udp客户端服务器程序c

Question

我正在用C编写一个简单的客户端和服务器程序。我能够从客户端发送日期到服务器。但是，我无法从服务器向客户端发送确认。

/*******************udpserver.c*****************/ 
int main(int argc, char *argv[]) 
{ 
    /* Variable and structure definitions. */ 
    int sd, rc; 
    struct sockaddr_in serveraddr, clientaddr; 
    clientaddrlen = sizeof(clientaddr); 
    int serveraddrlen = sizeof(serveraddr); 
    char buffer[100]; 
    char *bufptr = buffer; 
    int buflen = sizeof(buffer); 

    if((sd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) { 
     perror("UDP server - socket() error"); 
     exit(-1); 
    } 

    printf("UDP server - socket() is OK\n"); 

    memset(&serveraddr, 0x00, serveraddrlen); 
    serveraddr.sin_family = AF_INET; 
    serveraddr.sin_port = htons(0); 
    serveraddr.sin_addr.s_addr = htonl(INADDR_ANY); 
    if((rc = bind(sd, (struct sockaddr *)&serveraddr, serveraddrlen)) < 0) { 
     perror("UDP server - bind() error"); 
     close(sd); 
     exit(-1); 
    } 

    int addr_len = sizeof(serveraddr); 
    if (getsockname(sd, (struct sockaddr *) &serveraddr, &addr_len)<0) { 
     perror("Error getting socket name.\n"); 
     return -1; 
    } 
    printf("Using IP %s and port %d\n", inet_ntoa(serveraddr.sin_addr), ntohs(serveraddr.sin_port)); 
    printf("UDP server - Listening...\n"); 

    rc = recvfrom(sd, bufptr, buflen, 0, (struct sockaddr *)&clientaddr, &clientaddrlen); 
    if(rc < 0) { 
     perror("UDP Server - recvfrom() error"); 
     close(sd); 
     exit(-1); 
    } 

    printf("UDP Server received the following:\n \"%s\" message\n", bufptr); 

    printf("UDP Server replying to the UDP client...\n"); 
    rc = sendto(sd, bufptr, buflen, 0, (struct sockaddr *)&clientaddr, clientaddrlen); 
    if(rc < 0) { 
     perror("UDP server - sendto() error"); 
     close(sd); 
     exit(-1); 
    } 
    printf("UDP Server - sendto() is OK...\n"); 

    close(sd); 
    exit(0); 
} 

我UDPClient程序：

/****************udpclient.c********************/ 
int main(int argc, char *argv[]) 
{ 
    /* Variable and structure definitions. */ 
    int sd, rc; 
    struct sockaddr_in serveraddr, clientaddr; 
    int serveraddrlen = sizeof(serveraddr); 
    char server[255]; 
    char buffer[100]; 
    char *bufptr = buffer; 
    int buflen = sizeof(buffer); 
    struct hostent *hostp; 
    memset(buffer, 0x00, sizeof(buffer)); 
    /* 36 characters + terminating NULL */ 
    memcpy(buffer, "Hello! A client request message lol!", 37); 

    if((sd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) { 
     perror("UDP Client - socket() error"); 
     exit(-1); 
    } 
    else 
     printf("UDP Client - socket() is OK!\n"); 

    if(argc != 3) { 
     /*Use default hostname or IP*/ 
     printf("UDP Client - Usage <Server hostname or IP>\n"); 
     exit(0); 
    } 

    memset(&serveraddr, 0x00, sizeof(struct sockaddr_in)); 
    serveraddr.sin_family = AF_INET; 
    serveraddr.sin_port = htons(atoi(argv[2])); 

    hostp = gethostbyname(argv[1]); 
    if(hostp == (struct hostent *)NULL) { 
     printf("HOST NOT FOUND --> "); 
     printf("h_errno = %d\n", h_errno); 
     exit(-1); 
    } 
    else { 
     printf("UDP Client - gethostname() of the server is OK... \n"); 
     printf("Connected to UDP server\n"); 
    } 
    memcpy(&serveraddr.sin_addr, hostp->h_addr, sizeof(serveraddr.sin_addr)); 

    rc = sendto(sd, bufptr, buflen, 0, (struct sockaddr *)&serveraddr, sizeof(serveraddr)); 
    if(rc < 0) { 
     perror("UDP Client - sendto() error"); 
     close(sd); 
     exit(-1); 
    } 
    else 
     printf("UDP Client - sendto() is OK!\n"); 

    printf("Waiting a reply from UDP server...\n"); 

    rc = recvfrom(sd, bufptr, buflen, 0, (struct sockaddr *)&serveraddr, &serveraddrlen); 

    if(rc < 0) { 
     perror("UDP Client - recvfrom() error"); 
     close(sd); 
     exit(-1); 
    } else { 
     printf("UDP client received the following: \"%s\" message\n", bufptr); 
    } 

    close(sd); 
    exit(0); 
} 

当运行两个程序，我得到了以下的输出：

UdpServer：

$ ./UdpServer 
UDP server - socket() is OK 
Using IP 0.0.0.0 and port 49932 
UDP server - Listening... 
UDP Server received the following: 
"Hello! A client request message lol!" message 
UDP Server replying to the UDP client... 
UDP Server - sendto() is OK... 

UdpClient：

$ ./UdpClient MyPC 49932 
UDP Client - socket() is OK! 
UDP Client - gethostname() of the server is OK... 
Connected to UDP server 
UDP Client - sendto() is OK! 
Waiting a reply from UDP server... 

UdpClient程序停留在这一点。任何人都可以请解释什么是问题？

Answer 1

你可能想用select()使流程等待状态，直到数据avaibale阅读：

... 
{ 
    fd_set rfds; 
    int retval; 

    FD_ZERO(&rfds); 
    FD_SET(sd, &rfds); 

    retval = select(sd+1, &rfds, NULL, NULL, NULL); 
    if (retval == -1) 
    perror("select()"); 
    else 
    printf("Data is available for reading now.\n"); 

    ... 
} 
... 

Answer 2

如果服务器和客户端在同一台机器上运行，给

$ ./UdpClient localhost 49932

，而不是

$ ./UdpClient MyPC 49932

其他
$ ./UdpClient <server-IP-address> 49932

在服务器代码

而且，

clientaddrlen = sizeof(clientaddr);
应该
int clientaddrlen = sizeof(clientaddr);

但我想这只是一个复制粘贴错误。