POSIX equivalent of boost::thread::hardware_concurrency [duplicate]

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Programmatically find the number of cores on a machine

What is the POSIX or x86, x86-64 specific system call to determine the max number of threads the system can run without over-subscription? Thank you.

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It uses C-compatible constructs, so why not just use the actual code? [libs/thread/src/*/thread.cpp]

using pthread library:

unsigned thread::hardware_concurrency()
{
#if defined(PTW32_VERSION) || defined(__hpux)
    return pthread_num_processors_np();
#elif defined(__APPLE__) || defined(__FreeBSD__)
    int count;
    size_t size=sizeof(count);
    return sysctlbyname("hw.ncpu",&count,&size,NULL,0)?0:count;
#elif defined(BOOST_HAS_UNISTD_H) && defined(_SC_NPROCESSORS_ONLN)
    int const count=sysconf(_SC_NPROCESSORS_ONLN);
    return (count>0)?count:0;
#elif defined(_GNU_SOURCE)
    return get_nprocs();
#else
    return 0;
#endif
}

in windows:

unsigned thread::hardware_concurrency()
{
    SYSTEM_INFO info={{0}};
    GetSystemInfo(&info);
    return info.dwNumberOfProcessors;
}