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GETCPU(2) Linux Programmer's Manual 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     cpu and node is no longer current by the time the call returns.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.   (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.

     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 argu-
     ment is now ignored.