How to implement TCP sockets in C

TCP sockets are used for communication between a server and a client process. The server’s code runs first, which opens a port and listens for incoming connection requests from clients. Once a client connects to the same (server) port, the client or server may send a message. Once the message is sent, whoever receives it (server or client) will process it accordingly.

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Server-side

#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <arpa/inet.h>
int main(void)
{
    int socket_desc, client_sock, client_size;
    struct sockaddr_in server_addr, client_addr;
    char server_message[2000], client_message[2000];
    
    // Clean buffers:
    memset(server_message, '\0', sizeof(server_message));
    memset(client_message, '\0', sizeof(client_message));
    
    // Create socket:
    socket_desc = socket(AF_INET, SOCK_STREAM, 0);
    
    if(socket_desc < 0){
        printf("Error while creating socket\n");
        return -1;
    }
    printf("Socket created successfully\n");
    
    // Set port and IP:
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(2000);
    server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
    
    // Bind to the set port and IP:
    if(bind(socket_desc, (struct sockaddr*)&server_addr, sizeof(server_addr))<0){
        printf("Couldn't bind to the port\n");
        return -1;
    }
    printf("Done with binding\n");
    
    // Listen for clients:
    if(listen(socket_desc, 1) < 0){
        printf("Error while listening\n");
        return -1;
    }
    printf("\nListening for incoming connections.....\n");
    
    // Accept an incoming connection:
    client_size = sizeof(client_addr);
    client_sock = accept(socket_desc, (struct sockaddr*)&client_addr, &client_size);
    
    if (client_sock < 0){
        printf("Can't accept\n");
        return -1;
    }
    printf("Client connected at IP: %s and port: %i\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));
    
    // Receive client's message:
    if (recv(client_sock, client_message, sizeof(client_message), 0) < 0){
        printf("Couldn't receive\n");
        return -1;
    }
    printf("Msg from client: %s\n", client_message);
    
    // Respond to client:
    strcpy(server_message, "This is the server's message.");
    
    if (send(client_sock, server_message, strlen(server_message), 0) < 0){
        printf("Can't send\n");
        return -1;
    }
    
    // Closing the socket:
    close(client_sock);
    close(socket_desc);
    
    return 0;
}

Explanation

Include the header files sys/socket.h and arpa/inet.h:

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

Create a socket that returns a socket descriptor; this will be used to refer to the socket later on in the code:

int socket_desc = socket(AF_INET, SOCK_STREAM, 0);
  • The server-side code keeps the address information of both the server and the client in a variable of type sockaddr_in, which is a struct.

Initialize the server address by the port and IP:

struct sockaddr_in server_addr;
server_addr.sin_family = AF_INET;
server_addr.sin_port = htons(2000);
server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");

Bind the socket descriptor to the server address:

bind(socket_desc, (struct sockaddr*)&server_addr, sizeof(server_addr);

Turn on the socket to listen for incoming connections:

listen(socket_desc, 1);

Store the client’s address and socket descriptor by accepting an incoming connection:

struct sockaddr client_addr;
int client_size = sizeof(client_addr);
int client_sock = accept(socket_desc, (struct sockaddr*)&client_addr, &client_size);
  • The server-side code stops and waits at accept() until a client calls connect().

Communicate with the client using send() and recv():

recv(client_sock, client_message, sizeof(client_message), 0);

send(client_sock, server_message, strlen(server_message), 0);
  • When recv() is called, the code stops and waits for a message from the client.

Close the server and client socket to end communication:

close(client_sock);
close(socket_desc);

Client-side

#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <arpa/inet.h>
int main(void)
{
    int socket_desc;
    struct sockaddr_in server_addr;
    char server_message[2000], client_message[2000];
    
    // Clean buffers:
    memset(server_message,'\0',sizeof(server_message));
    memset(client_message,'\0',sizeof(client_message));
    
    // Create socket:
    socket_desc = socket(AF_INET, SOCK_STREAM, 0);
    
    if(socket_desc < 0){
        printf("Unable to create socket\n");
        return -1;
    }
    
    printf("Socket created successfully\n");
    
    // Set port and IP the same as server-side:
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(2000);
    server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
    
    // Send connection request to server:
    if(connect(socket_desc, (struct sockaddr*)&server_addr, sizeof(server_addr)) < 0){
        printf("Unable to connect\n");
        return -1;
    }
    printf("Connected with server successfully\n");
    
    // Get input from the user:
    printf("Enter message: ");
    gets(client_message);
    
    // Send the message to server:
    if(send(socket_desc, client_message, strlen(client_message), 0) < 0){
        printf("Unable to send message\n");
        return -1;
    }
    
    // Receive the server's response:
    if(recv(socket_desc, server_message, sizeof(server_message), 0) < 0){
        printf("Error while receiving server's msg\n");
        return -1;
    }
    
    printf("Server's response: %s\n",server_message);
    
    // Close the socket:
    close(socket_desc);
    
    return 0;
}

Explanation

Include header files, create a socket, and initialize the server’s address information in a variable of type sockaddr_in, similar to how it was done at the server-side:

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

int socket_desc = socket(AF_INET, SOCK_STREAM, 0);

struct sockaddr_in server_addr;
server_addr.sin_family = AF_INET;
server_addr.sin_port = htons(2000);
server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");

Send a connection request to the server, which is waiting at accept():

connect(socket_desc, (struct sockaddr*)&server_addr, sizeof(server_addr));

Communicate with the server using send() and recv():

send(socket_desc, client_message, strlen(client_message),0);

recv(socket_desc, server_message, sizeof(server_message),0);
  • The client waits for the server to send a message when recv() is called.

Close the socket:

close(socket_desc);

A deadlock will occur if both the client and the server are waiting for each other’s message at recv().

Control-flow of a client-server program
Control-flow of a client-server program

Client-server app

The following client-server application enables a client to connect to a server and send only one message. The server replies with "This is the server’s message" and the communication terminates.

Instructions

  1. Click the Run button in the widget below and execute the command for the Server. If the socket is created successfully, the message Listening for incoming connections… will be displayed.
  2. Press the + button to open another terminal tab and execute the Client’s command.
  3. Enter a message in the Client tab which is sent to the Server.
  4. The Server’s response will be shown in the Client’s tab.
Server Client
usercode/server usercode/client 
#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <arpa/inet.h>

int main(void)
{
    int socket_desc, client_sock, client_size;
    struct sockaddr_in server_addr, client_addr;
    char server_message[100], client_message[100];
    
    // Clean buffers:
    memset(server_message, '\0', sizeof(server_message));
    memset(client_message, '\0', sizeof(client_message));
    
    // Create socket:
    socket_desc = socket(AF_INET, SOCK_STREAM, 0);
    
    if(socket_desc < 0){
        printf("Error while creating socket\n");
        return -1;
    }
    printf("Socket created successfully\n");
    
    // Set port and IP:
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(2000);
    server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
    
    // Bind to the set port and IP:
    if(bind(socket_desc, (struct sockaddr*)&server_addr, sizeof(server_addr))<0){
        printf("Couldn't bind to the port\n");
        return -1;
    }
    printf("Done with binding\n");
    
    // Listen for clients:
    if(listen(socket_desc, 1) < 0){
        printf("Error while listening\n");
        return -1;
    }
    printf("\nListening for incoming connections.....\n");
    
    // Accept an incoming connection:
    client_size = sizeof(client_addr);
    client_sock = accept(socket_desc, (struct sockaddr*)&client_addr, &client_size);
    
    if (client_sock < 0){
        printf("Can't accept\n");
        return -1;
    }
    printf("Client connected at IP: %s and port: %i\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));
    
    // Receive client's message:
    if (recv(client_sock, client_message, sizeof(client_message), 0) < 0){
        printf("Couldn't receive\n");
        return -1;
    }
    printf("Msg from client: %s\n", client_message);
    
    // Respond to client:
    strcpy(server_message, "This is the server's message.");
    
    if (send(client_sock, server_message, strlen(server_message), 0) < 0){
        printf("Can't send\n");
        return -1;
    }
    
    // Closing the socket:
    close(client_sock);
    close(socket_desc);
    
    return 0;
}
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