套接字发送/接收在小数据传输时返回 0

Question

在我的客户中：

 numbytes = send(sockfd, argv[2], strlen(argv[2]), 0)

在我的服务器中：

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

在这两种情况下，numbytes 都是 0，argv[2]="test" 和 MAXBUFLEN=100。我不知道为什么要发送/接收 0 个字节。我正在通过 cygwin 将数据发送到虚拟机。

编辑：我已经使用之前工作过的单独客户端测试了代码，但我遇到了同样的问题，所以我认为问题出在服务器上

客户：

$ ./talker.exe 155.26.37.55 test
argv[2]: test
talker: sent 0 bytes to 155.26.37.55

服务器：

Maxbuflen: 100
listener: got packet from 155.26.37.55
listener: packet is 0 bytes long
listener: packet contains ""

代码中心：

#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#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 SERVERPORT "4951"    // the port users will be connecting to

int main(int argc, char *argv[])
{
    int sockfd;
    struct addrinfo hints, *servinfo, *p;
    int rv;
    int numbytes;

    if (argc != 3) {
        fprintf(stderr,"usage: talker hostname message\n");
        exit(1);
    }

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;

    if ((rv = getaddrinfo(argv[1], SERVERPORT, &hints, &servinfo)) != 0) {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(rv));
        return 1;
    }

    // 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("talker: socket");
            continue;
        }
        if (connect(sockfd, p->ai_addr, p->ai_addrlen) == -1) {
              close(sockfd);
              perror("client: connect");
              continue;
          }

        break;
    }

    if (p == NULL) {
        fprintf(stderr, "talker: failed to bind socket\n");
        return 2;
    }
    std::cout<<"argv[2] "<<argv[2]<<std::endl;
    if ((numbytes = send(sockfd, argv[2], strlen(argv[2]), 0) == -1)) {
        perror("talker: send");
        exit(1);
    }

    freeaddrinfo(servinfo);

    printf("talker: sent %d bytes to %s\n", numbytes, argv[1]);
    close(sockfd);

    return 0;
}

代码服务器：

#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 "4951"    // the port users will be connecting to

#define MAXBUFLEN 100
#define BACKLOG 10

// 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 sockfd;
    struct addrinfo hints, *servinfo, *p;
    int rv;
    int numbytes;
    int new_fd;
    socklen_t addr_size;
    struct sockaddr_storage their_addr;
    char buf[MAXBUFLEN];
    char s[INET6_ADDRSTRLEN];

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC; // set to AF_INET to force IPv4
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE; // use my IP

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

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

        int yes=1;

        // lose the pesky "Address already in use" error message
        if (setsockopt(sockfd,SOL_SOCKET,SO_REUSEADDR,&yes,sizeof(int)) == -1) {
            perror("setsockopt");
            exit(1);
        }

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

        if (listen(sockfd,BACKLOG) == -1){
            close(sockfd);
            perror("listener:listen");
            continue;
        }

        break;
    }

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

    freeaddrinfo(servinfo);

    printf("listener: waiting to recv..\n");
    while(1){
        addr_size = sizeof their_addr;
        if ((new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &addr_size))==-1){
            perror("accept");
            exit(1);
        }
        printf("Maxbuflen: %d\n",MAXBUFLEN);
        if ((numbytes = recv(new_fd, buf, MAXBUFLEN-1 , 0) == -1)) {
            perror("recv");
            exit(1);
        }

        printf("listener: got packet from %s\n",
            inet_ntop(their_addr.ss_family,
                get_in_addr((struct sockaddr *)&their_addr),
                s, sizeof s));
        printf("listener: packet is %d bytes long\n", numbytes);
        buf[numbytes] = '\0';
        printf("listener: packet contains \"%s\"\n", buf);

        close(new_fd);
    }

    return 0;
}

Answer 1

recv() 在对方关闭套接字时返回 0，在这种情况下是您的客户端。