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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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getcpu(2) System Calls Manual getcpu(2)
getcpu - determine CPU and NUMA node on which the calling thread
is running
Standard C library (libc, -lc)
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sched.h>
int getcpu(unsigned int *_Nullable cpu, unsigned int *_Nullable node);
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information returned in cpu and node is no longer current by the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.
On success, 0 is returned. On error, -1 is returned, and errno is
set to indicate the error.
EFAULT Arguments point outside the calling process's address
space.
Linux.
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information would be out of date.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.
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.
mbind(2), sched_setaffinity(2), set_mempolicy(2), sched_getcpu(3),
cpuset(7), vdso(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 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)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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