Python extension using a C library with one 'hello' function

Jason Swails jason.swails at gmail.com
Tue Nov 4 16:30:00 CET 2014


On Tue, 2014-11-04 at 16:22 +0630, Veek M wrote:
> https://github.com/Veek/Python/tree/master/junk/hello
> doesn't work.
> I have:
> hello.c which contains: int hello(void);
> hello.h
> 
> To wrap that up, i have:
> hello.py -> _hello (c extension) -> pyhello.c -> method py_hello()
> 
> People using this will do:
> python3.2>> import hello
> python3.2>> hello.hello()
> 
> It doesn't compile/work. 
> 
> deathstar> python setup.py build_ext --inplace
> running build_ext
> building '_hello' extension
> creating build
> creating build/temp.linux-x86_64-3.2
> gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -
> D_FORTIFY_SOURCE=2 -g -fstack-protector --param=ssp-buffer-size=4 -Wformat -
> Werror=format-security -fPIC -I/usr/include/python3.2mu -c pyhello.c -o 
> build/temp.linux-x86_64-3.2/pyhello.o
> pyhello.c:15:6: warning: character constant too long for its type [enabled 
> by default]
> pyhello.c:15:5: warning: initialization makes pointer from integer without a 
> cast [enabled by default]
> pyhello.c:15:5: warning: (near initialization for 'hellomethods[0].ml_name') 
> [enabled by default]
> pyhello.c:15:5: warning: initialization from incompatible pointer type 
> [enabled by default]
> pyhello.c:15:5: warning: (near initialization for 'hellomethods[0].ml_meth') 
> [enabled by default]
> pyhello.c:15:5: warning: initialization makes integer from pointer without a 
> cast [enabled by default]
> pyhello.c:15:5: warning: (near initialization for 
> 'hellomethods[0].ml_flags') [enabled by default]
> gcc -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions -Wl,-z,relro 
> build/temp.linux-x86_64-3.2/pyhello.o -o 
> /root/github/junk/hello/_hello.cpython-32mu.so
> 
> 
> >>> import hello
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
>   File "hello.py", line 1, in <module>
>     from _hello import *
> ImportError: ./_hello.cpython-32mu.so: undefined symbol: hello

When I try your code, I get the error:

ImportError: dynamic module does not define init function (PyInit__hello)

There were a couple other problems as well.  Like 'hello' in
hellomethods instead of "hello" (note the double quotes).  Also, NULL is
no longer acceptable as a METH_XXXARGS replacement, you need to set it
to METH_NOARGS (or MET_VARARGS if you plan on accepting arguments).

I've found that you also need a NULL sentinel in the hellomethods array
to avoid segfaults on my Linux box.

After fixing these problems, you still need to add "hello.c" to the list
of sources in setup.py to make sure that module is built.

I've submitted a PR to your github repo showing you the changes
necessary to get your module working on my computer.

Hope this helps,
Jason




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