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POPropagating exceptions through dlsym cython
 

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    <p>I am unable to propagate exceptions through dlsym. I use dlsym to load a cythonized python file. I made a minimal working example below so you can try it yourself:</p> <p>I have a pyx file, c_fun.pyx, which I compile to a C file using Cython. Then I'm using dlsym to load in the so file in another program, say use_fun.c++. You can use ./compile.sh to compile the files. On executing ./test, the program crashes with a segmentation fault.</p> <pre><code>#c_fun.pyx: cdef public double _myfunction(double x) except*: a=1/0 # This does not propagate an exception. Comment to make the example work return x**2-x # This works. #use_fun.c++ #include &lt;dlfcn.h&gt; #include &lt;stdio.h&gt; int main(int argc, char **argv) { void* handle = dlopen("./c_fun.so", RTLD_NOW | RTLD_GLOBAL); if(handle==NULL) { printf("%s \nat line number: %i\n",dlerror(),__LINE__); return; } double (*f)(double) = (double (*)(double))dlsym(handle, "_myfunction"); if(f==NULL) { printf("%s\n",dlerror()); return; } double res = 0; try { res = (*f)((double)99); } catch(char *err) { printf("Got exception: %s.\n", err); } printf("res = %f\n", res); } #setup.py from distutils.core import setup from distutils.extension import Extension from Cython.Distutils import build_ext ext_modules = [Extension("c_fun", ["c_fun.pyx"], libraries = ['python2.7'], extra_compile_args= ['-fexceptions'])] setup( name = 'Hello world app', cmdclass = {'build_ext': build_ext}, ext_modules = ext_modules ) # compile.sh python setup.py build_ext --inplace echo "gcc use_fun.c++ -g -O0 -o test -ldl" g++ use_fun.c++ -g -O0 -o test -ldl </code></pre> <p>Initially, I tried without the "except*" at the end of my function, and without the "-fexceptions" compiler flag. But adding those do not change the behavior. gdb doesn't even allow me to backtrace the problem, and says: "Cannot find new threads: generic error ". I have looked on the internet for exception handling in combination with dlsym, but have found little information.</p> <p>The rest of the story: To make it more complicated, actually this use_fun.c++ file is a Python module which I imported. So: I'm using Python to load in a the module use_fun, and in that C++ module, _myfunction is called. But also then, I cannot get the exception handling right. However, in that case, I did successfully use gdb to get into the C code and see that PyErr_Format was successfully called. But the error is not triggered nor caught in the C++ code.</p> <p>Note that to be able to expose the file _myfunction, I have specified the 'public' keyword in the pyx file. Without it, the name mangling would cause the dlsym function call to fail. I tried looking for documentation on these two links: </p> <p><a href="http://docs.cython.org/src/userguide/external_C_code.html#using-cython-declarations-from-c" rel="nofollow">http://docs.cython.org/src/userguide/external_C_code.html#using-cython-declarations-from-c</a></p> <p><a href="http://docs.cython.org/src/userguide/language_basics.html#error-return-values" rel="nofollow">http://docs.cython.org/src/userguide/language_basics.html#error-return-values</a></p> <p>Edit: I have found the solution. I'll keep the original question above so it can help others. There were essentially 2 problems</p> <p>1) Of course, since C does not have exceptions, it was wrong to set a try/catch over the function! What does work, is checking whether an error occurred in Python using PyErr_Occurred().</p> <p>2) Since cython generates a module, it has to be initialized before you can properly use it. This means: calling Py_Initialize/Py_Finalize, and also calling the init_cfun method.</p> <p>The solution is shown below:</p> <pre><code>#include "Python.h" #include &lt;dlfcn.h&gt; #include &lt;stdio.h&gt; int main(int argc, char **argv) { Py_Initialize(); void* handle = dlopen("./c_fun.so", RTLD_NOW | RTLD_GLOBAL); if(handle==NULL) { printf("%s \nat line number: %i\n",dlerror(),__LINE__); return -1; } void (*init_func)(); *(void**)(&amp;init_func) = dlsym(handle, "initc_fun"); init_func(); double (*f)(double) = (double (*)(double))dlsym(handle, "_myfunction"); if(f==NULL) { printf("%s\n",dlerror()); return -1; } double res = 0; res = (*f)((double)99); if(PyErr_Occurred()) { PyErr_Print(); } printf("res = %f\n", res); Py_Finalize(); return 0; } </code></pre>
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