[C++知识库]c++检测cpu内核的数目

以下代码摘自OpenCV源码中的parallel.cpp文件，?翻看源码的时候看到，里面的注释说的事很重要，记录一下。

static
int getNumberOfCPUs_()
{
    /*
     * Logic here is to try different methods of getting CPU counts and return
     * the minimum most value as it has high probablity of being right and safe.
     * Return 1 if we get 0 or not found on all methods.
    */
#if defined CV_CXX11 \
    && !defined(__MINGW32__) /* not implemented (2020-03) */ \

    /*
     * Check for this standard C++11 way, we do not return directly because
     * running in a docker or K8s environment will mean this is the host
     * machines config not the containers or pods and as per docs this value
     * must be "considered only a hint".
    */
    unsigned ncpus = std::thread::hardware_concurrency(); /* If the value is not well defined or not computable, returns 0 */
#else
    unsigned ncpus = 0; /* 0 means we have to find out some other way */
#endif

#if defined _WIN32

    SYSTEM_INFO sysinfo = {};
#if (defined(_M_ARM) || defined(_M_ARM64) || defined(_M_X64) || defined(WINRT)) && _WIN32_WINNT >= 0x501
    GetNativeSystemInfo( &sysinfo );
#else
    GetSystemInfo( &sysinfo );
#endif
    unsigned ncpus_sysinfo = sysinfo.dwNumberOfProcessors;
    ncpus = minNonZero(ncpus, ncpus_sysinfo);

#elif defined __APPLE__

    int numCPU=0;
    int mib[4];
    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;
    }

    ncpus = minNonZero(ncpus, (unsigned)numCPU);

#elif defined CV_CPU_GROUPS_1

#if defined CV_HAVE_CGROUPS
    static unsigned ncpus_impl_cpuset = (unsigned)getNumberOfCPUsImpl("/sys/fs/cgroup/cpuset/cpuset.cpus");
    ncpus = minNonZero(ncpus, ncpus_impl_cpuset);

    static unsigned ncpus_impl_cfs = getNumberOfCPUsCFS();
    ncpus = minNonZero(ncpus, ncpus_impl_cfs);
#endif

    static unsigned ncpus_impl_devices = (unsigned)getNumberOfCPUsImpl("/sys/devices/system/cpu/online");
    ncpus = minNonZero(ncpus, ncpus_impl_devices);

#endif

#if defined _GNU_SOURCE \
    && !defined(__MINGW32__) /* not implemented (2020-03) */ \
    && !defined(__EMSCRIPTEN__) \
    && !defined(__ANDROID__)  // TODO: add check for modern Android NDK

    cpu_set_t cpu_set;
    if (0 == sched_getaffinity(0, sizeof(cpu_set), &cpu_set))
    {
        unsigned cpu_count_cpu_set = CPU_COUNT(&cpu_set);
        ncpus = minNonZero(ncpus, cpu_count_cpu_set);
    }

#endif

#if !defined(_WIN32) && !defined(__APPLE__)

    static unsigned cpu_count_sysconf = (unsigned)sysconf( _SC_NPROCESSORS_ONLN );
    ncpus = minNonZero(ncpus, cpu_count_sysconf);

#endif

    return ncpus != 0 ? ncpus : 1;
}