modified to accept an additional argument that indicates how much extra space should be allocated when reallocation of the string memory is required: 
static char *
my_sv_unchop(pTHX_ SV *sv, STRLEN size, STRLEN reserve) {
    STRLEN len;
    char *pv = SvPV(sv, len);
    IV off = SvOOK(sv) ? SvIVX(sv) : 0;
    if (!size)
        return pv;
 
    if (off >= size) {
        SvLEN_set(sv, SvLEN(sv) + size);
        SvCUR_set(sv, len + size);
        SvPV_set(sv, pv - size);
        if (off == size)
            SvFLAGS(sv) &= ~SVf_OOK;
        else
            SvIV_set(sv, off - size);
    }
    else {
        size += reserve;
        if ((size < reserve) || (len + size < size))
            Perl_croak(aTHX_ "panic: memory wrap");
        
        if (len + size <= off + SvLEN(sv)) {
            SvCUR_set(sv, len + size);
            SvPV_set(sv, pv - off);
            Move(pv, pv + size - off, len, char);
            if (off) {
                SvLEN_set(sv, SvLEN(sv) + off );
                SvFLAGS(sv) &= ~SVf_OOK;
            }
        }
        else {
            SV *tmp = sv_2mortal(newSV(len + size));
            char *tmp_pv;
            SvPOK_on(tmp);
            tmp_pv = SvPV_nolen(tmp);
            Move(pv, tmp_pv + size, len, char);
            SvCUR_set(tmp, len + size);
            sv_setsv(sv, tmp);
        }
 
        if (reserve)
            sv_chop(sv, SvPVX(sv) + reserve);
    }
    return SvPVX(sv);
}

[download]

In reply to Re^2: XS Prepending space to an SVs PV by salva
in thread XS Prepending space to an SVs PV by BrowserUk

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