> Dear Garst,
>
> I can see that there are some friends in the list that had found a bug
from
> what Oleg has originally posted ( struct declararation and use ). I would
> like to respectfully ask this friends what the bug is an which is the
> interesting code it generates, so I ( and probably someone else ) could
> learn something new. It is clear that I missed something. Can the original
> Oleg's code work fine ? Why ? Thanks in advance.
>
The array v ( struct x *v[2] ) contains two elements, each of which is a
pointer to an "x" structure. I think everyone is clear on that one.
The original code was:
void swap(void)
{
struct x tmp;
tmp=*v[0];
*v[0]=*v[1];
*v[1]=tmp;
}
And the suggested change was:
void swap(void) {
struct x *tmp; /* this declaraction should be struct x *tmp ,
and
not the variable tmp of type struct x !!! */
*tmp = *v[0];
*v[0]= *v[1];
*v[1]= *tmp;
}
In the first case, tmp is declared to be a "struct x". The structure that
v[0] points to is copied into tmp. Then the structure that v[1] points to
is copied into the space that v[0] points to, and then the tmp structure is
copied into the space that v[0] points to.
I think you were trying to suggest that the pointers should be swapped, not
the structs they pointed to. In that case, the code would have been:
struct x *tmp;
tmp = v[0]; v[0] = v[1]; v[1] = tmp;
Note that there are no longer dereferencers (the *'s).
The code you have given will do pretty much the same thing as the original
code, except that the temporary area is accessed indirectly through a
pointer tmp, which is not initialised. In other words, the code will use a
random bit of memory somewhere (which may not physically exist) for the
scratch space, which is not what you want. The compiler should give you a
warning on that sort of thing (if not, add the -W and -Wall flags to your
compiler flags).
mvh.
David
>
>
> > Will this work?
> >
> > struct x{ /* struct of type x defined */
> > int a,b,c,d;
> > };
> >
> > struct x *v[2]; /* array of 2 pointers to structures of type x -
> > somewhere initialized, OK ?*/
> >
> > /* swap pointers, this means after call to swap(), v[0] holds the
> > pointer to v[1] and viceversa */
> > void swap(void) {
> > struct x *tmp; /* this declaraction should be struct x *tmp , and
> > not the variable tmp of type struct x !!! */
> > *tmp = *v[0];
> > *v[0]= *v[1];
> > *v[1]= *tmp;
> > }
> >
> > Garst
> > >
> > Luke Hoffmann wrote:
> > >
> > > Sorry, I'll repost this with HTML off ;-)
> > >
> > > Hi,
> > >
> > > I think Oleg has identified a real problem.
> > > Lets leave the question of "is this the best way to do it" aside and
> realise that "this is the best way to reproduce the problem"!
> > >
> > > The swap() function is not swapping pointers it is swapping the
contents
> of structures.
> > > This is a valid operation and it produces some interesting code!
> > > Clearly the code attached is wrong.
> > >
> > > Regards
> > > Luke
> > >
> > >
> > > -----Original Message-----
> > > From: Roberto G. Berner [mailto:[email protected]]
> > > Sent: Thursday, 23 January 2003 4:36 AM
> > > To: [email protected]
> > > Subject: RE: [Mspgcc-users] Struct assignment BUG...
> > >
> > > Dear Oleg:
> > >
> > > Accept my apologies, but I think you should check this. If I am wrong,
> please excuse me.
> > >
> > >
> > > struct x{ /* struct of type x defined */
> > > int a,b,c,d;
> > > };
> > >
> > > struct x *v[2]; /* array of 2 pointers to structures of type x -
> somewhere initialized, OK ?*/
> > >
> > > /* swap pointers, this means after call to swap(), v[0] holds the
> pointer to v[1] and viceversa */
> > > void swap(void) {
> > > struct x tmp; /* this declaraction should be struct x *tmp , and
not
> the variable tmp of type struct x !!! */
> > > tmp=*v[0]; /* tmp should be a pointer in order to hold
pointers
> properly */
> > > *v[0]=*v[1];
> > > *v[1]=tmp;
> > > }
> > >
> > > Kind regards,
> > >
> > > Roberto G. Berner
> > > [email protected]
> > > [email protected]
> > > 4308 3500 tel
> > > 4308 3700 fax
> > > 15 5122 6095 cel
> > > ----- Original Message -----
> > > From: Oleg Skydan
> > > To: [email protected]
> > > Sent: Wednesday, January 22, 2003 4:27 PM
> > > Subject: Re: [Mspgcc-users] Struct assignment BUG...
> > >
> > > Dear Roberto,
> > >
> > > The swap function exchanges the contents of the two structs,
> > > pointed by elements of the array v, so there a significant BUG.
> > >
> > > I have omitted the initialization section for simplicity (I have found
> this BUG in
> > > a real program, which is very large).
> > >
> > > Anyway the stack pointer should point to the same location at the
> function
> > > exit and entry (unless it is explicitly changed, using asm statement
for
> example).
> > >
> > > Thanks,
> > > Oleg.
> > > ----- Original Message -----
> > > From: Roberto G. Berner
> > > To: [email protected]
> > > Sent: Wednesday, January 22, 2003 6:25 PM
> > > Subject: RE: [Mspgcc-users] Struct assignment BUG...
> > >
> > > Dear Oleg:
> > >
> > > I think that there is no bug. Should I suggest trying ...
> > >
> > > struct x *tmp;
> > >
> > > Kind regards,
> > >
> > > Roberto G. Berner
> > > [email protected]
> > > [email protected]
> > > 4308 3500 tel
> > > 4308 3700 fax
> > > 15 5122 6095 cel
> > > ----- Original Message -----
> > > From: Oleg Skydan
> > > To: MSP430 GCC
> > > Sent: Wednesday, January 22, 2003 12:12 PM
> > > Subject: [Mspgcc-users] Struct assignment BUG...
> > >
> > > Hello, All !
> > >
> > > I found a significant BUG when using struct assignment.
> > >
> > > Here is the sample:
> > >
> > > struct x{
> > > int a,b,c,d;
> > > };
> > >
> > > struct x *v[2];
> > >
> > > void swap(void)
> > > {
> > > struct x tmp;
> > > tmp=*v[0];
> > > *v[0]=*v[1];
> > > *v[1]=tmp;
> > > }
> > >
> > > And here is GCC's listing (with -O option):
> > >
> > > /***********************
> > > * Function `swap'
> > > ***********************/
> > > swap:
> > > /* prologue: frame size = 8 */
> > > .L__FrameSize_swap=0x8
> > > .L__FrameOffset_swap=0x8
> > > sub #8, r1 ; 8, fpn 0
> > > /* prologue end (size=1) */
> > > mov &v, r15
> > > mov @r15+, @r1
> > > mov @r15+, 2(r1)
> > > mov @r15+, 4(r1)
> > > mov @r15+, 6(r1)
> > > mov &v+2, r14
> > > mov @r14+, 0(r15)
> > > mov @r14+, 2(r15)
> > > mov @r14+, 4(r15)
> > > mov @r14+, 6(r15)
> > > mov &v+2, r15
> > > mov @r1+, 0(r15) ;We should not change the stack pointer here !
> > > mov @r1+, 2(r15)
> > > mov @r1+, 4(r15)
> > > mov @r1+, 6(r15)
> > > /* epilogue: frame size=8 */
> > > add #8, r1 ;After this command the stack pointer
> points to the illegal location !
> > > ret
> > > /* epilogue end (size=2) */
> > > /* function swap size 33 (30) */
> > >
> > > I use win32 version (from 10.12.2002).
> > > Thanks, Oleg.
> > >
> > >
> > >
> >
> >
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>
>
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