inline
- C course
- Advanced
- Lesson 76/83
- 5 min
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.
Practice 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));.
Starter code
#include <stdio.h>
// Write inline int square_inline(int x) returning x*x
// Define the SQUARE macro: ((x) * (x))
// main: print the square of 5 both ways
int main(void) {
return 0;
}Sample 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;
}Expected output
inline: 25 macro: 25
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is the main difference between macro and inline?
- Macros are slower
- inline lacks type safety
- They are identical
- Macros double side effects Correct answer
Explanation: Macros are text substitution and evaluate side effects twice.
2 Match inline and macro concepts
| Term | Meaning |
|---|---|
inline |
Type safe |
#define |
Text substitution |
call overhead |
Removed by inline |
side effect |
Doubled in macro |
3 square_inline signature
inline int square_inline(____) { return x * x; }inline int square_inline(int x) { return x * x; }Explanation: The int x parameter carries the value to square.