TCP Server (server.c)

This program creates a simple TCP server that listens on a user-specified port. It's designed to accept a single client connection, read a message, send a reply, and then exit.

/*
 * Simple TCP Server
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>

// Function to handle errors and exit
void error(const char *msg) {
    perror(msg);
    exit(1);
}

int main(int argc, char *argv[]) {
    if (argc < 2) {
        fprintf(stderr, "Usage: %s <port>\n", argv[0]);
        exit(1);
    }

    int sockfd, newsockfd;
    int portno;
    socklen_t clilen;
    char buffer[256];
    struct sockaddr_in serv_addr, cli_addr;

    // 1. Create a new socket
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        error("ERROR opening socket");
    }

    bzero((char *) &serv_addr, sizeof(serv_addr));
    portno = atoi(argv[1]);

    // 2. Set up the server address structure
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = INADDR_ANY;
    serv_addr.sin_port = htons(portno);

    // 3. Bind the socket to the server address
    if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
        error("ERROR on binding");
    }

    // 4. Listen for incoming connections
    listen(sockfd, 5);
    printf("Server listening on port %d...\n", portno);

    clilen = sizeof(cli_addr);

    // 5. Accept a connection from a client
    newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);
    if (newsockfd < 0) {
        error("ERROR on accept");
    }

    printf("Connection accepted from %s:%d\n", inet_ntoa(cli_addr.sin_addr), ntohs(cli_addr.sin_port));

    bzero(buffer, 256);

    // 6. Read data from the client
    int n = read(newsockfd, buffer, 255);
    if (n < 0) {
        error("ERROR reading from socket");
    }
    printf("Message from client: %s\n", buffer);

    // 7. Write a response to the client
    n = write(newsockfd, "Message received successfully!", 28);
    if (n < 0) {
        error("ERROR writing to socket");
    }

    // 8. Close the sockets
    close(newsockfd);
    close(sockfd);

    return 0;
}

Explanation of server.c

• Includes: The necessary headers for standard I/O, string manipulation, and socket programming functions and structures.
• Step 1: socket(): Creates a socket endpoint. AF_INET specifies the IPv4 protocol family, and SOCK_STREAM specifies a reliable TCP connection.
• Step 2: Setup Address: The sockaddr_in struct serv_addr is configured. INADDR_ANY tells it to accept connections from any network interface on the machine, and htons() converts the port number to network byte order.
• Step 3: bind(): Assigns the address and port to the socket created in step 1. This is how the server "claims" a port to listen on.
• Step 4: listen(): Puts the socket into a passive mode to wait for incoming client connections. The 5 indicates the size of the pending connection queue.
• Step 5: accept(): This is a blocking call—the program waits here until a client connects. It then creates a new socket descriptor (newsockfd) for communicating with this specific client.
• Step 6 & 7: read() and write(): Uses the new socket (newsockfd) to receive a message from the client and send a reply back.
• Step 8: close(): Shuts down both the client-specific connection (newsockfd) and the main listening socket (sockfd) to release system resources.

TCP Client (client.c)

This program connects to the server at a specified hostname and port. It reads a line of text from the user, sends it to the server, and prints the server's reply.

/*
 * Simple TCP Client
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h> // For gethostbyname

// Function to handle errors and exit
void error(const char *msg) {
    perror(msg);
    exit(1);
}

int main(int argc, char *argv[]) {
    if (argc < 3) {
        fprintf(stderr, "Usage: %s <hostname> <port>\n", argv[0]);
        exit(1);
    }

    int sockfd;
    int portno;
    int n;
    struct sockaddr_in serv_addr;
    struct hostent *server;
    char buffer[256];

    portno = atoi(argv[2]);

    // 1. Create a socket
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        error("ERROR opening socket");
    }

    // 2. Resolve the hostname to an IP address
    server = gethostbyname(argv[1]);
    if (server == NULL) {
        fprintf(stderr, "ERROR, no such host\n");
        exit(0);
    }

    bzero((char *) &serv_addr, sizeof(serv_addr));
    serv_addr.sin_family = AF_INET;
    bcopy((char *)server->h_addr,
          (char *)&serv_addr.sin_addr.s_addr,
          server->h_length);
    serv_addr.sin_port = htons(portno);

    // 3. Connect to the server
    if (connect(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
        error("ERROR connecting");
    }

    // 4. Send a message to the server
    printf("Please enter the message: ");
    bzero(buffer, 256);
    fgets(buffer, 255, stdin);

    n = write(sockfd, buffer, strlen(buffer));
    if (n < 0) {
        error("ERROR writing to socket");
    }

    // 5. Receive the response from the server
    bzero(buffer, 256);
    n = read(sockfd, buffer, 255);
    if (n < 0) {
        error("ERROR reading from socket");
    }
    printf("Server response: %s\n", buffer);

    // 6. Close the socket
    close(sockfd);

    return 0;
}

Explanation of client.c

• Step 1: socket(): Creates the client's socket endpoint, just like the server.
• Step 2: gethostbyname(): This function resolves a hostname (e.g., "localhost") into an IP address that the network can understand. The address is then copied into the serv_addr struct.
• Step 3: connect(): This is the key client function. It attempts to establish a TCP connection with the server at the IP address and port specified in serv_addr.
• Step 4 & 5: write() and read(): After a successful connection, the client sends a message to the server and waits to receive a reply.
• Step 6: close(): Terminates the connection and releases the socket.

Makefile

This Makefile provides rules to automate the compilation of the server and client programs.

# Makefile for Simple TCP Client/Server

# Compiler
CC = gcc

# Compiler flags
# -g: Add debug information
# -Wall: Turn on all warnings
CFLAGS = -g -Wall

# Target executables
TARGETS = server client

# Default rule: build all targets
all: $(TARGETS)

# Rule to build the server
server: server.c
	$(CC) $(CFLAGS) -o server server.c

# Rule to build the client
client: client.c
	$(CC) $(CFLAGS) -o client client.c

# Rule to clean up compiled files
clean:
	rm -f $(TARGETS)

Explanation of Makefile

• all: This is the default rule. Running make will build both server and client.
• server & client rules: These specify how to compile each source file into an executable.
• clean: A utility rule. Running make clean will remove the compiled executable files.

How to Compile and Run

1. Save the Files: Save the three code blocks above as server.c, client.c, and Makefile in the same directory.
2. Compile: Open a terminal in that directory and run the command make.
3. Start the Server: In one terminal window, start the server:

   ./server 8888

4. Run the Client: Open a second terminal window and connect to the server:

   ./client localhost 8888

5. Communicate: The client terminal will prompt for a message. Type something and press Enter. You'll see the message appear on the server and a confirmation reply on the client.

Changelog

8/4/25, 11:40 AM

View All Changelog

• 2b7ff-web-deploy(Auto): Update base URL for web-pages branchon 8/4/25

Copyright

Copyright Ownership:WARREN Y.F. LONG

License under:Attribution-NonCommercial-NoDerivatives 4.0 International (CC-BY-NC-ND-4.0)