OpenMP with CFFI

[Jeppe Druedahl]

I get an load library error when a try to use OpenMP with cffi. Compiling the c-file to an exe-file works just fine.

Any ideas what the problem is?

I am on a Windows 10 machine, I believe I have done something very similar on a Windows 8.1 before where it worked.

test_openmp.c:

#include <stdio.h>
#include <stdlib.h>
#include <omp.h>

void test_openmp()
{
    FILE* log = fopen("log_openmp.txt","w");
    fprintf(log, "I am working (openmp with %d threads)\n",
    omp_get_max_threads());
    fclose(log);
}

int main()
{
 test_openmp();
 return 1;
}

python-fil:

import os
from cffi import FFI

# 1. test that it works (i)
os.system('gcc -fopenmp test_openmp.c -o test_openmp.exe')
os.system('test_openmp.exe')
         
# 2. cffi
cmd = 'gcc -shared -fopenmp test_openmp.c -o test_openmp.so'
os.system(cmd)                            

ffi = FFI()
ffi.cdef(r''' void test_openmp(); ''')

cfuncs = ffi.dlopen("test_openmp.so")
cfuncs.test_openmp()

error:

OSError                                   Traceback (most recent call last)
<ipython-input-4-0b2f5b870168> in <module>()
      8 ffi.cdef(r''' void test_openmp(); ''')
      9
---> 10 cfuncs = ffi.dlopen("test_openmp.so")
     11 cfuncs.test_openmp()


C:\Users\gmf123\AppData\Local\Continuum\Anaconda3\lib\site-packages\cffi\api.py in dlopen(self, name, flags)
    137         assert isinstance(name, basestring) or name is None
    138         with self._lock:
--> 139             lib, function_cache = _make_ffi_library(self, name, flags)
    140             self._function_caches.append(function_cache)
    141             self._libraries.append(lib)


C:\Users\gmf123\AppData\Local\Continuum\Anaconda3\lib\site-packages\cffi\api.py in _make_ffi_library(ffi, libname, flags)
    767     import os
    768     backend = ffi._backend
--> 769     backendlib = _load_backend_lib(backend, libname, flags)
    770     #
    771     def accessor_function(name):


C:\Users\gmf123\AppData\Local\Continuum\Anaconda3\lib\site-packages\cffi\api.py in _load_backend_lib(backend, name, flags)
    756         if '.' not in name and '/' not in name:
    757             raise OSError("library not found: %r" % (name,))
--> 758         return backend.load_library(name, flags)
    759     except OSError:
    760         import ctypes.util


OSError: cannot load library test_openmp.so: error 0x45a

[Armin Rigo]

Hi,

On 28 January 2017 at 01:25, Jeppe Druedahl <jeppe.d...@gmail.com> wrote:
> cmd = 'gcc -shared -fopenmp test_openmp.c -o test_openmp.so'
> os.system(cmd)

Are you compiling main() into the .so, too? Maybe that's the problem:
the .so is not supposed to have a main().

Otherwise, you'll have to use google to figure out what the "error
0x45a" means...


A bientôt,

Armin.

[Jeppe Druedahl]

Thanks for the suggestion. I have tried without the main() without any change.

I havn't been able to figure out what 0x45a means in this context... 

[Armin Rigo]

Hi,

On 28 January 2017 at 13:10, Jeppe Druedahl <jeppe.d...@gmail.com> wrote:
> Thanks for the suggestion. I have tried without the main() without any
> change.

No clue, but maybe it should have the extension ".dll" and not ".so",
as we're on Windows?

> I havn't been able to figure out what 0x45a means in this context...

I can only recommend to look more. Maybe it means some form of
``invalid .dll``, and maybe it occurs because of the extension, as I
said above.


A bientôt,

Armin.