MATLAB not running threads from C++/C and JAVA

Given the attached example (which is actually a *.c file, not *.m), this does not seem to run properly on Windows Matlab. On Linux it's fine and uses its designated eight threads. On Windows, I'm using the mingw64 compiler and it simply won't work. I've been trying other flavors of C/C++ import (using a precompiled library instead; using pthreads) and it still won't use more than one CPU core. The funny thing is, if I write a dummy program to test the library directly (feed it with some large arrays directly from an "int main()" in a separate *.cpp), it works fine at takes up all the cores...

Is there some way in which MATLAB prevents me from making actual threads? Or does it prevent the OS from distributing them?

Edit: Using JAVA Threads doesn't work either... :( (works fine on Linux, however)