C Macro

Summary: in this tutorial, you will learn about C macro. We will show you two different kinds of macros in C including object-like macros and function-like macros.

What is a C macro

A C macro is a piece of code that has a specific name called macro name. Whenever the macro name is used, C preprocessor will replace it with the body of the macro. C allows you to define a macro for any valid identifier, even C keyword.

C provides you with two kinds of macros: object-like macros and function-like macros.

Let’s examine each type of macro and context where you are going to use it.

Object-like macros

An object-like macro is an identifier that will be replaced by a sequence of token ( or piece of code) in the program. As its name implied, the object-like macro is similar to data object in code in term of its usage. You typically use an object-like macro to give a meaningful name to a constant.

To define a new macro in C, you use the #define directive. The syntax of the object-like macro is as follows:

#define macro_name macro_body
Code language: C++ (cpp)

In the syntax above:

• You use the #define directive followed by the macro name ( macro_name) and macro body ( macro_body).
• Macro body is a token sequence that macro name stands for. The macro body is also known as replacement list or expansion.
• No semicolon ( ;) is used at the end of the statement.

The following example creates a new object-like macro:

#define MAX_SIZE 1000
Code language: C++ (cpp)

The macro name is MAX_SIZE and it is abbreviated for the constant 1000. From now on, you can use this macro “like” an identifier in code such as:

int list[MAX_SIZE];
Code language: C++ (cpp)

The C preprocessor will replace the MAX_SIZE by 1000 as follows:

int list[1000];
Code language: C++ (cpp)

By convention, you put the macro name in uppercase to make your code is easier to read and maintain.

You can define a macro onto multiple lines using backslash-newline. C preprocessor will treat it as one line when the macro is expanded, for example:

#define RGB_COLORS red, \ 
 green,\ 
 blue
enum RGB{ RGB_COLORS };
Code language: C++ (cpp)

Another important point you should keep in mind that C preprocessor processes program sequentially. Therefore, the macro is effective from the position where it is defined. Consider the following example:

int SIZE = 10;
#define SIZE 100
int x = SIZE
Code language: C++ (cpp)

The value of x is 100.

Function-like Marcos

C also allows you to define a macro that look like a function call. Therefore, this macro is referred as a function-like macro.

The syntax of creating a function-like macro is similar to object-like macro except you have to put the parentheses () right after the macro name.

The following example demonstrates how to use function-like macros:

/*
 * File:   main.c
 * Author: learnc.net
 * C Macro Demo
 */
#include <stdio.h>
#include <stdlib.h>

#define min(a, b)  ((a) < (b) ? (a) : (b))
int main()
{
    int x = 20;
    int y = 30;
    int result = min(x,y);

    printf("Min of %d and %d is %d\n",x,y,result);
    return 0;
}
Code language: C++ (cpp)

In the code, the preprocessor only replaces function-like macro when the macro name appears with the parentheses () right after it.

In this tutorial, you have learned how to create two different kinds of macros in C: object-like macro and function-like macro. It is important to use C macro properly to increase your code readability.