near Pointer to far Pointer Conversions Use DS

MS-DOS                 | OS/2       | WINDOWS

The information in this article applies to:

• The Microsoft C/C++ Compiler (CL.EXE), included with:

      - Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax
      - Microsoft C for OS/2, versions 6.0 and 6.0a
      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft Visual C++ for Windows, versions 1.0 and 1.5
  

SUMMARY

When an application uses more than one memory model, pointer conversions must be performed correctly. The compiler converts a near pointer to a far pointer using the current value of the DS (data segment) register as the segment value.

MORE INFORMATION

This behavior may cause a problem when the application converts a near pointer returned from a function to a far pointer and compares it to NULL. If the value of the near pointer is NULL, when the application converts it to a far pointer its value is DS:0000h, which cannot be equal to NULL. If the application must use more than one memory model, it can force the segment value to be 0h with the C run-time macro FP_SEG, as demonstrated below.

Sample Code

/*
 * Compile options needed: none
 */

#include <stdio.h>
#include <dos.h>

char __near *func(void)

   return NULL;

void main(void)

   char _far *cptr;

   cptr = func();    // Set segment value to DS
   FP_SEG(cptr) = 0; // Set segment value back to 0