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

ID: Q47497

5.10 6.00 6.00a 6.00ax 7.00 | 5.10 6.00 6.00a | 1.00 1.50

MS-DOS                      | OS/2            | WINDOWS

kbprg

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)))

The first step of the macro is to take the address of the pointer that is passed to it. Because of this, the code

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

the compiler generates the following error:

   C2102:  '&' requires lvalue

QuickC 2.0 produces the following warning, which does not hinder compilation:

   C4046:  '&' on function/array, ignored

These messages occur because the macro cannot take the address of an address. However, the code

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

returns the offset of the variable val, as the address of valptr can be taken.

Additional reference words: kbinf 1.00 1.50 5.10 6.00 6.00a 6.00ax 7.00 KBCategory: kbprg KBSubcategory: CRTIss Keywords : kb16bitonly

Last Reviewed: July 18, 1997