Re: [Chicken-users] c-string return question

[Jörg F . Wittenberger]

On Oct 13 2011, John Cowan wrote:

> Jörg F. Wittenberger scripsit:
>
> > (define integer->utf8string
> >  (foreign-lambda*
> >   c-string ((unsigned-integer ch))
> >   "static C_uchar off[6]={0xFC,0xF8,0xF0,0xE0,0xC0,0x00};
> >  int size=5; C_uchar buf[7];
> >  buf[6]='\\0';
> >  if (ch < 0x80) {
> >    buf[5]=ch;
> >  } else {
> >    buf[size--]=(ch&0x3F)|0x80; ch=ch>>6;
> >    while (ch) { buf[size--]=(ch&0x3F)|0x80; ch=ch>>6; }
> >    /* Write the size information into the first byte */
> >    ++size;
> >    buf[size]=off[size]|buf[size];
> >  }
> >  return(buf+size);
> > "))
>
> This code is not good C, because it returns a pointer into a stack
> frame which has already been exited.  It may just so happen that
> there is still a correct value there, but there are no guarantees.
> I'd guess that the corruption happens when there is a minor GC.
> See http://c-faq.com/~scs/cclass/int/sx5.html .

Wait!

The chicken manual does not mention this restriction.  For a reason.

When you read the expanded C code as Chicken produces,
you will find, that it does through some magic to make sure
this restriction shall not apply.

(Watch out for C_cblock and C_cblockend #defines in chicken.h
, which depend on the C compiler in use.)

It does a local #define return(x) to insert a block wherein it
saves the to-be-returned string *before* the actual return statement
is seen by the C compiler.

> > 1.) static C_uchar buf[7];
> >    ^^^^^^
> >    does the trick.
>
> That's absolutely the Right Thing.  You are now returning a pointer to
> the static data region, which will always be available.

Not exactly.  While your explanation of my reasoning how to circumvent
the none-working situation is correct, this means that
the trick as deployed in the Chicken source does not work under certain
C compilers.


Best Regards

/Jörg