python - Using mkl_set_num_threads with numpy

Question

I'm trying to set the number of threads for numpy calculations with mkl_set_num_threads like this

import numpy
import ctypes
mkl_rt = ctypes.CDLL('libmkl_rt.so')
mkl_rt.mkl_set_num_threads(4)

but I keep getting an segmentation fault:

Program received signal SIGSEGV, Segmentation fault.
0x00002aaab34d7561 in mkl_set_num_threads__ () from /../libmkl_intel_lp64.so

Getting the number of threads is no problem:

print mkl_rt.mkl_get_max_threads()

How can I get my code working? Or is there another way to set the number of threads at runtime?

Answer 1

Ophion led me the right way. Despite the documentation, one have to transfer the parameter of mkl_set_num_thread by reference.

Now I have defined to functions, for getting and setting the threads

import numpy
import ctypes
mkl_rt = ctypes.CDLL('libmkl_rt.so')
mkl_get_max_threads = mkl_rt.mkl_get_max_threads
def mkl_set_num_threads(cores):
    mkl_rt.mkl_set_num_threads(ctypes.byref(ctypes.c_int(cores)))

mkl_set_num_threads(4)
print mkl_get_max_threads() # says 4

and they work as expected.

Edit: according to Rufflewind, the names of the C-Functions are written in capital-case, which expect parameters by value:

import ctypes

mkl_rt = ctypes.CDLL('libmkl_rt.so')
mkl_set_num_threads = mkl_rt.MKL_Set_Num_Threads
mkl_get_max_threads = mkl_rt.MKL_Get_Max_Threads