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A trick to prevent a file being included multiple times

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• Inclusion of header files
  • Large programming project:

    Large programming projects will have many "header files" that are being included into various program files Nested inclusion with such a large number of header file can sometimes create recursive inclusion

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  • Fact: Header files usually contains only the following:

    macros definitions (#define) data type definitions variable definitions

    You should define these items only once !!!

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  • Therefore:

    A header file must included for processing at most once !!!

    $64,000 question:

    How can we ensure that a header file will be processed at most once ????

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• Trick using #ifndef to prevent a header file being included multiple times
  • Consider the following header file:

    line 1 line 2 .... line n

  • We add the following conditional include lines to the header file:

    #ifndef KEY #define KEY line 1 line 2 .... line n #endif

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  • Suppose this header file is somehow included multiple times:

    #include "this-header-fiile.h" .... .... #include "this-header-fiile.h"

    Then:

    The first time we process this include file: The symbol KEY was not defined, so we process the text between #ifndef KEY .... #endif in the include file: #define KEY line 1 line 2 .... line n Result: We process the include file one time We define the symbol KEY The second time we process this include file: The symbol KEY has been defined !!!! Therefore: the while include file will be skipped (not processed !!) #ifndef KEY --- KEY is defined !!! FALSE !!! SKIP !!! #define KEY line 1 line 2 .... line n #endif

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• Stopping recursive include
  • This include trick is often used by C programmers to stop accidental recursive include files

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  • Example:

    /* (1) use conditional include --- the key must be unique !!! */ #ifndef RECURSE_H /* (2) inside the conditional include: define the key !!! */ #define RECURSE_H /* ======================================================== We define RECURSE_H to STOP the processing if this header file is INCLUDED AGAIN !!! ======================================================== */ // This is a recursive include: #include "recurse.h" #define MAX 99999 /* (3) this ends the conitional include from above*/ #endif

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255000/demo/c/Cprep/recurse-sol.h

    How to run the demo:

    Edit and change the /home/cs255000/demo/c/Cprep/recurse.h to the content of /home/cs255000/demo/c/Cprep/recurse-sol.h Then: gcc -E include-r.c