The Preprocessor

systems The C Preprocessor is a text substitution tool that processes your source code before the actual compilation begins. All preprocessor directives start with #.

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1. File Inclusion (#include)

Literally copy-pastes the contents of the specified file into the current file.

• #include <stdio.h>: Search standard system paths.
• #include "myheader.h": Search the current directory first.

2. Macro Substitution (#define)

Replaces occurrences of a token with specific text.

• Constants: #define PI 3.14159
• Function-like Macros:

  #define SQUARE(x) ((x) * (x))

• Warning: Macros are unaware of types and syntax. Always wrap arguments in parentheses to avoid operator precedence issues.
• Side Effects: SQUARE(i++) will increment i twice!

3. Conditional Compilation (#ifdef)

Compiles blocks of code only if certain conditions are met. Used heavily for:

• Cross-platform code: #ifdef _WIN32 ... #endif
• Debugging: #ifdef DEBUG ... #endif

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Header Guards

To prevent a header file from being included multiple times (which causes “redefinition” errors), use Header Guards:

#ifndef MY_HEADER_H
#define MY_HEADER_H
 
struct MyStruct { int x; };
 
#endif

Modern Alternative: #pragma once (Supported by most compilers).

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