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PTHREAD_SETAFFINITY_NP(3)           Linux Programmer's Manual           PTHREAD_SETAFFINITY_NP(3)

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

       #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.

       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  cur‐
       rently 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.  Typ‐
       ically, 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.)

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

       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 sup‐
              ported 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.

       These functions are provided by glibc since version 2.3.4.

   Multithreading (see pthreads(7))
       The pthread_setaffinity_np() and pthread_getaffinity_np() functions are thread-safe.

       These functions are nonstandard GNU extensions; hence the suffix  "_np"  (nonportable)  in
       the names.

       After  a  call to pthread_setaffinity_np(), the set of CPUs on which the thread will actu‐
       ally 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_getaffin‐
       ity(2) system calls.

       In glibc 2.3.3 only, versions of these functions were provided that did not have a cpuset‐
       size argument.  Instead the CPU set size given to the underlying system calls  was  always

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

       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

       #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)

       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 */

           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);


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

       This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2014-05-23                  PTHREAD_SETAFFINITY_NP(3)

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