Programmatically find the number of cores on a machine

C++11

#include <thread>

//may return 0 when not able to detect
const auto processor_count = std::thread::hardware_concurrency();

Reference: std::thread::hardware_concurrency

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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.