Writing an External Procedure

Using a third-generation programming language, you can write functions to be built into DLLs and started by EXTPROC.

The following is a simple Microsoft Visual C++ example of an external procedure called FIND_MAX:

Note:

Because external procedures are built into DLLs, they must be explicitly exported. In this example, the DLLEXPORT storage class modifier exports the function FIND_MAX from a dynamic link library.

#include <windows.h>
#define NullValue -1
/*
  This function tests if x is at least as big as y.
*/
long __declspec(dllexport) find_max(long 	x, 
 				short 	x_indicator, 
long 	y, 
short y_indicator, 
 				short *ret_indicator)
{
   /* It can be tricky to debug DLL's that are being called by a process
      that is spawned only when needed, as in this case.  
      Therefore try using the DebugBreak(); command.  
      This starts your debugger.  Uncomment the line with DebugBreak(); 
      in it and you can step right into your code.
   */
   /* DebugBreak();  */

   /* First check to see if you have any nulls. */
   /* Just return a null if either x or y is null. */

   if ( x_indicator==NullValue || y_indicator==NullValue) {
      *ret_indicator = NullValue;   
      return(0);
   } else { 
      *ret_indicator = 0;       /* Signify that return value is not null. */
      if (x >= y) return x;
      else return y;
   }
}

Parent topic: About Building External Procedures