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Calling cv::cuda::HOG from python

I need to access the cuda modules of opencv in C++ from python. I tried wrapping the c++ functions using Boost however I get the following error.
ImportError: ../build/lib.linux-x86_64-2.7/pyBoostHOG.so: undefined
symbol: _ZN2cv4cuda6Stream4NullEv.
Is there any way by which I can access the opencv cuda modules from python?
Thanks in advance..

I was facing the same error. And this is how I was compiling the .so file:
g++ `pkg-config opencv4 --cflags --libs` -fPIC -c flow.cpp
g++ -Wall -O3 -shared -o flow.so flow.o `pkg-config opencv4 --cflags --libs`
The error was resolved when I changed my make file to the following:
g++ `pkg-config opencv4 --cflags --libs` -fPIC -c flow.cpp
g++ -Wall -O3 -shared flow.o -o flow.so `pkg-config opencv4 --cflags --libs`

I had the same error.
I figured out that I must include opencv2/core/cuda.hpp file (symbol _ZN2cv4cuda6Stream4NullEv reffers to: cv::cuda::Stream::Null()
But this doesn't solve the problem, because when I tried to make wrap thought swig and include these file, new error:
.../include/opencv2/core/cuda.hpp:317: Error: Syntax error in input(1).
is displayed.
Compiling another program using cv::cuda::Stream::Null() cause not error.

Cython: Compiling and Cannot Find Library Mac OSX 10.12

I am just getting started with Cython and am trying to compile a "Hello World" script. I am trying to use gcc -Os /User/Documents/Python/Test\ Python/helloCopy.c -I/Library/Frameworks/Python.framework/Versions/3.5/include/python3.5m -l, but I don't know what to add after the -l. Other forum pages say to "include -lpython2.7 (or whatever version of Python you're using) on the linker command-line" but that produces ld: library not found for -lpython3.5
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Should I be directing the -l to a particular folder?

I do not know what resource you're using, but this does not say anything about a -l flag. It suggests
cython -a helloCopy.pyx
This creates a yourmod.c file, and the -a switch produces an annotated html file of the source code. Pass the -h flag for a complete list of supported flags.
gcc -shared -pthread -fPIC -fwrapv -O2 -Wall -fno-strict-aliasing -I/usr/include/python2.7 -o helloCopy.so helloCopy.c
(Linux)
On macOS I would try to compile with
gcc -I/usr/bin/python -o helloCopy.so helloCopy.c
to use the standard version of Python.

Fatal Python error when using SWIG under OSX

I have troubles using a Python interface generated with SWIG (I have OSX 10.11.12). After compiling and linking everything together as such:
swig -python erk_integrator.i
gcc -c -fPIC -O3 model.c auxiliary_functions.c timing_functions.c
gcc -c -fPIC -O3 erk_integrator.c erk_integrator_wrap.c -I. -I/usr/local/include/python2.7
gcc -lpython -dynamiclib model.o erk_integrator.o erk_integrator_wrap.o auxiliary_functions.o timing_functions.o -o _erk_integrator.so
I try a test script, but Python throws a fatal error:
/usr/local/bin/python test_erk.py
Fatal Python error: PyThreadState_Get: no current thread
Abort trap: 6
But when I run
/System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7 test_erk.py
everything works as it should. However, I need to use /usr/local/bin/python (from Homebrew) instead of the system Python.
I think something goes wrong in the linking step. Many thanks for helping!

This blog post helped me solve it: blog.tim-smith.us/2015/09/python-extension-modules-os-x
It turns out on OSX you need
-undefined dynamic_lookup
instead of
-lpython

g++ linking and swig

I have a cpp file with functions that I'm using in python with SWIG. I use the following commands to compile the source and create the file to use with python.
swig -c++ -python mini.i
g++ -O2 -c mini.cpp -I/usr/include/python2.4 -I/usr/lib/python2.4
g++ -O2 -c mini_wrap.cxx -I/usr/include/python2.4 -I/usr/lib/python2.4
g++ -shared mini.o mini_wrap.o -o _mini.so
I'm trying now to use GSL in my source cpp source file. If I was just compiling the GSL file I would do
g++ -lgsl -lgslcblas -lm -o mini.o mini.cpp
I've tried adding the -lgsl -lgslcblas -lm to the lines for the swig compile but I get
g++: -lgsl: linker input file unused because linking not done
g++: -lgslcblas: linker input file unused because linking not done
g++: -lm: linker input file unused because linking not done
How can I link the gsl libraries? Thanks

Swig does no linking, as the warning message states. Put the -lgsl etc. on the link command, which is your last g++ command.

how to link python static library with my c++ program

I am implementing a C++ program that uses python/C++ Extensions. As of now I am explicitly linking my program to python static library I compiled. I am wondering is there any way to link my program with system installed python(i mean the default python installation that comes with linux)

Yes. There is a command line utility called python-config:
Usage: /usr/bin/python-config [--prefix|--exec-prefix|--includes|--libs|--cflags|--ldflags|--help]
For linkage purposes, you have to invoke it with --ldflags parameter. It will print a list of flags you have to pass to the linker (or g++) in order to link with system installed python libraries:
$ python-config --ldflags
-L/usr/lib/python2.6/config -lpthread -ldl -lutil -lm -lpython2.6
It also can give you flags to compilation with --cflags parameter:
$ python-config --cflags
-I/usr/include/python2.6 -I/usr/include/python2.6 -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes
Say you have a test program in test.cpp file, then you can do something like this in order to compile and link:
g++ $(python-config --cflags) -o test $(python-config --ldflags) ./test.cpp
That will link your program with shared libraries. If you want to go static, you can pass -static option to the linker. But that will link with all static stuff, including a runtime. If you want to go just with static python, you have to find those libraries yourself. One of the option is to parse python-config --ldflags output and look for libraries with .a extensions. But I'd rather stick to all dynamic or all static.
Hope it helps. Good luck!