c++ - 服务器只从客户端接收空字符串？

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所以我有一个基本的客户端/服务器 C++ 程序。现在，当客户端连接到服务器时，我希望服务器发送一条“Hello，world”消息，并且客户端响应“Hello，server”，以确保我正确发送和转发消息。

当我运行时，客户端从服务器接收消息，但服务器只从客户端接收空字符串。

这是客户端的代码

int main(int argc, char *argv[]) {
int sockfd, numbytes;  
char buf[MAXDATASIZE];
struct addrinfo hints, *servinfo, *p;
int rv;
char s[INET6_ADDRSTRLEN];

if (argc != 2) {
    fprintf(stderr,"usage: client hostname\n");
    exit(1);
}

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

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

// loop through all the results and connect 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("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 connect\n");
    return 2;
}

inet_ntop(p->ai_family, get_in_addr((struct sockaddr *)p->ai_addr),
        s, sizeof s);
printf("client: connecting to %s\n", s);

freeaddrinfo(servinfo); // all done with this structure

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

if (send(sockfd, "Hello, client!", 13, 0) == -1) {
            perror("send");
}

buf[numbytes] = '\0';

printf("client: received '%s'\n",buf);

close(sockfd);

return 0;
}

这是服务器的代码

int main(void)
{
    int sockfd, new_fd, numbytes;  // listen on sock_fd, new connection on    new_fd
char buf[MAXDATASIZE];
struct addrinfo hints, *servinfo, *p;
struct sockaddr_storage their_addr; // connector's address information
socklen_t sin_size;
struct sigaction sa;
int yes=1;
char s[INET6_ADDRSTRLEN];
int rv;

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

if ((rv = getaddrinfo(NULL, PORT, &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("server: socket");
        continue;
    }

    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("server: bind");
        continue;
    }

    break;
}

freeaddrinfo(servinfo); // all done with this structure

if (p == NULL)  {
    fprintf(stderr, "server: failed to bind\n");
    exit(1);
}

if (listen(sockfd, BACKLOG) == -1) {
    perror("listen");
    exit(1);
}

sa.sa_handler = sigchld_handler; // reap all dead processes
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_RESTART;
if (sigaction(SIGCHLD, &sa, NULL) == -1) {
    perror("sigaction");
    exit(1);
}

printf("server: waiting for connections...\n");

while(1) {  // main accept() loop
    sin_size = sizeof their_addr;
    new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);
    if (new_fd == -1) {
        perror("accept");
        continue;
    }

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

    if (!fork()) { // this is the child process
        close(sockfd); // child doesn't need the listener
        if (send(new_fd, "Hello, world!", 13, 0) == -1)
            perror("send");
        if (numbytes = recv(new_fd, &buf, MAXDATASIZE-1, 0) == -1) {
            perror("recv");
            exit(1);
        } 

        buf[numbytes] = '\0';
        printf("server: received '%s'\n",buf);
        close(new_fd);
        exit(0);
    }
    close(new_fd);  // parent doesn't need this
}

return 0;
}

Answer 1

现在我明白了:

if (numbytes = recv(new_fd, &buf, MAXDATASIZE-1, 0) == -1) {

recv(new_fd, &buf, MAXDATASIZE-1, 0) == -1 成功时为 0，错误时为 1。然后它被分配给numbytes。因此，您需要添加这样的括号:

if ((numbytes = recv(new_fd, &buf, MAXDATASIZE-1, 0)) == -1) {

或者你想将其分成两行，如下所示:

numbytes = recv(new_fd, &buf, MAXDATASIZE-1, 0);
if (numbytes == -1) {

哦，下次请决定选择 C ​​还是 C++，两者都是不同的语言。