C Preprocessor Directives

This document covers preprocessor directives.

The C Preprocessor is a tool that processes the source code before compilation. Preprocessor directives start with # and do not end with a semicolon (;).

In this section, we will cover:

• Macro definitions (#define)
• Including files (#include)
• Conditional Compilation (#ifdef, #ifndef, #endif)
• Optimization and compiler-specific directives (#pragma)

Macro Definitions (#define and #undef)

Defining Constants

The #define directive is used to define constants and macros.

#define PI 3.14159
#define SQUARE(x) ((x) * (x))

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• PI is replaced with 3.14159 during preprocessing.

• SQUARE(x) is an inline macro function.

Undefining a Macro (#undef)

To remove a macro definition, use #undef:

#undef PI

File Inclusion (#include)

The #include directive is used to include header files.

#include <stdio.h>  // Standard Library Header
#include "myheader.h"  // User-defined Header

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• Angle brackets (<>) search in standard system directories.
• Quotes ("") search in the current directory first.

Conditional Compilation (#ifdef, #ifndef, #endif)

Conditional compilation allows compiling specific parts of code based on macro definitions.

Using #ifdef and #ifndef

#define DEBUG
#ifdef DEBUG
    #define LOG(x) printf("Debug: %s\n", x)
#else
    #define LOG(x)
#endif

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If DEBUG is defined, LOG(x) prints a debug message. Otherwise, LOG(x) does nothing.

Preventing Multiple Inclusions (#ifndef)

Header files should be wrapped with an inclusion guard:

#ifndef MYHEADER_H
#define MYHEADER_H

void myFunction();

#endif // MYHEADER_H

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Prevents multiple inclusion of the same file.

The #pragma Directive

#pragma provides compiler-specific instructions and optimizations.

Disabling Warnings (#pragma warning)

#pragma warning(disable: 4996)  // Disable specific warnings (MSVC only)

Optimization (#pragma optimize)

#pragma optimize("g", off)  // Disable global optimizations

This preprocessor directive is usefull for OpenMP-based parallel programming.