• Preprocessor – This is a macroprocessor that processes the source program before it is compiled. It is a collection of special statements called preprocesor statements or preprocessor directives.
• These directives are executed before the C program passes through the compiler.

• Preprocessor Directives – These are statements which begin with the # symbol placed in the first coloumn.
• They are placed before the function main().
• There will be no smicolon at the end of such statements.
• Three types -

1. Macro substitution directives
2. File inclusion directives
3. Conditional compilation directives

Macro Substitution

• This is a process of replacing an identifier of a C program by a constant or a symolic constant.
• This can be accomplished by the directive #define.

File Inclusion

• This is a process of inserting the external files containing functions or macro definitions into a C program.
• This process eliminates the job of rewriting the functions or macro definitions.

Conditional Compilation Directives

• C preprocessor provides a conditional compilation directive which is used to select alternate segments of code in a C program depending upon the condition.
• The #ifdef, #else, #elis, #endif directives are used for conditional compilation of the program. These are also used with the #define, #ifndef and #undef directives.

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1. Control Statements(Decision Making & Branching)
2. Constants and Variables
3. Operators & Expressions
4. Pointers
5. Data Files