Re: gnuplot numerical errors on Intel MacBook

[Ethan M. <merritt@u.washington.edu>]

On Tuesday 26 September 2006 12:49 pm, Per Persson wrote:
> Hi all,
> sorry for being late to the party, but I've been busy buing a
> house...

Congratulations!

> I don't have access to an Intel Mac, so I can't really test anything
> but checkin 'man tgamma' makes me wonder if  it would help here:
> DESCRIPTION
>       tgamma() calculates the gamma function of x.  lgamma()
> calculates the
>       natural logorithm of the absolute value of the gamma function
> of x.
>       gamma() is the same function as tgamma. Its use is deprecated.
>
> If gamma in fact *is* tgamma then tgamma is likely broken, no?

No, it's more complicated than that.
The problem is that the name "gamma" has historically been used
on some systems to refer to the function that is now called
"lgamma", and on other systems at other times to refer to a
function that is equivalent to "tgamma".  Thus we cannot tell
from the name exactly what its behaviour is.  

Gnuplot's auto-configure, as I understand it, is looking for a
function named "gamma" and assuming that it behaves like lgamma
with additional information about the sign of the result returned
in extern sgngam.  I am worried that on the Mac in question,
"gamma" is instead acting like "tgamma".  That's not broken;
it's just a different naming convention.  My thought was that
in this case we should test first for a function named "tgamma",
and use it along with its slightly different API.


> What confuses me a bit is Dan's configure run :
> There is no check for tgamma on my system.

Dan said he hacked the configure script, but I don't think
he posted a patch.

-- 
Ethan A Merritt
Biomolecular Structure Center
University of Washington, Seattle WA