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The select and time modules use functions from libm, but do not link against it.
* selectmodule.c calls ceil(3) in pyepoll_poll()
* timemodule.c calls fmod(3) and floor(3) in floatsleep()

Note: This causes problems at least when running on android, where the system is unable to find the symbols when loading the modules at runtime.

Additionally,
* audioop calls floor()
* _ctypes_test calls sqrt()
Patch attached.

> audioop_ctypes_test_link_with_libm.patch 
+ libraries=['m'])
Why not using math_libs here? It would also be nice to add a comment explaining why libm is needed in each module.
Can someone please combine both patches?

>> audioop_ctypes_test_link_with_libm.patch 
> + libraries=['m'])
> Why not using math_libs here?
math_libs is defined in detect_modules(). But the _ctypes_test
extension is defined in a different function: detect_ctypes().
The other option, would be to define math_libs=['m'] directly above this line and then use it once. I didn't think that added clarity, but I'd be happy to do it that way if it fits better with standard style.
> It would also be nice to add a comment explaining why libm is needed in each module.
Done.
> Can someone please combine both patches?
Done.

Bump. For rev c6930661599b timemodule.c and selectmodule.c no longer calls libm functions, while audioop.c and _ctypes_test.c still do. I have my updated patch here based on previous patches. Note that the introduction of detect_math_libs() is my naive try. Its naming and position in setup.py may need more consideration.
Here's the result before and after my patch.
Before:
shell@GT-N7000:/data/local/tmp $ python3
Python 3.6.0a0 (default:6a8fbb97c8d8+, Nov 26 2015, 18:42:29) 
[GCC 4.9 20140827 (prerelease)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import select 
>>> import time
>>> import audioop
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ImportError: dlopen failed: cannot locate symbol "floor" referenced by "audioop.cpython-36m-arm-linux-gnueabi.so"...
>>> import _ctypes_test
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ImportError: dlopen failed: cannot locate symbol "sqrt" referenced by "_ctypes_test.cpython-36m-arm-linux-gnueabi.so"...
>>> 
After:
shell@GT-N7000:/data/local/tmp $ python3
Python 3.6.0a0 (default:c6930661599b+, Nov 26 2015, 19:10:15) 
[GCC 4.9 20140827 (prerelease)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import time
>>> import selectr
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ImportError: No module named 'selectr'
>>> import select 
>>> import audioop
>>> import _ctypes_test
>>> 
Tested on Samsung Galaxy Note GT-N7000 with my custom build of CyanogenMod 11.0

Well, the hg revision of Python in two tests are different, because I `hg pull -u` each time I build.

The _datetime module also calls functions in libm: delta_new() uses
round() and accum() uses modf().
The attached patch updates setup.py for all the modules that use libm.

The approach with a helper function is better. See https://github.com/yan12125/python3-android/blob/038271d/mk/python/modules-link-libm.patch.
By the way, I have verified there are no libraries referencing libm functions with https://github.com/yan12125/python3-android/blob/cpython-hg/devscripts/import_all.py 

New changeset f92fea23161d by Victor Stinner in branch '3.5':
setup.py: add missing libm dependency
https://hg.python.org/cpython/rev/f92fea23161d
New changeset 3a9b47b062b9 by Victor Stinner in branch 'default':
Merge 3.5: Issue #21668
https://hg.python.org/cpython/rev/3a9b47b062b9 

I pushed ext_modules_libm.patch with the helper function proposed by Chi Hsuan Yen in https://github.com/yan12125/python3-android/blob/038271d/mk/python/modules-link-libm.patch.
Tell me if it's not enough. I hope that it will make CPython a little bit more portable ;-)

The patch should be acknowledged in Misc/NEWS to Chi Hsuan Yen and not to me.

New changeset f76753f26982 by Victor Stinner in branch 'default':
Issue #21668: Fix author of the patch.
https://hg.python.org/cpython/rev/f76753f26982 

New changeset 7530caa5ed1a by Victor Stinner in branch 'default':
Issue #21668: Add also Chi Hsuan Yen to Misc/ACKS
https://hg.python.org/cpython/rev/7530caa5ed1a 

> Xavier de Gaye: "The patch should be acknowledged in Misc/NEWS to Chi Hsuan Yen and not to me."
Oh sorry. I see 4 authors in attached patches, I picked the latest. Thank you to all authors for your contribution on this issue ;-) I fixed the author in Misc/NEWS.