Re: Building Cygwin fails with "newlib-cygwin/winsup/cygwin/math/truncl.c:24:15: error: ‘FE_TOWARDZERO’ undeclared (first use in this function)" ...

[Jon Turney]

On 02/07/2026 04:44, Mark Geisert via Cygwin wrote:
> Apologies, but I have to correct what I wrote previously...
> 
> On 7/1/2026 4:40 PM, Mark Geisert via Cygwin wrote:
>> Hi Roland,
>>
>> On 7/1/2026 9:43 AM, Roland Mainz via Cygwin wrote:
>>> Hi!
>>>
>>> ----
>>>
>>> Is anyone else seeing this build failure:
>>> ---- snip ---
>>> $ uname -a
>>> CYGWIN_NT-10.0-19045 bloodbath039 3.6.9-1.x86_64 2026-04-21 15:46 UTC
>>> x86_64 Cygwin
>>> $ git clone -b cygwin-3_6-branch git://cygwin.com/git/newlib-cygwin.git
>>> $ cd newlib-cygwin/winsup/
>>> $ ./autogen.sh
>>> $ cd ../..
>>> $ mkdir testinstall && cd testinstall/
>>> $ /home/roland_mainz/work/cygwin/newlib-cygwin/winsup/configure
>>> --prefix=/home/roland_mainz/work/cygwin/testinstall
>>> ...
>>> $ time nice make
>> [...]
>> Looks like you're building in the source directory.
>    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> I was incorrect here; I misled myself about your directory structure.
> 
> I now believe the main problem is that you ran the incorrect 
> 'configure'.  It should be the one in /home/.../cygwin/newlib-cygwin/. 
> Both newlib and winsup trees need to be built to complete cygwin1.dll.

Yeah, you need to configure from the top level directory.

But you don't need to guess this, the instructions at [1] say that!

[1] https://cygwin.com/faq.html#faq.programming.building-cygwin

> Another problem is that the "--prefix=" argument to configure should 
> name yet another new directory for installing, not the one you're 
> building in.  Don't name your current Cygwin installation root here!

As an aside: using prefix to control the installation location (as this 
example does) is a bad habit to get into.

(because prefix-based paths can end up baked into the built objects. In 
our particular case it's not too bad, as <prefix>/etc/cygserver.conf is 
probably the only instance of this, but other projects might be more 
severely effected.)

Ideally, use make DESTDIR=<install-staging-location> instead.

This FAQ has been revised in git, but the results aren't deployed yet [2].

[2] 
https://cygwin.com/doc/preview/faq/faq.html#faq.programming.building-cygwin