FP_SEG(), FP_OFF() Need Pointer Rather Than Address

MS-DOS                      | OS/2            | WINDOWS

The information in this article applies to:

• The C Run-time (CRT), included with:

      - Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, and 6.0ax
      - Microsoft C for OS/2, versions 5.1, 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

In Microsoft C, since the FP_SEG() and FP_OFF() macros dereference the pointer values passed to them, the address of a variable cannot be passed as a parameter.

MORE INFORMATION

The FP_SEG() and FP_OFF() macros are defined in DOS.H as follows:

   #define FP_SEG(fp) (*((unsigned *)&(fp) + 1))
   #define FP_OFF(fp) (*((unsigned *)&(fp)))

   unsigned val;
   unsigned i;
   i = FP_OFF(&val);

   C2102:  '&' requires lvalue

   C4046:  '&' on function/array, ignored

   unsigned val, *valptr;
   unsigned i;
   valptr = &val;
   i = FP_OFF(valptr);