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getcpu(2)


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   cpu and node is no longer current by the time the call returns.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 4.09 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/.



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