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

[Trent Mick]

Discussion:

Fix overflow bug in ldexp(x, exp). The 'exp' argument maps to a C int for the
math library call [double ldexp(double, int)], however the 'd'
PyArg_ParseTuple formatter was used to yield a double, which was subsequently
cast to an int. This could overflow.

Actually, this patch depends on my earlier patch to make the 'i' formatter
throw an Overflow exception, see:
http://www.python.org/pipermail/patches/2000-May/000607.html

This is a very unlikely (ldexp with a huge exponent) case but...

>>> from math import *
>>> pow(2, 31) # max C 'int' value + 1
2147483648.0
>>>
>>> ldexp(1, 1)
2.0
>>> ldexp(1, 100)
1.26765060023e+30
>>> ldexp(1, 10000)
inf
>>> ldexp(1, 100000)
inf
>>> ldexp(1, 2147483647)
inf
>>> ldexp(1, 2147483648)
OverflowError: integer literal too large
>>> ldexp(1, 2147483648L) # this overflows the int to s large negative
0.0
>>>


With this patch (and an earlier one of mine)...

>>> from math import *
>>> ldexp(1, 100)
1.2676506002282294e+30
>>> ldexp(1, 2147483647)
inf
>>> ldexp(1, 2147483648L)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
OverflowError: long int too long to convert
>>>


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Patch:


diff -c3  /home/trentm/main/contrib/python/dist/src/Modules/mathmodule.c ./Modules/mathmodule.c
*** /home/trentm/main/contrib/python/dist/src/Modules/mathmodule.c	Fri May  5 10:53:32 2000
--- ./Modules/mathmodule.c	Fri May  5 15:41:43 2000
***************
*** 196,208 ****
  	PyObject *self;
  	PyObject *args;
  {
! 	double x, y;
! 	/* Cheat -- allow float as second argument */
!         if (! PyArg_Parse(args, "(dd)", &x, &y))
  		return NULL;
  	errno = 0;
  	PyFPE_START_PROTECT("ldexp", return 0)
! 	x = ldexp(x, (int)y);
  	PyFPE_END_PROTECT(x)
  	CHECK(x);
  	if (errno != 0)
--- 196,208 ----
  	PyObject *self;
  	PyObject *args;
  {
! 	double x;
! 	int exp;
! 	if (! PyArg_Parse(args, "(di)", &x, &exp))
  		return NULL;
  	errno = 0;
  	PyFPE_START_PROTECT("ldexp", return 0)
! 	x = ldexp(x, exp);
  	PyFPE_END_PROTECT(x)
  	CHECK(x);
  	if (errno != 0)



-- 
Trent Mick
trentm@activestate.com