Scroll to navigation



cpuset_getaffinity, cpuset_setaffinitymanage CPU affinity


Standard C Library (libc, -lc)


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

cpuset_getaffinity(cpulevel_t level, cpuwhich_t which, id_t id, size_t setsize, cpuset_t *mask);

cpuset_setaffinity(cpulevel_t level, cpuwhich_t which, id_t id, size_t setsize, const cpuset_t *mask);


() and cpuset_setaffinity() allow the manipulation of sets of CPUs available to processes, threads, interrupts, jails and other resources. These functions may manipulate sets of CPUs that contain many processes or per-object anonymous masks that effect only a single object.

The valid values for the level and which arguments are documented in cpuset(2). These arguments specify which object and which set of the object we are referring to. Not all possible combinations are valid. For example, only processes may belong to a numbered set accessed by a level argument of CPU_LEVEL_CPUSET. All resources, however, have a mask which may be manipulated with CPU_LEVEL_WHICH.

Masks of type cpuset_t are composed using the CPU_SET macros. The kernel tolerates large sets as long as all CPUs specified in the set exist. Sets smaller than the kernel uses generate an error on calls to () even if the result set would fit within the user supplied set. Calls to cpuset_setaffinity() tolerate small sets with no restrictions.

The supplied mask should have a size of setsize bytes. This size is usually provided by calling sizeof(mask) which is ultimately determined by the value of CPU_SETSIZE as defined in <sys/cpuset.h>.

() retrieves the mask from the object specified by level, which and id and stores it in the space provided by mask.

() attempts to set the mask for the object specified by level, which and id to the value in mask.


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 level or which argument was not a valid value.
The mask argument specified when calling cpuset_setaffinity() was not a valid value.
The cpuset_setaffinity() 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 mask pointer passed was invalid.
The object specified by the id and which arguments could not be found.
The cpusetsize was either preposterously large or smaller than the kernel set size.
The calling process did not have the credentials required to complete the operation.
The calling process attempted to act on a process other than itself, while in capability mode. See capsicum(4).


capsicum(4), cpuset(1), cpuset(2), cpuset_getid(2), cpuset_setid(2), cpuset_getdomain(2), cpuset_setdomain(2), pthread_affinity_np(3), pthread_attr_affinity_np(3), cpuset(9)


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


Jeffrey Roberson <>

May 23, 2017 Debian