服务器无法接收来自客户端的消息

Question

我正在使用Ubuntu 12.05.I一直试图通过C套接字编程实现停止和等待协议。我创建了两个程序，一个是服务器和另一个客户端。代码预期的工作是通过意见

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解释

#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; 
} 

现在从服务器发送信息到客户端工作正常，但客户机到服务器（收到消息）正在产生问题。它既没有给出错误，也没有正确的结果，它只是给出了一个空白屏幕。

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，你有三个不同的问题，在你的代码：

1. 字符串比较（用K斯科特固定彼尔）。
2. 收到的消息的打印不出来。您需要在所有printf函数中添加'\n'。
3. 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，您正在调用服务器代码中的accept函数。从man accept page，我们可以阅读

的accept()功能必须提取关于 队列挂起连接的第一连接，创建具有相同插座 类型的协议和地址族新的套接字与指定插座，并为该套接字分配一个新的文件描述符 。

所以，在首次通讯后，您的服务器正在等待新的连接和您的客户端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。

下面是一个使用固定提案fork()：

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; 
}