inline

Learn the inline keyword and macro difference.

inline

The inline keyword suggests the compiler replace a function call with the body. The call overhead disappears; small functions gain performance.

Example: inline int square_inline(int x) { return x * x; }. A macro has a similar effect: #define SQUARE(x) ((x) * (x)). The difference: inline offers type safety and evaluates the argument once; a macro is a text substitution and doubles side effects. Modern C prefers inline.

Square example

The square of 5 is 25. square_inline(5) and SQUARE(5) both return 25; the difference is at compile time.

Function definition: inline int square_inline(int x) { return x * x; }. Macro definition: #define SQUARE(x) ((x) * (x)). The parentheses in the macro are required; without them SQUARE(a+b) gives a wrong result. In main you write printf("inline: %d\n", square_inline(5)); and printf("macro: %d\n", SQUARE(5));. The output is 25 for both.

Exercise inline and macro square

Write inline int square_inline(int x) that returns x * x. Define the macro #define SQUARE(x) ((x) * (x)). In main print printf("inline: %d\n", square_inline(5)); and printf("macro: %d\n", SQUARE(5));.

Show example solution

#include <stdio.h>

inline int square_inline(int x) { return x * x; }
#define SQUARE(x) ((x) * (x))

int main(void) {
    printf("inline: %d\n", square_inline(5));
    printf("macro: %d\n", SQUARE(5));
    return 0;
}

Test yourself

What is the main difference between macro and inline?
- Macros are slower
- inline lacks type safety
- They are identical
- Macros double side effects
Show answer

Macros double side effects — Macros are text substitution and evaluate side effects twice.
Match inline and macro concepts

inline

Type safe
#define

Text substitution
call overhead

Removed by inline
side effect

Doubled in macro

square_inline signature

inline int square_inline(____) { return x * x; }

Show answer

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

Next lesson: Memory alignment

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