Re: [Rd] precision when calling a C function; presence of Fortran call

From: Duncan Murdoch <murdoch_at_stats.uwo.ca>
Date: Tue 19 Dec 2006 - 21:24:42 GMT

On 12/19/2006 4:16 PM, Benjamin Tyner wrote: > Thanks; I'm on 32-bit linux, but it's good to know the behavior on > Windows. Do you know a way to force "floor" to use this reduced precision?

Force the value to be stored to a "double" and it will get the reduced precision. You can do this by assigning it to a numeric variable in R and then passing it back; you can presumably do it by some trickery in your compiler, but watch out for optimizations that leave it in an 80 bit register the whole time.

Duncan Murdoch

> 
> Ben
> 
> Duncan Murdoch wrote:

>> On Windows, it's fairly common for runtime libraries to switch the
>> precision from 80 bit to 64 bit. R on Windows tries to guard against
>> this, and on your test package, I get
>>
>> when f computed by R, C says 40 by itself
>> when f computed by R, C says 40, Fortran says 40
>> when f computed by C, C says 40 by itself
>> when f computed by C, C says 40, Fortran says 40
>>
>> It's possible that the floating point change is happening on your
>> platform, or happened before you even got to this call.
>>
>> Here's code that works in Windows to detect such a change across a
>> LoadLibrary call:
>>
>> rcw = _controlfp(0,0) & ~_MCW_IC; /* Infinity control is ignored */
>> _clearfp();
>> tdlh = LoadLibrary(path);
>> dllcw = _controlfp(0,0) & ~_MCW_IC;
>>
>> No idea if _controlfp() exists on your system.
>>
>> Duncan Murdoch


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