget(CENTRAL_MAILBOX, 0600 | IPC_CREAT);

printf("[+] Creating client mailbox\n");
int client_msgid = msgget(CLIENT_MAILBOX, 0600 | IPC_CREAT);

printf("[+] Initializing the data to be sent across\n");
cmbox.priority = 2;
cmbox.value = 1337;
cmbox.pid = uid;

length = ( sizeof(msgp) > sizeof(long) ? sizeof(msgp)-sizeof(long) : sizeof(long)-sizeof(msgp) );
printf("[+] Calculating the size of the message we are about to send across=%d\n", length);

printf("[+] Sending message to server\n");
result = msgsnd(server_msgid, &cmbox, length, 0);
printf("Result = %d\n", result);

result = msgrcv(client_msgid, &msgp, length, 1, 0);
printf("[+] Received message pid=%d, value=%d, priority=%ld\n", msgp.pid, msgp.value, msgp.priority);

printf("[+] Removing the mailbox\n");
status = msgctl(client_msgid, IPC_RMID, 0);

if ( status != 0 ) {
printf("Error when closing mailbox\n");
}

}

Answer 1

服务器在第一个msgrcv()上阻塞，因为它需要类型1的消息(在msgp, cmbox结构中称为优先级)：

// msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg);
result = msgrcv(server_msgid, &cmbox, length, 1, 0);

当客户端发送类型2的消息时：

printf("[+] Initializing the data to be sent across\n");
cmbox.priority = 2;

// [...]

printf("[+] Sending message to server\n");
result = msgsnd(server_msgid, &cmbox, length, 0);

查看如何在手册页中使用msgtyp值：

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)； ..。 msgp参数是指向以下一般形式的调用方定义结构的指针： 结构msgbuf { long mtype；/*消息类型，必须为>0 */ char mtext1；/*消息数据*/ }； ..。 msgrcv() ..。 参数msgtyp指定请求的消息类型如下：

• 如果msgtyp为0，则读取队列中的第一条消息。
• 如果msgtyp大于0，则读取msgtyp类型队列中的第一条消息，除非在msgflg中指定了MSG_EXCEPT，在这种情况下，类型不等于msgtyp的队列中的第一条消息将被读取。
• 如果msgtyp小于0，则将读取最低类型小于或等于msgtyp绝对值的队列中的第一条消息。