pthread_setaffinity_np(3) — Linux manual page


pthread_...inity_np(3)  Library Functions Manual  pthread_...inity_np(3)

NAME         top

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

LIBRARY         top

       POSIX threads library (libpthread, -lpthread)

SYNOPSIS         top

       #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);

DESCRIPTION         top

       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

       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         top

       On success, these functions return 0; on error, they return a
       nonzero error number.

ERRORS         top

       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.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       │ Interface                           Attribute     Value   │
       │ pthread_setaffinity_np(),           │ Thread safety │ MT-Safe │
       │ pthread_getaffinity_np()            │               │         │

STANDARDS         top

       GNU; hence the suffix "_np" (nonportable) in the names.

HISTORY         top

       glibc 2.3.4.

       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

NOTES         top

       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.

       A new thread created by pthread_create(3) inherits a copy of its
       creator's CPU affinity mask.

EXAMPLES         top

       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 s;
           cpu_set_t cpuset;
           pthread_t thread;

           thread = pthread_self();

           /* Set affinity mask to include CPUs 0 to 7. */

           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);


SEE ALSO         top

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

Linux man-pages (unreleased)     (date)           pthread_...inity_np(3)

Pages that refer to this page: sched_setaffinity(2)CPU_SET(3)pthread_attr_setaffinity_np(3)sched(7)