Programmatically find the number of cores on a machine

c c++


Is there a way to determine how many cores a machine has from C/C++ in a platform-independent way? If no such thing exists, what about determining it per-platform (Windows/*nix/Mac)?

1/4/2015 7:16:56 PM

Accepted Answer


//may return 0 when not able to detect
unsigned concurentThreadsSupported = 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;
    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];
6/27/2016 8:57:26 PM

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.

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