C - PreProcessor

C - PreProcessor process our source code before it is passed to the compiler. Each preprocessor statement begins with hash '#' symbol.

#include: The #include preprocessor directive is used to include system-defined and user-defined header files. If included file is not found, compiler renders error.

#define: The #define preprocessor directive is used to define constant or micro. It can use any basic data type.

#define pi 3.14  declare constant variable pi with 3.14 value.
#define and &&  declare && operator with english word and.
#define or ||  declare || operator with english word or.

Macro with Argument: Macro's can have argument just like functions. Syntax to define Macro's with argument is:

#define sum(a,b) a+b  declare a macro sum with two arguments to calculate sum of two numbers.
#define area(x) 3.14*r*r  declare a macro area with one argument and also define its defination in one line.
#define sq(a) a*a  declare a marcro sq with one argument to calculate square of a number.
#define greater(a,b) (a>b)?a:b  Input 2 number and find greater
#define greater3(a,b,c) (a>b && a>c)?a:(b>a && b>c)?b:c  Input 3 number and find greater.

#undef: The #undef preprocessor directive is used to undefine the constant or macro. Syntax:

#undef  FILE_SIZE //undefine the macro
#define FILE_SIZE 42 //redefine the macro

#ifdef: The #ifdef preprocessor directive checks if macro is defined by #define. If yes, it executes the code otherwise #else code is executed, if present. Syntax:

#define NOINPUT //remove this line and execute code again
void main() 
{  
    int a=0;  
    #ifdef NOINPUT  
        a=2;  
    #else  
        printf("Enter a:");  
        scanf("%d", &a);  
    #endif         
    printf("Value of a: %d\n", a);  
    getch();  
}

#ifndef: The #ifndef preprocessor directive checks if macro is not defined by #define. If yes, it executes the code otherwise #else code is executed, if present. Syntax:

#define INPUT  //remove this line and execute code again
void main() 
{  
    int a=0;  
    #ifndef INPUT  
        a=2;  
    #else  
        printf("Enter a:");  
        scanf("%d", &a);  
    #endif         
    printf("Value of a: %d\n", a);  
    getch();  
}

Predefined Macros: ANSI C defines many predefined macros that can be used in c program.

Example: Predefined Macro

void main()
{
    printf("File :%s\n", __FILE__ );
    printf("Date :%s\n", __DATE__ );
    printf("Time :%s\n", __TIME__ );
    printf("Line :%d\n", __LINE__ );
}

#if preprocessor: The #if preprocessor directive evaluates the expression or condition. If condition is true, it executes the code otherwise #elseif or #else or #endif code is executed.

#define NUMBER 1  
void main() 
{
    #if NUMBER==0  
        printf("Value of Number is: %d",NUMBER);  
    #else  
        print("Value of Number is non-zero");  
    #endif         
    getch();  
}

#error preprocessor: The #error preprocessor directive indicates error. The compiler gives fatal error if #error directive is found and skips further compilation process.

#include<stdio.h>
#ifndef __MATH_H  
    #error First include then compile  
#else  
void main()
{  
   float a;  
   a=sqrt(7);  
   printf("%f",a);  
}  
#endif

Miscellaneous Directives

#pragma directive
a) #pragma startup This directive allow us to execute function before main() function. Syntax:

void fun()
{
    printf("Welcome, user");
}
#pragma startup fun
void main()
{
    printf("\nMain Message");
}

We can use more than one function to be executed at startup but they will be executed in reverse order.

#include<stdio.h>
#include<conio.h>

void fun1()
{
    printf("\nWelcome, user1");
}
void fun2()
{
    printf("\nWelcome, user2");
}
#pragma startup fun1
#pragma startup fun2
void main()
{
    printf("\nMain Message");
}

0 = Highest priority
0-63 = Used by C libraries
64 = First available user priority
100 = Default priority
255 = Lowest priority

#include<stdio.h>
#include<conio.h>

void fun1()
{
    printf("\nWelcome, user1");
}
void fun2()
{
    printf("\nWelcome, user2");
}
#pragma startup fun1 64
#pragma startup fun2
void main()
{
    clrscr();
    printf("\nMain Message");
    getch();
}

b) #pragma exit This directive allow us to execute function after main () function.

#include<stdio.h>
#include<conio.h>

void fun()
{
    printf("Thankyou for using the program");
}
#pragma exit fun
void main()
{
    clrscr();
    printf("\nMain Message");
    getch();
}

Both startup and exit functions should neither receive nor return any value.
#pragma warn This directive tells the compiler whether the warning will be shown or not.
a) #pragma warn –rvl return value warning does not display even if return keyword is not used in a function.
b) #pragma warn – par parameter not used warning does not display even if variable not used in a parameter.
c) #pragma warn – rch unreachable code warning does not display even if any code of statement is unreachable.