DAY 14 Function Pointers, typedef, Double Pointers, and const Pointer Usage

Advanced C Programming: Function Pointers, typedef, Double Pointers, and const Pointer Usage

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1. Function Pointers

What is a function pointer?

A function pointer is a pointer that stores the address of a function.

A function name in C is itself a pointer:

int add(int a, int b);

The type of this function is:

int (*)(int, int)

Therefore, we define a function pointer as:

int (*op)(int, int) = NULL;

This means op is a pointer to a function that:

• returns int
• takes two int parameters

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1.1 Example: Select Addition or Subtraction Dynamically

#include <stdio.h>

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

int sub(int a,int b)
{
    return a-b;
}

int main()
{
    int (*op)(int ,int)=NULL;
    int a=10, b=20, num=0;

    printf("1:+, 2:-\ninput num:");
    scanf("%d",&num);

    switch(num)
    {
        case 1: op = add; break;
        case 2: op = sub; break;
        default: op = add;
    }

    int ret = op(a,b);   // Calling the function via pointer
    printf("op result = %d\n", ret);

    return 0;
}

This is a typical use case of callback functions, where the actual operation is determined at runtime.

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2. Callback Functions

A callback function is a function that is passed as an argument and invoked inside another function.

Why use callbacks?

✔ Reduce coupling
✔ Increase flexibility
✔ Enable runtime decision-making

Example: in sorting, you can pass a comparison function such as:

lessthen() → ascending  
greathen() → descending

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3. typedef — Creating Type Aliases

Using typedef makes complex pointer types more readable.

Basic usage:

typedef unsigned int u32;
typedef int s32;

Function pointer alias:

typedef int (*PFUN)(int,int);

PFUN op1 = NULL;  // Same as: int (*op1)(int,int) = NULL;

This greatly reduces complexity in large codebases.

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4. Double Pointers (pointer to pointer)

What is a double pointer?

A double pointer stores the address of another pointer:

char** → pointer to char*

Why use double pointers?

Because only a double pointer can modify another pointer inside a function.

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Example: Change a pointer’s value inside a function

#include <stdio.h>

void fun(char** arg_p)
{
    *arg_p = "ok";
}

int main()
{
    char *p = "hello";
    fun(&p);
    printf("p is %s\n", p);   // p changed to "ok"
}

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5. const and Pointers — Four Combinations (Important)

Form	Meaning
const char *p	Content cannot be modified
char const *p	Same as above
char * const p	Pointer cannot change
const char * const p	Both pointer and content cannot change

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Common Mistake: Modifying string literals (dangerous!)

char *p = "hello";
p[0] = 'a'; // ❌ runtime error

This attempts to modify read-only memory.

Correct version:

static char a[50] = "hello";
a[0] = 'a';   // ✔ Works, stored in writable memory

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6. Common Mistakes with String Pointers

Incorrect example (modifying constant memory)

char *fun2()
{
    char *p="hello";
    p[0]='a';  // ❌ error
    return p;
}

✔ Correct example (using static array)

char *fun()
{
    static char a[50]="hello";
    a[0]='a';
    return a;
}

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7. Implementing strcpy with Pointer Operations

char *mystrcpy(char *dst,char *src)
{
    char *tmp = dst;

    while (*src)
    {
        *dst = *src;
        dst++;
        src++;
    }

    *dst = '\0';
    return tmp;
}

This manually implements the logic of strcpy().

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8. Sorting Array Using Function Pointers (Callback)

Comparison functions:

int lessthen(int a,int b) { return a < b; }
int greathen(int a,int b) { return a > b; }

Sorting function that accepts callback:

void sort_array(int *a, int len, PFUN fun)
{
    int i=0, j=0;

    for(j = len-1; j > 0; j--)
    {
        if(fun(a[0], a[1]))
        {
            int t = a[0];
            a[0] = a[1];
            a[1] = t;
        }
    }
}

Usage:

PFUN op1 = lessthen;
sort_array(a, 10, op1);

Switching comparison function changes the sorting order.

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9. Helper Functions: Fill and Display Arrays

void fill_array(s32 *a,u32 len)
{
    srand(time(NULL));
    for(int i=0;i<len;i++)
        a[i] = rand() % 50;
}

void show_array(int *a,int len)
{
    for(int i=0;i<len;i++)
        printf("%d ", a[i]);
    printf("\n");
}

Simple and clean pointer-based array helpers.