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
       #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  repre-
              sent 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.

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() 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 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);
       }

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 3.25 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/.



Linux                                                      2008-11-27                                  PTHREAD_SETAFFINITY_NP(3)