无法从 C 中的 TCP 客户端接收消息

Question

我正在尝试创建一个可以从多个连接的客户端接收消息的服务器，但在我的代码中，recv() 函数始终返回 -1。如何让我的服务器接收来自客户端的代码？

这是我的服务器。在底部的while循环中调用了recv()函数。

我尝试使用read()，但找不到解除阻塞的方法。有什么方法可以使用 read() 来解释这一点吗？如果没有，尽管我连接了一个新客户端并且发送了一条新消息，但如何解决这个问题，让recv()总是返回-1？

    /*------------------Header Files----------------------------------------------------------------------*/
#define _GNU_SOURCE

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<string.h>
#include<errno.h>
#include<fcntl.h>

#include<sys/types.h>
#include<sys/socket.h>
#include<sys/un.h>
#include<sys/stat.h>

#include<netinet/in.h>
#include<netinet/ip.h>

#include<arpa/inet.h>

/*-------------------Linked List------------------------------------------------------------------------*/
struct client_online{

        int client_socket;
        char* name;
        struct client_online* next;
};

/*-------------------Global Variables-------------------------------------------------------------------*/
struct client_online* first_client = NULL;

/*-------------------Main Function----------------------------------------------------------------------*/
int main(int argc, char* argv[]){

    // Cite: get help from office hour code
    // Creating the socket
    int server_socket;
    server_socket = socket(AF_INET, SOCK_STREAM | SOCK_NONBLOCK, 0);
    if(server_socket < 0){
        perror("Error creating server socket");
        return -1;
    }

    // Server address
    struct sockaddr_in server_address;
    memset(&server_address, 0, sizeof(struct sockaddr_in));
    server_address.sin_family = AF_INET;
    server_address.sin_port = htons(atoi(argv[2]));
    server_address.sin_addr.s_addr = inet_addr(argv[1]);

    // Binding the socket
    int binder = bind(server_socket, (const struct sockaddr *) &server_address, sizeof(server_address));
    if(binder == -1){
            perror("Error calling bind");
            return -1;
    }

    // Listen to the socket
    int listener = listen(server_socket, 50);
    if(listener == -1){
            perror("Error calling listen");
            return -1;
    }

    // Client address
    struct sockaddr_in client_address;

    // Reading and writing
    int quit = 0;
    while(!quit){

        // Check if there is a new connection
        int client_address_size = sizeof(client_address);
        int each_client_socket = accept4(server_socket,
                     (struct sockaddr*) &client_address,
                         &client_address_size,
                     SOCK_NONBLOCK);

        // If there is a new connection, add client to linked list
        if(each_client_socket >= 0){

            if(first_client == NULL){
                first_client = malloc(sizeof(*first_client));
                first_client->client_socket = each_client_socket;
                first_client->name = (char*)malloc(100);
                strcpy(first_client->name,"User");
                first_client->next = NULL;

                free(first_client->name);
                free(first_client);
            }

            else{
                struct client_online* tracker = first_client;
                while(tracker->next != NULL){
                    tracker = tracker->next;
                }

                struct client_online* new_client;
                new_client = malloc(sizeof(*new_client));
                new_client->client_socket = each_client_socket;

                new_client->name = (char*)malloc(100);
                strcpy(new_client->name, "User");
                new_client->next = NULL;
                tracker->next = new_client;

                free(new_client->name);
                free(new_client);
            }
        }

        // Check if there are messages recieved
        struct client_online* checker = first_client;
        while(checker != NULL){

            char client_response[256];
            char toSend[256];

            bzero(client_response, sizeof(client_response));

            int receiver = recv(checker->client_socket,
                                client_response,
                                sizeof(client_response),
                                MSG_DONTWAIT);

            printf("%d\n", receiver);
            if(receiver > 0){
                printf("message received\n");
                snprintf(toSend, sizeof(toSend), "%s: %s\n", checker->name, client_response);

                struct client_online* sender = first_client;
                while(sender != NULL){

                    send(sender->client_socket, toSend, sizeof(toSend), MSG_DONTWAIT);
                }
            }

            checker = checker->next;
        }

        sleep(1);

    }

        return 0;
}

这是我的客户

/*----------------------------------------------------Header Files----------------------------------------*/
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<unistd.h>

#include<fcntl.h>

#include<sys/types.h>
#include<sys/socket.h>
#include<sys/un.h>
#include<sys/stat.h>

#include<netinet/ip.h>
#include<arpa/inet.h>

#define _GNU_SOURCE
/*----------------------------------------------------Main Function--------------------------------------*/
int main(int argc, char* argv[]){

    // Seek help from office hour codes
    // Create the client socket
    int client_socket = socket(AF_INET, SOCK_STREAM, 0);
    if(client_socket < 0){
        perror("Error creating client socket");
        return -1;
    }

    // Construct the address of the connection
    struct sockaddr_in client_address;
    memset(&client_address, 0, sizeof(struct sockaddr_in));
    client_address.sin_family = AF_INET;
    client_address.sin_port = htons(atoi(argv[2]));
    client_address.sin_addr.s_addr = inet_addr(argv[1]);

    // Connect the client to the server
    int connecter = connect(client_socket,
                    (const struct sockaddr*) &client_address,
                    sizeof(struct sockaddr_in));
    if(connecter < 0){
        perror("Error connecting to server");
    }

    // Reading and writing
    int quit = 0;
    while(!quit){

        // Read from command line
        char client_message[256];
        memset(&client_message, 0, sizeof(client_message));

        int flags = fcntl(STDIN_FILENO, F_GETFL, 0);
        fcntl(STDIN_FILENO, F_SETFL, flags | O_NONBLOCK);
        int reader = read(STDIN_FILENO, client_message, sizeof(client_message));

        if (reader > 0) {

            // Writing normal message
            send(client_socket, client_message, 1, MSG_WAITALL);
        }
        //
        sleep(1);

    }

    return 0;
}

Answer 1

由于您将 MSG_DONTWAIT 传递给 recv 调用，因此如果尚未从客户端接收到任何内容，则该调用将返回一个错误来表明这一事实。

如果您使用轮询机制，例如 epoll ，那么您通常会等待可读的通知。收到通知后，您可以重试 recv 调用。我在您的代码中没有看到任何轮询机制的使用。

或者，您可以为每个新连接生成一个线程，并使用对 recv 的阻塞调用(省略 MSG_DONTWAIT 标志)。