C – Preprocessor and Macros

Before learning the C macro, you should know about C preprocessor directives in C language. In this post, you can clearly understand the following

• what preprocess commands are available in C?
• How do you declare the preprocessor command.?
• what is a macro?
• What is the macro syntax?
• how to declare the macro?
• how to assign the macro value?
• how can use the macro function in c.

Here your answer, if you have any doubts please post in doubts about C Preprocessor and Macros.

In C language, totally 11 pre-processor commands are there,

Preprocessor Command	Description
#define	Substitutes a preprocessor macro.
#include	Inserts a particular header from another file.
#undef	Undefines a preprocessor macro.
#ifdef	Returns true if this macro is defined.
#ifndef	Returns true if this macro is not defined.
#if	Tests if a compile time condition is true.
#else	The alternative for #if.
#elif	else and #if in one statement.
#endif	Ends preprocessor conditional.
#error	Prints error message on stderr.
#pragma	Issues special commands to the compiler, using a standardized method.

The below table is the syntax, rule and compiling stage of preprocessor commands

Syntax	#include: For header file #include <stdio.h> #define: For macro #include MACRO_NAME or #include MACRO_NAME MACRO_VALUE
Rule	All the preprocessor command should start with "#" (hash) and followed by an input file or macro. Preprocessor declaration should be a non-blank character, If you do not mention the # for include files or macro then the compiler will give an error.
Preprocessor command in Compiling stage	#include: While compiling stage, it will remove the #Include and replace the header file PATH in the source code. #define: While compiling stage, also remove  #define (Macro)  with replacing macro name with code

What is MACRO in C?

Macros are the names of text/ literal values/ string (constant values) or code fragments, which will expand when the pre-processor processes the macro. The pre-processor processes the macros at compile-time; hence that macros replace with the corresponding code fragments.

A macro can execute faster than a function: As the code of macro gets expanded at the line of call, therefore macro gets executed faster with no overhead of context switch.

Syntax

• #define <MACRO_NAME> <MACRO_VALUE>
• Here #define denote user declaring a macro in c
• <MACRO_NAME> user can provide any char, String
• <MACRO_VALUE> user can provide any value to assign the value

Rules

• You should give the space in between #define and Macro Name.
• You should give the space in between Macro Name and Macro Value.
• No need to add the ; semicolon in end of the declaration as like variable value assignment.

Declaration

You should declare the macro with proper syntax like #define MACRO_NAME

Assign Macro Value

you can assign any values, Stings and athematic actions in macro

• Assign macro as string value: #define WEBSITE "www.ArunEworld.com" Here WEBSITE is the macro and www.arunEworld.com is the macro string value.
• Assign macro as char value: #define LETTER 'A' Here LETTER is the macro and 'A' is the macro char value.
• Assign macro as digits value: #define DIGIT 2, Here DIGIT is the macro and 2 is the macro digit value.

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C Preprocessor and Macros Examples

Here You can find many more examples related to C Preprocessor and Macros

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Understand of Macro while compilation time

#define MAX_NUM 15  

Referring to the sample above, what is MAX_NUM?

(A) MAX_NUM is a precompiler constant.

(B) MAX_NUM is a preprocessor macro.

(C) MAX_NUM is an integer variable.

(D) MAX_NUM is a linker constant.

Answer : MAX_NUM is a preprocessor macro.

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How to print the Macro value in C

While assign the macro value, user should know the value type in order to print the macro using printf() function

• If Macro is like #define WEBSITE "ArunEworld.com" then you can print the macro using printf("%s", WEBSITE). If you try to print as a printf("%d", WEBSITE) then compiler give the warning [warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘char *’ [-Wformat=]] but it will print the corresponding putput value 4195812 This 4195812 is belong to address of the "www.ArunEworld.com" string storing address in memory.
• If Macro is like #define VALUE 30 then you can print the macro using printf("%d", VALUE). If you try to print as a printf("%c", VALUE) then corresponding ASCII value will be print.

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Macro basic example-1

In this below code you can clearly understood what happening when we use the macro

Example-1: Actual Code:

#include <stdio.h>

#define OPEN_CURLY_BRACE    {
#define CLOSE_CURLY_BRACE   }
    
int main()
OPEN_CURLY_BRACE
    printf("ArunEworld");
    return 0;
CLOSE_CURLY_BRACE

Example-1:While Compiling the above code(Actual Code)

#include <stdio.h>

#define OPEN_CURLY_BRACE    {
#define CLOSE_CURLY_BRACE   }

int main()
{
    printf("ArunEworld");
    return 0;
}

Example-2:Actual Code

#include <stdio.h>

#define INT_TYPE int
INT_TYPE main()
{
    printf("ArunEworld");
    return 0;
}

Example-2: While compiling above code(Actual Code)

#include <stdio.h>

int main()
{
    printf("ArunEworld");
    return 0;
}

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Two time declare the same macro but different value assignment

If your declared the same macro two times but different value then warning [warning: “MACRO_NAME” redefined] should be generated by compiler and print the last declared macro value in console

Code

#include <stdio.h>

#define MACRO_NAME "ArunEworld"
#define MACRO_NAME "ArunEworld Macro re-defined example"

int main()
{
    printf("%s", MACRO_NAME);

    return 0;
}

Output:

main.c:12:0: warning: "MACRO_NAME" redefined
main.c:11:0: note: this is the location of the previous definition
ArunEworld Macro re-defined example   

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How you define Macro for how many seconds in Year?

• #define SECONDS_PER_YEAR (60 * 60 * 24 * 365)UL

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How you define Marco using find small number?

• Answer : #define MIN(x,y) ((x<y)?x:y)

Example

#include <stdio.h>
#define MIN(x,y) ((x<y)?x:y)

int main()
{
    int a,b,min;
    printf("Enter first number: ");
    scanf("%d",&a);
    printf("Enter second number: ");
    scanf("%d",&b); 
    
    min=MIN(a,b);
    printf("Minimum number is: %d\n",min);
    
    return 0;
}
/*
Enter first number: 100 
Enter second number: 200
Minimum number is: 100
*/

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Find out MIN & MAX using MACRO in C?

• Answer : #define MIN(X,Y) (X<Y ? X:Y)  or #define MAX(X,Y) (X>Y ? X:Y)

Example code:

#include <stdio.h>
#define MIN(x,y) ((x<y)?x:y)

int main()
{
    int a,b,min;
    printf("Enter first number: ");
    scanf("%d",&a);
    printf("Enter second number: ");
    scanf("%d",&b); 
    
    min=MIN(a,b);
    printf("Minimum number is: %d\n",min);
    
    return 0;
}

Output:

/*
Enter first number: 100 
Enter second number: 200
Minimum number is: 100
*/

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Write a macro to find the smallest number among 3 given numbers?

Answer :

Example Code :

#include<stdio.h>
#define MIN(X,Y)(X<Y ? X:Y)
#define SMALL(X,Y,Z)(MIN(X,Y)<Z ? MIN(X,Y):Z)
void main()
{
    int FIRST,SECOND,THIRD;
    clrscr();
    printf("ENTER NUMBERS TO COMPARE\n");
    printf("\nFIRST NUMBER : ");
    scanf("%d", &FIRST);
    printf("\nSECOND NUMBER : ");
    scanf("%d", &SECOND);
    printf("\nTHIRD NUMBER : ");
    scanf("%d", &THIRD);
    printf("\nThe SMALLER NUMBER IS : %d", SMALL(FIRST,SECOND,THIRD));
    getch();
}

Output

ENTER NUMBERS TO COMPARE 
FIRST NUMBER : 11
SECOND NUMBER : 22
THIRD NUMBER : 33
The SMALLER NUMBER IS : 11

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Swap Two Numbers using MACRO

• #define SWAP(x, y) (x ^= y ^= x ^= y)

MACRO Example of complete code

#include<stdio.h>
#define S printf("ArunEworld");
#define AEW(S) int main()\
             {\
                S\
                return 0;\
             }
AEW(S)

/* Output: ArunEworld */