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cpuset, cpuset_getid, cpuset_setid — manage CPU affinity sets
Standard C Library (libc, -lc)
cpuset_setid(cpuwhich_t which, id_t id, cpusetid_t setid);
cpuset_getid(cpulevel_t level, cpuwhich_t which, id_t id,
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 set.
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() 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 fork(2).
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
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()
[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 operation.
[ENFILE] There was no free cpusetid_t for allocation.
cpuset(1), cpuset_getaffinity(2), cpuset_setaffinity(2), CPU_SET(3),
The cpuset family of system calls first appeared in FreeBSD 7.1.
Jeffrey Roberson ⟨jeff@FreeBSD.org⟩