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GeezerVison is showing github.com — bpo-21536: On Cygwin, C extensions must be linked with libpython by embray · Pull Request #13549 · python/cpython

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@embray

@embray embray commented May 24, 2019 •
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It is also possible to link against a library or executable with a
statically linked libpython, but not both with the same DLL. In fact
building a statically linked python is currently broken on Cygwin
for other (related) reasons.

The same problem applies to other POSIX-like layers over Windows
(MinGW, MSYS) but Python's build system does not seem to attempt
to support those platforms at the moment.

Follow-up to #12946

https://bugs.python.org/issue21536

@vstinner

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@embray
bpo-21536: On Cygwin, C extensions must be linked with libpython in
effceb0 
general.

It is also possible to link against a library or executable with a
statically linked libpython, but not both with the same DLL.  In fact
building a statically linked python is currently broken on Cygwin
for other (related) reasons.

The same problem applies to other POSIX-like layers over Windows
(MinGW, MSYS) but Python's build system does not seem to attempt
to support those platforms at the moment.
@the-knights-who-say-ni the-knights-who-say-ni added the CLA signed label May 24, 2019
@bedevere-bot bedevere-bot added the awaiting review label May 24, 2019
embray
embray commented May 24, 2019
View reviewed changes
Comment thread Lib/distutils/command/build_ext.py
if get_config_var('ANDROID_API_LEVEL') != 0:
link_libpython = True
elif get_config_var('MACHDEP') == 'cygwin':
link_libpython = True

@embray embray May 24, 2019

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Here I just mostly "unwrapped" the single if condition above into multiple explicit branches, which I think makes the logic clearer, especially with more cases added on...

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@vstinner vstinner added the skip news label May 24, 2019
@vstinner
vstinner merged commit c994c8f into python:master May 24, 2019
@bedevere-bot bedevere-bot removed the awaiting review label May 24, 2019
@vstinner

vstinner commented May 24, 2019

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LGTM.I merged your PR, thanks!
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@embray

embray commented May 24, 2019

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Thank you, though perhaps it was too quick! There is a bug in my logic in configure.ac.
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@vstinner

vstinner commented May 24, 2019

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Thank you, though perhaps it was too quick! There is a bug in my logic in configure.ac.

Ah? I didn't notice. Write a second PR in that case ;-)
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embray
embray commented May 24, 2019
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Comment thread configure
# On Android and Cygwin the shared libraries must be linked with libpython.

if test -z "$ANDROID_API_LEVEL"; then
if test -z "$ANDROID_API_LEVEL" -o "$MACHDEP" != "cygwin"; then

@embray embray May 24, 2019

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This should be -a not -o technically, but it would probably be even clearer if the test weren't negated, and were instead

if test -n "$ANDROID_API_LEVEL" -o "$MACHDEP" = "cygwin";

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@vstinner vstinner May 24, 2019

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"even clearer if the test weren't negated" yeah, please invert the test conditions to avoid double negation ;-)

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@embray embray May 24, 2019

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Glad we're on the same page. It's the double-negation that tripped me up in the first place, and probably you too.

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@vstinner vstinner May 24, 2019

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probably you too.

yes

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@vstinner

vstinner commented May 24, 2019

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Hum, is if test -z "$ANDROID_API_LEVEL" -o "$MACHDEP" != "cygwin"; then the issue?

I'm not comfortable with double negation :-) Maybe write:

if test -n "$ANDROID_API_LEVEL" -o "$MACHDEP" = "cygwin"; then
   LIBPYTHON=...
else
  LIBPYTHON=
fi
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@embray embray mentioned this pull request May 24, 2019
bpo-21536: Follow-up to #13549 #13552
Merged
DinoV pushed a commit to DinoV/cpython that referenced this pull request Jan 14, 2020
@embray @DinoV
bpo-21536: On Cygwin, C extensions must be linked with libpython (pyt…
8789997 
…honGH-13549)

It is also possible to link against a library or executable with a
statically linked libpython, but not both with the same DLL.  In fact
building a statically linked python is currently broken on Cygwin
for other (related) reasons.

The same problem applies to other POSIX-like layers over Windows
(MinGW, MSYS) but Python's build system does not seem to attempt
to support those platforms at the moment.
@pitrou pitrou mentioned this pull request Sep 2, 2022
extension built with a shared python cannot be loaded with a static python #65735
Closed
eli-schwartz
eli-schwartz reviewed Dec 20, 2022
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Comment thread Lib/distutils/command/build_ext.py
Comment on lines +720 to +725
# remain RTLD_LOCAL and so the shared libraries must be linked with
# libpython when python is built with a shared python library (issue
# bpo-21536).
# On Cygwin (and if required, other POSIX-like platforms based on
# Windows like MinGW) it is simply necessary that all symbols in
# shared libraries are resolved at link time.

@eli-schwartz eli-schwartz Dec 20, 2022

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@embray trying to better understand this change...

On Android, before this PR, it is technically possible to refrain from linking to libpython if and only if python itself is configured as a static build.

This Cygwin change applies here also within if get_config_var('Py_ENABLE_SHARED'): -- does Cygwin only need libpython in shared builds, like Cygwin, or all the time, like MSVC?

The commit message glosses over it by pointing out that static Cygwin builds are broken anyway. I am simply curious what the most correct approach is (when implementing similar logic outside of distutils).

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@vstinner vstinner vstinner left review comments
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@eli-schwartz eli-schwartz eli-schwartz left review comments
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@embray @vstinner @eli-schwartz @the-knights-who-say-ni @bedevere-bot
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