C++11

#include <thread>//may return 0 when not able to detectconst auto processor_count = std::thread::hardware_concurrency();

Reference: std::thread::hardware_concurrency

In C++ prior to C++11, there's no portable way. Instead, you'll need to use one or more of the following methods (guarded by appropriate #ifdef lines):

• Win32

  SYSTEM_INFO sysinfo;GetSystemInfo(&sysinfo);int numCPU = sysinfo.dwNumberOfProcessors;

• Linux, Solaris, AIX and Mac OS X >=10.4 (i.e. Tiger onwards)

  int numCPU = sysconf(_SC_NPROCESSORS_ONLN);

• FreeBSD, MacOS X, NetBSD, OpenBSD, etc.

  int mib[4];int numCPU;std::size_t len = sizeof(numCPU); /* set the mib for hw.ncpu */mib[0] = CTL_HW;mib[1] = HW_AVAILCPU;  // alternatively, try HW_NCPU;/* get the number of CPUs from the system */sysctl(mib, 2, &numCPU, &len, NULL, 0);if (numCPU < 1) {    mib[1] = HW_NCPU;    sysctl(mib, 2, &numCPU, &len, NULL, 0);    if (numCPU < 1)        numCPU = 1;}

• HPUX

  int numCPU = mpctl(MPC_GETNUMSPUS, NULL, NULL);

• IRIX

  int numCPU = sysconf(_SC_NPROC_ONLN);

• Objective-C (Mac OS X >=10.5 or iOS)

  NSUInteger a = [[NSProcessInfo processInfo] processorCount];NSUInteger b = [[NSProcessInfo processInfo] activeProcessorCount];

This functionality is part of the C++11 standard.

#include <thread>unsigned int nthreads = std::thread::hardware_concurrency();

For older compilers, you can use the Boost.Thread library.

#include <boost/thread.hpp>unsigned int nthreads = boost::thread::hardware_concurrency();

In either case, hardware_concurrency() returns the number of threads that the hardware is capable of executing concurrently based on the number of CPU cores and hyper-threading units.

OpenMP is supported on many platforms (including Visual Studio 2005) and it offers a

int omp_get_num_procs();

function that returns the number of processors/cores available at the time of call.