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nss · Guest

Hi

I have a program which works correctly under BC4.52 and BC5.02 but
it fails under BC5.5.1 (FreeCommandLineTools).

The program fails when I point a struct to some binary data
and then try to manipulate the data through the structure members.
Like the following:

char *BinBuff; /* binary data buffer */

struct mystruct *mst;

mst = (struct mystruct *)BinBuff;

mst->mem01 = 0x10;
mst->mem03 = 0x55;
....etc

I have narrowed it down to few lines of code to demonstrate the
behaviour. I compiled this code using the commandline:
"bcc32 -c -S sizetest.c" under both BC5.02 and BC5.5.1. to generate
assembly listings.

I am posting the source code and the two different .asm listings
which clearly show that the BC5.5.1 compiler is messing up the
structure member addressing.

I have marked the differences with //<<<<< which need to be looked at.

Please advise whether it is a fault on my part or something else.

/*******************************************************************/
/* source code */
/*******************************************************************/
struct mystruct {
short member1;
char member2;
short member3;
};

int main()
{
struct mystruct ss;
struct mystruct *s;

s = &ss;
s->member1 = 32767;
s->member2 = 'A';
s->member3 = 32767;

printf("s->member1 = %dn",s->member1 );
printf("s->member2 = %dn",s->member2 );
printf("s->member3 = %dn",s->member3 );

return 0;
}

/*******************************************************************/
/* asm listing for BC5.02 */
/*******************************************************************/
.386p
model flat
ifndef ??version
?debug macro
endm
endif
?debug S "sizetest.c"
?debug T "sizetest.c"
_TEXT segment dword public use32 'CODE'
_TEXT ends
_DATA segment dword public use32 'DATA'
_DATA ends
_BSS segment dword public use32 'BSS'
_BSS ends
DGROUP group _BSS,_DATA
_DATA segment dword public use32 'DATA'
align 4
__stklen label dword
dd 8192
_DATA ends
_TEXT segment dword public use32 'CODE'
_main proc near
?live1@0:
;
; int main()
;
push ebp
mov ebp,esp
add esp,-8
push ebx
;
; {
; struct mystruct ss;
; struct mystruct *s;
;
; s = &ss;
;
@1:
lea ebx,dword ptr [ebp-8]
;
; s->member1 = 32767;
;
?live1@32: ; EBX = s
mov word ptr [ebx],32767
;
; s->member2 = 'A';
;
mov byte ptr [ebx+2],65
;
; s->member3 = 32767;
;
mov word ptr [ebx+3],32767 //<<<<<
;
;
; printf("s->member1 = %dn",s->member1 );
;
movsx eax,word ptr [ebx]
push eax
push offset s@
call _printf
add esp,8
;
; printf("s->member2 = %dn",s->member2 );
;
movsx edx,byte ptr [ebx+2]
push edx
push offset s@+16
call _printf
add esp,8
;
; printf("s->member3 = %dn",s->member3 );
;
movsx ecx,word ptr [ebx+3] //<<<<<
push ecx
push offset s@+32
call _printf
add esp,8
;
;
; return 0;
;
?live1@128: ;
xor eax,eax
;
; }
;
@3:
@2:
pop ebx
pop ecx
pop ecx
pop ebp
ret
_main endp
_TEXT ends
_DATA segment dword public use32 'DATA'
s@ label byte
; s@+0:
db "s->member1 = %d",10,0
; s@+16:
db "s->member2 = %d",10,0
; s@+32:
db "s->member3 = %d",10,0
align 4
_DATA ends
_TEXT segment dword public use32 'CODE'
_TEXT ends
public __stklen
public _main
extrn _printf:near
?debug D "C:PBORLANDBC5INCLUDEio.h" 8825 10304
?debug D "C:PBORLANDBC5INCLUDEprocess.h" 8825 10304
?debug D "C:PBORLANDBC5INCLUDEtchar.h" 8825 10304
?debug D "C:PBORLANDBC5INCLUDEdir.h" 8825 10304
?debug D "C:PBORLANDBC5INCLUDEalloc.h" 8825 10304
?debug D "C:PBORLANDBC5INCLUDEctype.h" 8825 10304
?debug D "C:PBORLANDBC5INCLUDEstddef.h" 8825 10304
?debug D "C:PBORLANDBC5INCLUDEstring.h" 8825 10304
?debug D "C:PBORLANDBC5INCLUDEstdarg.h" 8825 10304
?debug D "C:PBORLANDBC5INCLUDEstdlib.h" 8825 10304
?debug D "C:PBORLANDBC5INCLUDEdos.h" 8825 10304
?debug D "C:PBORLANDBC5INCLUDE_null.h" 8825 10304
?debug D "C:PBORLANDBC5INCLUDE_nfile.h" 8825 10304
?debug D "C:PBORLANDBC5INCLUDE_defs.h" 8825 10304
?debug D "C:PBORLANDBC5INCLUDEstdio.h" 8825 10304
?debug D "sizetest.c" 12902 19215
end

/*******************************************************************/
/* asm listing for BC5.5.1 */
/*******************************************************************/
.386p
ifdef ??version
if ??version GT 500H
.mmx
endif
endif
model flat
ifndef ??version
?debug macro
endm
endif
?debug S "sizetest.c"
?debug T "sizetest.c"
_TEXT segment dword public use32 'CODE'
_TEXT ends
_DATA segment dword public use32 'DATA'
_DATA ends
_BSS segment dword public use32 'BSS'
_BSS ends
DGROUP group _BSS,_DATA
_DATA segment dword public use32 'DATA'
align 4
__stklen label dword
dd 8192
_DATA ends
_TEXT segment dword public use32 'CODE'
_main proc near
?live1@0:
;
; int main()
;
push ebp
mov ebp,esp
add esp,-8
push ebx
;
; {
; struct mystruct ss;
; struct mystruct *s;
;
; s = &ss;
;
@1:
lea ebx,dword ptr [ebp-8]
;
; s->member1 = 32767;
;
?live1@32: ; EBX = s
mov word ptr [ebx],32767
;
; s->member2 = 'A';
;
mov byte ptr [ebx+2],65
;
; s->member3 = 32767;
;
mov word ptr [ebx+4],32767 //<<<<<
;
;
; printf("s->member1 = %dn",s->member1 );
;
movsx eax,word ptr [ebx]
push eax
push offset s@
call _printf
add esp,8
;
; printf("s->member2 = %dn",s->member2 );
;
movsx edx,byte ptr [ebx+2]
push edx
push offset s@+16
call _printf
add esp,8
;
; printf("s->member3 = %dn",s->member3 );
;
movsx ecx,word ptr [ebx+4] //<<<<<
push ecx
push offset s@+32
call _printf
add esp,8
;
;
; return 0;
;
?live1@128: ;
xor eax,eax
;
; }
;
@3:
@2:
pop ebx
pop ecx
pop ecx
pop ebp
ret
_main endp
_TEXT ends
_DATA segment dword public use32 'DATA'
s@ label byte
; s@+0:
db "s->member1 = %d",10,0
; s@+16:
db "s->member2 = %d",10,0
; s@+32:
db "s->member3 = %d",10,0
align 4
_DATA ends
_TEXT segment dword public use32 'CODE'
_TEXT ends
public __stklen
public _main
extrn _printf:near
?debug D "c:PBorlandBc55includeio.h" 10459 10272
?debug D "c:PBorlandBc55includeprocess.h" 10459 10272
?debug D "c:PBorlandBc55includetchar.h" 10459 10272
?debug D "c:PBorlandBc55includedir.h" 10459 10272
?debug D "c:PBorlandBc55includealloc.h" 10459 10272
?debug D "c:PBorlandBc55includembctype.h" 10459 10272
?debug D "c:PBorlandBc55includectype.h" 10459 10272
?debug D "c:PBorlandBc55includemem.h" 10459 10272
?debug D "c:PBorlandBc55include_loc.h" 10459 10272
?debug D "c:PBorlandBc55includelocale.h" 10459 10272
?debug D "c:PBorlandBc55include_str.h" 10459 10272
?debug D "c:PBorlandBc55includestring.h" 10459 10272
?debug D "c:PBorlandBc55includestdarg.h" 10459 10272
?debug D "c:PBorlandBc55includesearch.h" 10459 10272
?debug D "c:PBorlandBc55includestdlib.h" 10459 10272
?debug D "c:PBorlandBc55includeerrno.h" 10459 10272
?debug D "c:PBorlandBc55includedos.h" 10459 10272
?debug D "c:PBorlandBc55include_nfile.h" 10459 10272
?debug D "c:PBorlandBc55include_null.h" 10459 10272
?debug D "c:PBorlandBc55include_defs.h" 10459 10272
?debug D "c:PBorlandBc55include_stddef.h" 10459 10272
?debug D "c:PBorlandBc55includestdio.h" 10459 10272
?debug D "sizetest.c" 12902 19215
end
/*******************************************************************/

nss · Guest

I have figured it out to be a structure alignment (padding) problem
and it can be fixed by using a one byte alignment. All compilers have
vendor implementation specific defaults for alignment.

How do I instruct the compiler to use a one byte alignment for my
struct declarations for all the different compiler versions (or
brands)?

Is there any portable method of instructing a compiler to use a
particular struct alignment?

Thanks