Preprocessor

Pictorial Explanation

While Processing the source file, the preprocessor replaces header files and macros with defined files and values.

Preprocessor Tasks

Source file inclusion

#include<stdio.h> will be replaced by the original implementation of stdio.h file. Basically, it will remove the #include statement from source code and includes stdio.h file content into the source program.

Macro expansion

#define size 5. Here size is a macro which holds the value 5. Before the compilation starts, the preprocessor will replace size by 5 all over the program.

Conditional compilation

With the help of preprocessor, we can decide which part of the source code should compile and which part of the code should be ignored while compilation. We will discuss conditional compilation in upcoming topics.

Source File - Before Preprocessing

#include<stdio.h> 
#define size 5 

int main()
{
    int i = 0;

    for(i = 0; i < size; i++)
        printf("Hello");

    return 0;
}

After Preprocessing

/* origianl code of stdio.h */   //replaced by preprocessor 

int main()
{
    int i = 0;

    for(i = 0; i < 5; i++)    //size will be replaced by 5 
        printf("Hello");

    return 0;
}

Useful Resources