In this tutorial, you will learn the C Macros with programming examples. We will discuss here the types of C macro (object and function). You will also learn to use #define with some predefined C Macros with the help of programming examples. So let’s get started.

In C programming macro is a segment of the code that has some unique name. Whenever in a program we used the macro name, it is replaced by the contents of the macro.

Consider the below example,

#define ATICLEWORLD  24

In my code where I will use “ATICLEWORLD”, it is replaced with 24.

These replacements occur before the compilation of the code which is the reason there is no type checking occurring with C macros. Basically, there are two types of macros, object-like macros, and function-like macros.

Object-like Macros in C

An object-like macro is a simple identifier that will be replaced by a code segment (a piece of code) in the program. It is mainly used to give a symbolic name to a numeric constant and it is a good habit to give a meaningful name to a numeric constant.

Like if you are creating a macro to represent the second, you should write the macro name SECOND. It increases the code readability.

To define a new macro in C, We can use the #define. A #define is a pre-processor directive.

Syntax of the object-like macro,

#define  MACRO_NAME  MACRO_VALUE

For Example,

#define ARRAY_SIZE 1024

Now ARRAY_SIZE is an alias of 1024, whenever in the program we will use ARRAY_SIZE it replaces with 1024.

//Declaration of array

int arr[ARRAY_SIZE];

Before the compilation of the code, the C preprocessor will replace the ARRAY_SIZE with 1024 and the array will look like the below expression,

int arr[1024];

Note: Macro definitions need not be terminated by a semi-colon(;)

The following program illustrates the use of macros in C/C++:

#include <stdio.h>

/*
  You can see semicolon (;)
  is not require.
*/
#define PI_VALUE 3.1415

int main()
{
    float radius, circleArea;
    printf("Enter the radius: ");
    scanf("%f", &radius);

    //We are using PI_VALUE
    circleArea = PI_VALUE*radius*radius;

    printf("Area=%.2f",circleArea);

    return 0;
}

Output:

Enter the radius: 3
Area=28.27

An object-like macro could have a multi-line. So to create a multi-line macro you have to use backslash-newline. See the below example,

#include <stdio.h> 


#define LIST    10, \
                20, \
                30 				

int main() 
{ 
    //Initialized array
    int arr[] = { LIST };
    
    printf("arr[0] = %d\n",arr[0]);
    printf("arr[1] = %d\n",arr[1]);
    printf("arr[2] = %d\n",arr[2]);
    
    return 0; 
}

OutPut:

arr[0] = 10
arr[1] = 20
arr[2] = 30

Function like Macro in C:

A function-like macro looks like the function is because it has a pair of parentheses like the function. Similar to the object like macro #define use to create a function-like macro. You need to put a pair of parentheses immediately after the macro name.

For example,

#define MAX(A, B) (((A) > (B)) ? (A) : (B))

Let’s see an example code,

#include <stdio.h>
 
#define MAX(A, B)  (((A) > (B)) ? (A) : (B))


int main()
{
    int a = 10;
    int b = 20;
    
    //check max value
    int ret = MAX(a,b);
 
    printf("Max value %d\n", ret);
    
    return 0;
}

Output:

Max value 20

Note: You can See, Token Pasting Operator

You should remember the following points before using the function-like a macro in c.

• Always use proper parentheses in macro.
• Use each macro parameter once to avoid the undesired side effect.
• You should never use a function like macro if you can use the function.
• Always remember there is no type checking occur in macro.
• You should remember the following points before using the function-like a macro in c

We can also create a function as a macro in multi-line, See the below example in which I am swapping the value of two variables.

#include <stdio.h>

#define swap(x,y,T) do { \
    T temp = (*x);\
    (*x) = (*y); \
    (*y) = temp; \
} while (0)


int main(void)
{
    int a = 5;
    int b = 9;

    printf("Value of a and b before swaping\n");
    printf("a = %d\n",a);
    printf("b = %d\n",b);

    //Swap the number
    swap(&a,&b,int);


    printf("\n\nValue of a and b After swaping\n");
    printf("a = %d\n",a);
    printf("b = %d\n",b);

    return 0;
}

Output:

Some C Predefined Macros:

The following is the list of some predefined Macros which you should know.

Macro    Description

_DATE_	 current date in "MM DD YYYY" format.

_TIME_	 current time in "HH:MM:SS" format.

_FILE_	 current file name.

_LINE_	 current line number.

See the example code,

#include<stdio.h>

int main()
{
    printf("File Name :%s\n", __FILE__ );

    printf("Current Date(MMM DD YYYY)  :%s\n", __DATE__ );

    printf("Current Time(HH:MM:SS)  :%s\n", __TIME__ );

    printf("Line Number  :%d\n", __LINE__ );

    return 0;
}

Output:

File Name :pointermain.c
Current Date(MM DD YYYY) :Jan 1 2019
Current Time(HH:MM:SS) :14:34:08
Line Number :11

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