Introduction

Event-Driven Programming in C is a programming style where the program’s control flow is determined by events — actions or occurrences such as user inputs (keyboard, mouse), timer expiration, or messages from other programs. Rather than running in a strict, linear sequence, event-driven programs react to specific events.

In C, which is not inherently event-driven like some modern languages, you can still implement event-driven behavior using various techniques such as function pointers (callbacks), signals, state machines, and event loops.

Event-Driven Programming in C
Event-Driven Programming in C

Why Use Event-Driven Programming?

Event-driven design is useful when:

  • The application needs to respond to external inputs dynamically.
  • You are developing interactive systems, such as games, user interfaces, or embedded device software.
  • You want to separate the core logic from how events are handled.

Core Concepts in Event-Driven Programming

Let’s explore the key elements that define an event-driven program.

1. Event

Definition: An event is an occurrence or action that the program needs to respond to.

Examples:

  • A mouse click or key press.
  • A timer reaching a certain value.
  • A hardware signal or sensor trigger.

2. Event Handler

Definition: An event handler is a function or code block that executes when a specific event occurs.

Think of it like a response routine. For example, when you press a key, the event handler might display a character on the screen.

3. Event Loop

Definition: The event loop is a loop that waits for events, checks if any have occurred, and then calls the corresponding handler.

It’s the central piece that keeps checking for events and dispatching them for processing.

4. Callback Function

Definition: A callback is a function passed as a parameter to another function. The calling function will invoke (call back) this function when a specific event occurs.

Callbacks allow the program to handle events dynamically.

How to Implement Event-Driven Programming in C

C doesn’t offer built-in support for events, but we can simulate event-driven behavior using several techniques:

1. Using Function Pointers (Callbacks)

Function pointers allow us to store the address of a function and call it dynamically.

Example:
#include <stdio.h>

void on_click() {
    printf("Button clicked\n");
}

void trigger_event(void (*event_handler)()) {
    event_handler();
}

int main() {
    trigger_event(on_click);
    return 0;
}

Explanation: Click me!

  • on_click() is the event handler.
  • trigger_event() calls the handler function when an event is simulated.

2. Using State Machines

A state machine represents different states of a system and how it transitions between them based on events.

Example:
#include <stdio.h>

enum State { IDLE, ACTIVE };
enum Event { START, STOP };

int main() {
    enum State current = IDLE;
    enum Event input = START;

    if (current == IDLE && input == START) {
        current = ACTIVE;
        printf("Switched to ACTIVE state.\n");
    }
    return 0;
}

Explanation: Click me!

  • The system moves from IDLE to ACTIVE based on the START event.

3. Using Signal Handlers

Signals are software interrupts delivered to a program by the OS. In C, you can catch these using signal().

Example:
#include <stdio.h>
#include <signal.h>
#include <unistd.h> // For sleep()

void handle_signal(int sig) {
    printf("Caught signal: %d\n", sig);
}

int main() {
    signal(SIGINT, handle_signal); // Setup handler

    printf("Simulating signal in 2 seconds...\n");
    sleep(2); // Wait for 2 seconds (simulate delay)

    raise(SIGINT); // Manually raise SIGINT (simulate Ctrl+C)

    printf("Program continues after signal.\n");

    return 0;
}

Explanation: Click me!

  • Pressing Ctrl+C sends SIGINT, which triggers the handle_signal() function.

Complete Example: Simple Event Handler

#include <stdio.h>
#include <string.h>

void start_event() {
    printf("Start event triggered.\n");
}

void stop_event() {
    printf("Stop event triggered.\n");
}

void handle_event(const char* input) {
    if (strcmp(input, "start") == 0)
        start_event();
    else if (strcmp(input, "stop") == 0)
        stop_event();
    else
        printf("Unknown event.\n");
}

int main() {
    // Simulate 3 inputs
    char* events[] = {"start", "stop", "exit"};
    int n = 3;

    for (int i = 0; i < n; i++) {
        printf("Input: %s\n", events[i]);

        if (strcmp(events[i], "exit") == 0)
            break;

        handle_event(events[i]);
    }

    printf("Program ended.\n");
    return 0;
}

Output: Click me!

Enter event (start/stop/exit): start
Start event triggered.
Enter event (start/stop/exit): stop
Stop event triggered.
Enter event (start/stop/exit): exit

Advantages of Event-Driven Programming in C

  • Efficient Use of Resources: Code only runs when an event occurs.
  • Improved Responsiveness: Especially suitable for real-time and interactive applications.
  • Better Separation of Logic: Event handling is separate from core application logic.
  • Scalable for Complex Systems: Useful in systems with many possible events.

Disadvantages

  • Harder to Debug: Non-linear execution makes debugging more complex.
  • Difficult to Follow: Flow of control may not be obvious.
  • Callback Complexity: Too many callbacks can reduce code readability.
  • State Explosion: Managing states in large systems can become challenging.

Where is Event-Driven Programming Used?

  • Embedded Systems: Micro controller-based applications like sensors, alarms, and automation.
  • User Interfaces: Buttons, sliders, mouse movement, and key presses.
  • Networking: Servers handling multiple client requests (event loop).
  • Games: Event loops used to update player positions, detect collisions, etc.

Conclusion

Event-driven programming in C is a powerful approach for building applications that need to respond to real-time inputs, user interactions, or system-level signals. Although C does not offer built-in event support, developers can implement it effectively using function pointers, signal handlers, and state machines.

By structuring the code to handle events as they occur, programmers can build more responsive, modular, and maintainable systems — especially useful in embedded systems, networking, and real-time applications.

Interview Questions

1. What is event-driven programming in C?
Company: Infosys
Answer:
Event-driven programming is a style where the program flow is determined by events like user input, sensor output, or messages. Instead of running line by line, the program waits for events and responds using functions.

2. How do you handle events in C?
Company: TCS
Answer:
In C, we handle events using callback functions, state machines, or the signal() function. A common method is to assign a function that gets called when a specific event happens.

3. What is a callback function in C?
Company: Wipro
Answer:
A callback function is a function that is passed as an argument to another function, and it is called when an event occurs. It allows flexible handling of events.

4. Can C support GUI-based event-driven applications?
Company: Cognizant
Answer:
Yes, C can support GUI-based event-driven applications using external libraries like GTK or WinAPI. These libraries allow you to handle mouse clicks, key presses, and other user interactions.

5. Why is event-driven programming useful?
Company: IBM
Answer:
It makes programs more responsive and efficient. Instead of checking for input all the time, the program reacts only when needed, making it ideal for GUIs, games, and real-time systems.

Quizzes

Event-driven Programming in C Quiz

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