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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 suc-
     cessful, 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 out-
            side 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).

     allbox;  lbw25   lb   lb   l   l   l.    Interface Attribute Value   T{
     pthread_setaffinity_np(),      pthread_getaffinity_np()     T}   Thread
     safety  MT-Safe

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  cre-
     ator'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); }