C – Preprocessor

Introduction:

• Pre-processor program that modifies the text of a C program before it is compiled.
• Pre-processing includes
  • Inclusion of other files
  • Definition of symbolic constants and macros
  • Conditional compilation of program code
  • Conditional execution of preprocessor directives

Preprocessing, Compiling and Linking:

one.c  –>  PREPROCESSOR ->  tmp.c(temporary)  ->  COMPILER  -> one.obj   -> LINKER -> one.exe (final Executable)

Inclusion of header files:

• C application is a collection of Source files(.c) and Header files(.h)
• #include directive is used to connect the files in C application

We include Library file as well as Custom defined Files:

#include <filename>

• Searches standard library for file
• Use for standard library files

#include “filename”

• Searches current directory, then standard library
• Use for user-defined files

if you have a header file ‘header.h’ as follows,

int x; int biggest(int x, int y) { retrun x>y?x:y; }

and a main program called ‘program.c’ that uses the header file, like this,

#include “header.h” int main (void) {             x=biggest(10,20);             printf(“big :%d”,x);             return 0; }

Macros(#define):

• We use #define to create Macro constants.
• Constants recommended to define in Upper case.
• At the time or pre-processing all Macros replaced with their values

Syntax:             #define identifier replacement-text   Example:             #define PI 3.14159

Function-like Macros

• You can also define macros looks like a function call.
• For example,

#define CIRCLE_AREA( x ) ( PI * ( x ) * ( x ) )

• Would cause

                        area = CIRCLE_AREA( 4 ); 

• To become

                        area = ( 3.14159 * ( 4 ) * ( 4 ) );

Conditional Compilation (#if, #ifdef, #ifndef, #else, #elif, #endif)

• Six directives are available to control conditional compilation.
• They delimit blocks of program text that are compiled only if a specified condition is true.

#if directive:

• It is conditional compilation directive.
• If the condition is valid then the code defined inside the block gets compiled.

Syntax:

#if <Constant_expression>                ————-                 ————-             #endif

If constant expression will return 0 then condition will FALSE if it will return any non-zero number condition will TRUE.

#include<stdio.h> #if 0 int main() {             printf(“HELLO WORLD”);             return 0; } #endif Runtime Error: undefined symbol main

#else directive:The code which is defined in else-block gets compiled if the condition fails.

#include<stdio.h> #if(-4)             int main()             {                         printf(“WELCOME TO C-WORLD “);                         return 0;             }             #else             int main()             {                         printf(“HELLO WORLD”);                         return 0;             } #endif Output: WELCOME TO C-WORLD

Program to display System date using pre-defined Macro constant:

#include<stdio.h> int main() {             #ifdef __DATE__                         printf(“%s”,__DATE__);             #else                         printf(“First define the __DATE__”);             #endif             return 0; }

Output: It will print current system date.

Explanation: __DATE__ is global identifier. It has already defined in the header file stdio.h and it keeps the current system date.

Program to display System Time using pre-defined Macro

#include<stdio.h> #define I 30 int main() {             #ifndef __TIME__                         printf(“%d”,I);             #else                         printf(“%s”,__TIME__);             #endif             return 0; }

Output: It will print current system time.

Explanation:  __TIME__ is global identifier. It has been defined in the header file stdio.h. Compiler doesn’t compile the c codes which are inside the any conditional preprocessor directive if its condition is false. So we can write anything inside it.