Re: Returning a pointer to local data

[Steve Hay]

Tassilo von Parseval wrote:

On Fri, Oct 31, 2003 at 12:39:14PM +1100 Sisyphus wrote:
 

Steve Hay wrote:

   

So I tried rewriting my original example like this:

SV *
hello()
 PREINIT:
     char    *chartext;
     SV    *svtext;
 CODE:
     Newz(0, chartext, 13, char);
     strcpy(chartext, "Hello, world");
     sv_usepvn(svtext, chartext, 13);
     RETVAL = svtext;
 OUTPUT:
     RETVAL
However, I now find that when I call that from Perl ($greeting = 
Foo::hello()) then I get this error:

 Modification of a read-only value attempted
     

I get a segfault, which is what I would expect. You only allocate memory
for the string but not for the SV to be returned.
[snip]
 

One could change it to:

   SV *
   hello()
       PREINIT:
           char *chartext;
           SV   *svtext;
       CODE:
           Newz(0, chartext, 13, char);
           svtext = newSV(0);
           strcpy(chartext, "Hello, world");
           sv_usepvn(svtext, chartext, strlen(chartext));
           RETVAL = svtext;
       OUTPUT:
           RETVAL
 

Cool - thanks for the explanation.  I've simplified the above slightly 
to use RETVAL itself as the SV *:

   SV *
   hello()
       PREINIT:
           char    *chartext;
       CODE:
           Newz(0, chartext, 13, char);
           strcpy(chartext, "Hello, world");
           RETVAL = newSV(0);
           sv_usepvn(RETVAL, chartext, 13);
       OUTPUT:
           RETVAL
This still works fine.

That contrasts interestingly with the void xsub case:

   void
   hello()
       PREINIT:
           char    *chartext;
       CODE:
           Newz(0, chartext, 13, char);
           strcpy(chartext, "Hello, world");
           sv_usepvn(ST(0), chartext, 13);
           XSRETURN(1);
which works without having to say "ST(0) = newSV(0)".  Does that mean 
that ST(0) is already an allocated SV *?

If so, is that always the case?  In a previous thread discussing 
returning lists from xsubs 
(http://www.perldiscuss.com/article.php?id=1112&group=perl.xs) it was 
pointed out that when using EXTEND / PUSHs you get one free push if the 
xsub was called from Perl, but don't if it was called from another 
xsub.  What I'm wondering is would ST(0) still be a preallocated SV * if 
the xsub was called from another xsub?

(I would try it out to see, but I don't know how to call an xsub from 
another xsub!  How does one do that?)

- Steve