Re: Limitation of the mpz_get_str function

[Paul Zimmermann]

   Hi Christophe,

this issue is known since several years:

https://gmplib.org/list-archives/gmp-bugs/2019-September/004633.html

Paul

> From: Christophe Clavier 
> Date: Tue, 16 Feb 2021 14:22:20 +0100
> 
> Dear GMP developers,
> 
> I currently have to deal with huge numbers (several billions decimal 
> digits) which I need to convert in strings of digits with mpz_get_str.
> I noticed that this function produces an incorrect result when the 
> number of digits exceeds 2^31.
> The reason lies in the loop that scans the buffer computed by 
> mpn_get_str and transforms each digit from an integer to an ascii code :
> 
>    for (i = 0; i < str_size; i++)
>      res_str[i] = num_to_text[(int) res_str[i]];
>    res_str[str_size] = 0;
> 
> As i is of type int, when it is incremented from 2^31-1 to 2^31 it 
> becomes negative.
> For the comparison with str_size, which is of type size_t (i.e. unsigned 
> long int), i is casted in unsigned long int and becomes a large positive 
> value near 2^64.
> Thus the comparison is evaluated to false and the loop early terminates.
> 
> I suggest to modify the type of i to long int or to unsigned long int.
> 
> Regards
> 
> Christophe
> 
> 
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Re: Limitation of the mpz_get_str function

[Vincent Lefevre]

On 2021-02-16 14:22:20 +0100, Christophe Clavier wrote:
> I suggest to modify the type of i to long int or to unsigned long int.

long or unsigned long may still be on 32 bits, e.g. under MS Windows.

I think that a more appropriate type in the context of a string
(or more generally an array of char) would be size_t.

-- 
Vincent Lefèvre  - Web: 
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
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Limitation of the mpz_get_str function

[Christophe Clavier]

Dear GMP developers,

I currently have to deal with huge numbers (several billions decimal 
digits) which I need to convert in strings of digits with mpz_get_str.
I noticed that this function produces an incorrect result when the 
number of digits exceeds 2^31.
The reason lies in the loop that scans the buffer computed by 
mpn_get_str and transforms each digit from an integer to an ascii code :


  for (i = 0; i < str_size; i++)
    res_str[i] = num_to_text[(int) res_str[i]];
  res_str[str_size] = 0;

As i is of type int, when it is incremented from 2^31-1 to 2^31 it 
becomes negative.
For the comparison with str_size, which is of type size_t (i.e. unsigned 
long int), i is casted in unsigned long int and becomes a large positive 
value near 2^64.

Thus the comparison is evaluated to false and the loop early terminates.

I suggest to modify the type of i to long int or to unsigned long int.

Regards

Christophe


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