Question

我正在使用Ubuntu 12.05。我一直在尝试通过C套接字编程实现Stop和wait协议。我创建了两个程序，一个是服务器，另一个是客户端。通过评论

解释了代码的预期工作

serversocket.c

    #include <sys/socket.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <netinet/in.h>
    #include <arpa/inet.h>
    #include <unistd.h>
    #include <error.h>
    #include <string.h>
    #include <sys/types.h>
    #include <time.h>

int main(int argc,char *argv[])
{
    int listenfd = 0,qlength=10,connfd = 0,t=5;
    int n;
    struct sockaddr_in serv;
        struct sockaddr_in dest;
        socklen_t socksize = sizeof(struct sockaddr_in);
    char sendBuff[1024]="hi\n";
    //time_t ticks;
     listenfd = socket(AF_INET,SOCK_STREAM,0); //socket for listening to connection requests

     memset(&serv,'0',sizeof(serv));
     serv.sin_family = AF_INET;
     serv.sin_addr.s_addr = htonl(INADDR_ANY);
     serv.sin_port=htons(5000);

     bind(listenfd,(struct sockaddr *)&serv,sizeof(serv)); //binding the socket to a port
     listen(listenfd,2);    
     connfd=accept(listenfd,(struct sockaddr *)&dest,&socksize);  //another socket for sending and receiving on the previously built socket
         while(connfd){
         printf("Incoming connection from %s-sending a hi...\n",inet_ntoa(dest.sin_addr));
         send(connfd,sendBuff,strlen(sendBuff),0);         //at first my server sends a hi
         sleep(3);
         n=recv(connfd,sendBuff,1024,0);                  //if hi received by the client,then server should receive "Message Received" from the client
         sendBuff[n]='\0';
         printf("%s",sendBuff);
        connfd=accept(listenfd,(struct sockaddr *)&dest,&socksize); }

    return 0;
}

clientsocket.c

#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <arpa/inet.h> 

int main(int argc, char *argv[])
{
    int sockfd = 0, n = 0,m=2;
    char recvBuff[1024];
    struct sockaddr_in serv_addr; 

    if(argc != 2)
    {
        printf("\n Usage: %s <ip of server> \n",argv[0]);
        return 1;
    } 

    memset(recvBuff, '0',sizeof(recvBuff));
    if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
    {
        printf("\n Error : Could not create socket \n");
        return 1;
    } 

    memset(&serv_addr, '0', sizeof(serv_addr)); 

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(5000); 

    if(inet_pton(AF_INET, argv[1], &serv_addr.sin_addr)<=0)
    {
        printf("\n inet_pton error occured\n");
        return 1;
    } 

    if( connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)
    {
       printf("\n Error : Connect Failed \n");
       return 1;
    } 
   while(m--){                                      //m is 2 initially,I want that sending and receiving should be done 2 times
    n = recv(sockfd,recvBuff,1024,0);
    recvBuff[n]='\0';                               //"hi" is received
    printf("%s",recvBuff);                           

    if(recvBuff=="hi\n")
      send(sockfd,"Message Received",strlen("Message Received\n"),0);} //sending "Message received"


    return 0;
}

现在从服务器向客户端发送消息的工作正常，但是客户端到服务器（收到消息）正在产生问题。无论是给出错误还是正确结果，它只是给出一个空白屏幕。

Answer 1

您无法使用“==”运算符比较字符串。

您需要使用strcmp()

...所以

if( strcmp( recvBuff, "hi\n" ) == 0 )
    send(sockfd,"Message Received",strlen("Message Received\n"),0);
else
    printf( "[%s] != [hi\n]\n", recvBuff );

Answer 2

基本上，您的代码中有三个不同的问题：

字符串比较（由K Scott Piel修复）。
收到的消息的打印件没有出来。您需要在所有'\n'个功能中添加printf。
这是最重要的问题：在服务器和客户端之间的第一次通信之后，您在服务器代码中调用accept函数。从man accept page开始，我们可以阅读

accept()函数将提取第一个连接挂起连接队列，创建一个具有相同套接字的新套接字键入协议和地址族作为指定的套接字，并分配该套接字的新文件描述符。

因此，在第一次通信之后，您的服务器正在等待新连接，并且您的客户端正在等待来自服务器的消息。第二次沟通永远不会发生。

要解决此问题，您可以使用fork() API。

以下是使用fork()的修正提案：

clientsocket.c

#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <arpa/inet.h> 

int main(int argc, char *argv[])
{
    int sockfd = 0, n = 0,m=2;
    char recvBuff[1024];
    struct sockaddr_in serv_addr; 

    if(argc != 2)
    {
        printf("\n Usage: %s <ip of server> \n",argv[0]);
        return 1;
    } 

    memset(recvBuff, '0',sizeof(recvBuff));
    if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
    {
        printf("\n Error : Could not create socket \n");
        return 1;
    } 

    memset(&serv_addr, '0', sizeof(serv_addr)); 

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(5000); 

    if(inet_pton(AF_INET, argv[1], &serv_addr.sin_addr)<=0)
    {
        printf("\n inet_pton error occured\n");
        return 1;
    } 

    if( connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)
    {
       printf("\n Error : Connect Failed \n");
       return 1;
    } 

   //m is 2 initially,I want that sending and receiving should be done 2 times
   while(m--)
   {
        printf("Waiting from the server ...\n");
        n = recv(sockfd,recvBuff,1024,0);
        recvBuff[n]= '\0';                               //"hi" is received
        printf("%s\n", recvBuff);                           

        if(!strncmp(recvBuff, "hi\n", strlen("hi\n")))
        {
            printf("Send ACK\n");
            n = send(sockfd,"Message Received",strlen("Message Received\n"),0);
            printf("%d Sent ... \n", n);
        }
    } //sending "Message received"


    return 0;
}

serversocket.c

#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <error.h>
#include <string.h>
#include <sys/types.h>
#include <time.h>

int main(int argc,char *argv[])
{
    int listenfd = 0,qlength=10,connfd = 0,t=5;
    int n;
    struct sockaddr_in serv;
    struct sockaddr_in dest;
    socklen_t socksize = sizeof(struct sockaddr_in);
    char sendBuff[1024]="hi\n";
    pid_t  pid;
    int m = 2;
    //time_t ticks;
    listenfd = socket(AF_INET,SOCK_STREAM,0); //socket for listening to connection requests

    memset(&serv,'0',sizeof(serv));
    serv.sin_family = AF_INET;
    serv.sin_addr.s_addr = htonl(INADDR_ANY);
    serv.sin_port=htons(5000);

    bind(listenfd,(struct sockaddr *)&serv,sizeof(serv)); //binding the socket to a port

    printf("Server started ...\n");
    listen(listenfd,2);
    connfd=accept(listenfd,(struct sockaddr *)&dest,&socksize);  //another socket for sending and receiving on the previously built socket
    while(connfd > 0)
    {   
        printf("Incoming connection from %s-sending a hi...\n",inet_ntoa(dest.sin_addr));
        pid = fork();
        if(pid) {
            /* shall continue to listen to new client */
            printf("Parent shall continue listening ... \n");
            connfd=accept(listenfd,(struct sockaddr *)&dest,&socksize);
        } else {
            printf("Chlid process: Communication with the client ... \n");
            while(m--)
            {
                printf("try %d - Sending %s ... \n", m, sendBuff);
                send(connfd,sendBuff,strlen(sendBuff),0);         //at first my server sends a hi
                sleep(3);
                n=recv(connfd, sendBuff, 1024, 0);                  //if hi received by the client,then server should receive "Message Received" from the client
                sendBuff[n]='\0';
                printf("Data received: [%s]\n", sendBuff); /* need to add '\n' so the print will be displayed */
                strcpy(sendBuff, "hi\n");
            }
            printf("Child process will exit\n");
            return 0;
        }        
    }


    return 0;
}

服务器无法从客户端接收消息

2 个答案: