[FIXED] How to tell distutils to use gcc?

Issue

I want to wrap a test project containing C++ and OpenMP code with Cython, and build it with distutils via a setup.py file. The content of my file looks like this:

from distutils.core import setup
from distutils.extension import Extension
from Cython.Build import cythonize
from Cython.Distutils import build_ext


modules = [Extension("Interface",
                     ["Interface.pyx", "Parallel.cpp"],
                     language = "c++",
                     extra_compile_args=["-fopenmp"],
                     extra_link_args=["-fopenmp"])]

for e in modules:
    e.cython_directives = {"embedsignature" : True}

setup(name="Interface",
     cmdclass={"build_ext": build_ext},
     ext_modules=modules)

The -fopenmp flag is used with gcc to compile and link against OpenMP. However, if I just invoke

cls ~/workspace/CythonOpenMP/src $ python3 setup.py build

this flag is not recognized, because the compiler is clang:

running build
running build_ext
skipping 'Interface.cpp' Cython extension (up-to-date)
building 'Interface' extension
cc -Wno-unused-result -fno-common -dynamic -DNDEBUG -g -O3 -Wall -Wstrict-prototypes -I/usr/local/include -I/usr/local/opt/sqlite/include -I/usr/local/Cellar/python3/3.3.0/Frameworks/Python.framework/Versions/3.3/include/python3.3m -c Interface.cpp -o build/temp.macosx-10.8-x86_64-3.3/Interface.o -fopenmp
clang: warning: argument unused during compilation: '-fopenmp'
cc -Wno-unused-result -fno-common -dynamic -DNDEBUG -g -O3 -Wall -Wstrict-prototypes -I/usr/local/include -I/usr/local/opt/sqlite/include -I/usr/local/Cellar/python3/3.3.0/Frameworks/Python.framework/Versions/3.3/include/python3.3m -c Parallel.cpp -o build/temp.macosx-10.8-x86_64-3.3/Parallel.o -fopenmp
clang: warning: argument unused during compilation: '-fopenmp'
Parallel.cpp:24:10: warning: unknown pragma ignored [-Wunknown-pragmas]
        #pragma omp parallel for
                ^
1 warning generated.
c++ -bundle -undefined dynamic_lookup -L/usr/local/lib -L/usr/local/opt/sqlite/lib build/temp.macosx-10.8-x86_64-3.3/Interface.o build/temp.macosx-10.8-x86_64-3.3/Parallel.o -o build/lib.macosx-10.8-x86_64-3.3/Interface.so -fopenmp
ld: library not found for -lgomp
clang: error: linker command failed with exit code 1 (use -v to see invocation)
error: command 'c++' failed with exit status 1

I've unsucessfully tried to specify gcc:

cls ~/workspace/CythonOpenMP/src $ python3 setup.py build --compiler=g++-4.7
running build
running build_ext
error: don't know how to compile C/C++ code on platform 'posix' with 'g++-4.7' compiler

How can I tell distutils to use gcc?

Solution

Try setting the "CC" environment variable from inside the setup.py with os.environ.

Answered By - timotimo

Answer Checked By - Terry (PHPFixing Volunteer)

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[FIXED] What is this 'bad:' label generated by Cython?

Issue

While cythonizing my Cython source code files, I can see a dozen of warnings about a label named 'bad:' generated by Cython, for example:

read_input.cpp:30037:3: warning: label ‘bad’ defined but not used [-Wunused-label]

The C++ generated function is like this:

static PyObject* __pyx_convert__to_py_struct__VehicleCaps(struct VehicleCaps s) {
  PyObject* res;
  PyObject* member;
  res = __Pyx_PyDict_NewPresized(0); if (unlikely(!res)) return NULL;
  return res;
  bad:
  Py_XDECREF(member);
  Py_DECREF(res);
  return NULL;
}

The 'bad:' label is in there, I don't get it why Cython is generating this unused label and it shows warnings.

Do I have really fix these warnings? or it's safe to leave them untouched?

Solution

It's for goto bad if something fails in the function, but it doesn't look like anything can fail, so it's unused.

It isn't a problem so you can ignore it. But Cython generally tries not to generate unused labels, so feel free to report it as a (small) bug

Answered By - DavidW

Answer Checked By - David Marino (PHPFixing Volunteer)

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