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))
-
PIis 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
- 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
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
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.