Read Array Manipulation Functions. Use av_fetch and a for loop and i. Either use av_len once, or check the return of av_fetch for NULL before depointering. Perl's AVs are not C arrays. You must use the published function calls and macros for almost everything. Perl's AVs internally are C arrays of SV *s with metadata. So the return of av_fetch is not a double * but a SV **. If you want a C array of doubles, you have to build it yourself, it doesn't exist in the Perl engine. sv_newmortal a SV. Then do a "doublearr = sv_grow(mymortalsv, sizeof(double) * numofelements);" (not SvGROW, SvGROW will probably crash on you, I'm not explaining why right now) then do a loop using av_fetch and SvNV to fill doublearr. doublearr's memory will be freed when your XSUB returns since mymortalsv is "mortalized" and will free at the next perl lang scope or line change. The other choice is an algorithm that just uses av_fetch and SvNV and feeds doubles 1 by 1 to the C do_nothing, not keeping a C array of doubles around.

return para[1]
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will never work. Is your XS code for yourself, or it is for others to see and use?

A hackish, but fine but slower way of making an array of doubles is,

$str = pack('ddd', 1.0, 1.1, 1.3);
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then

void
func(doublearrsv)
    SV * doublearrsv
PREINIT:
    STRLEN len;
    double (*doublearr)[3];//I hope this works
CODE:
    doublearr = SvPV(doublearrsv, len);//cast warning here
    if(len != sizeof(*doublearr)) croak("bad len");
    do_nothing(&doublearr[2]); //THIS WILL UPDATE IN PERL LAND
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then after the XS call in perl

@array = unpack('ddd', $str);
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An array of arrays, "$array13", is a SV reference to an AV, and each slice of the AV is a SV reference to another AV, which contains non reference SVs. Since you used AV * as a XS syntax prototype, the first SV reference you didn't see and it was automated away for you. If you do

@arr = ([11,12,13], [21,22,23], [31,32,33]);
myxsub(@arr);
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then you didn't get an AV or a SV reference. You would need a vararg XSUB.

void
myxsub(...)
PREINIT:
    int i;
CODE:
    for(i=0; i< items; i++){
         SV * sv =  ST(i);
         //do something
    }
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All code untested.