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Conditional compilation

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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

    The clause #if clause is rarely used....

    I will skip it.

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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 DEBUG (This defines DEBUG to be the empty string !) int main( ) { int x, y; x = 3; y = 4; #ifdef DEBUG printf("x = %d\n", x); // Print variable x for debugging printf("y = %d\n", y); // Print variable x for debugging #endif }

    Output of the C pre-processor:

    int main( ) { int x, y; x = 3; y = 4; printf("x = %d\n", x); printf("y = %d\n", y); }

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/c/Cprep/ifdef1.c

    How to run the program:

    To compile:   gcc -E ifdef1.c (The -E option tells the C compiler to quit after running the C pre-processor)

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  • Experiment:

    Change the #define line to: #define NODEBUG "Re-compile" the program file with: gcc -E ifdef1.c You will now get a program without the print statements !!!

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• Defining symbol using the C-compiler option
  • The C-compiler has an option to define a symbol:

    gcc -Dmacro[=defn] ......

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  • Illustrative example:

    Program: ifdef2.c int main( ) { int x, y; x = 3; y = 4; #ifdef DEBUG printf("x = %d\n", x); // Print variable x for debugging printf("y = %d\n", y); // Print variable x for debugging #endif } Output of gcc -E ifdef2.c: int main( ) { int x, y; x = 3; y = 4; } because the symbol DEBUG was not defined However, the output of gcc -E -DDEBUG ifdef2.c: int main( ) { int x, y; x = 3; y = 4; printf("x = %d\n", x); printf("y = %d\n", y); } because the symbol DEBUG was defined by the compiler option -DDEBUG

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/c/Cprep/ifdef2.c

    How to run the program:

    To compile: see above

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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)