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

ID Number: Q47497

5.00 5.10 6.00 6.00a 6.00ax 7.00 | 5.00 5.10 6.00 6.00a

MS-DOS | OS/2

Summary:

In Microsoft C versions 5.0, 5.1, 6.0, 6.0a, 6.0ax, and C/C++ version

7.0, since 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);

produces the following error in C 5.0 and 5.1, as well as QuickC

1.01:

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: 5.00 5.10 6.00 6.00a 6.00ax 7.00