Section: Linux Programmer's Manual (2)
getcpu - determine CPU and NUMA node on which the calling thread is running
int getcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *tcache);
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
The processor is a unique small integer identifying a CPU.
The node is a unique small identifier identifying a NUMA node.
is NULL nothing is written to the respective pointer.
The third argument to this system call is nowadays unused,
and should be specified as NULL
unless portability to Linux 2.6.23 or earlier is required (see NOTES).
The information placed in
is guaranteed to be current only at the time of the call:
unless the CPU affinity has been fixed using
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
is no longer current by the time the call returns.
On success, 0 is returned.
On error, -1 is returned, and
is set appropriately.
Arguments point outside the calling process's address space.
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 system call,
necessitating the use of
Linux makes a best effort to make this call as fast as possible.
(On some architectures, this is done via an implementation in the
The intention of
is to allow programs to make optimizations with per-CPU data
or for NUMA optimization.
argument is unused since Linux 2.6.24.
In earlier kernels,
if this 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
Use of the cache could speed
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the returned information would be out of date.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.
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