1GETCPU(2) Linux Programmer's Manual GETCPU(2)
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6 getcpu - determine CPU and NUMA node on which the calling thread is
7 running
8
10 #define _GNU_SOURCE /* See feature_test_macros(7) */
11 #include <sched.h>
12 int getcpu(unsigned int *cpu, unsigned int *node);
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16 The getcpu() system call identifies the processor and node on which the
17 calling thread or process is currently running and writes them into the
18 integers pointed to by the cpu and node arguments. The processor is a
19 unique small integer identifying a CPU. The node is a unique small
20 identifier identifying a NUMA node. When either cpu or node is NULL
21 nothing is written to the respective pointer.
22 The information placed in cpu is guaranteed to be current only at the
24 time of the call: unless the CPU affinity has been fixed using
25 sched_setaffinity(2), the kernel might change the CPU at any time.
26 (Normally this does not happen because the scheduler tries to minimize
27 movements between CPUs to keep caches hot, but it is possible.) The
28 caller must allow for the possibility that the information returned in
29 cpu and node is no longer current by the time the call returns.
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32 On success, 0 is returned. On error, -1 is returned, and errno is set
33 to indicate the error.
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36 EFAULT Arguments point outside the calling process's address space.
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39 getcpu() was added in kernel 2.6.19 for x86-64 and i386. Library sup‐
40 port was added in glibc 2.29 (Earlier glibc versions did not provide a
41 wrapper for this system call, necessitating the use of syscall(2).)
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44 getcpu() is Linux-specific.
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47 Linux makes a best effort to make this call as fast as possible. (On
48 some architectures, this is done via an implementation in the vdso(7).)
49 The intention of getcpu() is to allow programs to make optimizations
50 with per-CPU data or for NUMA optimization.
51 C library/kernel differences
53 The kernel system call has a third argument:
54 int getcpu(unsigned int *cpu, unsigned int *node,
56 struct getcpu_cache *tcache);
57 The tcache argument is unused since Linux 2.6.24, and (when invoking
59 the system call directly) should be specified as NULL, unless portabil‐
60 ity to Linux 2.6.23 or earlier is required.
61 In Linux 2.6.23 and earlier, if the tcache argument was non-NULL, then
63 it specified a pointer to a caller-allocated buffer in thread-local
64 storage that was used to provide a caching mechanism for getcpu(). Use
65 of the cache could speed getcpu() calls, at the cost that there was a
66 very small chance that the returned information would be out of date.
67 The caching mechanism was considered to cause problems when migrating
68 threads between CPUs, and so the argument is now ignored.
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71 mbind(2), sched_setaffinity(2), set_mempolicy(2), sched_getcpu(3),
72 cpuset(7), vdso(7)
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75 This page is part of release 5.13 of the Linux man-pages project. A
76 description of the project, information about reporting bugs, and the
77 latest version of this page, can be found at
78 https://www.kernel.org/doc/man-pages/.
79Linux 2021-03-22 GETCPU(2)