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

[John Cowan]

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 .

> 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.

-- 
John Cowan  address@hidden   http://ccil.org/~cowan
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