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A commonly used C preprocessor trick to handle definition and declaration of global variables

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• How to organize the global variables in a multi-files C program
  • The global variables in a multi-files C program are organized as follows:

    Each global variable must be defined inside exactly one of the files Each global variable must be declared inside every C program files

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  • A simple way to achieve this is:

    Define each global variable inside one of the C program files Declare each global variable inside every C program files (except in the one in which they have been defined)

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  • Schematically:

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  • In order to understand how to organize the definitions and declarations of the global variables in a C program with ease, we must first learn about a new feature of the C pre-processor.

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• Conditionals in the C pre-processor
  • Conditional clause of the C pre-processor:

    The C pre-processor has a conditional clause that tells the C processor whether it should skip the processing of a portion of the C program file

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  • The C pre-processor has the following 3 conditional clauses:

    #if #ifdef #ifndef

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  • We will study the #ifdef conditional first, because this conditional is used in the programming trick to manage definitions/declarations of the global variables.

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• The #ifdef conditional

  • Syntax of the #ifdef conditional:

    Syntax	Meaning
    #ifdef MACRO-NAME text segment #endif	If the MACRO-NAME is defined, then the text segment will be kept (not removed) by the C pre-processor Otherwise, (the MACRO-NAME is not defined), then the text segment will be omitted (removed) by the C pre-processor
    #ifdef MACRO-NAME text segment 1 #else text segment 2 #endif	If the MACRO-NAME is defined, then the text segment 1 will be kept (not removed) and the text segment 2 will be omitted (removed) by the C pre-processor Otherwise, (the MACRO-NAME is not defined), then the text segment 2 will be kept (not removed) and the text segment 1 will be omitted (removed) by the C pre-processor
    #ifdef MACRO-NAME-1 text segment 1 #elif MACRO-NAME-2 text segment 2 [#else text segment 3] #endif	If the MACRO-NAME-1 is defined, then the text segment 1 will be kept (not removed) and all other text segments will be omitted (removed) by the C pre-processor Otherwise, if the MACRO-NAME-2 is not defined, then the text segment 2 will be kept (not removed) and all other text segments will be omitted (removed) by the C pre-processor And so on.

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  • Example:

    #define COMPILE_IT (This defines COMPILE_IT to be the empty string !) #ifdef COMPILE_IT You can put any text here. The C-preprocessor will keep it when the above value is NON-ZERO #else You can put any text here. The C-preprocessor will keep it when the above value is ZERO #endif

    Output of the C pre-processor:

    You can put any text here. The C-preprocessor will keep it when the above value is NON-ZERO

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   gcc -E cprep1.c (The -E option tells the C compiler to quit after running the C pre-processor)

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  • Experiment:

    Delete the line: #define COMPILE_IT "Re-compile" the program file with: gcc -E cprep1.c You will see the text in the else part

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• The #ifndef conditional
  • #ifndef:

    The #ifndef conditional is similar to the #ifdef, except the meaning is reversed: #ifndef MACRO-NAME text segment #endif the text segment is kept when the macro name MACR-NAME is not defined The text segment is omitted otherwise (when the macro name MACR-NAME is defined)

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• A programming trick to manage definitions/declarations of global variables used in multiple C program files

  • Recal, we want make it easy to achieve this simple way to organize definitions/declarations of global variables:

    Define each global variable inside one of the C program files Declare each global variable inside every C program files (except in the one in which they have been defined)

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  • Schematically:

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  • Programming trick:

    Create a header file containing all the global variables as follows: #ifndef EXTERN #define EXTERN extern #endif EXTERN type1 globalvar1; EXTERN type2 globalvar2; .... Include the header file vars.h in every C program file that make use of the global variables In one of the C program files, add this definition at the start of the file: #define EXTERN (This defines EXTERN as the empty string).

    Example:

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  • How the trick works:

    In the one program file that contains the definition: #define EXTERN the content of #include "vars.h" will be processed as follows: EXTERN double x; ⇒ double x; EXTERN int y; ⇒ int y; Because: The macro EXTERN was defined (as the empty string) Hence, the #ifndef EXTERN will be skipped and the C pre-processor will replace EXTERN with the empty string in the rest of the file Result: The global variables are defined inside this one program file. In all other program files the content of #include "vars.h" will be processed as follows: EXTERN double x; ⇒ extern double x; EXTERN int y; ⇒ extern int y; Because: The macro EXTERN is not defined Hence, the #ifndef EXTERN will be keep the then portion and the header file vars.h becomes: #define EXTERN extern EXTERN double x; EXTERN int y; and the C pre-processor will replace EXTERN with extern !!!! Result: The global variables are declared inside all other program files.

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• A common usage of the conditional clauses of the C pre-processor
  • The conditional clauses are often used to write platform dependent code:

    Different implementation of the C compiler make of different libraries Or there may be subtle differences between various library functionalities in different UNIX systems (

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  • How to write a C program for multiple computational platforms:

    /* -------------------------------------------------------------- When here are no platform differences (standard C statements): just write the code without C pre-processor conditionals --------------------------------------------------------------- */ .... Standard C staments .... /* ----------------------------------------------------------------- When here are platform differences (uses platform dependent library function calls): write code for each platform with C pre-processor conditionals ------------------------------------------------------------------ */ #ifdef Linux Linux code #endif #ifdef Solaris Solaris code #endif ....

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  • C compiler option to define a symbol:

    gcc -Dsymbol .....

    will tricket the C pre-processor to include the following definition:

    #define symbol

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  • Therefore:

    If you wan to compile the program on Linux, you will use this command: gcc -DLinux .... If you wan to compile the program on Solaris, you will use this command: gcc -DSolaris .... And so on.

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  • BTW:

    The way it is set up, you usually create a Makefile where you can change the platform at the start of the Makefile Example: PLATFORM=Linux CC=gcc -D$(PLATFORM) .... (other options)

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• The #if condition

  • Syntax of the #if conditional:

    Syntax	Meaning
    #if EXPRESSION text segment #endif	If the EXPRESSION evaluates to non-zero (≠ 0), then the text segment will be kept (not removed) by the C pre-processor Otherwise, (the EXPRESSION evaluates to zero (= 0)), then the text segment will be omitted (removed) by the C pre-processor
    #if EXPRESSION text segment 1 #else text segment 2 #endif	If the EXPRESSION evaluates to non-zero (≠ 0), then the text segment 1 will be kept (not removed) and the text segment 2 will be omitted (removed) by the C pre-processor Otherwise, (the EXPRESSION evaluates to zero (= 0)), then the text segment 2 will be kept (not removed) and the text segment 1 will be omitted (removed) by the C pre-processor
    #if EXPRESSION-1 text segment 1 #elif EXPRESSION-2 text segment 2 [#else text segment 3] #endif	If the EXPRESSION-1 evaluates to non-zero (≠ 0), then the text segment 1 will be kept (not removed) and all other text segments will be omitted (removed) by the C pre-processor Otherwise, if the EXPRESSION-2 evaluates to non-zero (≠ 0), then the text segment 2 will be kept (not removed) and all other text segments will be omitted (removed) by the C pre-processor And so on.

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  • Example:

    #define COMPILE_IT 1 #if COMPILE_IT You can put any text here. The C-preprocessor will keep it when the above value is NON-ZERO #else You can put any text here. The C-preprocessor will keep it when the above value is ZERO #endif

    Output of the C pre-processor:

    You can put any text here. The C-preprocessor will keep it when the above value is NON-ZERO

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   gcc -E cprep1.c (The -E option tells the C compiler to quit after running the C pre-processor)