Compiler control directives

Yogesh Khanal

 


Compiler control directive allows the compiler to compile a corresponding code on some condition. These can specifically use to control the compiler to run/escape a program. 

Syntax   

 

#ifdef:- we use this pre-processor to check if the provided macro exists before including the subsequent code in the compilation process. 

 

#ifndef:- it checks macro substitution preprocessor,  if the provided macro does not exist then the corresponding code will execute in the compilation process. 

 

#else:- it is used to write an alternative part of a code  

 

#endif:- it checks whether the given token has been defined earlier in the or in an included file; if not, it includes the code between it and the closing #else or, if no #else is present, #endif statement. 

 

#if:- it is similar with #ifdef where #ifdef check the macro definition exists or not but #if check the value of define function. 

Let's see here to understand the code in more depth  

 

Initially we have define x with 100 and we check x is present in program or not in following  lines, if not we have define x with now value I.e. 200, but we have already define x with 100 so, it will execute 100 when program will run. 

#define x 100 

#ifndef x  // checking x is present or not 

#define x 200 // if not defining x 

#endif // closing if statement.   

 

 

In the following program, we have defined a y with 100 and include macro. c in the next line we have to check macro y is present or not, wherein next to that we have undefined y if it is present in the program and we end up with that.  

#deine y 100 

#ifdef y   // checking y is present or not 

#undef y // undefine y 

#endif   // closing if statement  

 

 

           QUESTION:- write a program to check either a macro variable Is define or not in the program, if yes print a message. 

  


#include <stdio.h>
#include <conio.h>
#define x 100
void main()
{
#ifndef x
#define x 200
    printf("new value defined");
#else
    printf("x is defined ");
#endif
    getch();
}

 

Output  

X is defined  

 

Let's discuss another program here to undefine a code. 


#include <stdio.h>
#include <conio.h>
#define x 100 // if we keep a 0 value then undefine function will not execute,
 to execute a code we have keep the value more than 0 as we see in the program.
void main()
{
#if x // checking x is define or now
#undef x // if its define, it will undefine
    printf("the value of x is undefined");
#endif
    getch();
}

  

output 

The value of x is undefined.


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