linux - 为什么来自套接字的 recv() 不阻塞

Question

我创建了一个 TCP 服务器套接字来监听 connect() 请求并接受它们。接受客户端套接字后，它会从中接收数据。我希望 recv() 函数超时阻塞，但它似乎是非阻塞的。

我已经实现了客户端在收到服务器响应时处于阻塞模式，但它似乎与服务器端的工作方式不同。

下面我复制粘贴我想以阻塞模式接收数据的服务器端:

int clientSocket = accept(_serverSocket, (struct sockaddr *)NULL, NULL);

if (clientSocket < 0)
    return -1;

// set TIMEOUT option to server socket
struct timeval tv;
tv.tv_sec = 0;
tv.tv_usec = 500 * 1000; 
int sockOptRet = setsockopt(clientSocket, SOL_SOCKET, SO_RCVTIMEO, (const char*)&tv, sizeof tv);

struct linger sl;
sl.l_onoff = 1;     /* non-zero value enables linger option in kernel */
sl.l_linger = 0;    /* timeout interval in seconds */
setsockopt(clientSocket, SOL_SOCKET, SO_LINGER, &sl, sizeof(sl));

char _rcvBuffer[sizeof(can_frame)];
int numBytesRcv = recv(clientSocket, _rcvBuffer, sizeof(can_frame), 0); 

我也尝试过使用 MSG_WAITALL 标志，但它没有改变任何东西......

Answer 1

您的代码显然应该阻塞，但您的 0.5 秒超时并不能体现它的公正性。

如果您将超时增加到更显眼的值(例如 2 秒)并将代码移植到例如 beej's tpc server template 上，你会得到类似的东西:

/*
** server.c -- a stream socket server demo
*/

#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 <netdb.h>
#include <arpa/inet.h>
#include <sys/wait.h>
#include <signal.h>

#define PORT "3490"  // the port users will be connecting to

#define BACKLOG 10     // how many pending connections queue will hold

void sigchld_handler(int s)
{
    // waitpid() might overwrite errno, so we save and restore it:
    int saved_errno = errno;

    while(waitpid(-1, NULL, WNOHANG) > 0);

    errno = saved_errno;
}


// 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, new_fd;  // listen on sock_fd, new connection on new_fd
    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


            // set TIMEOUT option to server socket
            struct timeval tv;
            tv.tv_sec = 2;
            tv.tv_usec = 0 * 500 * 1000; 
            int sockOptRet = setsockopt(new_fd, SOL_SOCKET, SO_RCVTIMEO, (const char*)&tv, sizeof tv);

            struct linger sl;
            sl.l_onoff = 1;     /* non-zero value enables linger option in kernel */
            sl.l_linger = 0;    /* timeout interval in seconds */
            setsockopt(new_fd, SOL_SOCKET, SO_LINGER, &sl, sizeof(sl));

            char can_frame[1024];
            char _rcvBuffer[sizeof(can_frame)];
            int numBytesRcv = recv(new_fd, _rcvBuffer, sizeof(can_frame), 0); 



            if (send(new_fd, "Hello, world!", 13, 0) == -1)
                perror("send");
            close(new_fd);
            exit(0);
        }
        close(new_fd);  // parent doesn't need this
    }

    return 0;
}

现在如果你运行这个然后做:

nc localhost 3490

无需输入要发送的行，应该有明显的 2 秒等待在服务器放弃你之前，表明 recv 确实在阻塞。