Preprocessor Directives in c

 Preprocessor Directives in c, pre-processor instruction call as ‘directives’, directives always start with ‘#’

1. Macro replacement
2. File Inclusion
3. Conditional Compilation
4. Replace Comments with empty space

1. Macro: 

Macros are symbolic names give to a constant. At the stage of Preprocessor, macros replace with their definitions. There are two types of Macros.
Syntax: #define MACRO_NAME Definition

1. object-like macros: The object-like macros look like a constant. They can’t take arguments.

#define NUM  10

2. function-like macros: Function like same as object-like macros but it can take arguments.

#define        SET_BIT(a, p)      (a | (1<<p))

Macro Definitions end with newline char. If you want to continue the macro definition with a new line we can use this one ‘\’ end of the line. For Example

#define PRINT_BITS(no, size_bits) {\             
                    int i;\
                    for(i=(size_bits * 8)-1;i>=0;i--)\
                        printf("%d ",(no>>i) & 0x01);\
                    printf("\n");\
                }

2. File Inclusion:

        File inclusion makes it easy to handle collections of #defines and declarations (among other things). Any source line of the form 

             #include “filename” or (c pre-processor will look for the specified file in the current directory)

             #include<filename> (c pre-processor will look for the specified file in fixed locations)

3. Conditional Compilation:

 It is possible to control pre-processing itself with conditional statements that evaluate during pre-processing. This provides a way to include code selectively, depending on the value of conditions evaluated during compilation.

The preprocessor has provided a set of conditional directives which can use to implement conditional compilation.

#ifdef: #ifdef is return true if MACRO_NAME is define, return false if not defined.

Syntax:  #ifdef MACRO_NAME
               body;
         #else 
               body;

#include <stdio.h>

#define INPUT 1
int main()
{
	#ifdef INPUT
		int i;

                i=2;
                printf("%d\n",i);
	#else	
		int i;
                printf("Enter a value\n");
                scanf("%d",&i);

                printf("%d\n",i);
	#endif
}

#ifndef: #ifndef is return true if MACRO_NAME is not define, return false if defined.

Syntax:  #ifndef MACRO_NAME
               body;
         #else 
               body;

#include <stdio.h>

int main()
{
	#ifdef INPUT
		int i;
                i=2;
                printf("%d\n",i);
	#else	
		int i;
                printf("Enter a value\n");
                scanf("%d",&i);
                printf("%d\n",i);
	#endif
}

#if: #if is return true if the given expression returns a non-zero value, return false if the given expression returns zero.

Syntax:  #if EXPRESSION
               body;
         #else 
               body;

#include <stdio.h>

int main()
{
	#if 1
		int i;
                i=2;
                printf("%d\n",i);
	#else	
		int i;
                printf("Enter a value\n");
                scanf("%d",&i);
                printf("%d\n",i);
	#endif
}

Preprocessor Directives in c, “defined” is a unary operator which can use with #if to achieve the same behavior as #ifdef. “!defined” is a unary operator which can use with #if to achieve the same behavior as #ifndef.

Syntax: #ifdef MACRO1
	      body;
	#elif defined MACRO2  /* returns true if macro is defined */
	      body;
	#elif MACRO3		/* returns true if expression returns true */
	      body;
	#elif !defined MACRO4 /* returns true if macro is not defined */
	      body;
	#endif

Operators:

# is a unary operator, # operator will return the given operand enclosed in ” “

## operator will concatenate given operands and returns the concatenated value.

#include<stdio.h>
#define VAL(x,y) x##y
int main(){
        printf("%d\n",VAL(10,30));
        return 0;
}