NAME

pthread_setaffinity_np, pthread_getaffinity_np - set/get CPU affinity of a thread

SYNOPSIS

#define _GNU_SOURCE
#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() 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.

The argument cpusetsize is the length (in bytes) of the buffer pointed to by cpuset. Normally this argument would be specified as sizeof(cpu_set_t). The constant CPU_SETSIZE specifies a value one greater than the maximum CPU number that can be stored in a CPU set.

For more details on CPU affinity masks, as well as a description of a set of macros that can be used to manipulate and inspect CPU sets, see sched_setaffinity(2) for details.

RETURN VALUE

On success, these functions return 0; on error, they return a non-zero error number.

ERRORS

EFAULT

A supplied memory address was invalid.

EINVAL

(pthread_setaffinity_np()) The affinity bit mask mask contains no processors that are physically on the system.

EINVAL

(pthread_setaffinity_np()) cpuset specified a CPU that was outside the range permitted by the kernel data type used to represent CPU sets. This range is determined by the kernel configuration option CONFIG_NR_CPUS.

EINVAL

(pthread_getaffinity_np()) cpusetsize is smaller than the size of the affinity mask used by the kernel.

ESRCH

There is no thread matching thread (e.g., perhaps that thread has already terminated and been joined).

VERSIONS

These functions are provided by glibc since version 2.3.4.

CONFORMING TO

These functions are non-standard GNU extensions; hence the suffix "_np" (non-portable) in the names.

NOTES

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() 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 errExitEN(en, msg)      { errno = en; perror(msg); \

                                  exit(EXIT_FAILURE); }
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)

        errExitEN(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)

        errExitEN(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_getcpu(3), sched_setaffinity(2), sched_setscheduler(2), pthread_attr_setaffinity_np(3), pthread_self(3), cpuset(7), pthreads(7)

COLOPHON

This page is part of release 3.13 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.