pthread_setaffinity_np

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 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.)

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 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.

VERSIONS
       These functions are provided by glibc since version 2.3.4.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────────────┬───────────────┬─────────┐
       │Interface                 Attribute     Value   │
       ├──────────────────────────┼───────────────┼─────────┤
       │pthread_setaffinity_np(), │ Thread safety │ MT-Safe │
       │pthread_getaffinity_np()  │               │         │
       └──────────────────────────┴───────────────┴─────────┘
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 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.

EXAMPLES
       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;
           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:\n");
           for (int j = 0; j < CPU_SETSIZE; j++)
               if (CPU_ISSET(j, &cpuset))
                   printf("    CPU %d\n", j);

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       sched_setaffinity(2), CPU_SET(3), pthread_attr_setaffinity_np(3),
       pthread_self(3), sched_getcpu(3), cpuset(7), pthreads(7), sched(7)

COLOPHON
       This page is part of release 5.09 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
       https://www.kernel.org/doc/man-pages/.



Linux                              2020-11-01          PTHREAD_SETAFFINITY_NP(3)