Q32837 Bad Code Generated when Accessing Data in the Code Segment
C Compiler
5.10   | 5.10
MS-DOS | OS/2

Summary:
   The C compiler generates incorrect code for a program that accesses
data in the code segment. It "forgets" that the data is in a far
segment and tries to access it with a near (16-bit) pointer.
   A workaround to the problem is to compile with the optimizer
disabled (/Od).
   Microsoft has confirmed this to be a problem in Version 5.10. We
are researching this problem and will post new information as it
becomes available.

More Information:
   The following is the program that demonstrates the problem,
along with the generated code; it is compiled in medium model:

extern void far SomeProc();

main()
  {
  typedef struct {
    char patch;
    } TEMPLATE;

  TEMPLATE far *lpSomeProc;

  lpSomeProc = (TEMPLATE far *)SomeProc; /* point pointer into code segment*/
  lpSomeProc->patch = 123;        /* Bad code generated for this statement */
  }                               /* when optimizer used */

/*
;BAD CODE:
; Line 11
mov     WORD PTR [bp-4],OFFSET _SomeProc        ;lpSomeProc
mov     WORD PTR [bp-2],SEG _SomeProc
; Line 12
mov     BYTE PTR _SomeProc,123        <== THIS IS INCORRECT; A NEAR PTR


;GOOD CODE: (produced with -Od option)
; Line 11
        mov     WORD PTR [bp-4],OFFSET _SomeProc        ;lpSomeProc
        mov     WORD PTR [bp-2],SEG _SomeProc
; Line 12
        les     bx,DWORD PTR [bp-4]     ;lpSomeProc
        mov     BYTE PTR es:[bx],123      <== THIS IS CORRECT; A FAR PTR
; Line 13
*/



Keywords:  buglist5.10 tar78670
Updated  88/07/29 12:16