Reverting the internals of an IV to their original states

syphilis has asked for the wisdom of the Perl Monks concerning the following question:

Hi,

As revealed in this demo by Devel::Peek::Dump(), calling foo($sv) can make significant alterations to the internals of $sv:

use strict;
use warnings;
use Devel::Peek;


use Inline C =><<'EOC';

void foo(SV * sv) {
  printf("\n%s\n\n", SvPV_nolen(sv));
/*
  SvPOK_off(sv);
  SvPV_set(sv, NULL);
*/
}

EOC

my $sv = 12345;

Dump $sv;
foo($sv);
Dump($sv);

__END__

Outputs:

SV = IV(0x4ecd70) at 0x4ecd80
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 12345

12345

SV = PVIV(0x4f0088) at 0x4ecd80
  REFCNT = 1
  FLAGS = (IOK,POK,pIOK,pPOK)
  IV = 12345
  PV = 0x25a4448 "12345"\0
  CUR = 5
  LEN = 10
[download]

Inside that XS sub foo() is there a simple way that, having called SVPV_nolen(), the internals of the provided argument can be restored to the states they held just prior to the calling of foo($sv) ?

Inclusion of the commented-out C-code in foo() goes some way to doing that ... but not all the way.
It will change the second Dump() to:

SV = PVIV(0x4f0088) at 0x4ecd80
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 12345
  PV = 0
[download]

That looks about right except that the SV is still PVIV, and the PV slot, although cleared, is still being displayed. Perhaps the PV slot is being displayed simply because the SV is PVIV, and the solution lies in getting the "PVIV" to revert to "IV".

Cheers,
Rob

Comment on Reverting the internals of an IV to their original states Select or Download Code

Replies are listed 'Best First'.
Re: Reverting the internals of an IV to their original states by dave_the_m (Monsignor) on Dec 27, 2021 at 10:31 UTC
In general, perl doesn't have APIs to downgrade SVs. If you're really bothered, make a temporary copy of the value, print using that, then free it. By the way, your code with the SvPV_set() will leak the string buffer each time. Dave.	[reply]
Re^2: Reverting the internals of an IV to their original states by syphilis (Archbishop) on Dec 27, 2021 at 14:15 UTC
If you're really bothered, make a temporary copy of the value, print using that, then free it Yes, that's pretty much my goto approach when I have a sub that can potentially alter the flags of its arguments && I want to avoid doing so. In some cases it can be a bit tedious, and I've often wondered whether it's feasible to instead just allow the flag alteration(s) to occur and then revert it/them. (Hence my question.) For the demo case I provided, just calling `SvPOK_off(sv);` would be sufficient to restore the flags (as reported by Devel::Peek::Dump) to their original values. That would then have $sv Dump()ing to: `SV = PVIV(0x33fea8) at 0x33d0e0 REFCNT = 1 FLAGS = (IOK,pIOK) IV = 12345 PV = 0x2838458 "12345"\0 CUR = 5 LEN = 10` [download] If the objective is to simply keep those flags unchanged, then that objective has now been met. But are there any gotchas associated with leaving $sv in that state ? I'm not really all that bothered about flags being changed - except for when it happens without good reason. As an example of flags being changed for no good reason, consider: `> perl -MStorable -MDevel::Peek -le "$v = 2**70; Dump($v); $f = Stora +ble::freeze(\$v); Dump($v)" SV = NV(0x47e370) at 0x47e388 REFCNT = 1 FLAGS = (NOK,pNOK) NV = 1.18059162071741e+21 SV = PVNV(0x30a3e8) at 0x47e388 REFCNT = 1 FLAGS = (NOK,pIOK,pNOK,IsUV) UV = 18446744073709551615 NV = 1.18059162071741e+21 PV = 0` [download] I don't think we should assume that such an unannounced and unexpected alteration to the flags of `$v` would be of no consequence to the user. Cheers, Rob	[reply] [d/l] [select]
Re^3: Reverting the internals of an IV to their original states by dave_the_m (Monsignor) on Dec 27, 2021 at 15:23 UTC
Well the entire philosophy of the perl core for the last 25 years has been that whenever you want to access the value of an SV, you request the perl core to update that SV into the form you want, then access the appropriate slot for the value (the string or IV or whatever). For the vast majority of use cases this "just works". It's only a few edge cases (like data serialisers and bitwise operators) where this becomes an issue. Note that perl will only set the private versions of the flags if the conversion isn't fully accurate / reversible, such as the pIOK but not IOK in your example above. This is very much tied to the perl language philosophy that values are type agnostic and the type of operation is specified by the operator, e.g. eq versus ==. Although those flag changes are "unannounced", it's the sort of thing that happens to most SVs in most perl programs. I think you'd have to give a practical example of real harm being caused by these flag changes. Dave.	[reply]
Re^4: Reverting the internals of an IV to their original states by syphilis (Archbishop) on Dec 28, 2021 at 00:45 UTC
Re^5: Reverting the internals of an IV to their original states by NERDVANA (Priest) on Dec 30, 2021 at 22:21 UTC
Some notes below your chosen depth have not been shown here
Re^5: Reverting the internals of an IV to their original states by Anonymous Monk on Dec 30, 2021 at 22:23 UTC
Re: Reverting the internals of an IV to their original states by ikegami (Patriarch) on Dec 27, 2021 at 17:49 UTC
calling foo($sv) can make significant alterations to the internals of $sv More precisely, it's stringifying `$sv` that did this. `"$sv"` would have the save effect.	[reply] [d/l] [select]
Re: Reverting the internals of an IV to their original states by etj (Priest) on Dec 27, 2021 at 07:14 UTC
I have to ask: what are you trying to achieve? :-)	[reply]