Review:   Nested include files can lead to recursion !!!

  • Warning

      • The nested include mechanism can lead to recursive inclusion

  • Example of a recusive file inclusion chain:

    recurse.h include-r.c 
     #include "recurse.h"  // Recursive    
                       // include chain    

 #define  MAX  99999



 
    #include "header2.h" //Header file

int main( int argc, char* argv[] ) 
{
   int x;

   x = MAX;
}

  • I will now show you how to break a recursive include chain in this set of slides

DEMO: /home/cs255001/demo/C/1-C-pre/include-r.c

How to make sure that a header file in include only once by the C pre-processor

  • Consider the header file named recurse.h (or any header file):                                                      

    


  #include "recurse.h"  // Recursive include chain

  #define MAX   99999

  • We will use the associated macro name recurse_h to break the recursive include chain

  • Fact:

      • When the C pre-processor reads a header file for the first time, the name recurse_h is not #defined

DEMO: /home/cs255001/demo/C/1-C-pre/include-r.c

How to make sure that a header file in include only once by the C pre-processor

  • Therefore, the following condition directive will allow the C pre-processor the process the header file for the first time: 

      #ifndef  recurse_h  


  #include "recurse.h"  // Recursive include chain

  #define MAX   99999 

  #endif

  • Next, we need to prevent the C pre-processor from processing the same include file again

  • Trick:

      • If the name recurse_h is #defined, the #ifndef conditional directive will skip the entire header file !!

DEMO: /home/cs255001/demo/C/1-C-pre/include-r.c

How to make sure that a header file in include only once by the C pre-processor

  • Here is the trick:   insert a #define recurse_h line into the header file:                              

      #ifndef  recurse_h  
  #define  recurse_h 
  
  #include "recurse.h"  // Recursive include chain

  #define MAX   99999 

  #endif

  • When the C pre-processor read this header file for the 2nd time, it will detect that recurse_h has been #defined !

  • Result:

      • The header file will not be processed in the 2nd reading of the same header file !!!

DEMO: /home/cs255001/demo/C/1-C-pre/include-r.c