I wrote a fragment of code dealing with passing pointers to pointers to data for some FFI calls. Reducing it now to SSCC(?)E: if written as 

pack 'P', pack 'P', $data

[download]

then Perl warns "Attempt to pack pointer to temporary value", so it's easy to debug. But in my case, it was something like:

sub foo { pack 'P', $_[0] } foo pack 'P', $data;

[download]

-- no warnings, correct result for single call, but incorrect if return values are not used immediately but e.g. stored. Then I ran some tests:

>perl -MDevel::Peek -we "Dump 1; sub foo{Dump $_[0];$_[0]=1} foo(1)"
SV = IV(0x653478) at 0x653488
  REFCNT = 1
  FLAGS = (IOK,READONLY,PROTECT,pIOK)
  IV = 1
SV = IV(0x653358) at 0x653368
  REFCNT = 1
  FLAGS = (IOK,READONLY,PROTECT,pIOK)
  IV = 1
Modification of a read-only value attempted at -e line 1.

>perl -MDevel::Peek -we "Dump 1+1; sub foo{Dump $_[0];$_[0]=1} foo(1+1
+)"
SV = IV(0x2612cf8) at 0x2612d08
  REFCNT = 1
  FLAGS = (PADTMP,IOK,READONLY,PROTECT,pIOK)
  IV = 2
SV = IV(0xfcb140) at 0xfcb150
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 2

[download]

Why is not PADTMP flag copied to $_[0], and why, if present, it also prevents copying READONLY?

In reply to When aliasing sub arguments to @_ elements, PADTMP, READONLY flags are copied inconsistently by vr

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.