[cfp-interest 3998] Re: signgam

[Jerome Coonen]

Hi, Paul, thanks for your question about the signgam variable associated
with the lgamma family of functions in some systems. In our 26 August
meeting, we determined that this is not a 754 issue, putting it outside the
scope of CFP. We will take no action here.

Thanks again.

-Jerome Coonen


On Mon, Aug 17, 2026 at 4:03 AM Paul Zimmermann <Paul.Zimmermann at inria.fr>
wrote:

>        Hi,
>
> the C standard says nothing about the "signgam" variable that concrete
> implementations of lgamma use, for example with the GNU libc:
>
>      extern int signgam;
>
>      signgam:
>          _XOPEN_SOURCE
>              || /* Since glibc 2.19: */ _DEFAULT_SOURCE
>              || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
>
>      The lgamma(), lgammaf(), and lgammal() functions return the natural
> loga‐
>      rithm  of the absolute value of the Gamma function.  The sign of the
> Gamma
>      function is returned in the external integer signgam declared in
> <math.h>.
>      It is 1 when the Gamma function is positive or zero, -1 when it  is
> nega‐
>      tive.
>
> Should the CFP deal about signgam? Or ignore it?
>
> Paul
>
> PS: I know there is also lgamma_r which returns signgam in the pointer from
> its 2nd argument.
>
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