Microsoft KB Archive/47497

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FP_SEG, FP_OFF Need Pointer Rather Than Address

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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

Issue type :

Technology : kbVCsearch kbAudDeveloper kbCRT