man3/pthread_setaffinity_np.3

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.TH PTHREAD_SETAFFINITY_NP 3 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
pthread_setaffinity_np, pthread_getaffinity_np \- set/get
CPU affinity of a thread
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
.B #include <pthread.h>
.PP
.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 );
.PP
Compile and link with \fI\-pthread\fP.
.fi
.SH DESCRIPTION
The
.BR pthread_setaffinity_np ()
function
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.
.PP
The
.BR pthread_getaffinity_np ()
function returns the CPU affinity mask of the thread
.I thread
in the buffer pointed to by
.IR cpuset .
.PP
For more details on CPU affinity masks, see
.BR sched_setaffinity (2).
For a description of a set of macros
that can be used to manipulate and inspect CPU sets, see
.BR CPU_SET (3).
.PP
The argument
.I cpusetsize
is the length (in bytes) of the buffer pointed to by
.IR cpuset .
Typically, this argument would be specified as
.IR sizeof(cpu_set_t) .
(It may be some other value, if using the macros described in
.BR CPU_SET (3)
for dynamically allocating a CPU set.)
.SH RETURN VALUE
On success, these functions return 0;
on error, they return a nonzero 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 currently physically on the system
and permitted to the thread according to any restrictions that
may be imposed by the "cpuset" mechanism described in
.BR cpuset (7).
.TP
.BR EINVAL
.RB ( pthread_setaffinity_np ())
.I cpuset
specified a CPU that was outside the set supported by the kernel.
(The kernel configuration option
.BR CONFIG_NR_CPUS
defines the range of the set supported 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.
.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
No thread with the ID
.I thread
could be found.
.SH VERSIONS
These functions are provided by glibc since version 2.3.4.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR pthread_setaffinity_np (),
.BR pthread_getaffinity_np ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
These functions are nonstandard GNU extensions;
hence the suffix "_np" (nonportable) in the names.
.SH NOTES
After a call to
.BR pthread_setaffinity_np (),
the set of CPUs on which the thread will actually run is
the intersection of the set specified in the
.I 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
.BR 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.
.PP
These functions are implemented on top of the
.BR sched_setaffinity (2)
and
.BR sched_getaffinity (2)
system calls.
.PP
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) .
.PP
A new thread created by
.BR pthread_create (3)
inherits a copy of its creator's CPU affinity mask.
.SH EXAMPLES
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.
.PP
.EX
#define _GNU_SOURCE
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>

#define handle_error_en(en, msg) \e
        do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)

int
main(int argc, char *argv[])
{
    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 (int j = 0; j < 8; j++)
        CPU_SET(j, &cpuset);

    s = pthread_setaffinity_np(thread, sizeof(cpuset), &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(cpuset), &cpuset);
    if (s != 0)
        handle_error_en(s, "pthread_getaffinity_np");

    printf("Set returned by pthread_getaffinity_np() contained:\en");
    for (int j = 0; j < CPU_SETSIZE; j++)
        if (CPU_ISSET(j, &cpuset))
            printf("    CPU %d\en", j);

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR sched_setaffinity (2),
.BR CPU_SET (3),
.BR pthread_attr_setaffinity_np (3),
.BR pthread_self (3),
.BR sched_getcpu (3),
.BR cpuset (7),
.BR pthreads (7),
.BR sched (7)