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);

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       in cpu and node is no longer current by the time the call returns.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.  Library support was added in glibc 2.29
       (Earlier glibc versions did not provide a  wrapper  for  this  system  call,  necessitating  the  use  of
       syscall(2).)

CONFORMING TO

       getcpu() is Linux-specific.

NOTES

       Linux  makes  a  best effort to make this call as fast as possible.  (On some architectures, this is done
       via an implementation in the  vdso(7).)   The  intention  of  getcpu()  is  to  allow  programs  to  make
       optimizations with per-CPU data or for NUMA optimization.

       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 5.10 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
       https://www.kernel.org/doc/man-pages/.

Linux                                              2019-03-06                                          GETCPU(2)