Question

我有一个小程序，可以从服务器和客户端发送和接收数据，反之亦然。一切正常，但我看不到双方收到的消息，它总是0字节。它没有给出任何编译错误但是没有按照我想要的方式工作。你能不能看看我做错了什么？感谢

//客户端

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

int main(int argc, char* argv[])
{
    int sockfd;

    struct addrinfo hints, *servinfo, *p;
    int rv;
    int numbytes;
    char* hostname = "localhost";
    char* server = "localhost";

    memset(&hints, 0, sizeof(hints));
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    if ((rv = getaddrinfo(hostname, "5000", &hints, &servinfo)) != 0)
    {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
        return 0;
    }


    // loop through all the results and make a socket
    for(p = servinfo; p != NULL; p = p->ai_next) 
    {
        if ((sockfd = socket(p->ai_family, p->ai_socktype, p->ai_protocol)) == -1) 
        {
            perror("client: socket");
            continue;
        }

        if (connect(sockfd, p->ai_addr, p->ai_addrlen) == -1) 
        {
            close(sockfd);
            perror("client: connect");
            continue;
        }
        break;
    }
    if (p == NULL) 
    {
        fprintf(stderr, "client: failed to bind socket\n");
        return 2;
    }

    char message[] = "hi";
    if ((numbytes = send(sockfd, message, 2, 0)) == -1) 
    {
        perror("client: send");
        exit(1);
    }

    int numbytesRecv;
    char buf[100];
    if ((numbytesRecv = recv(sockfd, buf, 99, 0)) == -1) 
    {
        perror("client: recv");
        exit(1);
    }

    freeaddrinfo(servinfo);
    printf("client: sent %d bytes to %s\n", numbytes, server);
    printf("client: received %d bytes from %s\n", numbytesRecv, server);
    buf[numbytesRecv] = '\0';
    printf("client: message received is %s\n",buf);
    close(sockfd);
    return 0;
}

// server

#include   <stdio.h>
#include   <stdlib.h>
#include   <unistd.h>
#include   <errno.h>
#include   <string.h>
#include   <sys/types.h>
#include   <sys/socket.h>
#include   <netinet/in.h>
#include   <arpa/inet.h>
#include   <netdb.h>
#define MYPORT "5000"     // the port users will be connecting to
#define MAXBUFLEN 100
// get sockaddr, IPv4 or IPv6:
void *get_in_addr(struct sockaddr *sa)
{
     if (sa->sa_family == AF_INET) {
          return &(((struct sockaddr_in*)sa)->sin_addr);
     }
     return &(((struct sockaddr_in6*)sa)->sin6_addr);
}
int main(void)
{
    int sockfdTCP;
    struct addrinfo hintsTCP, *servinfoTCP, *pTCP;
    int rv;
    int numbytes;
    struct sockaddr_storage their_addr;
    char buf[MAXBUFLEN];

    size_t addr_len;
    char s[INET6_ADDRSTRLEN];

    memset(&hintsTCP, 0, sizeof hintsTCP);
    hintsTCP.ai_family = AF_UNSPEC;
    hintsTCP.ai_socktype = SOCK_STREAM;
    hintsTCP.ai_flags = AI_PASSIVE;

    if ((rv = getaddrinfo(NULL, MYPORT, &hintsTCP, &servinfoTCP)) != 0) {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
        return 1;
    }

    // loop through all the results and bind to the first we can for TCP socket
    for(pTCP = servinfoTCP; pTCP != NULL; pTCP = pTCP->ai_next) 
    {
        if ((sockfdTCP = socket(pTCP->ai_family, pTCP->ai_socktype,pTCP->ai_protocol)) == -1) 
        {
            perror("listener: socket");
            continue;
        }

        if (bind(sockfdTCP, pTCP->ai_addr, pTCP->ai_addrlen) == -1) 
        {
            close(sockfdTCP);
            perror("listener: bind");
            continue;
        }
        break;
    }

    if (pTCP == NULL) 
    {
        fprintf(stderr, "listener: failed to bind socket\n");
        return 2;
    }
    freeaddrinfo(servinfoTCP);

    printf("listener: waiting to recvfrom...\n");
    addr_len = sizeof their_addr;

    const int BACKLOG = 10;
    if (listen(sockfdTCP, BACKLOG) == -1) {
        perror("listen");
        exit(1);
    }

    socklen_t sin_size;
    int new_fd;
    sin_size = sizeof their_addr;
    new_fd = accept(sockfdTCP, (struct sockaddr *)&their_addr, &sin_size);
    printf("I am here\n");
    if (new_fd == -1) 
    {
        perror("accept");
    }

    if ((numbytes = recv(new_fd, buf, MAXBUFLEN, 0) == -1)) 
{
    perror("recv");
    exit(1);
}

char* msg = " hi i am server";
int numbytesSend;
if ((numbytesSend = send(new_fd, msg,strlen(msg), 0) == -1)) 
{
    perror("recv");
    exit(1);
}

inet_ntop(their_addr.ss_family,get_in_addr((struct sockaddr *)&their_addr), s, sizeof s);
printf("server: got connection from %s\n", s);

printf("listener: packet is %d bytes long\n", numbytes);
buf[numbytes] = '\0';
printf("listener: packet contains : %s\n", buf);

close(sockfdTCP);
close(new_fd);
return 0;

}

//输出客户端

./clientTCP 
client: sent 2 bytes to localhost
client: received 0 bytes from localhost
client: message received is

//输出服务器

./serverTCP 
listener: waiting to recvfrom...
I am here
server: got connection from 127.0.0.1
listener: packet is 0 bytes long
listener: packet contains :

Answer 1

您的问题是服务器中的这一行：

if ((numbytes = recv(new_fd, buf, MAXBUFLEN, 0) == -1))

应该是：

if ((numbytes = recv(new_fd, buf, MAXBUFLEN, 0)) == -1)

这将允许正确分配numbytes，这允许

buf[numbytes] = '\0';

做你想做的事情并让我们

printf("listener: packet contains : %s\n", buf);

打印你想要的东西。

同样处理

if ((numbytesSend = send(new_fd, msg, strlen(msg), 0) == -1))

但该错误并未在示例程序中显示出来。

Answer 2

自从我完成原始套接字编程以来已经有一段时间了，但我认为你的recv()调用有些可疑。 IIRC，recv()阻塞，直到它读取缓冲区的完整大小或连接已关闭。由于两端都没有发送足够的数据来填充缓冲区，并且套接字在recv()调用之后才关闭，我认为您的客户端/服务器中的一个或两个应该挂起。既然他们没有，就会发生两件事之一。套接字配置为非阻塞模式，您需要将它们配置为阻止，或者某些东西正在关闭套接字。对不起，我不能提供比这更详细的帮助，但希望它会指出你正确的方向。您可能还想调查在套接字上使用的shutdown()调用，以便在发送缓冲区中的所有数据之前不要关闭它们。不确定这是否与您的问题有关。

Answer 3

编辑：我刚刚在OS X上测试过。您的代码适用于我。现在，我真的认为你必须有一些东西消耗你的字节。杀死你的流程并做：

$ netstat -na | grep 5000

查看是否有任何侦听端口5000的内容。如果是，则需要终止服务器。

另外，请确保超级服务器（inetd / xinetd）没有产生处理端口5000上的数据的东西。

Answer 4

看起来你被Nagle's Algorithm咬了。尝试设置TCP_NODELAY以允许立即发送小数据包。

请注意，对于生产代码，您应该在决定禁用Nagle算法之前进行配置。

客户端 - 服务器程序，问题从客户端发送消息到服务器，反之亦然

4 个答案: