c - 在理解套接字 getaddrinfo 方面需要帮助

Question

我一直在努力学习套接字编程，并偶然发现了这个网站:http://beej.us/guide/bgnet/output/html/multipage/index.html这真的很好。我能够理解，但是在我自己编写了一个测试程序之后，我遇到了一个问题，我在 3 小时后无法理解。时间有限，所以想在这里请教高手。

这是我的服务器程序:

/**
 * Program showing how sockets can be used
 */

#include <stdio.h>          // For printf
#include <strings.h>        // For bzero, memset
#include <stdlib.h>         // For exit, atoi
#include <unistd.h>         // For close

#include <sys/socket.h>     // For socket
#include <sys/types.h>      // For types

#include <arpa/inet.h>      // For inet_addr
#include <netdb.h>          // Import module network database

#include <errno.h>          // To access the global errno that holds last system call error
#include <assert.h>         // For asserting

#define ADDRESSINFO_GET_SUCCESS 0
const int   kSuccess            = 0;
const char *kMyPort             = "3490"; // Can be either port num or name of the service
const int   kMaxListenConn      = 10;     // How many connections queue will hold

// Utility function to get socket address, IPv4 or IPv6:
void* getSocketAddress(const struct sockaddr *sa)
{
    // Cast socketaddr to sockaddr_in to get address and port values from data

    if (sa->sa_family == PF_INET) {
        return &(((struct sockaddr_in *)sa)->sin_addr);
    }

    return &(((struct sockaddr_in6 *)sa)->sin6_addr);
}

// Utility function to get socket address string
void getSocketAddressString(const struct sockaddr *sd, char *ipAddr)
{
    inet_ntop(sd->sa_family, getSocketAddress(sd), ipAddr, INET6_ADDRSTRLEN);
}

int createAndBindSocket(const struct addrinfo *addrList)
{
    int status = -1;            // Invalid status
    int socketFileDesc = -1;    // Invalid descriptor

    const struct addrinfo *addrIt;

    /*
     * STEP 2.1: Loop through all the addrinfo nodes and bind to the one we can
     */

    for (addrIt = addrList; addrIt != NULL; addrIt = addrIt->ai_next)
    {
        char ipAddr[INET6_ADDRSTRLEN];
        inet_ntop(addrIt->ai_family, getSocketAddress(addrIt->ai_addr), ipAddr, sizeof ipAddr);
        printf("IP: %s\n", ipAddr);

        /*
         * STEP 2.2: Crete the socket file descriptor for our IP address
         */

        socketFileDesc = socket(addrIt->ai_family, addrIt->ai_socktype, addrIt->ai_protocol);

        if (socketFileDesc == -1) {
            fprintf(stderr, "Failed to create socket with error: %d\n", errno);
            perror("socket");   // Get error desc
            continue;           // Try next address
        }

        /*
         * STEP 2.3: Set socket behaviour by making ip address to be re-used if used already
         */

        int yes=1;

        if (setsockopt(socketFileDesc, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1)
        {
            perror("setsockopt");
            exit(1);
        }

        printf("Port %d\n",((struct sockaddr_in *)addrIt->ai_addr)->sin_port);

        /*
         * STEP 2.4: Bind our socket with ip address and port number to listen
         */

        status = bind(socketFileDesc, addrIt->ai_addr, addrIt->ai_addrlen);
        if (status != kSuccess) {
            fprintf(stderr, "Failed to bind socket with error:%d\n", errno);
            perror("bind");  // Get error desc

            // Clear socket
            close(socketFileDesc);
            socketFileDesc = -1;

            continue;       // Try next address
        }
    }

    return socketFileDesc;
}


int main()
{
    int status = -1; // Status is invalid

    struct addrinfo  hints;         // Holds our hints to get address info
    struct addrinfo *addrList;      // Contains our address info

    /*
     * STEP 1: Setup service details
     */

    // Make sure struct is empty
    bzero(&hints, sizeof hints); // memset(&hints, 0, sizeof(hints));

    hints.ai_family     = AF_UNSPEC;    // Don't care IPv4 or v6
    hints.ai_socktype   = SOCK_STREAM;  // Use TCP stream sockets
    hints.ai_flags      = AI_PASSIVE;   // Use my local IPs or you igore this and provide IP manually in first arg


    status = getaddrinfo(NULL, kMyPort, &hints, &addrList);
    if (status != kSuccess) {
        fprintf(stderr, "Failed to get address info with error: %s\n", gai_strerror(status));
        exit(1);
    }

    /*
     * STEP 2: Create a socket and bind it
     */

    int socketFileDesc;
    socketFileDesc = createAndBindSocket(addrList);

    freeaddrinfo(addrList);   // Done with list

    if (socketFileDesc == -1) {
        exit(1);
    }

    /*
     * STEP 3: Listen to the port for incoming connections
     */

    status = listen(socketFileDesc, kMaxListenConn);     // Second arg is number of incoming connections in queue
    if (status != kSuccess) {
        fprintf(stderr, "Failed to listen to the port\n");
        perror("listen");
        goto exit;
    }

    printf("Server is listening at the port %s\n", kMyPort);

    struct sockaddr_storage inConnAddr;     // Big enough to hold both IPv4 and v6
    socklen_t inConnAddrLen = sizeof inConnAddr;

    const size_t kMaxBufferSize = 50;
    char buff[kMaxBufferSize] = {0};

    long bytesReceived = -1;
    long bytesSent = 0;
    int clientSockfd;

    while (1) {

        /*
         * STEP 4: Accept incoming connections
         */

        inConnAddrLen = sizeof inConnAddr;

        clientSockfd = accept(socketFileDesc, (struct sockaddr *)&inConnAddr, &inConnAddrLen);

        // Got new connection ?
        if (clientSockfd == -1) {
            perror("accept");   // Print error description
            continue;           // Continue to look for new connections
        }

        //
        // Got connection, create child process to handle request
        //

        if (!fork()) {
            close(socketFileDesc); // No need with child

            char ipAddr[INET6_ADDRSTRLEN];
            getSocketAddressString((struct sockaddr *)&inConnAddr, ipAddr);
            printf("Child process created for hanlding request from %s\n", ipAddr);

            /*
             * STEP 5: Receive and Send data to requests
             */

            bytesReceived = recv(clientSockfd, &buff, kMaxBufferSize - 1, 0);
            if (bytesReceived > 0)
            {
                printf("Data from client %s\n", buff);
                while (bytesSent < bytesReceived) {
                    bytesSent = send(clientSockfd, buff, bytesReceived, 0);
                    printf("Bytes sent %ld\n", bytesSent);
                }
            }

            if (bytesReceived < 0) {
                perror("recv");
            }
            else if (bytesReceived == 0) {
                printf("Connection closed by server\n");
            }

            close(clientSockfd); // Close socket
            exit(0);
        }
    }

exit:
    /*
     * STEP 5: Close the socket
     */
    close(socketFileDesc);


    return 0;
}

这是我的客户端程序:

#include <stdio.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
#include <netdb.h>
#include <strings.h>
#include <arpa/inet.h>

const char  kSuccess = 0;

// Utility function to get socket address
void* getSocketAddress(const struct sockaddr *sa)
{
    if (sa->sa_family == AF_INET) {
        return &( ((struct sockaddr_in *)sa)->sin_addr );
    }

    return &( ((struct sockaddr_in6 *)sa)->sin6_addr );
}

// Utility function to get socket address string
void getSocketAddressString(const struct sockaddr *sd, char *ipAddr)
{
    inet_ntop(sd->sa_family, getSocketAddress(sd), ipAddr, INET6_ADDRSTRLEN);
}

int createAndConnectSocket(const struct addrinfo *addrList)
{
    int socketFileDesc = -1;    // Invalid descriptor

    const struct addrinfo *addrIt;

    /*
     * STEP 2.1: Loop through all the addrinfo nodes and bind to the one we can
     */

    for (addrIt = addrList; addrIt != NULL; addrIt = addrIt->ai_next)
    {
        /*
         * STEP 2.2: Crete the socket file descriptor for our IP address
         */

        socketFileDesc = socket(addrIt->ai_family, addrIt->ai_socktype, addrIt->ai_protocol);

        if (socketFileDesc == -1) {
            fprintf(stderr, "Failed to create socket with error: %d\n", errno);
            perror("socket");   // Get error desc
            continue;           // Try next address
        }

        /*
         * STEP 2.3: Set socket behaviour by making ip address to be re-used if used already
         */

        int yes=1;

        if (setsockopt(socketFileDesc, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1)
        {
            perror("setsockopt");
            exit(1);
        }

        char ipAdd[INET6_ADDRSTRLEN];
        getSocketAddressString(addrIt->ai_addr, ipAdd);
        ((struct sockaddr_in *)addrIt->ai_addr)->sin_port = atoi("3490");

        printf("IP is %s::%d\n", ipAdd,((struct sockaddr_in *)addrIt->ai_addr)->sin_port);

        // Connect to the socket
        int status;
        status = connect(socketFileDesc, addrIt->ai_addr, addrIt->ai_addrlen);
        if (status != kSuccess) {
            perror("connect");
            close(socketFileDesc);
            socketFileDesc = -1;
            continue;
        }
    }

    return socketFileDesc;
}

int main(int argc, char* argv[])
{
    // Check we have data from arguments
    if (argc < 3 || argc > 3) {
        perror("Invalid command");
        printf("Usage: %s hostname portnumber\n", argv[0]);
        printf("       %s 192.168.1.2 3490\n", argv[0]);
    }

    // Setup server address info
    struct addrinfo *serverInfo;
    struct addrinfo hints;

    int status = -1;

    memset(&hints, 0, sizeof hints);  // Make sure it is empty

    hints.ai_family     = AF_INET;      // IPv4
    hints.ai_socktype   = SOCK_STREAM;  // Use socket stream

    ((struct sockaddr_in *)&hints.ai_addr)->sin_port = atoi(argv[2]);

    status = getaddrinfo(argv[1], "3490", &hints, &serverInfo);
    if (status != kSuccess) {
        fprintf(stderr, "Failed to get address info %s\n", gai_strerror(status));
        exit(1);
    }

    printf("Connecting to %s::%s...\n", argv[1], argv[2]);

    // Create and bind socket
    int sockfd = createAndConnectSocket(serverInfo); 

    freeaddrinfo(serverInfo);   // We are done with serverinfo

    if (sockfd == -1) {
        exit(1);
    }

    // Send and receive data from server

    long bytesReceived = -1;
    long bytesSent  = 0;

    const size_t kMaxBufferSize = 50;
    char buff[kMaxBufferSize];

    const char *msg = "Hi! I am client!";
    size_t msgLen = strlen(msg);

    printf("Connected to server\n");

    // Loop to send and receive data
    while (1) {

        while (bytesSent < msgLen) {
            bytesSent = send(sockfd, msg, msgLen, 0);
            printf("Bytes sent %ld\n", bytesSent);
        }

        bytesReceived = recv(sockfd, &buff, kMaxBufferSize - 1, 0);
        if (bytesReceived > 0)
        {
            printf("Data received from server: %s\n", buff);
        }
        else if (bytesReceived < 0) {
            perror("Read error");
            break;
        }
        else if (bytesReceived == 0) {
            printf("Connection closed by server\n");
            break;
        }
    }

    // Close socket
    close(sockfd);

    return 0;
}

我的问题是:即使我在 getaddrinfo() 调用中设置了端口和 IP，但是当它绑定(bind)或连接时，在 sockaddr 结构中发现的端口号是错误的，因为它是不同的值。我确实知道这里发生了什么，因此当我这样做时我收到连接被拒绝的消息。谁能看看我的程序并告诉我为什么连接被拒绝？如果有人可以对我的代码提出任何改进建议，我将不胜感激。

谢谢

Answer 1

您没有连接到您认为正在连接的端口。

((struct sockaddr_in *)addrIt->ai_addr)->sin_port = atoi("3490");

sin_port 的值必须是网络字节顺序，即大端。您改为直接分配值 3490(通过 atoi)，因此该值按主机字节顺序排列。您的平台很可能使用小端字节序。

因此，您连接的不是端口 3490(十六进制 0DA2)，而是端口 41485(十六进制 A2 0D)。

您需要使用 htons 函数，它将 16 位值(因为 sin_port 是一个 16 位字段)从主机字节顺序转换为网络字节顺序。另外，这里不需要使用 atoi。只需使用数字常量即可。

((struct sockaddr_in *)addrIt->ai_addr)->sin_port = htons(3490);