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.\" Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
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.TH PTHREAD_SETAFFINITY_NP 3 2008-11-11 "Linux" "Linux Programmer's Manual"
.SH NAME
pthread_setaffinity_np, pthread_getaffinity_np \- set/get
CPU affinity of a thread
.SH SYNOPSIS
.nf
.B #define _GNU_SOURCE
.B #include <pthread.h>

.BI "int pthread_setaffinity_np(pthread_t " thread ", size_t " cpusetsize ,
.BI "                           const cpu_set_t *" cpuset );
.BI "int pthread_getaffinity_np(pthread_t " thread ", size_t " cpusetsize ,
.BI "                           cpu_set_t *" cpuset );
.sp
Compile and link with \fI\-pthread\fP.
.SH DESCRIPTION
The
.BR pthread_setaffinity_np ()
sets the CPU affinity mask of the thread
.I thread
to the CPU set pointed to by
.IR cpuset .
If the call is successful,
and the thread is not currently running on one of the CPUs in
.IR cpuset ,
then it is migrated to one of those CPUs.

The
.BR pthread_getaffinity_np ()
function returns the CPU affinity mask of the thread
.I thread
in the buffer pointed to by
.IR cpuset .

The argument
.I cpusetsize
is the length (in bytes) of the buffer pointed to by
.IR cpuset .
Normally this argument would be specified as
.IR sizeof(cpu_set_t) .
The constant
.B 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
.BR sched_setaffinity (2)
for details.
.SH RETURN VALUE
On success, these functions return 0;
on error, they return a non-zero error number.
.SH ERRORS
.TP
.B EFAULT
A supplied memory address was invalid.
.TP
.B EINVAL
.RB ( pthread_setaffinity_np ())
The affinity bit mask
.I mask
contains no processors that are physically on the system.
.TP
.BR EINVAL
.RB ( pthread_setaffinity_np ())
.I cpuset
specified a CPU that was outside the range
permitted by the kernel data type
.\" cpumask_t
used to represent CPU sets.
.\" The raw sched_getaffinity() system call returns the size (in bytes)
.\" of the cpumask_t type.
This range is determined by the kernel configuration option
.BR CONFIG_NR_CPUS .
.TP
.B EINVAL
.RB ( pthread_getaffinity_np ())
.I cpusetsize
is smaller than the size of the affinity mask used by the kernel.
.TP
.B ESRCH
There is no thread matching
.IR thread
(e.g., perhaps that thread has already terminated and been joined).
.SH VERSIONS
These functions are provided by glibc since version 2.3.4.
.SH CONFORMING TO
These functions are non-standard GNU extensions;
hence the suffix "_np" (non-portable) in the names.
.SH NOTES
These functions are implemented on top of the
.BR sched_setaffinity (2)
and
.BR sched_getaffinity (2)
system calls.

In glibc 2.3.3 only,
versions of these functions were provided that did not have a
.I cpusetsize
argument.
Instead the CPU set size given to the underlying system calls was always
.IR sizeof(cpu_set_t) .

A new thread created by
.BR pthread_create ()
inherits a copy of its creator's CPU affinity mask.
.SH EXAMPLE
In the following program, the main thread uses
.BR 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
.BR pthread_getaffinity_np ()
to check the resulting CPU affinity mask of the thread.

.nf
#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);
}
.fi
.SH SEE ALSO
.BR sched_getcpu (3),
.BR sched_setaffinity (2),
.BR sched_setscheduler (2),
.BR pthread_attr_setaffinity_np (3),
.BR pthread_self (3),
.BR cpuset (7),
.BR pthreads (7)