# Calling Julia shared library from Python, linking libraries

**URL:** <https://discourse.julialang.org/t/calling-julia-shared-library-from-python-linking-libraries/9169>\
**Category:** General Usage\
**Created:** [February 19, 2018, 3:43pm UTC](https://discourse.julialang.org/t/calling-julia-shared-library-from-python-linking-libraries/9169 "2018-02-19T15:43:45Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![sdanisch](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sdanisch/32/1406_2.png) [@sdanisch](https://discourse.julialang.org/u/sdanisch)\
**Post date:** [February 19, 2018, 3:43pm UTC](https://discourse.julialang.org/t/calling-julia-shared-library-from-python-linking-libraries/9169/1 "2018-02-19T15:43:45Z")

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Hi all,  
I’m trying to get PackageCompiler to emit Python wrappers for shared libraries compiled from Julia packages.

I got as far as this minimal test:

[https://github.com/JuliaLang/PackageCompiler.jl/tree/sd/py\_so/test/pytest](https://github.com/JuliaLang/PackageCompiler.jl/tree/sd/py_so/test/pytest)

Which actually works fine and registers a c-extension with python, but when I import it in python it throws:

```julia
>>> import hello

/usr/local/lib/python2.7/dist-packages/hello-1.0-py2.7-linux-x86_64.egg/hello.py:3: UserWarning: Module hello was already imported from /usr/local/lib/python2.7/dist-packages/hello-1.0-py2.7-linux-x86_64.egg/hello.pyc, but /home/s/.julia/v0.6/PackageCompiler/test/pytest is being added to sys.path

fatal: error thrown and no exception handler available.

Base.InitError(mod=:Sys, error=ErrorException("could not load symbol "jl_cpu_cores":

python: undefined symbol: jl_cpu_cores"))

```

The linking + compilation commands setuptools generate from the above linked setup.py are:

```julia
creating build/temp.linux-x86_64-2.7

x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fno-strict-aliasing -Wdate-time -D_FORTIFY_SOURCE=2 -g -fstack-protector-strong -Wformat -Werror=format-security -fPIC -DJULIA_ENABLE_THREADING=1 -I/home/s/juliastuff/julia6_2/include/julia -I/usr/include/python2.7 -c pymodule.c -o build/temp.linux-x86_64-2.7/pymodule.o

creating build/lib.linux-x86_64-2.7

x86_64-linux-gnu-gcc -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions -Wl,-Bsymbolic-functions -Wl,-z,relro -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -Wdate-time -D_FORTIFY_SOURCE=2 -g -fstack-protector-strong -Wformat -Werror=format-security -Wl,-Bsymbolic-functions -Wl,-z,relro -Wdate-time -D_FORTIFY_SOURCE=2 -g -fstack-protector-strong -Wformat -Werror=format-security build/temp.linux-x86_64-2.7/pymodule.o /home/s/.julia/v0.6/PackageCompiler/test/pytest/pyshared.o -L/home/s/juliastuff/julia6_2/lib -Wl,-R/home/s/juliastuff/julia6_2/lib -Wl,-R/home/s/juliastuff/julia6_2/lib/julia -Wl,-R/home/s/.julia/v0.6/PackageCompiler/test/pytest/ -ljulia -o build/lib.linux-x86_64-2.7/hello.so -DJULIA_ENABLE_THREADING=1 -std=gnu99 -fPIC -Wl,--export-dynamic

creating build/bdist.linux-x86_64

creating build/bdist.linux-x86_64/egg

copying build/lib.linux-x86_64-2.7/hello.so -> build/bdist.linux-x86_64/egg

creating stub loader for hello.so

byte-compiling build/bdist.linux-x86_64/egg/hello.py to hello.pyc

```

I basically try to link [pyshared.o](https://github.com/JuliaLang/PackageCompiler.jl/blob/sd/py_so/test/pytest/setup.py#L6) to the [c extension](https://github.com/JuliaLang/PackageCompiler.jl/blob/sd/py_so/test/pytest/pymodule.c). pyshared.o is generated from [pyshared.jl](https://github.com/JuliaLang/PackageCompiler.jl/blob/sd/py_so/test/pytest/pyshared.jl) with those [commands](https://github.com/JuliaLang/PackageCompiler.jl/blob/sd/py_so/src/static_julia.jl#L182).

I hope, that there is just a wrong flag somewhere, and someone with a bit more experience with julia internals and linking libraries can quickly spot the mistake 😉

Best,  
Simon

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<div class="post-metadata">

**Author:** ![dm3](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dm3/32/2755_2.png) [@dm3](https://discourse.julialang.org/u/dm3)\
**Post date:** [May 13, 2018, 7:43am UTC](https://discourse.julialang.org/t/calling-julia-shared-library-from-python-linking-libraries/9169/2 "2018-05-13T07:43:17Z")

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Running into the same error when embedding Julia in Java:

```julia
g++ -I$JNI_ARCH_INC -I$JNI_INC -I$JULIA_INC -fPIC -Wall -c -g -o target/interop.o interop.cpp
g++ -o target/libinterop.so target/interop.o -L$JULIA_LIB -ljulia -ldl -shared -Wl,-rpath,"$JULIA_LIB" -Wl,-rpath,"$JULIA_LIB/julia"

```

@sdanisch - did you ever find out what was going on?

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<div class="post-metadata">

**Author:** ![dm3](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dm3/32/2755_2.png) [@dm3](https://discourse.julialang.org/u/dm3)\
**Post date:** [May 14, 2018, 8:23am UTC](https://discourse.julialang.org/t/calling-julia-shared-library-from-python-linking-libraries/9169/3 "2018-05-14T08:23:20Z")

</div>

Mitigated by explicitly specifying the `LD_PRELOAD=libjulia.so`. It’s a hack, so I’d like to solve the issue properly. Somehow the shared Julia lib isn’t picked up even though all the information is present in the `libinterop.so` which I load from Java.

Would be grateful for any hints.