Limitation of the mpz_get_str function
christophe.clavier at unilim.fr
Tue Feb 16 13:22:20 UTC 2021
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
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.
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