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C Preprocessor directives:

Before a C program is compiled in a compiler, source code is processed by a program called preprocessor. This process is called preprocessing.
Commands used in preprocessor are called preprocessor directives and they begin with “#” symbol.

Below is the list of preprocessor directives that C programming language offers.

Preprocessor	Syntax/Description
Macro	Syntax: #define This macro defines constant value and can be any of the basic data types.
Header file inclusion	Syntax: #include <file_name> The source code of the file “file_name” is included in the main program at the specified place.
Conditional compilation	Syntax: #ifdef, #endif, #if, #else, #ifndef Set of commands are included or excluded in source program before compilation with respect to the condition.
Other directives	Syntax: #undef, #pragma #undef is used to undefine a defined macro variable. #Pragma is used to call a function before and after main function in a C program.

A program in C language involves into different processes. Below diagram will help you to understand all the processes that a C program comes across.

There are 4 regions of memory which are created by a compiled C program. They are,

First region – This is the memory region which holds the executable code of the program.
2^nd region – In this memory region, global variables are stored.
3^rd region – stack
4^th region – heap

Do you know difference between stack & heap memory in C language?

Stack	Heap
Stack is a memory region where “local variables”, “return addresses of function calls” and “arguments to functions” are hold while C program is executed.	Heap is a memory region which is used by dynamic memory allocation functions at run time.
CPU’s current state is saved in stack memory	Linked list is an example which uses heap memory.

Do you know difference between compilers VS Interpreters in C language?

Compilers	Interpreters
Compiler reads the entire source code of the program and converts it into binary code. This process is called compilation. Binary code is also referred as machine code, executable, and object code.	Interpreter reads the program source code one line at a time and executing that line. This process is called interpretation.
Program speed is fast.	Program speed is slow.
One time execution. Example: C, C++	Interpretation occurs at every line of the program. Example: BASIC

Key points to remember:

Source program is converted into executable code through different processes like precompilation, compilation, assembling and linking.
Local variables uses stack memory.
Dynamic memory allocation functions use the heap memory.

Example program for #define, #include preprocessors in C language:

#define – This macro defines constant value and can be any of the basic data types.
#include <file_name> – The source code of the file “file_name” is included in the main C program where “#include <file_name>” is mentioned.

#include <stdio.h>

#define height 100
#define number 3.14
#define letter 'A'
#define letter_sequence "ABC"
#define backslash_char '\?'

void main()
{
   printf("value of height    : %d \n", height );
   printf("value of number : %f \n", number );
   printf("value of letter : %c \n", letter );
   printf("value of letter_sequence : %s \n", letter_sequence);
   printf("value of backslash_char  : %c \n", backslash_char);

}

#include <stdio.h>

#define height 100

#define number 3.14

#define letter 'A'

#define letter_sequence "ABC"

#define backslash_char '\?'

void main()

{

printf("value of height : %d \n", height );

printf("value of number : %f \n", number );

printf("value of letter : %c \n", letter );

printf("value of letter_sequence : %s \n", letter_sequence);

printf("value of backslash_char : %c \n", backslash_char);

}

Output:

value of height : 100

value of number : 3.140000

value of letter : A

value of letter_sequence : ABC

value of backslash_char : ?

Example program for conditional compilation directives:

a) Example program for #ifdef, #else and #endif in C:

“#ifdef” directive checks whether particular macro is defined or not. If it is defined, “If” clause statements are included in source file.
Otherwise, “else” clause statements are included in source file for compilation and execution.

#include <stdio.h>
#define RAJU 100

int main()
{
   #ifdef RAJU
   printf("RAJU is defined. So, this line will be added in " \
          "this C file\n");
   #else
   printf("RAJU is not defined\n");
   #endif
   return 0;
}

#include <stdio.h>

#define RAJU 100

int main()

{

#ifdef RAJU

printf("RAJU is defined. So, this line will be added in " \

"this C file\n");

#else

printf("RAJU is not defined\n");

#endif

return 0;

}

Output:

RAJU is defined. So, this line will be added in this C file

b) Example program for #ifndef and #endif in C:

#ifndef exactly acts as reverse as #ifdef directive. If particular macro is not defined, “If” clause statements are included in source file.
Otherwise, else clause statements are included in source file for compilation and execution.

#include <stdio.h>
#define RAJU 100
int main()
{
   #ifndef SELVA
   {
      printf("SELVA is not defined. So, now we are going to " \
             "define here\n");
      #define SELVA 300
   }
   #else
   printf("SELVA is already defined in the program”);

   #endif
   return 0;

}

#include <stdio.h>

#define RAJU 100

int main()

{

#ifndef SELVA

{

printf("SELVA is not defined. So, now we are going to " \

"define here\n");

#define SELVA 300

}

#else

printf("SELVA is already defined in the program”);

#endif

return 0;

}

Output:

SELVA is not defined. So, now we are going to define here

c) Example program for #if, #else and #endif in C:

“If” clause statement is included in source file if given condition is true.
Otherwise, else clause statement is included in source file for compilation and execution.

#include <stdio.h>
#define a 100
int main()
{
   #if (a==100)
   printf("This line will be added in this C file since " \
          "a \= 100\n");
   #else
   printf("This line will be added in this C file since " \
          "a is not equal to 100\n");
   #endif
   return 0;
}

#include <stdio.h>

#define a 100

int main()

{

#if (a==100)

printf("This line will be added in this C file since " \

"a \= 100\n");

#else

printf("This line will be added in this C file since " \

"a is not equal to 100\n");

#endif

return 0;

}

Output:

This line will be added in this C file since a = 100

Example program for undef in C language:

This directive undefines existing macro in the program.

#include <stdio.h>

#define height 100
void main()
{
   printf("First defined value for height    : %d\n",height);
   #undef height          // undefining variable
   #define height 600     // redefining the same for new value
   printf("value of height after undef \& redefine:%d",height);
}

#include <stdio.h>

#define height 100

void main()

{

printf("First defined value for height : %d\n",height);

#undef height // undefining variable

#define height 600 // redefining the same for new value

printf("value of height after undef \& redefine:%d",height);

}

Output:

First defined value for height : 100

value of height after undef & redefine : 600

Example program for pragma in C language:

Pragma is used to call a function before and after main function in a C program.

#include <stdio.h>

void function1( );
void function2( );

#pragma startup function1
#pragma exit function2

int main( )
{
   printf ( "\n Now we are in main function" ) ;
   return 0;
}

void function1( )
{
   printf("\nFunction1 is called before main function call");
}

void function2( )
{
   printf ( "\nFunction2 is called just before end of " \
            "main function" ) ;"
}

#include <stdio.h>

void function1( );

void function2( );

#pragma startup function1

#pragma exit function2

int main( )

{

printf ( "\n Now we are in main function" ) ;

return 0;

}

void function1( )

{

printf("\nFunction1 is called before main function call");

}

void function2( )

{

printf ( "\nFunction2 is called just before end of " \

"main function" ) ;"

}

Output:

Function1 is called before main function call

Now we are in main function

Function2 is called just before end of main function

More on pragma directive in C language:

Pragma command	Description
#Pragma startup <function_name_1>	This directive executes function named “function_name_1” before
#Pragma exit <function_name_2>	This directive executes function named “function_name_2” just before termination of the program.
#pragma warn – rvl	If function doesn’t return a value, then warnings are suppressed by this directive while compiling.
#pragma warn – par	If function doesn’t use passed function parameter , then warnings are suppressed
#pragma warn – rch	If a non reachable code is written inside a program, such warnings are suppressed by this directive.

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