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Dear all,
I am compiling Python 3.4 on Cygwin 1.7.17.
The following has been done in order to reach the point where _struct module failed.
> A clean Python 3.4
> Applied patches:
 cygwin_si_band.patch in Issue21085
 0001-CYGWIN-issue13756-Python-make-fail-on-cygwin.patch in issue13756
 0019-MINGW-export-_PyNode_SizeOf-as-PyAPI-for-parser-modu.patch in issue186373
> configure + make
The issue happened during make:
building '_struct' extension
gcc -Wno-unused-result -Werror=declaration-after-statement -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I./Include -I. -IInclude -I/cygdrive/c/temp/Python-3.4.0/Include -I/cygdrive/c/temp/Python-3.4.0 -c /cygdrive/c/temp/Python-3.4.0/Modules/_struct.c -o build/temp.cygwin-1.7.17-i686-3.4/cygdrive/c/temp/Python-3.4.0/Modules/_struct.o
/cygdrive/c/temp/Python-3.4.0/Modules/_struct.c:1630:5: error: initializer element is not constant
/cygdrive/c/temp/Python-3.4.0/Modules/_struct.c:1630:5: error: (near initialization for ‘unpackiter_type.ob_base.ob_base.ob_type’)
Please feel free to find the build.log and the output of _struct.c.txt (gcc with -dD -E -DPy_BUILD_core) for more information.
Thanks in advance,
Br,
Dellair

The full build log

This looks like a duplicate of Issue6672.

Neil,
It doesn't look like a duplicate of Issue6672.
The one in Issue6672 was for Mingw, all the patches simply added __MINGW32__ to __CYGWIN__ build structure.
While my issue is, the build failed with _struct.c on Cygwin. So the module is recognized, just didn't pass the compilation.
Br,
Dellair

I wrote a patch file.
In other extention module source codes, global variable PyTypeObject has initialized to using "PyVarObject_HEAD_INIT(NULL, 0)".
And so, as with other modules, I tried to edit and compiling _struct.c in Cygwin 1.7.28.
The module compiling was passing, And struct module passed a test "python3.4 -m test test_struct".

Yamamoto,
Thanks, the patch you offered did make the _struct error disappeared. I will do a bit more testing.
Dellair

Hello masamoto,
The patch you provided works quite well.
The build passed and Python calls are successfully.
Please let me know what else you need me to test in order to have the patch accepted or else feel free to close it with Resolution Fixed.
Thanks,
Dellair

I have solved about compiling _struct module too.
I have no authority of this issues, So please close of issues.

Fixed with the patch.

New changeset 3bde312ae936 by Zachary Ware in branch 'default':
Issue #21124: Fix building _struct on Cygwin.
https://hg.python.org/cpython/rev/3bde312ae936 

For future reference, having a patch attached to the issue does not mean the issue is fixed, and it should not be closed.

PyType_Ready() should be called for unpackiter_type.
See also issue26906.

I wrote a patch to add the unpackiter_type initialization into PyInit__struct function. I has confirmed solve #28337 on ubuntu x86 16.04.

Do we not have a unit test for that?

unpack_iterator type has not been registered into _struct module. And all users get only unpack_iterator object from function returning iterator. The object iterating doesn't need reference to type. Therefore, I think issue finish by PyType_Ready fix because this segfault doesn't have big impact.
However, I think if possible, it is preferable that extension module having un-initialize type objects gets a compile error at build time. And impossible case for compile error is better that wrong extension module always fails import.

New changeset c4eb211fb38b by Zachary Ware in branch 'default':
Closes #21124, #28337: Call PyType_Ready on unpackiter_type.
https://hg.python.org/cpython/rev/c4eb211fb38b 

Thanks for the patch again :).
The last idea you mentioned sounds somewhat interesting, please open a new issue for it if you'd like to pursue it.