mpz_limbs interface

Marc Glisse marc.glisse at
Tue Jan 21 15:30:38 UTC 2014

On Tue, 21 Jan 2014, Niels Möller wrote:

>>   MPFR_ASSERTN (nbits >= 1);
>>   /* To be sure to avoid the potential allocation of mpz_urandomb */
>>   ALLOC(z) = SIZ(z) = MPFR_PREC2LIMBS (nbits);
>>   PTR(z)   = mp;
> [...]
>>   mpz_urandomb (z, rstate, nbits);
> I think this is brittle, and I don't think this style should be
> officially supported in the public gmp interface. mpz_urandomb is not
> included in mini-gmp, but if you, e.g., try the same with other
> functions, it will most likely break with mini-gmp which assigns many of
> the output parameters using mpz_swap.

We already have function mpz_array_init which encourages thinking of the 
buffer as belonging to a variable, with all other functions keeping this 
association as long as the buffer is large enough. Gratuitous extra 
alloc+dealloc pairs can have a horrible performance effect. That's not an 
issue for mini-gmp because performance is not a concern, but it is for 
mpfr and many users, and even if they rewrite the code to accomodate 
mini-gmp, they will probably want to keep the current one for regular gmp 
(with suitable #if).

Marc Glisse

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