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Subject: Re: Floating point exception in strtod()
To: cygwin@cygwin.com
References: <dd5e98ea-8f39-1c4b-116c-37968cb3a52d@cornell.edu> <20180409094706.GO18998@calimero.vinschen.de>
From: Ken Brown <kbrown@cornell.edu>
Message-ID: <c4afb320-e963-adf8-ea74-4c625729336c@cornell.edu>
Date: Mon, 9 Apr 2018 08:09:20 -0400
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On 4/9/2018 5:47 AM, Corinna Vinschen wrote:
> On Apr  7 13:40, Ken Brown wrote:
>> $ cat strtod_test.c
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <fenv.h>
>>
>> int
>> main ()
>> {
>>    /* The following number comes from /usr/share/asymptote/ode.asy.  */
>>    const char *str = "121645100408832000.0";
>>    char *ptr;
>>
>>    feenableexcept (FE_INVALID);
>>    strtod (str, &ptr);
>>
>>    /* If there was an exception, the following will not get executed.  */
>>    printf ("No exception.\n");
>> }
>>
>> $ gcc strtod_test.c
>>
>> $ ./a
>> Floating point exception (core dumped)
>>
>> [The above was on x86.  On x86_64 there's simply no output.]
>>
>> I have no idea what's special about the number 121645100408832000.0, but the
>> problem goes away if, for example, I replace the leading 1 by 2.
> 
> GDB shows that the exception occurs in newlib/libc/stdlib/strtod.c
> line 1189, in this statment, which looks rather inconspicious at
> first glance:
> 
>     L = (Long)aadj;
> 
> L is of type Long == int32_t, aadj is of type double.  The
> value of aadj at this time is 2529648000.0 == 0x96c75d80 which
> appears to be perfectly valid for a 32 bit int.
> 
> However, on 64 bit for example the assembler statement generating the FP
> exception is
> 
>    cvttsd2si %xmm0,%eax
> 
> It is documented that this statemnt may raise FE_INVALID or FE_INEXACT
> exceptions.  The problem is that the generated 32 bit value is a
> negative signed int value, while the source operand is positive.  So the
> conversion is, in fact, invalid.
> 
> I applied a fix to newlib's strtod, to always use 64 bit ints in this
> place.  This fixes the problem and no exception is raised.

Thanks for the explanation and the quick fix.  I'm sorry for causing 
confusion by also sending this to the newlib list.

Ken


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