C Code Examples for TCP, UDP, and FTP Sockets

Posted on Nov 30, 2025 in Legal Sciences of Public Administrations

TCP Client-Server C Code

TCP Server

This C code sets up a basic TCP server that listens on a specified port, accepts a client connection, receives a message, sends a reply, and then closes the connection, waiting for the next client.

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

int main() {
    char *ip = "127.0.0.1";
    int port = 5566;

    int server_sock, client_sock;
    struct sockaddr_in server_addr, client_addr;
    socklen_t addr_size;
    char buffer[1024];
    int n;

    server_sock = socket(AF_INET, SOCK_STREAM, 0);
    if (server_sock < 0) {
        perror("[-] Socket error");
        exit(1);
    }
    printf("[+] TCP server socket created.\n");

    memset(&server_addr, '\0', sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = port;
    server_addr.sin_addr.s_addr = inet_addr(ip);

    n = bind(server_sock, (struct sockaddr*)&server_addr, sizeof(server_addr));
    if (n < 0) {
        perror("[-] Bind error");
        exit(1);
    }
    printf("[+] Bind to the port number: %d\n", port);

    listen(server_sock, 5);
    printf("Listening...\n");

    while (1) {
        addr_size = sizeof(client_addr);
        client_sock = accept(server_sock, (struct sockaddr*)&client_addr, &addr_size);
        printf("[+] Client connected.\n");

        bzero(buffer, 1024);
        recv(client_sock, buffer, sizeof(buffer), 0);
        printf("Client: %s\n", buffer);

        bzero(buffer, 1024);
        strcpy(buffer, "Hi, this is the server. Have a nice day!");
        printf("Server: %s\n", buffer);
        send(client_sock, buffer, strlen(buffer), 0);

        close(client_sock);
        printf("[+] Client disconnected.\n\n");
    }

    return 0;
}

TCP Client

This C code creates a TCP client that connects to the server, sends a message, receives a response, and then closes the connection.

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

int main() {
    char *ip = "127.0.0.1";
    int port = 5566;

    int sock;
    struct sockaddr_in addr;
    char buffer[1024];

    sock = socket(AF_INET, SOCK_STREAM, 0);
    if (sock < 0) {
        perror("[-] Socket error");
        exit(1);
    }
    printf("[+] TCP client socket created.\n");

    memset(&addr, '\0', sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = port;
    addr.sin_addr.s_addr = inet_addr(ip);

    connect(sock, (struct sockaddr*)&addr, sizeof(addr));
    printf("Connected to the server.\n");

    bzero(buffer, 1024);
    strcpy(buffer, "HELLO, THIS IS THE CLIENT");
    printf("Client: %s\n", buffer);
    send(sock, buffer, strlen(buffer), 0);

    bzero(buffer, 1024);
    recv(sock, buffer, sizeof(buffer), 0);
    printf("Server: %s\n", buffer);

    close(sock);
    printf("Disconnected from the server.\n");

    return 0;
}

UDP Client-Server C Code

UDP Server

This C code demonstrates a simple connectionless UDP server that waits for a datagram from a client, receives it, and sends a response back.

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

int main(int argc, char **argv) {
    if (argc != 2) {
        printf("Usage: %s <port>\n", argv[0]);
        exit(0);
    }

    char *ip = "127.0.0.1";
    int port = atoi(argv[1]);

    int sockfd;
    struct sockaddr_in server_addr, client_addr;
    char buffer[1024];
    socklen_t addr_size;
    int n;

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        perror("[-] Socket error");
        exit(1);
    }

    memset(&server_addr, '\0', sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(port);
    server_addr.sin_addr.s_addr = inet_addr(ip);

    n = bind(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr));
    if (n < 0) {
        perror("[-] Bind error");
        exit(1);
    }

    bzero(buffer, 1024);
    addr_size = sizeof(client_addr);
    recvfrom(sockfd, buffer, 1024, 0, (struct sockaddr*)&client_addr, &addr_size);
    printf("[+] Data received: %s\n", buffer);

    bzero(buffer, 1024);
    strcpy(buffer, "Welcome to the UDP server.");
    sendto(sockfd, buffer, 1024, 0, (struct sockaddr*)&client_addr, sizeof(client_addr));
    printf("[+] Data sent: %s\n", buffer);

    return 0;
}

UDP Client

This C code is for a UDP client that sends a single datagram to the server and waits to receive a response.

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

int main(int argc, char **argv) {
    if (argc != 2) {
        printf("Usage: %s <port>\n", argv[0]);
        exit(0);
    }

    char *ip = "127.0.0.1";
    int port = atoi(argv[1]);

    int sockfd;
    struct sockaddr_in addr;
    char buffer[1024];
    socklen_t addr_size;

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    
    memset(&addr, '\0', sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(port);
    addr.sin_addr.s_addr = inet_addr(ip);

    bzero(buffer, 1024);
    strcpy(buffer, "Hello, World!");
    sendto(sockfd, buffer, 1024, 0, (struct sockaddr*)&addr, sizeof(addr));
    printf("[+] Data sent: %s\n", buffer);

    bzero(buffer, 1024);
    addr_size = sizeof(addr);
    recvfrom(sockfd, buffer, 1024, 0, (struct sockaddr*)&addr, &addr_size);
    printf("[+] Data received: %s\n", buffer);

    return 0;
}

Simple FTP C Code for File Transfer

FTP Server

This C code implements a basic FTP-like server that accepts a connection and receives a file from a client, writing the contents to a local file.

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

#define SIZE 1024

void write_file(int sockfd) {
    int n;
    FILE *fp;
    char *filename = "received_file.txt";
    char buffer[SIZE];

    fp = fopen(filename, "w");
    if (fp == NULL) {
        perror("[-] Error in creating file.");
        exit(1);
    }

    while (1) {
        n = recv(sockfd, buffer, SIZE, 0);
        if (n <= 0) {
            break;
        }
        fprintf(fp, "%s", buffer);
        bzero(buffer, SIZE);
    }
    fclose(fp);
    return;
}

int main() {
    char *ip = "127.0.0.1";
    int port = 8080;
    int e;

    int sockfd, new_sock;
    struct sockaddr_in server_addr, new_addr;
    socklen_t addr_size;
    
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        perror("[-] Error in socket");
        exit(1);
    }
    printf("[+] Server socket created successfully.\n");

    server_addr.sin_family = AF_INET;
    server_addr.sin_port = port;
    server_addr.sin_addr.s_addr = inet_addr(ip);

    e = bind(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr));
    if (e < 0) {
        perror("[-] Error in bind");
        exit(1);
    }
    printf("[+] Binding successful.\n");

    if (listen(sockfd, 10) != 0) {
        perror("[-] Error in listening");
        exit(1);
    }
    printf("[+] Listening....\n");

    addr_size = sizeof(new_addr);
    new_sock = accept(sockfd, (struct sockaddr*)&new_addr, &addr_size);

    write_file(new_sock);
    printf("[+] Data written to the file successfully.\n");

    return 0;
}

FTP Client

This C code is for a simple FTP-like client that reads a local file and sends its contents to the server over a TCP connection.

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

#define SIZE 1024

void send_file(FILE *fp, int sockfd) {
    char data[SIZE] = {0};

    while (fgets(data, SIZE, fp) != NULL) {
        if (send(sockfd, data, sizeof(data), 0) == -1) {
            perror("[-] Error in sending file.");
            exit(1);
        }
        bzero(data, SIZE);
    }
}

int main() {
    char *ip = "127.0.0.1";
    int port = 8080;
    int e;

    int sockfd;
    struct sockaddr_in server_addr;
    FILE *fp;
    char *filename = "source_file.txt";

    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        perror("[-] Error in socket");
        exit(1);
    }
    printf("[+] Server socket created successfully.\n");

    server_addr.sin_family = AF_INET;
    server_addr.sin_port = port;
    server_addr.sin_addr.s_addr = inet_addr(ip);

    e = connect(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr));
    if (e == -1) {
        perror("[-] Error in connect");
        exit(1);
    }
    printf("[+] Connected to Server.\n");

    fp = fopen(filename, "r");
    if (fp == NULL) {
        perror("[-] Error in reading file.");
        exit(1);
    }

    send_file(fp, sockfd);
    printf("[+] File data sent successfully.\n");

    printf("[+] Closing the connection.\n");
    close(sockfd);

    return 0;
}