XS leakage

Simon Cozens simon at simon-cozens.org
Thu Feb 24 13:54:23 GMT 2011

It has been altogether too long since I wrote XS code. I have some code which 
goes through an array and splices out a certain value:

void _drop_node(HV* a, SV* activelist) {
     AV* newlist = newAV();
     I32 i = 0;
     while (i <= av_len((AV*)SvRV(activelist))) {
         SV** x = av_fetch((AV*)SvRV(activelist), i, 0);
         if (*x && SvRV(*x) && SvRV(*x) != (SV*)a)
             av_push(newlist, newSVsv(*x));
     SvRV_set(activelist, (SV*)newlist);

This works, but it's horrible and it leaks like a sieve. I'd like to fix the 
leak, but ideally I'd prefer to replace it with more sensible code. So I tried 
this instead:

void _drop_node(HV* a, SV* activelist) {
     SV** src;
     AV* ary = (AV*)SvRV(activelist);
     I32 i = 0;
     while (i <= AvFILLp(ary)) {
         SV** x = AvARRAY(ary) + i;
         if (*x && SvRV(*x) && SvRV(*x) == (SV*)a) break;
     src = AvARRAY(ary) + i + 1;
     Move(src,src - 1, AvFILLp(ary) - i + 1, SV*);

(Last three lines stolen from pp.c:pp_splice.) Unfortunately, this seems to do 
the right thing to the array but it messes up existing pointers to things that 
used to be in the array - tests are giving strangely wrong answers.

I don't know what to do at this point. Can anyone help? (The module is 
Text::KnuthPlass, if anyone's interested, and there's a similarly leaky 
function called _insert_before that I'd also like to fix up.)

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