[Patches] fix overflow bug in ldexp(x, exp) (original) (raw)

Guido van Rossum guido@python.org
Tue, 09 May 2000 10:05:27 -0400

[me]

> On Solaris 2.7, I'm now getting this: > > >>> ldexp(1, 100000000) > Traceback (most recent call last): > File "", line 1, in ? > OverflowError: math range error > >>> ldexp(1, sys.maxint-1) > Traceback (most recent call last): > File "", line 1, in ? > OverflowError: math range error > >>> ldexp(1, sys.maxint) # this takes a second or two... > Infinity # ...expected OverflowError! > >>> > > I have a feeling though that this is a bug in the C math library -- > not sure if it's worth fixing. On Linux I get an OverflowError as > expected for sys.maxint.

[Trent]

It seems to me, as well, that that is a C math library bug. It is not returning an ERANGE errno. Can you reproduce this with a simple C program, on Solaris?

#include <stdio.h> #include <math.h> #include <errno.h> #include <linits.h>

<limits.h> !

int main(void) { double x; printf("ldexp(1.0, %d)=%lf\n", INTMAX, ldexp(1.0, INTMAX)); printf("errno = %d (ERANGE=%d)\n", errno, ERANGE); }

This takes over a second to run and then prints

ldexp(1.0, 2147483647)=Inf errno = 0 (ERANGE=34)

Clearly a C library bug. I won't bother to report it to Sun.

> Note that ldexp(1, 3.14) is still accepted! The 'i' format calls > tpint for floats and int(3.14) is 3.

That is another matter. I don't know what I should expect the 'i' formatter to do. It calls PyIntAsLong. Should PyIntAsLong complain if it is given a PyFloatObject? That is a philosophical question that is kind of moot here (its behaviour is not going to change).

See other discusssion. I agree.

I think that ldexp() should not accept a float for the 'exp' argument because the rounding to an int is not obvious to the user. I suppose the 'O&' formatter could be used with a special converter that raises a TypeError if the argument is not an integral type (i.e. PyInt or PyLong). Is it worth making that change? Is it even desired to make that change because it might break some code out there.

See other discussion. It's not worth the change -- too many other places suffer in the same way.

--Guido van Rossum (home page: http://www.python.org/~guido/)