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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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pthread_s...finity_np(3) Library Functions Manualpthread_s...finity_np(3)
pthread_setaffinity_np, pthread_getaffinity_np - set/get CPU
affinity of a thread
POSIX threads library (libpthread, -lpthread)
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <pthread.h>
int pthread_setaffinity_np(pthread_t thread, size_t cpusetsize,
const cpu_set_t *cpuset);
int pthread_getaffinity_np(pthread_t thread, size_t cpusetsize,
cpu_set_t *cpuset);
The pthread_setaffinity_np() function sets the CPU affinity mask
of the thread thread to the CPU set pointed to by cpuset. If the
call is successful, and the thread is not currently running on one
of the CPUs in cpuset, then it is migrated to one of those CPUs.
The pthread_getaffinity_np() function returns the CPU affinity
mask of the thread thread in the buffer pointed to by cpuset.
For more details on CPU affinity masks, see sched_setaffinity(2).
For a description of a set of macros that can be used to
manipulate and inspect CPU sets, see CPU_SET(3).
The argument cpusetsize is the length (in bytes) of the buffer
pointed to by cpuset. Typically, this argument would be specified
as sizeof(cpu_set_t). (It may be some other value, if using the
macros described in CPU_SET(3) for dynamically allocating a CPU
set.)
On success, these functions return 0; on error, they return a
nonzero error number.
EFAULT A supplied memory address was invalid.
EINVAL (pthread_setaffinity_np()) The affinity bit mask 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 cpuset(7).
EINVAL (pthread_setaffinity_np()) cpuset specified a CPU that was
outside the set supported by the kernel. (The kernel
configuration option CONFIG_NR_CPUS defines the range of
the set supported by the kernel data type used to represent
CPU sets.)
EINVAL (pthread_getaffinity_np()) cpusetsize is smaller than the
size of the affinity mask used by the kernel.
ESRCH No thread with the ID thread could be found.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ pthread_setaffinity_np(), │ Thread safety │ MT-Safe │
│ pthread_getaffinity_np() │ │ │
└──────────────────────────────────────┴───────────────┴─────────┘
GNU; hence the suffix "_np" (nonportable) in the names.
glibc 2.3.4.
In glibc 2.3.3 only, versions of these functions were provided
that did not have a cpusetsize argument. Instead the CPU set size
given to the underlying system calls was always sizeof(cpu_set_t).
After a call to pthread_setaffinity_np(), the set of CPUs on which
the thread will actually run is the intersection of the set
specified in the cpuset 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 cpuset(7) is being used. These restrictions on the actual set
of CPUs on which the thread will run are silently imposed by the
kernel.
These functions are implemented on top of the sched_setaffinity(2)
and sched_getaffinity(2) system calls.
A new thread created by pthread_create(3) inherits a copy of its
creator's CPU affinity mask.
In the following program, the main thread uses
pthread_setaffinity_np() to set its CPU affinity mask to include
CPUs 0 to 7 (which may not all be available on the system), and
then calls pthread_getaffinity_np() to check the resulting CPU
affinity mask of the thread.
#define _GNU_SOURCE
#include <err.h>
#include <pthread.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
int
main(void)
{
int s;
cpu_set_t cpuset;
pthread_t thread;
thread = pthread_self();
/* Set affinity mask to include CPUs 0 to 7. */
CPU_ZERO(&cpuset);
for (size_t j = 0; j < 8; j++)
CPU_SET(j, &cpuset);
s = pthread_setaffinity_np(thread, sizeof(cpuset), &cpuset);
if (s != 0)
errc(EXIT_FAILURE, s, "pthread_setaffinity_np");
/* Check the actual affinity mask assigned to the thread. */
s = pthread_getaffinity_np(thread, sizeof(cpuset), &cpuset);
if (s != 0)
errc(EXIT_FAILURE, s, "pthread_getaffinity_np");
printf("Set returned by pthread_getaffinity_np() contained:\n");
for (size_t j = 0; j < CPU_SETSIZE; j++)
if (CPU_ISSET(j, &cpuset))
printf(" CPU %zu\n", j);
exit(EXIT_SUCCESS);
}
sched_setaffinity(2), CPU_SET(3), pthread_attr_setaffinity_np(3),
pthread_self(3), sched_getcpu(3), cpuset(7), pthreads(7), sched(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⟩.
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
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man-pages@man7.org
Linux man-pages 6.15 2025-05-17 pthread_s...finity_np(3)
Pages that refer to this page: sched_setaffinity(2), CPU_SET(3), pthread_attr_setaffinity_np(3), sched(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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