Function Pointers in C

01What is a Function Pointer?

Simple Definition

A function pointer is a variable that stores the memory address of a function. Just like regular pointers store addresses of variables, function pointers store addresses of functions!

Visual: Regular Pointers vs Function Pointers

Regular Pointer

int *ptr

→

int x = 42

Points to a variable

Function Pointer

int (*fp)()

→

int add() {...}

Points to a function

Why Use Function Pointers?

Callbacks

Pass a function to another function to be called later. Used in event handlers, sorting, etc.

Dynamic Behavior

Choose which function to call at runtime. Like a menu: user picks option → call matching function.

State Machines

Each state has a different handler function. Current state pointer determines behavior.

Plugin Systems

Load functions at runtime. Extensible architecture without recompiling.

Real-World Example

The C standard library's qsort() function uses a function pointer to let YOU define how to compare elements. This makes it work with any data type - integers, strings, structs, anything!

02Function Pointer Syntax

Declaration Syntax

return_type (*pointer_name)(parameter_types);

The parentheses around *pointer_name are important!

function_pointer_basic.c

1#include <stdio.h>
2
3// A simple function
4int add(int a, int b) {
5    return a + b;
6}
7
8int subtract(int a, int b) {
9    return a - b;
10}
11
12int main() {
13    // Declare a function pointer
14    // It can point to any function that:
15    // - Returns int
16    // - Takes two int parameters
17    int (*operation)(int, int);
18    
19    // Point to 'add' function
20    operation = add;  // or: operation = &add;
21    
22    // Call through pointer
23    int result = operation(10, 5);
24    printf("add(10, 5) = %d\n", result);
25    
26    // Point to 'subtract' function
27    operation = subtract;
28    
29    result = operation(10, 5);
30    printf("subtract(10, 5) = %d\n", result);
31    
32    return 0;
33}

Output

add(10, 5) = 15

subtract(10, 5) = 5

03Callback Functions

What is a Callback?

A callback is a function passed to another function as an argument. The receiving function can then "call back" to it!

callback.c

1#include <stdio.h>
2
3// Callback function type for processing array elements
4void applyToArray(int* arr, int size, int (*callback)(int)) {
5    for (int i = 0; i < size; i++) {
6        arr[i] = callback(arr[i]);  // Apply callback to each element
7    }
8}
9
10// Functions to use as callbacks
11int doubleValue(int x) {
12    return x * 2;
13}
14
15int square(int x) {
16    return x * x;
17}
18
19int addTen(int x) {
20    return x + 10;
21}
22
23void printArray(int* arr, int size) {
24    for (int i = 0; i < size; i++) {
25        printf("%d ", arr[i]);
26    }
27    printf("\n");
28}
29
30int main() {
31    int numbers[] = {1, 2, 3, 4, 5};
32    int size = 5;
33    
34    printf("Original: ");
35    printArray(numbers, size);
36    
37    // Apply 'double' callback
38    applyToArray(numbers, size, doubleValue);
39    printf("Doubled:  ");
40    printArray(numbers, size);
41    
42    // Reset and apply 'square'
43    int nums2[] = {1, 2, 3, 4, 5};
44    applyToArray(nums2, size, square);
45    printf("Squared:  ");
46    printArray(nums2, size);
47    
48    // Reset and apply 'addTen'
49    int nums3[] = {1, 2, 3, 4, 5};
50    applyToArray(nums3, size, addTen);
51    printf("Add 10:   ");
52    printArray(nums3, size);
53    
54    return 0;
55}

Output

Original: 1 2 3 4 5

Doubled: 2 4 6 8 10

Squared: 1 4 9 16 25

Add 10: 11 12 13 14 15

04Array of Function Pointers

function_pointer_array.c

1#include <stdio.h>
2
3int add(int a, int b) { return a + b; }
4int subtract(int a, int b) { return a - b; }
5int multiply(int a, int b) { return a * b; }
6int divide(int a, int b) { return b != 0 ? a / b : 0; }
7
8int main() {
9    // Array of function pointers
10    int (*operations[4])(int, int) = {add, subtract, multiply, divide};
11    char* names[] = {"Add", "Subtract", "Multiply", "Divide"};
12    
13    int a = 20, b = 5;
14    
15    printf("Calculator Demo (a=%d, b=%d):\n\n", a, b);
16    
17    for (int i = 0; i < 4; i++) {
18        int result = operations[i](a, b);  // Call through array
19        printf("%s: %d\n", names[i], result);
20    }
21    
22    // Menu-driven selection
23    printf("\nEnter operation (0=Add, 1=Sub, 2=Mul, 3=Div): ");
24    int choice = 0;  // Simulating user input
25    printf("%d\n", choice);
26    
27    printf("Result: %d\n", operations[choice](a, b));
28    
29    return 0;
30}

Output

Calculator Demo (a=20, b=5):

Add: 25

Subtract: 15

Multiply: 100

Divide: 4

05Cleaner Syntax with typedef

Function pointer syntax is complex. Use typedef to make it cleaner!

Without typedef (hard to read):

int (*operation)(int, int);

With typedef (clean):

Operation op;

typedef_function_pointer.c

1#include <stdio.h>
2
3// Define a type for our function pointer
4typedef int (*Operation)(int, int);
5
6int add(int a, int b) { return a + b; }
7int multiply(int a, int b) { return a * b; }
8
9// Now function parameters are clean!
10int calculate(int a, int b, Operation op) {
11    return op(a, b);
12}
13
14int main() {
15    // Clean variable declaration
16    Operation myOp = add;
17    printf("add: %d\n", myOp(10, 5));
18    
19    myOp = multiply;
20    printf("multiply: %d\n", myOp(10, 5));
21    
22    // Pass to function easily
23    printf("calculate with add: %d\n", calculate(10, 5, add));
24    printf("calculate with multiply: %d\n", calculate(10, 5, multiply));
25    
26    return 0;
27}

Output

add: 15

multiply: 50

calculate with add: 15

calculate with multiply: 50

06Real Example: qsort() from Standard Library

The Power of Function Pointers

The C standard library's qsort() function is a perfect example of why function pointers are powerful. It can sort any type of data because YOU provide the comparison function!

qsort() Signature

void qsort(

void *base, // Array to sort

size_t num, // Number of elements

size_t size, // Size of each element

int (*compare)(const void*, const void*) // Your comparison function!

);

The last parameter is a function pointer — you decide how elements are compared!

qsort_example.c

1#include <stdio.h>
2#include <stdlib.h>
3#include <string.h>
4
5// Comparison function for ascending integers
6int compareAsc(const void* a, const void* b) {
7    return (*(int*)a - *(int*)b);
8}
9
10// Comparison function for descending integers
11int compareDesc(const void* a, const void* b) {
12    return (*(int*)b - *(int*)a);
13}
14
15// Comparison function for strings
16int compareStrings(const void* a, const void* b) {
17    return strcmp(*(char**)a, *(char**)b);
18}
19
20void printArray(int arr[], int n) {
21    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
22    printf("\n");
23}
24
25int main() {
26    // Sort integers ascending
27    int nums[] = {5, 2, 8, 1, 9, 3};
28    int n = sizeof(nums) / sizeof(nums[0]);
29    
30    printf("Original: ");
31    printArray(nums, n);
32    
33    qsort(nums, n, sizeof(int), compareAsc);
34    printf("Ascending: ");
35    printArray(nums, n);
36    
37    qsort(nums, n, sizeof(int), compareDesc);
38    printf("Descending: ");
39    printArray(nums, n);
40    
41    // Sort strings!
42    char* names[] = {"Charlie", "Alice", "Bob", "Diana"};
43    int count = 4;
44    
45    printf("\nNames before: ");
46    for (int i = 0; i < count; i++) printf("%s ", names[i]);
47    printf("\n");
48    
49    qsort(names, count, sizeof(char*), compareStrings);
50    
51    printf("Names after: ");
52    for (int i = 0; i < count; i++) printf("%s ", names[i]);
53    printf("\n");
54    
55    return 0;
56}

Output

Original: 5 2 8 1 9 3

Ascending: 1 2 3 5 8 9

Descending: 9 8 5 3 2 1

Names before: Charlie Alice Bob Diana

Names after: Alice Bob Charlie Diana

!Code Pitfalls: Common Mistakes & What to Watch For

These are the most common mistakes that trip up beginners. Study them carefully to avoid hours of debugging!

Missing Parentheses Around *

The parentheses around *pointer_name are required!

WRONG:

int *func(int, int);

→ Function returning int*

CORRECT:

int (*func)(int, int);

→ Pointer to function returning int

Calling NULL Function Pointer

Always check if a function pointer is NULL before calling it!

WRONG (crashes!):

int (*fp)(int) = NULL;
fp(5);  // CRASH!

CORRECT:

int (*fp)(int) = NULL;
if (fp != NULL) {
    fp(5);
}

Mismatched Signatures

The function pointer type must exactly match the function signature!

WRONG:

int add(int a, int b);
float (*fp)(int, int); // Wrong return type!
fp = add;  // Type mismatch

CORRECT:

int add(int a, int b);
int (*fp)(int, int); // Matches!
fp = add;  // OK

Watch Out When Copying Code!

Common Mistakes with Function Pointers

Copying code frequently generate broken function pointer code:

✗Wrong parentheses: Copied code int *fp(int) instead of int (*fp)(int)
✗Signature mismatches: Beginners often assign functions with wrong return types or parameters
✗Missing NULL checks: Beginners often dereference function pointers that could be NULL
✗Incorrect qsort comparators: Beginners often forget to cast void* or returns wrong comparison values

Always Understand Before Using

Function pointer syntax is tricky even for humans. Use typedef to make it clearer. Always verify the function signature matches exactly, and check for NULL before calling. Compile with -Wall -Werror to catch type mismatches.

09Frequently Asked Questions

Q:Why is the syntax for function pointers so complicated?

A: The parentheses around(*name) are needed because without them,int *fp(int) means "function returning int*", not "pointer to function." Use typedef to simplify:typedef int (*Calculator)(int, int);

Q:What is a callback function?

A: A function passed to another function to be called later. Example: qsort takes a comparison callback — you tell it HOW to compare, it handles the sorting. Callbacks allow generic, reusable code.

Q:Can I have arrays of function pointers?

A: Yes! Great for dispatch tables (menu systems, state machines). int (*ops[])(int,int) ={add, sub, mul} then ops[choice](a, b). Much cleaner than long switch statements.

Q:What's the difference between fp and &fp for functions?

A: For functions, both are equivalent! A function name automatically converts to a pointer to itself (like arrays).fp = add andfp = &add do the same thing.

Q:Where are function pointers used in real code?

A: Everywhere! qsort/bsearch (stdlib), signal handlers, GUI callbacks, plugin systems, virtual method tables (OOP in C), state machines, and more. They enable runtime flexibility and polymorphism in C.

09Summary

Key Takeaways

•Function pointer: Variable storing function address
•Syntax: return_type (*name)(params)
•Callbacks: Pass functions as arguments

•typedef: Makes syntax much cleaner
•Arrays: Store multiple function pointers
•qsort(): Classic real-world example

Quick Reference

Declare

int (*fp)(int, int);

Assign

fp = add;

Call

result = fp(5, 3);

typedef

typedef int (*Op)(int, int);

10Real-World Use Cases

Callback Functions

Event handlers, timers, and async operations use function pointers as callbacks. You register your function, and the library calls it when the event occurs.

Plugin Systems

Load shared libraries at runtime with dlsym() and store the function addresses in function pointers. This enables extensible applications.

State Machines

Function pointers are excellent for implementing state machines. Each state is a function, and the current state is stored as a function pointer. Transitioning states just means changing which function the pointer references.

01What is a Function Pointer?

Simple Definition

A function pointer is a variable that stores the memory address of a function. Just like regular pointers store addresses of variables, function pointers store addresses of functions!

Visual: Regular Pointers vs Function Pointers

Regular Pointer

int *ptr

→

int x = 42

Points to a variable

Function Pointer

int (*fp)()

→

int add() {...}

Points to a function

Why Use Function Pointers?

Callbacks

Pass a function to another function to be called later. Used in event handlers, sorting, etc.

Dynamic Behavior

Choose which function to call at runtime. Like a menu: user picks option → call matching function.

State Machines

Each state has a different handler function. Current state pointer determines behavior.

Plugin Systems

Load functions at runtime. Extensible architecture without recompiling.

Real-World Example

The C standard library's qsort() function uses a function pointer to let YOU define how to compare elements. This makes it work with any data type - integers, strings, structs, anything!

02Function Pointer Syntax

Declaration Syntax

return_type (*pointer_name)(parameter_types);

The parentheses around *pointer_name are important!

function_pointer_basic.c

1#include <stdio.h>
2
3// A simple function
4int add(int a, int b) {
5    return a + b;
6}
7
8int subtract(int a, int b) {
9    return a - b;
10}
11
12int main() {
13    // Declare a function pointer
14    // It can point to any function that:
15    // - Returns int
16    // - Takes two int parameters
17    int (*operation)(int, int);
18    
19    // Point to 'add' function
20    operation = add;  // or: operation = &add;
21    
22    // Call through pointer
23    int result = operation(10, 5);
24    printf("add(10, 5) = %d\n", result);
25    
26    // Point to 'subtract' function
27    operation = subtract;
28    
29    result = operation(10, 5);
30    printf("subtract(10, 5) = %d\n", result);
31    
32    return 0;
33}

Output

add(10, 5) = 15

subtract(10, 5) = 5

03Callback Functions

What is a Callback?

A callback is a function passed to another function as an argument. The receiving function can then "call back" to it!

callback.c

1#include <stdio.h>
2
3// Callback function type for processing array elements
4void applyToArray(int* arr, int size, int (*callback)(int)) {
5    for (int i = 0; i < size; i++) {
6        arr[i] = callback(arr[i]);  // Apply callback to each element
7    }
8}
9
10// Functions to use as callbacks
11int doubleValue(int x) {
12    return x * 2;
13}
14
15int square(int x) {
16    return x * x;
17}
18
19int addTen(int x) {
20    return x + 10;
21}
22
23void printArray(int* arr, int size) {
24    for (int i = 0; i < size; i++) {
25        printf("%d ", arr[i]);
26    }
27    printf("\n");
28}
29
30int main() {
31    int numbers[] = {1, 2, 3, 4, 5};
32    int size = 5;
33    
34    printf("Original: ");
35    printArray(numbers, size);
36    
37    // Apply 'double' callback
38    applyToArray(numbers, size, doubleValue);
39    printf("Doubled:  ");
40    printArray(numbers, size);
41    
42    // Reset and apply 'square'
43    int nums2[] = {1, 2, 3, 4, 5};
44    applyToArray(nums2, size, square);
45    printf("Squared:  ");
46    printArray(nums2, size);
47    
48    // Reset and apply 'addTen'
49    int nums3[] = {1, 2, 3, 4, 5};
50    applyToArray(nums3, size, addTen);
51    printf("Add 10:   ");
52    printArray(nums3, size);
53    
54    return 0;
55}

Output

Original: 1 2 3 4 5

Doubled: 2 4 6 8 10

Squared: 1 4 9 16 25

Add 10: 11 12 13 14 15

04Array of Function Pointers

function_pointer_array.c

1#include <stdio.h>
2
3int add(int a, int b) { return a + b; }
4int subtract(int a, int b) { return a - b; }
5int multiply(int a, int b) { return a * b; }
6int divide(int a, int b) { return b != 0 ? a / b : 0; }
7
8int main() {
9    // Array of function pointers
10    int (*operations[4])(int, int) = {add, subtract, multiply, divide};
11    char* names[] = {"Add", "Subtract", "Multiply", "Divide"};
12    
13    int a = 20, b = 5;
14    
15    printf("Calculator Demo (a=%d, b=%d):\n\n", a, b);
16    
17    for (int i = 0; i < 4; i++) {
18        int result = operations[i](a, b);  // Call through array
19        printf("%s: %d\n", names[i], result);
20    }
21    
22    // Menu-driven selection
23    printf("\nEnter operation (0=Add, 1=Sub, 2=Mul, 3=Div): ");
24    int choice = 0;  // Simulating user input
25    printf("%d\n", choice);
26    
27    printf("Result: %d\n", operations[choice](a, b));
28    
29    return 0;
30}

Output

Calculator Demo (a=20, b=5):

Add: 25

Subtract: 15

Multiply: 100

Divide: 4

05Cleaner Syntax with typedef

Function pointer syntax is complex. Use typedef to make it cleaner!

Without typedef (hard to read):

int (*operation)(int, int);

With typedef (clean):

Operation op;

typedef_function_pointer.c

1#include <stdio.h>
2
3// Define a type for our function pointer
4typedef int (*Operation)(int, int);
5
6int add(int a, int b) { return a + b; }
7int multiply(int a, int b) { return a * b; }
8
9// Now function parameters are clean!
10int calculate(int a, int b, Operation op) {
11    return op(a, b);
12}
13
14int main() {
15    // Clean variable declaration
16    Operation myOp = add;
17    printf("add: %d\n", myOp(10, 5));
18    
19    myOp = multiply;
20    printf("multiply: %d\n", myOp(10, 5));
21    
22    // Pass to function easily
23    printf("calculate with add: %d\n", calculate(10, 5, add));
24    printf("calculate with multiply: %d\n", calculate(10, 5, multiply));
25    
26    return 0;
27}

Output

add: 15

multiply: 50

calculate with add: 15

calculate with multiply: 50

06Real Example: qsort() from Standard Library

The Power of Function Pointers

The C standard library's qsort() function is a perfect example of why function pointers are powerful. It can sort any type of data because YOU provide the comparison function!

qsort() Signature

void qsort(

void *base, // Array to sort

size_t num, // Number of elements

size_t size, // Size of each element

int (*compare)(const void*, const void*) // Your comparison function!

);

The last parameter is a function pointer — you decide how elements are compared!

qsort_example.c

1#include <stdio.h>
2#include <stdlib.h>
3#include <string.h>
4
5// Comparison function for ascending integers
6int compareAsc(const void* a, const void* b) {
7    return (*(int*)a - *(int*)b);
8}
9
10// Comparison function for descending integers
11int compareDesc(const void* a, const void* b) {
12    return (*(int*)b - *(int*)a);
13}
14
15// Comparison function for strings
16int compareStrings(const void* a, const void* b) {
17    return strcmp(*(char**)a, *(char**)b);
18}
19
20void printArray(int arr[], int n) {
21    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
22    printf("\n");
23}
24
25int main() {
26    // Sort integers ascending
27    int nums[] = {5, 2, 8, 1, 9, 3};
28    int n = sizeof(nums) / sizeof(nums[0]);
29    
30    printf("Original: ");
31    printArray(nums, n);
32    
33    qsort(nums, n, sizeof(int), compareAsc);
34    printf("Ascending: ");
35    printArray(nums, n);
36    
37    qsort(nums, n, sizeof(int), compareDesc);
38    printf("Descending: ");
39    printArray(nums, n);
40    
41    // Sort strings!
42    char* names[] = {"Charlie", "Alice", "Bob", "Diana"};
43    int count = 4;
44    
45    printf("\nNames before: ");
46    for (int i = 0; i < count; i++) printf("%s ", names[i]);
47    printf("\n");
48    
49    qsort(names, count, sizeof(char*), compareStrings);
50    
51    printf("Names after: ");
52    for (int i = 0; i < count; i++) printf("%s ", names[i]);
53    printf("\n");
54    
55    return 0;
56}

Output

Original: 5 2 8 1 9 3

Ascending: 1 2 3 5 8 9

Descending: 9 8 5 3 2 1

Names before: Charlie Alice Bob Diana

Names after: Alice Bob Charlie Diana

!Code Pitfalls: Common Mistakes & What to Watch For

These are the most common mistakes that trip up beginners. Study them carefully to avoid hours of debugging!

Missing Parentheses Around *

The parentheses around *pointer_name are required!

WRONG:

int *func(int, int);

→ Function returning int*

CORRECT:

int (*func)(int, int);

→ Pointer to function returning int

Calling NULL Function Pointer

Always check if a function pointer is NULL before calling it!

WRONG (crashes!):

int (*fp)(int) = NULL;
fp(5);  // CRASH!

CORRECT:

int (*fp)(int) = NULL;
if (fp != NULL) {
    fp(5);
}

Mismatched Signatures

The function pointer type must exactly match the function signature!

WRONG:

int add(int a, int b);
float (*fp)(int, int); // Wrong return type!
fp = add;  // Type mismatch

CORRECT:

int add(int a, int b);
int (*fp)(int, int); // Matches!
fp = add;  // OK

Watch Out When Copying Code!

Common Mistakes with Function Pointers

Copying code frequently generate broken function pointer code:

✗Wrong parentheses: Copied code int *fp(int) instead of int (*fp)(int)
✗Signature mismatches: Beginners often assign functions with wrong return types or parameters
✗Missing NULL checks: Beginners often dereference function pointers that could be NULL
✗Incorrect qsort comparators: Beginners often forget to cast void* or returns wrong comparison values

Always Understand Before Using

09Frequently Asked Questions

Q:Why is the syntax for function pointers so complicated?

Q:What is a callback function?

A: A function passed to another function to be called later. Example: qsort takes a comparison callback — you tell it HOW to compare, it handles the sorting. Callbacks allow generic, reusable code.

Q:Can I have arrays of function pointers?

A: Yes! Great for dispatch tables (menu systems, state machines). int (*ops[])(int,int) ={add, sub, mul} then ops[choice](a, b). Much cleaner than long switch statements.

Q:What's the difference between fp and &fp for functions?

A: For functions, both are equivalent! A function name automatically converts to a pointer to itself (like arrays).fp = add andfp = &add do the same thing.

Q:Where are function pointers used in real code?

09Summary

Key Takeaways

•Function pointer: Variable storing function address
•Syntax: return_type (*name)(params)
•Callbacks: Pass functions as arguments

•typedef: Makes syntax much cleaner
•Arrays: Store multiple function pointers
•qsort(): Classic real-world example

Quick Reference

Declare

int (*fp)(int, int);

Assign

fp = add;

Call

result = fp(5, 3);

typedef

typedef int (*Op)(int, int);

10Real-World Use Cases

Callback Functions

Event handlers, timers, and async operations use function pointers as callbacks. You register your function, and the library calls it when the event occurs.

Plugin Systems

Load shared libraries at runtime with dlsym() and store the function addresses in function pointers. This enables extensible applications.

What You Will Learn

01What is a Function Pointer?

Simple Definition

Visual: Regular Pointers vs Function Pointers

Regular Pointer

Function Pointer

Why Use Function Pointers?

Real-World Example

02Function Pointer Syntax

Declaration Syntax

03Callback Functions

What is a Callback?

04Array of Function Pointers

05Cleaner Syntax with typedef

06Real Example: qsort() from Standard Library

The Power of Function Pointers

qsort() Signature

!Code Pitfalls: Common Mistakes & What to Watch For

Missing Parentheses Around *

Calling NULL Function Pointer

Mismatched Signatures

Watch Out When Copying Code!

Common Mistakes with Function Pointers

Always Understand Before Using

09Frequently Asked Questions

Q:Why is the syntax for function pointers so complicated?

Q:What is a callback function?

Q:Can I have arrays of function pointers?

Q:What's the difference between fp and &fp for functions?

Q:Where are function pointers used in real code?

09Summary

Key Takeaways

Quick Reference

10Real-World Use Cases

Callback Functions

Plugin Systems

State Machines

Test Your Knowledge

Related Tutorials

C Preprocessor Directives

Bitmasks and Bit Manipulation

Pointers in C

Have Feedback?

What You Will Learn

01What is a Function Pointer?

Simple Definition

Visual: Regular Pointers vs Function Pointers

Regular Pointer

Function Pointer

Why Use Function Pointers?

Real-World Example

02Function Pointer Syntax

Declaration Syntax

03Callback Functions

What is a Callback?

04Array of Function Pointers

05Cleaner Syntax with typedef

06Real Example: qsort() from Standard Library

The Power of Function Pointers

qsort() Signature

!Code Pitfalls: Common Mistakes & What to Watch For

Missing Parentheses Around *

Calling NULL Function Pointer

Mismatched Signatures

Watch Out When Copying Code!

Common Mistakes with Function Pointers

Always Understand Before Using

09Frequently Asked Questions

Q:Why is the syntax for function pointers so complicated?

Q:What is a callback function?

Q:Can I have arrays of function pointers?

Q:What's the difference between fp and &fp for functions?

Q:Where are function pointers used in real code?

09Summary

Key Takeaways

Quick Reference

10Real-World Use Cases

Callback Functions

Plugin Systems

State Machines