C Inline Functions

Inline Function

You might sometimes see the inline keyword used in other people's functions. It's not something you need to use often as a beginner, but it's good to know what it means.

An inline function is a small function that asks the compiler to insert its code directly where it is called, instead of jumping to it.

This can make short, frequently used functions a little faster, because it removes the small delay of a normal function call.

Let's compare a regular function with an inline function:

Regular Function

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

Inline Function

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

Both functions work the same way. The only difference is that the inline version suggests to the compiler to copy the function's code directly where it is used.

Inline Function Example

Here is a simple example that uses an inline function:

Example

#include <stdio.h>

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

int main() {
  printf("%d", add(5, 3));
  return 0;
}

Here, the compiler might replace add(5, 3) with the code 5 + 3 directly - saving the time it would take to call a separate function.

When Not to Use Inline

Inline functions are best for small, simple functions. Avoid using them for:

• Large functions (they make your program bigger)
• Recursive functions
• Functions that are rarely called

Too many inline functions can make your program slower and larger, a problem known as code bloat.

Inline vs Regular Function

In this tutorial, we will continue to use regular functions.

Summary

• Inline functions are defined with the inline keyword.
• They can make small, often-used functions faster.
• The compiler decides whether to inline a function or not.
• Mostly used for short, simple functions.