How to override -DNDEBUG compile flag when building a Cython module

Question

I have a Cython module that calls a C++ function via cdef extern. The C++ function has assert() statements, and I would like to check those assertions. However, when I create the module by calling python setup.py build_ext --inplace, GCC is always invoked with -DNDEBUG. Whenever the code is run, the assertions are not checked.

I can't find a way to override -DNDEBUG using setup.py. Is this possible?

Currently the only way I have found to deal with this is to manually call Cython, GCC, and g++ with the options that are used by python setup.py, but to take out -DNDEBUG. But there must be a simpler way.

See http://stackoverflow.com/questions/4541565/how-can-i-assert-from-python-c-code. — kirbyfan64sos, Jun 18 '14 at 01:27

score 12 · Answer 1 · edited Oct 30 '22 at 22:03

12

You can undefine NDEBUG in your setup.py file. Just use the undef_macros option when defining your extension:

extensions = [ Extension ( "myprog", [ mysrc.c ], undef_macros = [ "NDEBUG" ] ) ]

In the build output you'll see -DNDEBUG followed by -UNDEBUG, which overrides it. For more information on Extension options, see the distutils documentation.

Note, however, that an assert triggered in an extension module will cause the Python, or IPython, interpreter to exit.

edited Oct 30 '22 at 22:03

Peter Mortensen

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answered Jun 11 '15 at 17:47

orodbhen

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setting CFLAGS="-UNDEBUG" is also an alternative to editing setup.py – Bjoern Dahlgren Aug 20 '17 at 06:23
Is there a convention for undefining NDEBUG just for test/non-release builds? – TheNeuralBit Aug 31 '22 at 18:26

score 1 · Accepted Answer · answered Jun 18 '14 at 01:40

You can manually undefine NDEBUG, if it is defined, prior to including <cassert>. Add the following lines to the top of the cpp file which contains these assert statements. Make sure these are the very first statements in that file.

#ifdef NDEBUG
# define NDEBUG_DISABLED
# undef NDEBUG
#endif
#include <cassert>

#ifdef NDEBUG_DISABLED
# define NDEBUG        // re-enable NDEBUG if it was originally enabled
#endif

// rest of the file

This will ensure that NDEBUG is not defined when the processor includes <cassert>, which will result in the assertion checks being compiled into your code.

How to override -DNDEBUG compile flag when building a Cython module

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