Section: Linux Programmer's Manual (2)
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sched_setaffinity, sched_getaffinity - set and get a thread's CPU affinity mask
#define _GNU_SOURCE /* See feature_test_macros(7) */
int sched_setaffinity(pid_t pid, size_t cpusetsize,
const cpu_set_t *mask);
int sched_getaffinity(pid_t pid, size_t cpusetsize,
A thread's CPU affinity mask determines the set of CPUs on which
it is eligible to run.
On a multiprocessor system, setting the CPU affinity mask
can be used to obtain performance benefits.
by dedicating one CPU to a particular thread
(i.e., setting the affinity mask of that thread to specify a single CPU,
and setting the affinity mask of all other threads to exclude that CPU),
it is possible to ensure maximum execution speed for that thread.
Restricting a thread to run on a single CPU also avoids
the performance cost caused by the cache invalidation that occurs
when a thread ceases to execute on one CPU and then
recommences execution on a different CPU.
A CPU affinity mask is represented by the
structure, a "CPU set", pointed to by
A set of macros for manipulating CPU sets is described in
sets the CPU affinity mask of the thread whose ID is
to the value specified by
is zero, then the calling thread is used.
is the length (in bytes) of the data pointed to by
Normally this argument would be specified as
If the thread specified by
is not currently running on one of the CPUs specified in
then that thread is migrated to one of the CPUs specified in
writes the affinity mask of the thread whose ID is
structure pointed to by
argument specifies the size (in bytes) of
is zero, then the mask of the calling thread is returned.
On error, -1 is returned, and
is set appropriately.
A supplied memory address was invalid.
The affinity bit mask
contains no processors that are currently physically on the system
and permitted to the thread according to any restrictions that
may be imposed by the "cpuset" mechanism described in
and, in kernels before 2.6.9,
is smaller than the size of the affinity mask used by the kernel.
The calling thread does not have appropriate privileges.
The caller needs an effective user ID equal to the real user ID
or effective user ID of the thread identified by
or it must possess the
The thread whose ID is pid could not be found.
The CPU affinity system calls were introduced in Linux kernel 2.5.8.
The system call wrappers were introduced in glibc 2.3.
Initially, the glibc interfaces included a
argument, typed as
In glibc 2.3.3, the
argument was removed, but was then restored in glibc 2.3.4, with type
These system calls are Linux-specific.
After a call to
the set of CPUs on which the thread will actually run is
the intersection of the set specified in the
argument and the set of CPUs actually present on the system.
The system may further restrict the set of CPUs on which the thread
runs if the "cpuset" mechanism described in
is being used.
These restrictions on the actual set of CPUs on which the thread
will run are silently imposed by the kernel.
has a description of the Linux scheduling scheme.
The affinity mask is a per-thread attribute that can be
adjusted independently for each of the threads in a thread group.
The value returned from a call to
can be passed in the argument
as 0 will set the attribute for the calling thread,
and passing the value returned from a call to
will set the attribute for the main thread of the thread group.
(If you are using the POSIX threads API, then use
A child created via
inherits its parent's CPU affinity mask.
The affinity mask is preserved across an
C library/kernel ABI differences
This manual page describes the glibc interface for the CPU affinity calls.
The actual system call interface is slightly different, with the
being typed as
unsigned long *
reflecting the fact that the underlying implementation of CPU
sets is a simple bit mask.
On success, the raw
system call returns the size (in bytes) of the
data type that is used internally by the kernel to
represent the CPU set bit mask.
This page is part of release 3.74 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
- RETURN VALUE
- CONFORMING TO
- C library/kernel ABI differences
- SEE ALSO