Conditional pre-processing

  • The C preprocessor provide these conditional pre-processing directives: 

      Conditional directive         Meaning
  ==========================================================
  #ifdef   Name               Pre-process ... when 
  ...                         "Name" has been #defined
  #endif

  #ifndef   Name              Pre-process ... when 
  ...                         "Name" has not been #defined
  #endif

  #if Condition               Pre-process ... when 
  ...                         "Condition" is true
  #endif

  • I will not discuss the 

      #if Condition

    clause because it is rarely used.

The #ifdef conditional pre-processing directive

  • Syntax of the #ifdef conditional directive: 

      #ifdef   NAME-1
     text segment 1        
 [#elif    NAME-2
     text  segment 2]  
  ...     
 [#else
     text segment 3]
  #endif

  • Meaning:

      • If NAME-1 was #defined, then the C pre-processor will process text segnment1
      • Otherwise, if NAME-2 was #defined, then the C pre-processor will process text segnment2
      • And so on (if you have more #elif directives)
      • Otherwise, the the C pre-processor will process text segnment3

Example of using the #ifdef conditional directive

  • I personally like to use ifdef around print( ) statements that I use for debugging: 

      #define DEBUG   // This defines DEBUG = 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
  }

  • When I want the C program to print out debug information, I would #define DEBUG at the top of my program

Example of using the #ifdef conditional directive

  • The conditional directive will generate the following C source: 

     

  int main( )
  {
      int x, y;

      x = 3;
      y = 4;



      printf("x = %d\n", x);  // Print variable x for debugging
      printf("y = %d\n", y);  // Print variable x for debugging

  }

    and the C program will print out the variables so I can check and find bugs

DEMO: demo/C/1-C-pre/ifdef1.c

Example of using the #ifdef conditional directive

  • When I do not want to see the debug information, I would rename the macro-name: 

      #define DEBUGx  // Now DEBUG is not defined !

  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
  }

Comment: there is an even better way to manage printing of debug messages!

Example of using the #ifdef conditional directive

  • The conditional directive will generate the following C source: 

     

  int main( )
  {
      int x, y;

      x = 3;
      y = 4;



    
     

  }

    and the C program will not have print( ) statements for debugging

The define symbol C compiler option

  • The C compiler has a define symbol compile option that allow you to define a macro name before the C compiler translates a C program

  • C compiler syntax for the define symbol option: 

      gcc  -Dmacro[=pattern]  ......

  • The C program does not contain the #define DEBUG directive: 

    int main( )
{
   int 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
}

  • To include the print( ) statement, compile with:   gcc -DDEBUG prog.c

DEMO: demo/C/1-C-pre/ifdef2.c

The #ifndef conditional pre-processing directive

  • Syntax of the #ifndef conditional directive: 

      #ifndef   NAME-1
     text segment 1        
 [#elif    NAME-2
     text  segment 2]  
  ...     
 [#else
     text segment 3]
  #endif

  • Meaning:

      • If NAME-1 was not #defined, then the C pre-processor will process text segnment1
      • Otherwise, if NAME-2 was not #defined, then the C pre-processor will process text segnment2
      • And so on (if you have more #elif directives)
      • Otherwise, the the C pre-processor will process text segnment3

Common use of the #ifndef conditional directive

  • The #ifndef conditional directive is commonly used to prevent multiple inclusion of the same header file

  • Commonly used technique in C programming

    • The #ifndef conditional directive can break a recursive include chain of include files

    I will show you how in the next set of slides