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PTHREAD_SETAFFINITY_NP(3) Linux Programmer's Manual PTHREAD_SETAFFINITY_NP(3)
NAME
pthread_setaffinity_np, pthread_getaffinity_np - set/get CPU affinity of a thread
SYNOPSIS
#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);
Compile and link with -pthread.
DESCRIPTION
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 cur‐
rently 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. Typ‐
ically, 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.)
RETURN VALUE
On success, these functions return 0; on error, they return a nonzero error number.
ERRORS
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 sup‐
ported 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.
VERSIONS
These functions are provided by glibc since version 2.3.4.
ATTRIBUTES
Multithreading (see pthreads(7))
The pthread_setaffinity_np() and pthread_getaffinity_np() functions are thread-safe.
CONFORMING TO
These functions are nonstandard GNU extensions; hence the suffix "_np" (nonportable) in
the names.
NOTES
After a call to pthread_setaffinity_np(), the set of CPUs on which the thread will actu‐
ally 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_getaffin‐
ity(2) system calls.
In glibc 2.3.3 only, versions of these functions were provided that did not have a cpuset‐
size argument. Instead the CPU set size given to the underlying system calls was always
sizeof(cpu_set_t).
A new thread created by pthread_create(3) inherits a copy of its creator's CPU affinity
mask.
EXAMPLE
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 <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#define handle_error_en(en, msg) \
do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)
int
main(int argc, char *argv[])
{
int s, j;
cpu_set_t cpuset;
pthread_t thread;
thread = pthread_self();
/* Set affinity mask to include CPUs 0 to 7 */
CPU_ZERO(&cpuset);
for (j = 0; j < 8; j++)
CPU_SET(j, &cpuset);
s = pthread_setaffinity_np(thread, sizeof(cpu_set_t), &cpuset);
if (s != 0)
handle_error_en(s, "pthread_setaffinity_np");
/* Check the actual affinity mask assigned to the thread */
s = pthread_getaffinity_np(thread, sizeof(cpu_set_t), &cpuset);
if (s != 0)
handle_error_en(s, "pthread_getaffinity_np");
printf("Set returned by pthread_getaffinity_np() contained:\n");
for (j = 0; j < CPU_SETSIZE; j++)
if (CPU_ISSET(j, &cpuset))
printf(" CPU %d\n", j);
exit(EXIT_SUCCESS);
}
SEE ALSO
sched_setaffinity(2), pthread_attr_setaffinity_np(3), pthread_self(3), sched_getcpu(3),
cpuset(7), pthreads(7), sched(7)
COLOPHON
This page is part of release 3.74 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at http://www.kernel.org/doc/man-pages/.
Linux 2014-05-23 PTHREAD_SETAFFINITY_NP(3)
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