Simple Client/Server Application in C

Socket programming enables two programs to communicate over a network. Here, we create a simple client–server application in C where the server sends a message to the client when a connection is established. The communication uses TCP sockets for reliable data transmission.

Client

In socket programming, the client application initiates communication with the server. It establishes a connection, sends requests, and receives responses that are then presented to the user. Typically:

• A client communicates with one server at a time
• Interacts directly with the user
• Sends requests and receives responses

Client Socket Workflow

1. Create a socket:

• The socket() function creates a communication endpoint.
• Parameters:- Domain: AF_INET (IPv4), Type: SOCK_STREAM (TCP), Protocol: 0 (default)

2. Connect to the server: The connect() function establishes a connection with the server using its IP address and port number.

3.Receive data: The recv() function receives data sent by the server.

4. Display the message: The received data is printed on the client terminal.

Client Code

#include <netinet/in.h> //structure for storing address information
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h> //for socket APIs
#include <sys/types.h>

int main(int argc, char const* argv[])
{
    int sockD = socket(AF_INET, SOCK_STREAM, 0);

    struct sockaddr_in servAddr;

    servAddr.sin_family = AF_INET;
    servAddr.sin_port
        = htons(9001); // use some unused port number
    servAddr.sin_addr.s_addr = INADDR_ANY;

    int connectStatus
        = connect(sockD, (struct sockaddr*)&servAddr,
                  sizeof(servAddr));

    if (connectStatus == -1) {
        printf("Error...\n");
    }

    else {
        char strData[255];

        recv(sockD, strData, sizeof(strData), 0);

        printf("Message: %s\n", strData);
    }

    return 0;
}

 Server

In a client–server architecture, the server program runs continuously and listens for connection requests from clients. After establishing a connection, it processes the request and returns the required data. Servers typically:

• Run continuously
• Handle requests from multiple clients
• Provide services such as data access or computation

Server Socket Workflow

1. Create a socket: Similar to the client using the socket() function.

2. Bind the socket: The bind() function associates the socket with a specific IP address and port number.

3. Listen for connections: The listen() function puts the server in passive mode, waiting for client connections.

4. Accept a client connection: The accept() function accepts the incoming connection and returns a new socket for communication with the client.

5. Send data to the client: The send() function sends the message to the connected client.

Server Code

#include <netinet/in.h> //structure for storing address information
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h> //for socket APIs
#include <sys/types.h>

int main(int argc, char const* argv[])
{

    // create server socket similar to what was done in
    // client program
    int servSockD = socket(AF_INET, SOCK_STREAM, 0);

    // string store data to send to client
    char serMsg[255] = "Message from the server to the "
                       "client \'Hello Client\' ";

    // define server address
    struct sockaddr_in servAddr;

    servAddr.sin_family = AF_INET;
    servAddr.sin_port = htons(9001);
    servAddr.sin_addr.s_addr = INADDR_ANY;

    // bind socket to the specified IP and port
    bind(servSockD, (struct sockaddr*)&servAddr,
         sizeof(servAddr));

    // listen for connections
    listen(servSockD, 1);

    // integer to hold client socket.
    int clientSocket = accept(servSockD, NULL, NULL);

    // send's messages to client socket
    send(clientSocket, serMsg, sizeof(serMsg), 0);

    return 0;
}

Client–Server Communication Flow

1. Server creates a socket
2. Binds socket to IP address and port
3. Listens for incoming connections
4. Client creates a socket and sends connection request
5. Server accepts the connection
6. Server sends message to client
7. Client receives and displays message

Instructions to Execute

1. Open two terminals.

2. Compile the server program:

gcc server.c -o server

3. Compile the client program:

gcc client.c -o client

4. Run the server first:

./server

5. Run the client in another terminal:

./client

6. The client terminal will display:

Message: Message from the server to the client 'Hello Client'

Reference