Macros in C

In the C programming language, the macro is defined as a segment of the code replaced by the macro defined value at the beginning. ‘#define’ is the directive that defines the macro. A macro is used to define the constant values that are used constantly and repeatedly within the same program.

Macro is helpful if the tokens present are concatenated within the code in order to simplify more complex declarations. Macros provide the text replacement functionality at the time of pre-processing the program.

Syntax:

#define SIZE 10

In the above defined syntax, the macro of SIZE has the value of 10.

E.g.:

#include<stdio.h>
#include<conio.h>
#define SIZE 10
int main(void)
{
int size = 0;
size = size + SIZE;
printf (“The value of the size is %d \n”, size);
return 0;
}

Output:

The value of the size is 10.

Here, the macro SIZE was replaced by the value 10 in the stage of pre-processing the program by the compiler. Macros are handled by the compiler or the pre-compiler and are inlined. Macros are used for operations that are short and which avoid function call overhead. Hence it can be used if any short operation will be done in the program repeatedly.

There are two types of macros in the C standard. They are;

1. The object like macros: An object like macro is an identifier where the value will be replaced. This type of macro is widely used in order to represent the numeric constants such as

#define PI 3.14 

The value of the PI is the macro’s name that will be replaced by the value 3.14.

2. Function like macros: This type of macro looks like a function call present in the C standard such as;

#define MAX (a, b) ((a) < (b) ? (a):(b))

Where the MAX is the name of the macro function, and it further checks for the maximum number present within the variable. Function like macros is beneficial whenever the same code block should be executed numerous times since it avoids rewriting code multiple times.

Pre-defined macros in C

ANSI C defines several pre-defined macros that are used in the C programming language, such as;

1. _DATE_: As the name itself suggests, this macro represents the current date in the “MMM DD YYY” format.
2. _TIME_: It represents the current time in the “HH:MM:SS” format.
3. _FILE_: It indicates the current file name.
4. _LINE_: It represents the current line number.
5. _STDC_: It is represented as one when the compiler complies with the ANSI standard.

E.g.:

#include<stdio.h>  
#include<conio.h>


 int main()
{    
printf("File :%s\n", __FILE__ );    
printf("Date :%s\n", __DATE__ );    
printf("Time :%s\n", __TIME__ );    
printf("Line :%d\n", __LINE__ );    
printf("STDC :%d\n", __STDC__ );      
return 0;  
}

Output:

File :/tmp/2PPBdgfZRz.c
Date :Jun  5 2021
Time :11:02:31
Line :6
STDC :1

Multi-line macro:

Multi-line macro is same like function, but each statement ends with “\”.

Let us see an example.

Below is a simple macro, which accepts input numbers from users, and prints whether the entered number is even or odd.

E.g.:

#include <stdio.h>


#define MACRO(num, str) do 
{\
printf("%d", num);\
	printf(" is");\
	printf(" %s number", str);\
	printf("\n");\
} while(0)


int main(void)
{
	int num;


	printf("Enter a number: ");
	scanf("%d", &num);


	if (num & 1)
		MACRO(num, "Odd");
	else
		MACRO(num, "Even");


	return 0;
}

Output:

Enter a number: 2
is Even number
Enter a number: 3
is Odd number

Conditional Macros in C

Conditional macros are very helpful when it comes to applying conditions. Code snippets are guarded with a condition checking if a particular macro is defined or not. They are accommodating in large projects having code segregated as per releases of the project.

Syntax:

#ifdef MACRO
controlled text
#endif /* MACRO */

E.g.:

#include <stdio.h>


int main() 
{
#if 0
printf("commented code 1");
printf("commented code 2");
#endif


#define TEST1 1


#ifdef TEST1
printf("MACRO TEST1 is defined\n");
#endif


#ifdef TEST3
printf("MACRO TEST3 is defined\n");
#else
printf("MACRO TEST3 is NOT defined\n");
#endif


return 0;
}

Output:

MACRO TEST1 is defined
MACRO TEST3 is NOT define.