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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.

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

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       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.