The C Preprocessor not the compiler part, but it executes as compilation occurs.

C Preprocessor is just a text substitution tool that tells  the compiler to do required pre-processing before the actual compilation.

#define MAX_ARRAY_LENGTH 20

This will tell the compiler to replace Max_Array_length with 20.

#include <stdio.h>
#include "myheader.h"

The first line will include file from global library and from the set of header file defined for C compiler.

The second line will tell the compiler to include local library file from the location where header files are placed.

#ifndef MESSAGE
   #define MESSAGE "You wish!"
#endif

This is simple conditional statement that is used to do operation or display message based on some condition.

Predefined Macros

Macros are the group of statements that are used when we want to use it again and again.

#include <stdio.h>

main() {

   printf("File :%s\n", __FILE__ );
   printf("Date :%s\n", __DATE__ );
   printf("Time :%s\n", __TIME__ );
   printf("Line :%d\n", __LINE__ );
   printf("ANSI :%d\n", __STDC__ );

}