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       allow  for  the  possibility  that  the  information returned in cpu and node is no longer
       current by the time the call returns.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.

VERSIONS

       getcpu()  was  added  in  Linux  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).)

STANDARDS

       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)