C 언어에서 connect 함수로 서버에 연결하기: 실용 가이드

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

int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

#include <sys/socket.h>
#include <arpa/inet.h>

int sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0) {
    perror("Socket creation failed");
    return -1;
}

struct sockaddr_in server_addr;
server_addr.sin_family = AF_INET;             // IPv4
server_addr.sin_port = htons(8080);          // 서버 포트 (8080 예시)
server_addr.sin_addr.s_addr = inet_addr("192.168.1.1");  // 서버 IP 주소

if (connect(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
    perror("Connection failed");
    return -1;
}

if (connect(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
    perror("Connection failed");
    return -1;
}

if (connect(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
    if (errno == ECONNREFUSED) {
        fprintf(stderr, "Connection refused by the server\n");
    } else if (errno == ETIMEDOUT) {
        fprintf(stderr, "Connection timed out\n");
    } else {
        perror("Connection failed");
    }
    return -1;
}

int attempts = 0, max_attempts = 5;
while (connect(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0 && attempts < max_attempts) {
    perror("Retrying connection");
    sleep(1);  // 1초 대기
    attempts++;
}
if (attempts == max_attempts) {
    fprintf(stderr, "Failed to connect after multiple attempts\n");
    return -1;
}

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

int main() {
    int sockfd;
    struct sockaddr_in server_addr;
    char buffer[1024] = {0};
    const char *message = "Hello, Server!";

    // 1. 소켓 생성
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        perror("Socket creation failed");
        exit(EXIT_FAILURE);
    }

    // 2. 서버 주소 설정
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(8080);
    if (inet_pton(AF_INET, "127.0.0.1", &server_addr.sin_addr) <= 0) {
        perror("Invalid address or address not supported");
        close(sockfd);
        exit(EXIT_FAILURE);
    }

    // 3. 서버에 연결 요청
    if (connect(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("Connection failed");
        close(sockfd);
        exit(EXIT_FAILURE);
    }
    printf("Connected to the server.\n");

    // 4. 메시지 전송
    send(sockfd, message, strlen(message), 0);
    printf("Message sent to server: %s\n", message);

    // 5. 서버 응답 수신
    int valread = read(sockfd, buffer, sizeof(buffer) - 1);
    if (valread > 0) {
        buffer[valread] = '\0';
        printf("Message received from server: %s\n", buffer);
    } else {
        perror("Failed to receive message");
    }

    // 6. 소켓 종료
    close(sockfd);
    printf("Connection closed.\n");

    return 0;
}

Connected to the server.
Message sent to server: Hello, Server!
Message received from server: Hello, Client!
Connection closed.

#include <fcntl.h>

// 소켓을 비동기 모드로 설정
int flags = fcntl(sockfd, F_GETFL, 0);
fcntl(sockfd, F_SETFL, flags | O_NONBLOCK);

   if (connect(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
       if (errno == EINPROGRESS) {
           printf("Connection in progress...\n");
       } else {
           perror("Connection failed");
           close(sockfd);
           exit(EXIT_FAILURE);
       }
   }

   fd_set writefds;
   struct timeval timeout;

   FD_ZERO(&writefds);
   FD_SET(sockfd, &writefds);

   timeout.tv_sec = 5;   // 타임아웃: 5초
   timeout.tv_usec = 0;

   int ret = select(sockfd + 1, NULL, &writefds, NULL, &timeout);
   if (ret > 0 && FD_ISSET(sockfd, &writefds)) {
       printf("Connection established.\n");
   } else {
       perror("Connection timed out or failed");
       close(sockfd);
   }

int flags = fcntl(sockfd, F_GETFL, 0);
fcntl(sockfd, F_SETFL, flags | O_NONBLOCK);

if (connect(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0 && errno != EINPROGRESS) {
    perror("Connection failed immediately");
    close(sockfd);
    exit(EXIT_FAILURE);
}

fd_set writefds;
FD_ZERO(&writefds);
FD_SET(sockfd, &writefds);

struct timeval timeout = {5, 0};
int ret = select(sockfd + 1, NULL, &writefds, NULL, &timeout);

if (ret > 0 && FD_ISSET(sockfd, &writefds)) {
    printf("Connection established asynchronously.\n");
} else {
    perror("Connection timed out or failed");
    close(sockfd);
}

   import socket

   server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
   server.bind(("127.0.0.1", 8080))
   server.listen(1)
   print("Server is listening on port 8080...")

   conn, addr = server.accept()
   print(f"Connection established with {addr}")
   conn.send(b"Hello, Client!")
   conn.close()

   printf("Attempting to create socket...\n");
   printf("Socket created successfully.\n");
   printf("Attempting to connect to server...\n");

   if (connect(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
       perror("Connection failed");
       fprintf(stderr, "Error code: %d\n", errno);
   }

   tcpdump -i lo tcp port 8080

   netstat -an | grep 8080

   int attempts = 0, max_attempts = 3;
   while (connect(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0 && attempts < max_attempts) {
       perror("Retrying connection...");
       sleep(1);
       attempts++;
   }

   close(sockfd);

#!/bin/bash

gcc client.c -o client
for i in {1..10}; do
    ./client
    sleep 1
done