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.
These functions are provided since glibc 2.3.4.
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
pthread_setaffinity_np (), pthread_getaffinity_np () |
Thread safety |
MT-Safe |
These functions are nonstandard GNU extensions; hence the suffix "_np"
(nonportable) in the names.
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.
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).
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 <errno.h>
#include <pthread.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)