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CPUSET(2) System Calls Manual CPUSET(2)


cpuset, cpuset_getid, cpuset_setidmanage 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 memory domains and assign processes and threads to these sets. Processor sets contain lists of CPUs and domains 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 to bind to a specific CPU or subset of CPUs and memory domains.

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 and memory domains 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) and cpuset_setdomain(2).

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

Root set
Assigned set
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:

id is lwpid_t (thread id)
id is pid_t (process id)
id is jid (jail id)
id is a cpusetid_t (cpuset id)
id is an irq number
id is an irq number for an interrupt handler
id is an irq number for an ithread
id is a NUMA domain

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).

() 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).

() 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.

() 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), cpuset_setaffinity(2), cpuset_getdomain(2), and cpuset_setdomain(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:

The which or level argument was not a valid value.
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.
The setid pointer passed to cpuset_getid() or cpuset() was invalid.
The object specified by the id and which arguments could not be found.
The calling process did not have the credentials required to complete the operation.
There was no free cpusetid_t for allocation.


cpuset(1), cpuset_getaffinity(2), cpuset_setaffinity(2), cpuset_getdomain(2), cpuset_setdomain(2), pthread_affinity_np(3), pthread_attr_affinity_np(3), cpuset(9)


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


Jeffrey Roberson <>

May 3, 2017 Debian