getcpu(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

GETCPU(2)               Linux Programmer's Manual              GETCPU(2)

NAME         top

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

SYNOPSIS         top

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

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

DESCRIPTION         top

       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       the information returned in cpu and node is no longer current by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         top

       On success, 0 is returned.  On error, -1 is returned, and errno
       is set to indicate the error.

ERRORS         top

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

VERSIONS         top

       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         top

       getcpu() is Linux-specific.

NOTES         top

       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.

   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.

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.11 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                          2021-03-22                      GETCPU(2)

Pages that refer to this page: get_mempolicy(2)mbind(2)sched_setaffinity(2)set_mempolicy(2)syscalls(2)sched_getcpu(3)cpuset(7)