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     cpuset, cpuset_getid, cpuset_setid — manage CPU affinity sets


     Standard C Library (libc, -lc)


     #include <sys/param.h>
     #include <sys/cpuset.h>

     cpuset(cpusetid_t *setid);

     cpuset_setid(cpuwhich_t which, id_t id, cpusetid_t setid);

     cpuset_getid(cpulevel_t level, cpuwhich_t which, id_t id, cpusetid_t *setid);


     The cpuset family of system calls allow applications to control sets of processors and
     assign processes and threads to these sets.  Processor sets contain lists of CPUs that
     members may run on and exist only as long as some process is a member of the set.  All
     processes in the system have an assigned set.  The default set for all processes in the
     system is the set numbered 1.  Threads belong to the same set as the process which contains
     them, however, they may further restrict their set with the anonymous per-thread mask.

     Sets are referenced by a number of type cpuset_id_t.  Each thread has a root set, an
     assigned set, and an anonymous mask.  Only the root and assigned sets are numbered.  The
     root set is the set of all CPUs available in the system or in the system partition the
     thread is running in.  The assigned set is a subset of the root set and is administratively
     assignable on a per-process basis.  Many processes and threads may be members of a numbered

     The anonymous set is a further thread-specific refinement on the assigned set.  It is
     intended that administrators will manipulate numbered sets using cpuset(1) while application
     developers will manipulate anonymous sets using cpuset_setaffinity(2).

     To select the correct set a value of type cpulevel_t is used.  The following values for
     level are supported:

           CPU_LEVEL_ROOT      Root set
           CPU_LEVEL_CPUSET    Assigned set
           CPU_LEVEL_WHICH     Set specified by which argument

     The which argument determines how the value of id is interpreted and is of type cpuwhich_t.
     The which argument may have the following values:

           CPU_WHICH_TID       id is lwpid_t (thread id)
           CPU_WHICH_PID       id is pid_t (process id)
           CPU_WHICH_CPUSET    id is a cpusetid_t (cpuset id)
           CPU_WHICH_IRQ       id is an irq number

     An id of '-1' may be used with a which of CPU_WHICH_TID, CPU_WHICH_PID, or CPU_WHICH_CPUSET
     to mean the current thread, process, or current thread's cpuset.  All cpuset syscalls allow
     this usage.

     A level argument of CPU_LEVEL_WHICH combined with a which argument other than
     CPU_WHICH_CPUSET refers to the anonymous mask of the object.  This mask does not have an id
     and may only be manipulated with cpuset_setaffinity(2).

     cpuset() creates a new set containing the same CPUs as the root set of the current process
     and stores its id in the space provided by setid.  On successful completion the calling
     process joins the set and is the only member.  Children inherit this set after a call to

     cpuset_setid() attempts to set the id of the object specified by the which argument.
     Currently CPU_WHICH_PID is the only acceptable value for which as threads do not have an id
     distinct from their process and the API does not permit changing the id of an existing set.
     Upon successful completion all of the threads in the target process will be running on CPUs
     permitted by the set.

     cpuset_getid() retrieves a set id from the object indicated by which and stores it in the
     space pointed to by setid.  The retrieved id may be that of either the root or assigned set
     depending on the value of level.  level should be CPU_LEVEL_CPUSET or CPU_LEVEL_ROOT to get
     the set id from the process or thread specified by the id argument.  Specifying
     CPU_LEVEL_WHICH with a process or thread is unsupported since this references the unnumbered
     anonymous mask.

     The actual contents of the sets may be retrieved or manipulated using cpuset_getaffinity(2)
     and cpuset_setaffinity(2).  See those manual pages for more detail.


     Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and
     the global variable errno is set to indicate the error.


     The following error codes may be set in errno:

     [EINVAL]           The which or level argument was not a valid value.

     [EDEADLK]          The cpuset_setid() call would leave a thread without a valid CPU to run
                        on because the set does not overlap with the thread's anonymous mask.

     [EFAULT]           The setid pointer passed to cpuset_getid() or cpuset() was invalid.

     [ESRCH]            The object specified by the id and which arguments could not be found.

     [EPERM]            The calling process did not have the credentials required to complete the

     [ENFILE]           There was no free cpusetid_t for allocation.


     cpuset(1), cpuset_getaffinity(2), cpuset_setaffinity(2), pthread_affinity_np(3),


     The cpuset family of system calls first appeared in FreeBSD 7.1.


     Jeffrey Roberson <>