memory allocation

Paul Zimmermann Paul.Zimmermann at
Thu Jun 3 22:34:09 CEST 2010


> > Why does GMP allocate 3 limbs for z (which fits into 2 limbs), and allocate
> > 3 limbs for r (which fits into 1 limb)?
> Hello,
> does it cause trouble somehow? It is only a constant number (1 for the 
> string, 2 for the division, in the worst case) of extra limbs, the 
> documentation mentions this can happen and many gmp functions do it (the 
> worst being maybe z-z which allocates size(z)+1).

yes it causes trouble in MPFR, where we use mpz functions with our own
allocation function instead of mpz_init, to replace calls to malloc() by
calls to alloca(). However this approch works only if we are able to bound
the maximal allocated size, since if mpz_ functions call realloc() we are dead.
The problem is that the maximal allocated size (wrt actual size) in mpz is
not documented, and we can only guess it.


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