Inline Functions (C99)

What Are Inline Functions?

An inline function is a function where the compiler attempts to expand the function body at the point of call, instead of performing a normal function call.

Syntax:

inline return_type function_name(parameters) {
    // code
}

• Introduced in C99 standard
• Improves speed by avoiding function-call overhead
• Compiler may ignore inline request
• Best suited for short, frequently used functions

How Inline Works Internally?

Normally, function calls require:

• Pushing parameters to stack
• Jumping to function address
• Returning back to caller

Inline avoids this by directly inserting function code into the calling point.

When Inline Is Not Applied

• Functions with loops
• Functions with recursion
• Functions too large in size
• Functions using static variables
• Variadic functions (with ...)

Inline works best for **small mathematical functions, getters, setters**, etc.

Advantages of Inline Functions

• Faster execution (no function call overhead)
• No stack push/pop operations
• Improves optimization for small functions

Disadvantages

• Increases code size (code duplication)
• May lead to slow performance if overused
• Compiler can reject inline request

Example: Simple Inline Function

#include <stdio.h>

inline int square(int n){
    return n * n;
}

int main(){
    printf("Square: %d", square(6));
    return 0;
}

Example: Inline Function for Addition

#include <stdio.h>

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

int main(){
    printf("Sum: %d", add(10, 20));
    return 0;
}

Inline Function vs Macro

• Inline is type-safe; macros are not
• Inline functions are checked by compiler
• Macros only perform text replacement

Macro Example (Unsafe)

#define SQUARE(x) (x*x)

Inline Example (Safe)

inline int square(int x){ return x*x; }

Real Life Use Cases

• Math shortcut functions (min, max, square)
• Small utility functions
• Performance-critical embedded systems
• Functions used inside loops

Practice Questions

1. What is an inline function?
2. Difference between inline and macro?
3. Why compiler may ignore inline request?
4. Write an inline function for cube of a number.
5. Where are inline functions best used?

Practice Task