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Preprocessor Directives

command description
#define define a macro
#include include a source code file
#undef undefined macro
#ifdef Returns true if the macro is defined
#ifndef Returns true if the macro is not defined
#if Compile the following code if the given condition is true
#else Alternative to #if
#elif If the #if condition is false, the current condition is true
#endif End a #if...#else conditional compilation block
#error Print an error message when standard error is encountered
#pragma Issue special commands to the compiler using the standardized method 
// replace all MAX\_ARRAY\_LENGTH with 20
#define MAX\_ARRAY\_LENGTH 20
// Get stdio.h from the system library
#include <stdio.h>
// Get myheader.h in the local directory
#include "myheader.h"
#undef FILE\_SIZE
#define FILE\_SIZE 42 // undefine and define to 42
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