in reply to Re^3: XS/Inline::C concat *any* two SVs.
in thread XS/Inline::C concat *any* two SVs.

The code I posted in the OP now does what I need it to do. Basically the XS equivalent of Perl's 

$scalar1 .= $scalar2;

[download]

I don't want to create a new scalar, except in the specific instance of a readonly input. The routine will be called many times and I need to accumulate the results in the scalar.

Duplicating the (already appended to), scalar in order to avoid using SvREFCOUNT_inc() is extremely wasteful and completely negates the benefits of perl's dynamically allocated string management. You have a scalar with n bytes, you call sv_catsv() to add m bytes to it. Having done that, you make a complete copy of m+n bytes in order to avoid the unreferenced scalar warning? This makes no sense to me, especially as the scalar of n bytes originates from Perl and has never gone out of scope.

My question still remains, why am I getting that warning?

The question is are there any other things that I need to test for and handle in order that I can handle any pair of SVs as input.

And if I comment out the sv_catsv() call ...

That is the (other) problem. If you call an SV that is undef as the first parameter to sv_catsv(), (which it usually will be on the first call for a particular SV), you get the warning. Hence the need for 

    if( !SvPOK( a ) )      // Still nothing, must be undef?
        sv_setpv( a, "" ); // Make it the null string to stop (one pos
+sible)
        // Use of uninitialized value in subroutine entry from sv_cats
+v

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Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

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Re^5: XS/Inline::C concat *any* two SVs.
by creamygoodness (Curate) on May 28, 2006 at 14:44 UTC
    You can't concatenate onto a readonly scalar, in either Perl or XS. 
    #!/usr/bin/perl
use strict;
use warnings;

use Inline C => <<'END_C';

void 
readonly(SV *sv) {
    SvREADONLY_on(sv);
}

END_C

my $foo = "foo";
readonly($foo);

# This dies with: 
# "Modification of a read-only value attempted at cat_readonly.plx lin
+e 16."
$foo .= "bar";

# unreachable:
print "$foo\n";

    [download]

    So, if you have to deal with the possibility that your first scalar might be readonly, you have to do $a = $a . $b or its equivalent in XS rather than $a .= $b. It might be wasteful, but it's the only way.

    --
    Marvin Humphrey
    Rectangular Research ― http://www.rectangular.com
[reply]
[d/l]
[select]

      If you look at the OP, you'll see that is exactly what I do.

      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
[d/l]

      If you look at the OP, you'll see that is exactly what I do. But only for readonly scalars.

      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
[d/l]

        Your original post is pretty messed up, so please forgive me if I used your more recent explanation, however imprecise, as a reference instead. I'm trying to help, here.

        --
        Marvin Humphrey
        Rectangular Research ― http://www.rectangular.com
[reply]

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