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C – Preprocessors

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As the “pre” means beforehand, similarly it means processing something before passing it on further. The preprocessor is a program that processes the source program before it is passed to the compiler. So we can say that it is a separate process in the compilation.

Preprocessor commands (often known as directives) form what can almost be considered a language within the C language. We can certainly write C programs without knowing anything about the preprocessor or its facilities.

It gives specific features called directives. A preprocessor directive begins with “#” symbol at the starting of the program. These directives are processed before the compilation of the program.

The following are preprocessor directives:

  • Macro expansion
  • File inclusion
  • Conditional Compilation
  • Miscellaneous directives

Macro Definition

macro is defined by #define in a program. It is used to define some constant in a program that can be used anywhere during the entire program. This constant becomes global value.

#define PI 3.1415

This statement is called ‘macro definition’ or more commonly, just a ‘macro’.
Its purpose: during preprocessing, the preprocessor replaces every occurrence of PI in the program with a 3.1415 value.

You may try this program:

#include <stdio.h>

#define PI 3.1415 //macro

int main()
{
    float rad, area;
    printf("Enter the radius of a circle: ");
    scanf("%f", &rad);

    //PI is used here
    area = PI * rad * rad;

    printf("Area of a circle: %.2f", area);
    return 0;
}

File inclusion

#include is used to include some files into the program. These are usually header files, but sometimes maybe any text file. The command looks something like this:

#include <stdio.h>

stdio.h is the header file that contains the library function that is used in a program such as printf, scanf, etc. #include is a preprocessor directive.

Check the example of macro mentioned above where stdio.h file is defined by #include. If we do not include this header file then we will not be able to use the in-built function.

Conditional Compilation

The C preprocessor provides a series of directives for conditional compilation: #if, #elif, #else, #ifdef, #ifndef, and #endif. These commands cause the preprocessor to include or exclude sections of the source code from compilation depending on certain conditions.

Conditional compilation is used for three main purposes:

  • Optionally include debugging code
  • Enclose non-portable code
  • Guard against multiple inclusion of header files.

Syntax:

#ifdef, #endif, #if, #else, #ifndef

#ifndef

#ifndef TEXT
   #define TEXT "Hello World"
#endif

It tells the C Preprocessor to define TEXT only if TEXT isn’t already defined.

#ifdef Directive

#ifdef MACRO     
   // conditional codes
#endif
#if expression
   conditional codes if expression is non-zero
#else
   conditional if expression is 0
#endif

You can also add nested conditional to your #if...#else using #elif

#if expression
    // conditional codes if expression is non-zero
#elif expression1
    // conditional codes if expression is non-zero
#elif expression2
    // conditional codes if expression is non-zero
#else
    // conditional if all expressions are 0
#endif

Miscellaneous directives

#undef is used to undefine a defined macro variable.

#undef  FILE_SIZE
#define FILE_SIZE 38

It tells the C Preprocessor to undefine existing FILE_SIZE and define it as 38.

#pragma is used to call a function before and after the main function in a C program.

#undef, #pragma

Predefined Macros

MacroValue
__DATE__A string containing the current date
__FILE__A string containing the file name
__LINE__An integer representing the current line number
__STDC__If follows ANSI standard C, then the value is a nonzero integer
__TIME__A string containing the current date.

Example: print current time and date using predefined function.

#include <stdio.h>
int main()
{
   printf("Current time: %s",__TIME__);
   printf("Current date: %s",__DATE__);   
}

You can run the above program in the compiler and the Current time and date will be displayed on the screen.

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