getcpu

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



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

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

       int getcpu(unsigned int *cpu, unsigned int *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       cpu and node is no longer current by the time the call returns.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.

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.

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

COLOPHON
       This page is part of release 5.12 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)