Programmatically find the number of cores on a machine

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)?

C++11

#include 

//may return 0 when not able to detect
const 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];

Stack Overflow Q#150355, score: 872