C Socket编程：客户端输入与服务端交互代码问题排查

核心问题梳理 #

客户端问题 #

• scanf参数不匹配：原代码scanf("%c",&letter);中，letter是字符数组，&letter的类型是char(*)[1024]，与scanf期望的char*类型不符，导致编译警告。应改为scanf(" %c", letter);（数组名本身指向首地址，前面加空格跳过输入缓冲区的空白字符）。
• 逻辑顺序错误：原代码先执行read循环等待服务端数据，再发送指令，这会导致程序卡在read步骤（服务端需先收到指令才会返回数据），必须调整为先发送指令，再接收响应。
• 发送/输出错误：发送时不应传递整个数组长度sizeof(letter)，只需发送单个字符（长度为1）；输出服务端响应时，printf("%c", recvBuff);错误，应使用%s输出字符串。

服务端问题 #

• 字符比较错误：原代码from_client == 't'将数组指针与字符直接比较，逻辑完全错误，应取数组首元素from_client[0] == 't'进行判断。
• 字符串赋值错误：sendBuff == "Marcus Baker"是比较操作而非赋值，需用strcpy(sendBuff, "Marcus Baker");复制字符串。
• 发送长度错误：发送时不应传递整个数组长度sizeof(sendBuff)，应发送实际字符串长度strlen(sendBuff)，避免发送多余空字符。
• 退出逻辑不完善：收到'q'时直接return 0会导致服务端异常退出，需先发送响应、关闭当前连接，再退出循环。
• 未处理recv返回值：未判断recv的返回值，当客户端断开时会导致无效操作，需处理recv_len <= 0的情况。

修正后的服务端代码 #

#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <time.h> 

int main(int argc, char *argv[])
{
    int listenfd, connfd;
    struct sockaddr_in serv_addr; 

    char sendBuff[1025];
    char from_client[1025];
    time_t ticks; 
    ssize_t recv_len;

    listenfd = socket(AF_INET, SOCK_STREAM, 0);
    memset(&serv_addr, 0, sizeof(serv_addr));
    memset(sendBuff, 0, sizeof(sendBuff)); 

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    serv_addr.sin_port = htons(5000); 

    bind(listenfd, (struct sockaddr*)&serv_addr, sizeof(serv_addr)); 
    listen(listenfd, 10); 

    while(1)
    {
        connfd = accept(listenfd, (struct sockaddr*)NULL, NULL); 
        if (connfd < 0) {
            perror("accept error");
            continue;
        }

        recv_len = recv(connfd, from_client, sizeof(from_client)-1, 0);
        if (recv_len <= 0) {
            close(connfd);
            continue;
        }
        from_client[recv_len] = '\0'; // 添加字符串结束符
        printf("Received: %c\n", from_client[0]);

        if(from_client[0] == 't')
        {
            ticks = time(NULL);
            snprintf(sendBuff, sizeof(sendBuff), "%.24s\r\n", ctime(&ticks));
            write(connfd, sendBuff, strlen(sendBuff));
        }
        else if(from_client[0] == 'n')
        {
            strcpy(sendBuff, "Marcus Baker");
            send(connfd, sendBuff, strlen(sendBuff), 0);
        }
        else if(from_client[0] == 'i')
        {
            strcpy(sendBuff, "201604543");
            send(connfd, sendBuff, strlen(sendBuff), 0);
        }
        else if(from_client[0] == 'q')
        {
            strcpy(sendBuff, "Quit");
            send(connfd, sendBuff, strlen(sendBuff), 0);
            close(connfd);
            break; // 退出循环，关闭服务端
        }
        else
        {
            strcpy(sendBuff, "Invalid command");
            send(connfd, sendBuff, strlen(sendBuff), 0);
        }

        close(connfd);
        sleep(1);
     }
    close(listenfd);
    return 0;
}

修正后的客户端代码 #

#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <arpa/inet.h> 

int main(int argc, char *argv[])
{
    int sockfd, n = 0;
    char letter[2]; // 仅存储单个字符+字符串结束符
    char recvBuff[1024];
    struct sockaddr_in serv_addr; 

    if(argc != 2)
    {
        printf("\n Usage: %s <ip of server> \n", argv[0]);
        return 1;
    } 

    printf("Enter a Character (t, n, i, q): ");
    // 空格跳过空白字符，避免读取输入缓冲区残留的换行符
    scanf(" %c", letter);

    memset(recvBuff, 0, sizeof(recvBuff));
    if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
    {
        printf("\n Error : Could not create socket \n");
        return 1;
    } 

    memset(&serv_addr, 0, sizeof(serv_addr)); 

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(5000); 

    if(inet_pton(AF_INET, argv[1], &serv_addr.sin_addr) <= 0)
    {
        printf("\n inet_pton error occured\n");
        return 1;
    } 

    if( connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)
    {
       printf("\n Error : Connect Failed \n");
       return 1;
    } 

    // 先发送指令给服务端
    int toServer = send(sockfd, letter, 1, 0);
    if (toServer < 0) {
        perror("send error");
        close(sockfd);
        return 1;
    }
    printf("Letter Sent\n");

    // 接收服务端响应
    n = read(sockfd, recvBuff, sizeof(recvBuff)-1);
    if (n > 0) {
        recvBuff[n] = '\0';
        printf("Server Response: %s\n", recvBuff);
    } else if (n < 0) {
        printf("\n Read error \n");
    }

    close(sockfd);
    return 0;
}

内容的提问来源于stack exchange，提问作者Marcus Baker