Casting (signed long)(unsigned long)

[curious_one]

All,
I have a sample code that does something like this,

unsigned long i = SOME_HUGE_VALUE ;
unsigned long j = SOME_HUGE_VALUE ;

void decrement_count ()
{
if((signed long)j > 0)
j--
else
printf("Empty") ;
}

void add_count()
{
if((signed long)j < i)
{
j++;
}
else
{
printf("Error") ;
}
}

Will the casting in add_count truncate the j value while
comparision,as a result j++ will have a value more than i at some
point?

regds
curious

[Dan Pop]

Re: Casting (signed long)(unsigned long)

In <dee72238.04012 92201.486a36ec@ posting.google. com> curious.one@eud oramail.com (curious_one) writes:
[color=blue]
> I have a sample code that does something like this,
>
> unsigned long i = SOME_HUGE_VALUE ;
> unsigned long j = SOME_HUGE_VALUE ;
>
> void decrement_count ()
>{
> if((signed long)j > 0)
> j--
> else
> printf("Empty") ;
>}
>
>void add_count()
>{
> if((signed long)j < i)
> {
> j++;
> }
> else
> {
> printf("Error") ;
> }
>}[/color]

Most likely, this code was written by someone who didn't know what he
was doing. To behave correctly, both casts must be eliminated.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Dan.Pop@ifh.de

[Jack Klein]

Re: Casting (signed long)(unsigned long)

On 29 Jan 2004 22:01:17 -0800, curious.one@eud oramail.com
(curious_one) wrote in comp.lang.c:
[color=blue]
> All,
> I have a sample code that does something like this,
>
> unsigned long i = SOME_HUGE_VALUE ;
> unsigned long j = SOME_HUGE_VALUE ;
>
> void decrement_count ()
> {
> if((signed long)j > 0)[/color]

This may NEVER be true. An unsigned long with a value greater than
ULONG_MAX will result in a negative value when cast to signed long on
most implementations .
[color=blue]
> j--
> else
> printf("Empty") ;
> }
>
> void add_count()
> {
> if((signed long)j < i)
> {
> j++;
> }
> else
> {
> printf("Error") ;
> }
> }
>
> Will the casting in add_count truncate the j value while
> comparision,as a result j++ will have a value more than i at some
> point?[/color]

No, there is no such guarantee.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
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comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++

http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html

[Peter Nilsson]

Re: Casting (signed long)(unsigned long)

"Jack Klein" <jackklein@spam cop.net> wrote in message
news:m88m10dbvu uuv31gp83efkj2t aq79ict90@4ax.c om...[color=blue]
> On 29 Jan 2004 22:01:17 -0800, curious.one@eud oramail.com
> (curious_one) wrote in comp.lang.c:
>[color=green]
> > All,
> > I have a sample code that does something like this,
> >
> > unsigned long i = SOME_HUGE_VALUE ;
> > unsigned long j = SOME_HUGE_VALUE ;
> >
> > void decrement_count ()
> > {
> > if((signed long)j > 0)[/color]
>
> This may NEVER be true. An unsigned long with a value greater than
> ULONG_MAX will result in a negative value when cast to signed long on[/color]
^^^^^^^^^[color=blue]
> most implementations .[/color]

You mean: LONG_MAX

--
Peter

[curious_one]

Re: Casting (signed long)(unsigned long)

Dan,
Thanks for your reply, I am not entirely clear on why the casts
need to be eliminated. [Not that I know the reson why the casts are
there in the first place :)], may i ask you the reason why the code
will result in undefined behaviour ?

thanks
-curious

an.Pop@cern.ch (Dan Pop) wrote in message news:<bvdh31$qb i$3@sunnews.cer n.ch>...[color=blue]
> In <dee72238.04012 92201.486a36ec@ posting.google. com> curious.one@eud oramail.com (curious_one) writes:
>[color=green]
> > I have a sample code that does something like this,
> >
> > unsigned long i = SOME_HUGE_VALUE ;
> > unsigned long j = SOME_HUGE_VALUE ;
> >
> > void decrement_count ()
> >{
> > if((signed long)j > 0)
> > j--
> > else
> > printf("Empty") ;
> >}
> >
> >void add_count()
> >{
> > if((signed long)j < i)
> > {
> > j++;
> > }
> > else
> > {
> > printf("Error") ;
> > }
> >}[/color]
>
> Most likely, this code was written by someone who didn't know what he
> was doing. To behave correctly, both casts must be eliminated.
>
> Dan[/color]

[pete]

Re: Casting (signed long)(unsigned long)

curious_one wrote:[color=blue]
>
> All,
> I have a sample code that does something like this,
>
> unsigned long i = SOME_HUGE_VALUE ;
> unsigned long j = SOME_HUGE_VALUE ;
>
> void decrement_count ()
> {
> if((signed long)j > 0)
> j--
> else
> printf("Empty") ;
> }[/color]

void decrement_count ()
{
if (j != 0) j--;
else printf("Empty") ;
}
[color=blue]
>
> void add_count()
> {
> if((signed long)j < i)
> {
> j++;
> }
> else
> {
> printf("Error") ;
> }
> }[/color]


void add_count()
{
if (j != 0lu - 1) {
j++;
} else {
printf("Error") ;
}
}
[color=blue]
> Will the casting in add_count truncate the j value while
> comparision,as a result j++ will have a value more than i at some
> point?[/color]

Conversion of out of range values to signed types,
is implementation defined.

--
pete