echo_client.c

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

#define BUF_SIZE    1024

int main(int argc, char **argv) 
{
    int sock;
    char msg[BUF_SIZE];
    
    int str_len;
    struct sockaddr_in serv_addr;

    if(argc != 3) {
        printf("Usage : %s <IP> <port>\n", argv[0]);
        return -1;
    }
    
    sock = socket(AF_INET, SOCK_STREAM, 0);
    if(sock < 0) {
        perror("socket");
        return -1;
    }

    memset(&serv_addr, 0, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = inet_addr(argv[1]);
    serv_addr.sin_port = htons(atoi(argv[2]));
    
    if(connect(sock, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
        perror("connect");
        return -1;    
    }

    while(1) {
        fputs("Input msg(Q to quit): ", stdout);
        fgets(msg, BUF_SIZE, stdin);  // 개행과 같이 입

        if(!strcmp(msg, "q\n") || !strcmp(msg, "Q\n")) {
            break;   
        }
        
        write(sock, msg, strlen(msg));
        str_len = read(sock, msg, BUF_SIZE-1);
        msg[str_len] = 0;
        printf("Message from server :%s", msg);
    }

    close(sock);
    return 0;
}

echo_server.c

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

#define BUF_SIZE    1024

int main(int argc, char **argv) 
{
    int serv_sock, clnt_sock;
    char msg[BUF_SIZE];
    
    int str_len;
    struct sockaddr_in serv_addr, clnt_addr;
    socklen_t clnt_addr_sz;

    if(argc != 2) {
        printf("Usage : %s <port>\n", argv[0]);
        return -1;
    }
    
    serv_sock = socket(AF_INET, SOCK_STREAM, 0);
    if(serv_sock < 0) {
        perror("socket");
        return -1;
    }

    memset(&serv_addr, 0, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    serv_addr.sin_port = htons(atoi(argv[1]));

    if(bind(serv_sock, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
        perror("bind");
        return -1;
    }

    if(listen(serv_sock, 5) < 0) {
        perror("listen");
        return -1;
    }

    clnt_addr_sz = sizeof(clnt_addr);
    
    // 총 5번 실행
    for(int i=0; i<5; i++) {
        clnt_sock = accept(serv_sock, (struct sockaddr *)&clnt_addr, &clnt_addr_sz);
        if(clnt_sock < 0) {
            perror("accept");
            return -1;
        }

        printf("connected client : %d\n", i+1);
        
        // 널 문자를 만날때까지 읽는다
        while((str_len=read(clnt_sock, msg, BUF_SIZE)) != 0) {
            write(clnt_sock, msg, str_len);
        }

        close(clnt_sock);
    }

    close(serv_sock);
    return 0;
}