plucky (2) getcpu.2.gz

NAME

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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <sched.h>

       int getcpu(unsigned int *_Nullable cpu, unsigned int *_Nullable node);

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 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       in cpu and node is no longer current by the time the call returns.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 to indicate the error.

ERRORS

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

STANDARDS

       Linux.

HISTORY

       Linux 2.6.19 (x86-64 and i386), glibc 2.29.

   C library/kernel differences
       The kernel system call has a third argument:

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

       The tcache argument is unused since Linux 2.6.24, and (when invoking the system call directly) should  be
       specified as NULL, unless portability to Linux 2.6.23 or earlier is required.

       In  Linux  2.6.23  and  earlier,  if  the  tcache 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       that the returned information would be out of date.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.

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.

SEE ALSO

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