substr and vec are special in the fact that they are lvalue functions.

ref returns "LVALUE" for references to PVLV variables (except it lies SCALAR for tied scalars), so it's clear "LV" refers to "lvalue".

PVLV adds the following fields to its "base class", PVMG (scalar with magic): 

STRLEN      xlv_targoff;
STRLEN      xlv_targlen;
SV*         xlv_targ;
char        xlv_type;       /* k=keys .=pos x=substr v=vec /=join/re
                             * y=alem/helem/iter t=tie T=tied HE */

[download]

The extra fields only have meaning to the associated magic handlers (found in mg.c).

In reply to Re^3: XS: SvPVLV examples? by ikegami
in thread XS: SvPVLV examples? by BrowserUk

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