getcpu(2) System Calls Manual getcpu(2)
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cpu and node is no longer current by the time the call returns.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 sup-
port 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 portabil-
ity 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)
Linux man-pages 6.03 2022-12-04 getcpu(2)
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