Re: Comments please

From: Martin Dickopp (expires-2004-02-29_at_zero-based.org)
Date: 01/20/04 

Date: 20 Jan 2004 23:39:39 +0100

"Sean Kenwrick" <skenwrick@hotmail.com> writes:

> "Sean Kenwrick" <skenwrick@hotmail.com> wrote in message
> news:bujv72$olg$1@titan.btinternet.com...
> >
> > "Martin Dickopp" <expires-2004-02-29@zero-based.org> wrote in message
> > news:buj7ok$ka$01$2@news.t-online.com...
> > > "Sean Kenwrick" <skenwrick@hotmail.com> writes:
> > >
> > > > "Jeremy Yallop" <jeremy@jdyallop.freeserve.co.uk> wrote in message
> > > > news:slrnc0omn2.6ih.jeremy@hehe.cl.cam.ac.uk...
> > > > > Christopher Benson-Manica wrote:
> > > > > > This function is intended to print a string guaranteed to be
> > > > > > in the form MM/DD/YYYY (assume <stdio.h> and <string.h> are
> > > > > > included) as YYYY-MM-DD:
> > > > > >
> > > > > > void printdate( const char *date )
> > > > > > {
> > > > > > char tdate[10], *cp;
> > > > > > int mod=10, i;
> > > > > >
> > > > > > cp=tdate;
> > > > > > for( i=6 ; i%mod != 5 ; ++i, ++cp ) {
> > > > > > if( !(i%mod) )
> > > > > > *(cp++)='-';
> > > > > > *cp=date[i%mod];
> > > > > > }
> > > > > > *cp=0;
> > > > > > *strchr( tdate, '/' )='-';
> > > > > > printf( "New date is %s\n", tdate );
> > > > > > }
> > > > >
> > > > > /*
> > > > > This function is intended to print a string guaranteed to be in the
> > > > > form MM/DD/YYYY (assume <stdio.h> and <string.h> are included) as
> > > > > YYYY-MM-DD:
> > > > > */
> > > > > void printdate(const char *date)
> > > > > {
> > > > > printf( "New date is %s-%.2s-%.2s\n", strrchr(date, '/') + 1, date,
> > > > > strchr(date, '/') + 1);
> > > > > }
> > > > >
> > > > > Jeremy.
> > > >
> > > > I like this - very clever - but it relies on 'C' calling convention of
> > > > passing arguments to functions in reverse order (but so what).
> > >
> > > There is no such convention in C, and the code doesn't make the assumption
> > > that there is.
> >
> > Doh! Of course your right, I missed the fact that the first call was to
> > strrchr() rather than strchr() and so I couldn;t quite figure out the logic
> > until my brain suggested that the calls must be going in the reverse order
> > (which still wouldn't work (damn this stupid brain!)).

The order in which the `strrchr' and `strchr' calls happen is unspecified,
but that doesn't matter, since neither function has any side effects.

> Actually, I just thought about this again and realised I was right the first
> time about the C calling convention. Check out this code:
>
>
> #include <stdio.h>
>
> void myfunc(int, int);
>
> int main(){
> int i=0;
> myfunc(++i,i+=8);
> }
>
> void myfunc(int a,int b){
> printf("a=%d b=%d\n",a,b);
> }
>
> Do you think it will print out a=1, b=9 or a=9, b=8 ???

I think it will print nothing, but it will make daemons fly out of my
nose. ;)

> Its the latter,

No, it's not. `++i' and `i+=8' both modify the same object `i', and since
there is no intervening sequence point, the program invokes undefined
behavior. Virtually /anything/ can happen.

> (although I agree his solution didn't rely on this calling convention)

The only "calling convention" the C language has is that the arguments to
a function call are evaluated in unspecified order, and that there is
sequence point immediately before the function is called.

Martin

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