Building a Simple Chat App in C: A Linux Socket Programming Guide for Beginners

Architecture Diagram

+------------+          +-----------+
|   Client   | <------> |  Server   |
+------------+          +-----------+
     ^                        ^
     | stdin                  | stdin
     v                        v
+----------+            +----------+
|  User    |            |  User    |
| Input    |            | Input    |
+----------+            +----------+

Server Implementation Breakdown

Server Code Structure

#include <sys/socket.h>
#include <stdio.h>
#include <arpa/inet.h>
#include <poll.h>
#include <unistd.h>
 
int main() {
    // Socket creation and setup
    // Main loop with poll()
    // Communication handling
    return 0;
}

Step-by-Step Explanation

Step 1: Create Socket

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

AF_INET: IPv4 address family
SOCK_STREAM: TCP socket type
Returns file descriptor for the socket

Step 2: Bind Socket

struct sockaddr_in address = {
    AF_INET,
    htons(9999),
    0
};
bind(sockfd, &address, sizeof(address));

htons(9999): Convert port number to network byte order
0 in address field means bind to all interfaces

Step 3: Listen for Connections

listen(sockfd, 10);

Backlog of 10 pending connections
Server is now waiting for client connections

Step 4: Accept Client Connection

int clientfd = accept(sockfd, 0, 0);

Blocks until client connects
Returns new file descriptor for client communication

Step 5: Setup poll() Structure

struct pollfd fds[2] = {
    {0, POLLIN, 0},     // stdin
    {clientfd, POLLIN, 0} // client socket
};

Monitor 2 file descriptors:
1. Standard input (keyboard)
2. Client socket

Step 6: Main poll() Loop

for (;;) {
    char buffer[256] = {0};
    poll(fds, 2, 50000);
    
    // Handle events...
}

poll() waits for 50 seconds (50000 milliseconds)
Checks for input from either source

Step 7: Handle Input Events

if (fds[0].revents & POLLIN) {  // stdin input
    read(0, buffer, 255);
    send(clientfd, buffer, 255, 0);
} 
else if (fds[1].revents & POLLIN) {  // client message
    if (recv(clientfd, buffer, 255, 0) == 0) return 0;
    printf("%s\n", buffer);
}

Forward stdin input to client
Print received messages from client

Client Implementation Breakdown

Client Code Structure

#include <sys/socket.h>
#include <stdio.h>
#include <arpa/inet.h>
#include <poll.h>
#include <unistd.h>
 
int main() {
    // Socket creation and connection
    // Main loop with poll()
    // Communication handling
    return 0;
}

Step-by-Step Explanation

Step 1: Create Socket

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

Same as server socket creation

Step 2: Connect to Server

struct sockaddr_in address = {
    AF_INET,
    htons(9999),
    0
};
connect(sockfd, &address, sizeof(address));

Connect to server at port 9999
0 in address field means localhost (127.0.0.1)

Step 3: Setup poll() Structure

struct pollfd fds[2] = {
    {0, POLLIN, 0},    // stdin
    {sockfd, POLLIN, 0} // server socket
};

Similar to server's poll setup

Step 4: Main poll() Loop

for (;;) {
    char buffer[256] = {0};
    poll(fds, 2, 50000);
    
    // Handle events...
}

Same logic as server's main loop

Step 5: Handle Input Events

if (fds[0].revents & POLLIN) {  // stdin input
    read(0, buffer, 255);
    send(sockfd, buffer, 255, 0);
} 
else if (fds[1].revents & POLLIN) {  // server message
    if (recv(sockfd, buffer, 255, 0) == 0) return 0;
    printf("%s\n", buffer);
}

Mirror of server's message handling

Communication Flow Diagram

Client Process                      Server Process
     |                                   |
     |-------- TCP Connection ---------->|
     |                                   |
     |------- stdin Input -------------->|
     |<----- Received Message -----------|
     |                                   |
     |<----- stdin Input ----------------|
     |------- Received Message --------->|

Key Concepts Explained

poll() System Call

+----------------+
|    poll()      |
+----------------+
|                |       +------------+
|  File Descriptors <----| stdin (0)  |
|                |       | socket fd  |
+----------------+       +------------+
       |
       v
+---------------+
| Event Checking|
| POLLIN        |
| POLLOUT       |
| etc.          |
+---------------+

Monitors multiple file descriptors simultaneously
Returns when any fd is ready for I/O
More efficient than busy-waiting

Network Byte Order

Host Byte Order (e.g., 1234)  ->  htons()  ->  Network Byte Order (e.g., 3412)
Network Byte Order (e.g., 3412) -> ntohs() ->  Host Byte Order (e.g., 1234)

Ensures consistent data representation across different architectures
Critical for port numbers and IP addresses

Execution Flow

Start server first:

./server

Start client in another terminal:

./client

Communication test:

Type message in server terminal -> appears in client
Type message in client terminal -> appears in server

Important Notes

Port Number: Both use port 9999 (ensure it's available)
Localhost Only: 0 in address field binds to all interfaces
Buffer Size: Fixed 256-byte buffer (255 chars + null terminator)
Blocking Behavior: poll() with 50s timeout prevents busy-waiting

Limitations and Improvements

Single Client: Server handles only one connection at a time
No Error Handling: Add checks for socket/bind/listen errors
Message Truncation: Messages over 255 bytes will be split
Exit Condition: Closing connection terminates both ends

Full Code with Comments

server.c (Enhanced Comments)

#include <sys/socket.h>
#include <stdio.h>
#include <arpa/inet.h>
#include <poll.h>
#include <unistd.h>
 
int main() {
    // Create TCP socket
    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    
    // Configure server address
    struct sockaddr_in address = {
        AF_INET,          // IPv4
        htons(9999),      // Port
        0                 // Listen on all interfaces
    };
    
    // Bind socket to address
    bind(sockfd, (struct sockaddr*)&address, sizeof(address));
    
    // Start listening
    listen(sockfd, 10);
    
    // Accept incoming connection
    int clientfd = accept(sockfd, 0, 0);
    
    // Configure poll structure
    struct pollfd fds[2] = {
        {0, POLLIN, 0},      // stdin
        {clientfd, POLLIN, 0} // client socket
    };
    
    // Main communication loop
    for (;;) {
        char buffer[256] = {0};
        
        // Wait for events (50s timeout)
        poll(fds, 2, 50000);
        
        // Handle stdin input
        if (fds[0].revents & POLLIN) {
            read(0, buffer, 255);
            send(clientfd, buffer, 255, 0);
        }
        // Handle client message
        else if (fds[1].revents & POLLIN) {
            if (recv(clientfd, buffer, 255, 0) == 0) {
                // Connection closed
                return 0;
            }
            printf("Received: %s\n", buffer);
        }
    }
    
    return 0;
}

client.c (Enhanced Comments)

#include <sys/socket.h>
#include <stdio.h>
#include <arpa/inet.h>
#include <poll.h>
#include <unistd.h>
 
int main() {
    // Create TCP socket
    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    
    // Configure server address
    struct sockaddr_in address = {
        AF_INET,          // IPv4
        htons(9999),      // Port
        0                 // Connect to localhost
    };
    
    // Connect to server
    connect(sockfd, (struct sockaddr*)&address, sizeof(address));
    
    // Configure poll structure
    struct pollfd fds[2] = {
        {0, POLLIN, 0},    // stdin
        {sockfd, POLLIN, 0} // server socket
    };
    
    // Main communication loop
    for (;;) {
        char buffer[256] = {0};
        
        // Wait for events (50s timeout)
        poll(fds, 2, 50000);
        
        // Handle stdin input
        if (fds[0].revents & POLLIN) {
            read(0, buffer, 255);
            send(sockfd, buffer, 255, 0);
        }
        // Handle server message
        else if (fds[1].revents & POLLIN) {
            if (recv(sockfd, buffer, 255, 0) == 0) {
                // Connection closed
                return 0;
            }
            printf("Received: %s\n", buffer);
        }
    }
    
    return 0;
}

References & Further Reading

Essential Socket Programming Resources:

"UNIX Network Programming" by W. Richard Stevens
The definitive guide to network programming in UNIX environments
"Beej's Guide to Network Programming" (Free Online Resource)
https://beej.us/guide/bgnet/
Excellent beginner-friendly tutorial on socket programming
Linux Manual Pages
Use man 2 socket, man 2 bind, man poll in terminal
Official Linux system call documentation
"The Linux Programming Interface" by Michael Kerrisk
Comprehensive guide to Linux system programming
GNU C Library Documentation
https://www.gnu.org/software/libc/manual/
Official C standard library documentation

Support & Contact

☕ Support the Developer
If you found this guide helpful, consider supporting my work:
buymeacoffee.com/trish07

🐦 Connect on Twitter/X
For updates and programming tips follow :
Trish & Paul

Learning Path Recommendation

Screenshot 2025-04-04 182657.png
Visual roadmap from basic sockets to advanced network protocols

Path Explanation

Socket Basics: Understand fundamental socket APIs
TCP Client/Server: Master connection-oriented communication
Multiplexing: Learn efficient I/O handling with poll()/select()
Multi-client: Scale to handle concurrent connections
Advanced Protocols: Implement TLS, HTTP, or custom protocols

This visual roadmap helps you:

Track your progress through networking concepts
Understand prerequisite relationships between topics
Identify key milestones in network programming

For any questions or queries, feel free to reach out!

Building a Simple Chat App in C: A Linux Socket Programming Guide for Beginners

Table of contents

Architecture Diagram

Server Implementation Breakdown

Server Code Structure

Step-by-Step Explanation

Client Implementation Breakdown

Client Code Structure

Step-by-Step Explanation

Communication Flow Diagram

Key Concepts Explained

poll() System Call

Network Byte Order

Execution Flow

Important Notes

Limitations and Improvements

Full Code with Comments

server.c (Enhanced Comments)

client.c (Enhanced Comments)

References & Further Reading

Essential Socket Programming Resources:

Support & Contact

Learning Path Recommendation