Vipul Ved Prakash on Mon, 18 Sep 2000 13:00:58 +0530

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Re: Math::Pari segfaults?

On Sun, Sep 17, 2000 at 03:57:47PM -0400, Ilya Zakharevich wrote:
> This summer I have been debugging a similar problem.  My conclusion was:
> this is the problem sitting deeply in PARI semantic.  Maybe your
> problem is the same.
> PARI's Mod(a,b) may reference the b modulus in several ways, one of
> them being that the object for Mod(a,b) just keeps the pointer to b.
> *Without increasing the refcount of b* (there is no notion of refcount
> in PARI!).  The logic of Math::Pari is such that you cannot control
> whether this way of storing the modulus is used.
> Corollary for Math::Pari: if you use $c = Mod($a,$b), $b should live
> strictly longer than $b.  
> [snip]

Hi Ilya,

After a few wild goose chases, I discovered the bug.  makerandom_itv() (in
Crypt::Random) was doing $c = Mod(0, $b-$a); -- this statement was
triggering the segfault.  I changed it $d = $b-$a; $c = Mod(0, $d); and now
it seems to work fine.  ($a,$b,$c,$d are Math::Pari objects.)

While this seems to have something to do with PARI's non-existent
refcounting mechanism, I don't understand exactly why $c = Mod(0, $b-$a) was
breaking.  I would appreciate if you could explain.

best regards,


VIPUL VED PRAKASH               |  Cryptography                  |  Distributed Systems            |  Network Agents
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