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Macro functions

  • C course
  • Beginner
  • Lesson 109/116
  • 5 min

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.

Practice 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.

Starter code

main.c
#include <stdio.h>

// Define the SQUARE(x) macro
int main(void) {
    // Print the value of SQUARE(5) with printf
    return 0;
}

Sample solution

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

Expected output

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 a macro function and a real function?

  • A macro is text substitution before compilation Correct answer
  • A macro is safer
  • A macro has no type checking
  • A macro is slower

Explanation: A macro is not a function; it is text substitution before compilation.

2 Match macro to behavior

Term Meaning
#define PI 3.14 Constant
#define SQUARE(x) Macro function
((x) * (x)) Parenthesized body
#include Includes a header

3 Fill in the blank: macro function body

Question 
#define SQUARE(x) ____
Answer 
#define SQUARE(x) ((x) * (x))

Explanation: ((x) * (x)) parenthesizes each parameter; expands correctly.

All lessons in Beginner

1 C basics

  1. What is C?
  2. Your first program
  3. Compile and run
  4. Comments
  5. printf and escapes
  6. Summary - C basics

2 Variables

  1. Declaring variables
  2. Variable assignment
  3. const
  4. Variable scope
  5. typedef
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. char
  4. sizeof
  5. Type conversion
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Menu loop
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Bitwise operators
  6. Summary - Operators

6 I/O

  1. printf formatting
  2. scanf
  3. Multiple inputs
  4. getchar and putchar
  5. puts and fputs
  6. Summary - I/O

7 Conditionals

  1. The if statement
  2. else and else if
  3. The switch statement
  4. The ternary operator
  5. Nested conditionals
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. break and continue
  5. Nested loops
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding products
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Declaring arrays
  2. Array access
  3. Array size
  4. Multidimensional arrays
  5. Arrays and loops
  6. Summary - Arrays

11 Strings

  1. String basics
  2. String literals
  3. strlen
  4. strcpy and strcat
  5. strcmp
  6. Summary - Strings

12 Functions

  1. Defining functions
  2. Return values
  3. Parameters
  4. Function prototypes
  5. Recursion
  6. Summary - Functions

13 Pointer basics

  1. What is a pointer?
  2. The & address operator
  3. Pointer dereference
  4. NULL
  5. Pointer arithmetic
  6. Summary - Pointer basics

14 Arrays and pointers

  1. Array name as pointer
  2. Pointer to array
  3. Passing arrays to functions
  4. Pointer loop
  5. Multidim array pointer
  6. Summary - Arrays and pointers

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 String functions

  1. strchr
  2. strstr
  3. strtok
  4. sprintf
  5. sscanf
  6. Summary - String functions

17 Structs

  1. Defining structs
  2. Using structs
  3. typedef struct
  4. Array of structs
  5. Nested structs
  6. Summary - Structs

18 Memory and stack

  1. Stack and heap
  2. Automatic storage
  3. malloc
  4. free
  5. calloc and realloc
  6. Summary - Memory and stack

19 Preprocessor

  1. #define
  2. #include
  3. Conditional compilation
  4. Macro functions
  5. Header files
  6. Summary - Preprocessor

20 Beginner final

  1. Project design
  2. Data model
  3. Functions
  4. Memory management
  5. Putting it together