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

#define IPADDR "127.0.0.1" /* 伺服端 IP 位址 */
#define PORT 43866         /* 埠號 */
#define STRING_LEN_16 16
#define STRING_LEN_64 64
#define BACKLOG 10

char *_inet_ntoa(struct in_addr *addr, char *ipAddr, int len)
{
    // 將二進位 IP 位址轉換為可讀 IP 位址
    if (NULL == addr || NULL == ipAddr || 16 > len)
    {
        printf("invalid param\n");
        return NULL;
    }
    unsigned char *tmp = (unsigned char *)addr;
    snprintf(ipAddr, len, "%d.%d.%d.%d", tmp[0], tmp[1], tmp[2], tmp[3]);
    return ipAddr;
}

int main(int argc, char *argv[])
{
    // 1. 建立 socket socket()
    int sockfd = socket(AF_INET, SOCK_STREAM, 0); // TCP socket
    if (sockfd == -1)
    {
        printf("fail to call socket, errno[%d, %s]\n", errno, strerror(errno));
        exit(0);
    }

    // 手動建構 sockaddr_in 結構體
    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(addr));    // 將結構體初始化為 0
    addr.sin_family = AF_INET;         // IPv4
    addr.sin_port = htons(PORT);       // 埠號
    inet_aton(IPADDR, &addr.sin_addr); // 將可讀 IP 位址轉換為二進位 IP 位址
    // inet_aton 目前已不建議使用，因為它不支援 IPv6

    // 2. 將 socket 與特定的 IP 位址和埠號綁定 bind()
    if (bind(sockfd, (struct sockaddr *)&addr, sizeof(addr)) == -1)
    {
        printf("fail to call bind, errno[%d, %s]\n", errno, strerror(errno));
        close(sockfd);
        exit(0);
    }

    // 3. 讓 socket 進入被動監聽狀態 listen()
    if (listen(sockfd, BACKLOG) == -1)
    {
        printf("fail to call listen, errno[%d, %s]\n", errno, strerror(errno));
        close(sockfd);
        exit(0);
    }

    printf("server listen on %s:%u\n", IPADDR, PORT);

    // 4. 當 socket 處於監聽狀態時，可透過 accept 函式接收客戶端的請求 accept()
    struct sockaddr_in peerAddr;
    socklen_t peerAddrLen = sizeof(struct sockaddr_in);
    int connfd = accept(sockfd, (struct sockaddr *)&peerAddr, &peerAddrLen);
    if (connfd == -1)
    {
        printf("fail to call accept, errno[%d, %s]\n", errno, strerror(errno));
        close(sockfd);
        exit(0);
    }
    char peerIPAddr[STRING_LEN_16];
    _inet_ntoa(&peerAddr.sin_addr, peerIPAddr, STRING_LEN_16);
    printf("peer client address [%s:%u]\n", peerIPAddr, ntohs(peerAddr.sin_port));

    while (1)
    {
        // 5. 讀取客戶端傳送的資料 recv()
        char buf[STRING_LEN_64];
        int n = recv(connfd, buf, STRING_LEN_64 - 1, 0);
        buf[n] = '\0';

        if (0 == n) // n 為 0 表示對端關閉
        {
            printf("peer close\n");
            break;
        }

        printf("recv msg from client : %s\n", buf);

        sleep(2);

        // 6. 傳送資料給客戶端 send()
        char str[] = "recved!";
        printf("send msg to client : %s\n", str);
        send(connfd, str, strlen(str), 0);
    }

    // 7. 互動結束，關閉 socket close()
    close(connfd);
    close(sockfd);

    return 0;
}

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

#define IPADDR "127.0.0.1" /* 伺服端 IP 位址 */
#define PORT 43866         /* 伺服端埠號 */
#define STRING_LEN_64 64

int main()
{
    // 1. 建立 TCP socket socket()
    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    printf("sockfd = %d\n", sockfd);
    if (-1 == sockfd)
    {
        printf("fail to call socket, errno[%d, %s]\n", errno, strerror(errno));
        exit(0);
    }

    // 2. 將 socket 與特定的 IP 位址和埠號建立連線 connect()
    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(PORT);
    inet_aton(IPADDR, &addr.sin_addr);
    if (connect(sockfd, (struct sockaddr *)&addr, sizeof(addr)) == -1)
    {
        printf("fail to call connect, errno[%d, %s]\n", errno, strerror(errno));
        close(sockfd);
        exit(0);
    }

    // 3. 透過 socket 傳送資料給伺服端 send()
    char str[] = "hello world";
    printf("send msg to server : %s\n", str);
    send(sockfd, str, strlen(str), 0);

    // 4. 透過 socket 從伺服端接收資料 recv()
    char buf[STRING_LEN_64];
    int n = recv(sockfd, buf, STRING_LEN_64 - 1, 0);
    buf[n] = '\0';
    printf("recv msg from server : %s\n", buf);

    // 5. 互動結束，關閉 socket close()
    close(sockfd);
}

Compiling:
gcc client.c -o client
gcc server.c -o server

$ ./server # 在伺服端終端機執行伺服器
server listen on 127.0.0.1:43866
peer client address [127.0.0.1:33844]
recv msg from client : hello world
send msg to client : recved!
peer close

$ ./client # 在客戶端終端機執行客戶端
sockfd = 3
send msg to server : hello world
recv msg from server : recved!