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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
       #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  The caller must allow for the possibility that the information returned in cpu and node is no longer current by the time the call returns.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 sys‐
       tem 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 in‐
       formation 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)

Linux                                                                          2021-03-22                                                                      GETCPU(2)