Repost: [Fwd: Returning a list from an XSUB]

[Steve Hay]

Any comments on this, anyone?

- Steve

-------- Original Message --------
Subject:        Returning a list from an XSUB
Date:   Mon, 08 Sep 2003 15:05:20 +0100
From:   Steve Hay <[EMAIL PROTECTED]>
To:     [EMAIL PROTECTED]


Hi,

I have an XSUB that calls a C function to get three integers, and then 
returns them (in list context).  In scalar context it just returns a 
Boolean to indicate whether the C function succeeded or not.  It 
currently looks like this:

void
my_func()
  PREINIT:
      I32     gimme;
      int     one, two, three;
  PPCODE:
      gimme = GIMME_V;
      if (My_Func(&one, &two, &three) == 0) {
          if (gimme == G_SCALAR) {
              XSRETURN_YES;
          }
          else if (gimme == G_ARRAY) {
              EXTEND(SP, 3);
              PUSHs(sv_2mortal(newSViv(one)));
              PUSHs(sv_2mortal(newSViv(two)));
              PUSHs(sv_2mortal(newSViv(three)));
          }
      }
      else {
          if (gimme == G_SCALAR)
              XSRETURN_NO;
          else if (gimme == G_ARRAY)
              XSRETURN_EMPTY;
      }
I have a few questions about this:

- Am I correct in just ignoring the G_VOID case, or would it be better 
to return somehow immediately rather than waste time calling the C 
function (given that the C function has no side effects beyond setting 
the three integers)?

- Is the EXTEND(SP, 3) call correct for the 3 subsequent PUSHs() calls?  
I thought it was OK to call PUSHs() once without needing to call 
EXTEND(SP, 1), so perhaps I should only call EXTEND(SP, 2) when I have 3 
values to return?

- Do I need the "else" case of the main "if" block at all?  Presumably 
just falling off the end of the XSUB when the C function failed would 
achieve the same thing?  Or is it better style to code it explicitly as 
above?

- Are the XSRETURN_* macros OK in a PPCODE section?  The Perl manpages 
seem to say it's OK, but I read something somewhere (can't remember 
where) that it wasn't a good idea because the C code generated contains 
"return" statements in a void() subroutine.

Thanks in advance,
- Steve
.