Macro functions

Learn to write parameterized macro functions.

Macro functions

A #define directive can also take parameters; this is called a macro function. It is expanded as text at the call site before compilation.

#define SQUARE(x) ((x) * (x))

int r = SQUARE(5);  /* ((5) * (5)) = 25 */

Each parameter should be wrapped in parentheses; otherwise precedence issues arise. For example SQUARE(a+b) expands to ((a+b) * (a+b)). Macro functions are text substitution before compilation; they do not perform type checking; use them carefully.

The importance of parentheses

In macro functions each parameter should be wrapped in parentheses. Otherwise precedence errors occur.

#define BAD(x)  x * x
#define GOOD(x) ((x) * (x))

BAD(2+3)   /* 2+3*2+3 = 11 */
GOOD(2+3)  /* ((2+3) * (2+3)) = 25 */

The BAD macro does not parenthesize its parameters, so it expands to 2+3*2+3; this gives a wrong result. GOOD gives the expected 25 thanks to proper parenthesization. So always use parentheses when writing macros.

Exercise Compute square with the SQUARE macro

Define the macro #define SQUARE(x) ((x) * (x)). In main print the value of SQUARE(5) with printf using the %d format.

Show example solution

#define SQUARE(x) ((x) * (x))
printf("%d\n", SQUARE(5));

Test yourself

What is the main difference between a macro function and a real function?
- A macro is text substitution before compilation
- A macro is safer
- A macro has no type checking
- A macro is slower
Show answer

A macro is text substitution before compilation — A macro is not a function; it is text substitution before compilation.
Match macro to behavior

#define PI 3.14

Constant
#define SQUARE(x)

Macro function
((x) * (x))

Parenthesized body
#include

Includes a header

Fill in the blank: macro function body

#define SQUARE(x) ____

Show answer

#define SQUARE(x) ((x) * (x))

Next lesson: Header files

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