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:
1 | #define macro_name macro_body |
In the syntax above:
- You use the
#definedirective 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:
1 | #define MAX_SIZE 1000 |
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:
1 | int list[MAX_SIZE]; |
The C preprocessor will replace the MAX_SIZE by 1000 as follows:
1 | int list[1000]; |
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:
1 2 3 4 | #define RGB_COLORS red, \ green,\ blue enum RGB{ RGB_COLORS }; |
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:
1 2 3 | int SIZE = 10; #define SIZE 100 int x = SIZE |
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:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | /* * File: main.c * Author: zentut.com * 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; } |
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.