Hello all,
A couple of days ago I installed a 64-bit Linux distro instead of a
32-bit one, and discovered an interesting bug in some C++ code I'd written.
The bug comes down to this little bit of code:
unsigned long int M_ul = floor(log2(ULONG_MAX-1))-1;
unsigned long int M_rng = floor(log2(r->type->max-1));
unsigned long int M = min(M_ul,M_rng);
unsigned long int N = 1<<M;
... where min() is from the <algorithm> header and floor() is from <cmath>.
The aim here is that N should be the largest power of 2 such that
-- it is within the range of the random number generator, and
-- 2*N is within the range of the unsigned long int implementation
of the machine.
This is important because later on in the code I will call,
gsl_rng_uniform_int(rng,N);
... which is where the code, compiled on my 64-bit system, falls over
with an error,
gsl: ../gsl/gsl_rng.h:200: ERROR: invalid n, either 0 or exceeds
maximum value of generator
Further investigation reveals that despite the bit of code to make sure
N is within the desired limits, N is being set to 2^64, which is WAY
beyond the maximum range of the RNG. (I'm using mt19937; incidentally,
the reason the above code got written in the first place is because once
I used ranlux, which has a maximum value less than the range of a 32-bit
unsigned long int.)
More weirdly still, M = min(M_ul,M_rng) is being set to the correct
value of 31.
I suspect the solution to this problem is not really gsl-related, but
thought I'd post the query here as a likely source of insight and
interest ... :-)
Thanks & best wishes,
-- Joe
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