Programmatically find the number of cores on a machine

Question

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

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OpenMP is supported on many platforms  including Visual Studio 2005  and it offers a   int omp get num procs      function that returns the number of processors cores available at the time of call

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One more Windows recipe  use system-wide environment variable NUMBER OF PROCESSORS   printf   d n   atoi getenv  NUMBER OF PROCESSORS

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This functionality is part of the C  11 standard    include  lt thread gt   unsigned int nthreads   std  thread  hardware concurrency      For older compilers  you can use the Boost Thread library    include  lt boost thread hpp gt   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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you can use WMI in  net too but you re then dependent on the wmi service running etc  Sometimes it works locally  but then fail when the same code is run on servers  I believe that s a namespace issue  related to the  names  whose values you re reading

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Almost  Platform Independent function in c-code   ifdef  WIN32  include  lt windows h gt   elif MACOS  include  lt sys param h gt   include  lt sys sysctl h gt   else  include  lt unistd h gt   endif  int getNumCores      ifdef WIN32     SYSTEM INFO sysinfo      GetSystemInfo  amp sysinfo       return sysinfo dwNumberOfProcessors   elif MACOS     int nm 2       size t len   4      uint32 t count       nm 0    CTL HW  nm 1    HW AVAILCPU      sysctl nm  2   amp count   amp len  NULL  0        if count  lt  1            nm 1    HW NCPU          sysctl nm  2   amp count   amp len  NULL  0           if count  lt  1    count   1              return count   else     return sysconf  SC NPROCESSORS ONLN    endif

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If you have assembly-language access  you can use the CPUID instruction to get all sorts of information about the CPU  It s portable between operating systems  though you ll need to use manufacturer-specific information to determine how to find the number of cores  Here s a document that describes how to do it on Intel chips  and page 11 of this one describes the AMD specification

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On Linux  you can read the  proc cpuinfo file and count the cores

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C  11   include  lt thread gt     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  amp 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   amp numCPU   amp len  NULL  0    if  numCPU  lt  1         mib 1    HW NCPU      sysctl mib  2   amp numCPU   amp len  NULL  0       if  numCPU  lt  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

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On Linux  it s may not be safe to to use  SC NPROCESSORS ONLN as it s not part of POSIX standard and the sysconf manual states as much  So there s a possibility that  SC NPROCESSORS ONLN may not be present    These values also exist  but may not be standard                         -  SC NPROCESSORS CONF               The number of processors configured          -  SC NPROCESSORS ONLN               The number of processors currently online  available     A simple approach would be to read  proc stat or  proc cpuinfo and count them    include lt unistd h gt   include lt stdio h gt   int main void    char str 256   int procCount   -1     to offset for the first entry FILE  fp   if   fp   fopen   proc stat    r          while fgets str  sizeof str  fp     if   memcmp str   cpu   3    procCount       if   procCount    -1      printf  Unable to get proc count  Defaulting to 2    procCount 2     printf  Proc Count  d n   procCount   return 0      Using  proc cpuinfo    include lt unistd h gt   include lt stdio h gt   int main void    char str 256   int procCount   0  FILE  fp   if   fp   fopen   proc cpuinfo    r          while fgets str  sizeof str  fp     if   memcmp str   processor   9    procCount       if    procCount       printf  Unable to get proc count  Defaulting to 2    procCount 2     printf  Proc Count  d n   procCount   return 0        The same approach in shell using grep   grep -c  processor  proc cpuinfo   Or  grep -c  cpu  proc stat   subtract 1 from the result

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OS X alternative  The solution described earlier based on   NSProcessInfo processInfo  processorCount  is only available on OS X 10 5 0  according to the docs   For earlier versions of OS X  use the Carbon function MPProcessors     If you re a Cocoa programmer  don t be freaked out by the fact that this is Carbon   You just need to need to add the Carbon framework to your Xcode project and MPProcessors   will be available

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For Win32   While GetSystemInfo   gets you the number of logical processors  use GetLogicalProcessorInformationEx     to get the number of physical processors

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On linux the best programmatic way as far as I know is to use  sysconf  SC NPROCESSORS CONF    or  sysconf  SC NPROCESSORS ONLN    These aren t standard  but are in my man page for Linux

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You probably won t be able to get it in a platform independent way   Windows you get number of processors   Win32 System Information

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hwloc  http   www open-mpi org projects hwloc   is worth looking at  Though requires another library integration into your code but it can provide all the information about your processor  number of cores  the topology  etc

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OpenMP is supported on many platforms  including Visual Studio 2005  and it offers a   int omp get num procs      function that returns the number of processors cores available at the time of call

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Note that  number of cores  might not be a particularly useful number  you might have to qualify it a bit more   How do you want to count multi-threaded CPUs such as Intel HT  IBM Power5 and Power6  and most famously  Sun s Niagara UltraSparc T1 and T2  Or even more interesting  the MIPS 1004k with its two levels of hardware threading  supervisor AND user-level     Not to mention what happens when you move into hypervisor-supported systems where the hardware might have tens of CPUs but your particular OS only sees a few   The best you can hope for is to tell the number of logical processing units that you have in your local OS partition  Forget about seeing the true machine unless you are a hypervisor  The only exception to this rule today is in x86 land  but the end of non-virtual machines is coming fast

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Windows  x64 and Win32  and C  11 The number of groups of logical processors sharing a single processor core   Using GetLogicalProcessorInformationEx  see GetLogicalProcessorInformation as well  size t NumberOfPhysicalCores   noexcept        DWORD length   0      const BOOL result first   GetLogicalProcessorInformationEx RelationProcessorCore  nullptr   amp length       assert GetLastError      ERROR INSUFFICIENT BUFFER        std  unique ptr lt  uint8 t    gt  buffer new uint8 t length        const PSYSTEM LOGICAL PROCESSOR INFORMATION EX info                reinterpret cast lt  PSYSTEM LOGICAL PROCESSOR INFORMATION EX  gt  buffer get          const BOOL result second   GetLogicalProcessorInformationEx RelationProcessorCore  info   amp length       assert result second    FALSE        size t nb physical cores   0      size t offset   0      do           const PSYSTEM LOGICAL PROCESSOR INFORMATION EX current info               reinterpret cast lt  PSYSTEM LOGICAL PROCESSOR INFORMATION EX  gt  buffer get     offset           offset    current info- gt Size            nb physical cores        while  offset  lt  length                return nb physical cores     Note that the implementation of NumberOfPhysicalCores is IMHO far from trivial  i e   quot use GetLogicalProcessorInformation or GetLogicalProcessorInformationEx quot    Instead it is rather subtle if one reads the documentation  explicitly present for GetLogicalProcessorInformation and implicitly present for GetLogicalProcessorInformationEx  at MSDN  The number of logical processors   Using GetSystemInfo  size t NumberOfSystemCores   noexcept       SYSTEM INFO system info      ZeroMemory  amp system info  sizeof system info             GetSystemInfo  amp system info            return static cast lt  size t  gt  system info dwNumberOfProcessors      Note that both methods can easily be converted to C C  98 C  03

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Note that  number of cores  might not be a particularly useful number  you might have to qualify it a bit more   How do you want to count multi-threaded CPUs such as Intel HT  IBM Power5 and Power6  and most famously  Sun s Niagara UltraSparc T1 and T2  Or even more interesting  the MIPS 1004k with its two levels of hardware threading  supervisor AND user-level     Not to mention what happens when you move into hypervisor-supported systems where the hardware might have tens of CPUs but your particular OS only sees a few   The best you can hope for is to tell the number of logical processing units that you have in your local OS partition  Forget about seeing the true machine unless you are a hypervisor  The only exception to this rule today is in x86 land  but the end of non-virtual machines is coming fast

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Windows Server 2003 and later lets you leverage the GetLogicalProcessorInformation function  http   msdn microsoft com en-us library ms683194 aspx

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More on OS X  sysconf  SC NPROCESSORS ONLN  is available only versions    10 5  not 10 4   An alternative is the HW AVAILCPU sysctl   BSD code which is available on versions    10 2

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you can use WMI in  net too but you re then dependent on the wmi service running etc  Sometimes it works locally  but then fail when the same code is run on servers  I believe that s a namespace issue  related to the  names  whose values you re reading

User · Answer

One more Windows recipe  use system-wide environment variable NUMBER OF PROCESSORS   printf   d n   atoi getenv  NUMBER OF PROCESSORS

User · Answer

Unrelated to C    but on Linux I usually do   grep processor  proc cpuinfo   wc -l   Handy for scripting languages like bash perl python ruby

[c++] Programmatically find the number of cores on a machine

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