NAME

       getcpu - determine CPU and NUMA node on which the calling thread is running

SYNOPSIS

       #include <linux/getcpu.h>

       int getcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *tcache);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION

       The  getcpu() system call identifies the processor and node on which the calling thread or
       process is currently running and writes them into the integers pointed to by the  cpu  and
       node arguments.  The processor is a unique small integer identifying a CPU.  The node is a
       unique small identifier identifying a NUMA node.  When either cpu or node is NULL  nothing
       is written to the respective pointer.

       The third argument to this system call is nowadays unused, and should be specified as NULL
       unless portability to Linux 2.6.23 or earlier is required (see NOTES).

       The information placed in cpu is guaranteed to be current only at the time  of  the  call:
       unless the CPU affinity has been fixed using sched_setaffinity(2), the kernel might change
       the CPU at any time.  (Normally this does  not  happen  because  the  scheduler  tries  to
       minimize  movements between CPUs to keep caches hot, but it is possible.)  The caller must
       allow for the possibility that the information returned in  cpu  and  node  is  no  longer
       current by the time the call returns.

RETURN VALUE

       On success, 0 is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS

       EFAULT Arguments point outside the calling process's address space.

VERSIONS

       getcpu() was added in kernel 2.6.19 for x86_64 and i386.

CONFORMING TO

       getcpu() is Linux-specific.

NOTES

       Linux  makes  a  best  effort  to  make  this  call as fast as possible.  The intention of
       getcpu() is to allow programs  to  make  optimizations  with  per-CPU  data  or  for  NUMA
       optimization.

       Glibc  does  not  provide a wrapper for this system call; call it using syscall(2); or use
       sched_getcpu(3) instead.

       The tcache argument is unused since Linux 2.6.24.  In earlier kernels,  if  this  argument
       was  non-NULL,  then  it  specified a pointer to a caller-allocated buffer in thread-local
       storage that was used to provide a caching mechanism for getcpu().  Use of the cache could
       speed  getcpu()  calls,  at  the cost that there was a very small chance that the returned
       information would be out of date.  The caching mechanism was considered to cause  problems
       when migrating threads between CPUs, and so the argument is now ignored.

SEE ALSO

       mbind(2), sched_setaffinity(2), set_mempolicy(2), sched_getcpu(3), cpuset(7), vdso(7)

COLOPHON

       This  page  is  part of release 4.04 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.