🧩 C Function Pointers – Callback, Syntax, and Examples

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🧲 Introduction – What Are Function Pointers?

In C programming, function pointers are pointers that point to the address of a function rather than data. This enables powerful features like callback functions, dynamic execution, and function arrays.

🎯 In this guide, you’ll learn:

• How function pointers work in C
• Syntax to declare, assign, and call function pointers
• Real-world examples like callbacks and function tables
• Best practices and dangers to avoid

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🔍 Core Concept – Function as First-Class Address

In C, functions have addresses just like variables. You can:

• Store a function’s address in a pointer
• Pass the pointer as an argument
• Call the function using the pointer

✅ Basic Syntax:

return_type (*pointer_name)(parameter_list);

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💻 Code Examples – Function Pointer Usage

✅ Example 1: Simple Function Pointer

#include <stdio.h>

void greet() {
    printf("Hello from a function pointer!\n");
}

int main() {
    void (*func_ptr)() = greet;  // Assigning function address
    func_ptr();  // Calling function via pointer
    return 0;
}

🖨️ Output:

Hello from a function pointer!

📖 Explanation:
func_ptr holds the address of greet(). func_ptr() calls it.

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✅ Example 2: Pointer to Function with Parameters

#include <stdio.h>

int add(int a, int b) {
    return a + b;
}

int main() {
    int (*operation)(int, int) = add;
    printf("Sum = %d\n", operation(5, 3));
    return 0;
}

🖨️ Output:

Sum = 8

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✅ Example 3: Callback Function

#include <stdio.h>

void notify(const char* msg) {
    printf("NOTIFY: %s\n", msg);
}

void process(void (*callback)(const char*)) {
    callback("Processing complete.");
}

int main() {
    process(notify);  // Passing function as argument
    return 0;
}

🖨️ Output:

NOTIFY: Processing complete.

📌 Used commonly in libraries and event handlers.

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💡 Best Practices & Tips

📘 Best Practice: Always match the signature exactly—return type and parameters.

💡 Tip: Use typedef to simplify complex function pointer syntax.

typedef int (*math_op)(int, int);
math_op op = add;

⚠️ Pitfall: Dereferencing incorrect function pointer types leads to undefined behavior.

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📊 Function Pointer Use Cases

Use Case	Description
Callback Mechanism	Event handlers, GUI callbacks, comparators
Dynamic Dispatch	Select function at runtime from user input
Function Tables/Arrays	Replace switch-case with array of function calls
Plugin-like Systems	Dynamically register function implementations

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🛠️ Real-World Applications

• qsort() in standard library uses function pointers
• Operating systems (interrupt tables, syscall dispatch)
• GUI frameworks (event handling)
• Game engines (AI strategy switching)

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📌 Summary – Recap & Next Steps

Function pointers give C programmers flexibility to write modular, reusable, and dynamic code. They’re crucial for callbacks, dispatch tables, and runtime decision making.

🔍 Key Takeaways:

• A function pointer stores a function’s address
• It must match the exact function signature
• Can be passed as argument, returned from function, or stored in arrays
• Ideal for modular, flexible program design

⚙️ Real-World Relevance:

Used extensively in drivers, GUI systems, embedded firmware, OS kernels, and more.

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❓ Frequently Asked Questions (FAQ)

❓ What is a function pointer?

✅ A variable that holds the address of a function and can be used to invoke it.

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❓ Can you pass a function as an argument in C?

✅ Yes, using function pointers. This enables callbacks and dynamic behaviors.

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❓ What’s the syntax for a pointer to a function returning int and taking two int arguments?

✅ int (*ptr)(int, int);

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❓ How is qsort() related to function pointers?

✅ qsort() accepts a comparator function pointer to define sorting behavior dynamically.

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❓ Can function pointers point to main() or recursive functions?

✅ Technically yes, but doing so is discouraged and can result in undefined behavior depending on context.

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