sched_setaffinity, sched_getaffinity — set and get a thread's CPU affinity mask


#define _GNU_SOURCE              /* See feature_test_macros(7) */
#include <sched.h>
int sched_setaffinity( pid_t pid,
  size_t cpusetsize,
  const cpu_set_t *mask);
int sched_getaffinity( pid_t pid,
  size_t cpusetsize,
  cpu_set_t *mask);


A thread's CPU affinity mask determines the set of CPUs on which it is eligible to run. On a multiprocessor system, setting the CPU affinity mask can be used to obtain performance benefits. For example, by dedicating one CPU to a particular thread (i.e., setting the affinity mask of that thread to specify a single CPU, and setting the affinity mask of all other threads to exclude that CPU), it is possible to ensure maximum execution speed for that thread. Restricting a thread to run on a single CPU also avoids the performance cost caused by the cache invalidation that occurs when a thread ceases to execute on one CPU and then recommences execution on a different CPU.

A CPU affinity mask is represented by the cpu_set_t structure, a "CPU set", pointed to by mask. A set of macros for manipulating CPU sets is described in CPU_SET(3).

sched_setaffinity() sets the CPU affinity mask of the thread whose ID is pid to the value specified by mask. If pid is zero, then the calling thread is used. The argument cpusetsize is the length (in bytes) of the data pointed to by mask. Normally this argument would be specified as sizeof(cpu_set_t).

If the thread specified by pid is not currently running on one of the CPUs specified in mask, then that thread is migrated to one of the CPUs specified in mask.

sched_getaffinity() writes the affinity mask of the thread whose ID is pid into the cpu_set_t structure pointed to by mask. The cpusetsize argument specifies the size (in bytes) of mask. If pid is zero, then the mask of the calling thread is returned.


On success, sched_setaffinity() and sched_getaffinity() return 0. On error, −1 is returned, and errno is set appropriately.



A supplied memory address was invalid.


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).


(sched_getaffinity() and, in kernels before 2.6.9, sched_setaffinity()) cpusetsize is smaller than the size of the affinity mask used by the kernel.


(sched_setaffinity()) The calling thread does not have appropriate privileges. The caller needs an effective user ID equal to the real user ID or effective user ID of the thread identified by pid, or it must possess the CAP_SYS_NICE capability.


The thread whose ID is pid could not be found.


The CPU affinity system calls were introduced in Linux kernel 2.5.8. The system call wrappers were introduced in glibc 2.3. Initially, the glibc interfaces included a cpusetsize argument, typed as unsigned int. In glibc 2.3.3, the cpusetsize argument was removed, but was then restored in glibc 2.3.4, with type size_t.


These system calls are Linux-specific.


After a call to sched_setaffinity(), the set of CPUs on which the thread will actually run is the intersection of the set specified in the mask 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.

There are various ways of determining the number of CPUs available on the system, including: inspecting the contents of /proc/cpuinfo; using sysconf(3) to obtain the values of the _SC_NPROCESSORS_CONF and _SC_NPROCESSORS_ONLN parameters; and inspecting the list CPU directories under /sys/devices/system/cpu/.

sched(7) has a description of the Linux scheduling scheme.

The affinity mask is a per-thread attribute that can be adjusted independently for each of the threads in a thread group. The value returned from a call to gettid(2) can be passed in the argument pid. Specifying pid as 0 will set the attribute for the calling thread, and passing the value returned from a call to getpid(2) will set the attribute for the main thread of the thread group. (If you are using the POSIX threads API, then use pthread_setaffinity_np(3) instead of sched_setaffinity().)

A child created via fork(2) inherits its parent's CPU affinity mask. The affinity mask is preserved across an execve(2).

C library/kernel ABI differences

This manual page describes the glibc interface for the CPU affinity calls. The actual system call interface is slightly different, with the mask being typed as unsigned long *, reflecting the fact that the underlying implementation of CPU sets is a simple bit mask. On success, the raw sched_getaffinity() system call returns the size (in bytes) of the cpumask_t data type that is used internally by the kernel to represent the CPU set bit mask.


lscpu(1), nproc(1), taskset(1), clone(2), getcpu(2), getpriority(2), gettid(2), nice(2), sched_get_priority_max(2), sched_get_priority_min(2), sched_getscheduler(2), sched_setscheduler(2), setpriority(2), CPU_SET(3), pthread_setaffinity_np(3), sched_getcpu(3), capabilities(7), cpuset(7), sched(7)


This page is part of release 4.00 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at−pages/.

  Copyright (C) 2002 Robert Love
and Copyright (C) 2006 Michael Kerrisk

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2002-11-19 Robert Love <> - initial version
2004-04-20 mtk - fixed description of return value
2004-04-22 aeb - added glibc prototype history
2005-05-03 mtk - noted that sched_setaffinity may cause thread
migration and that CPU affinity is a per-thread attribute.
2006-02-03 mtk -- Major rewrite
2008-11-12, mtk, removed CPU_*() macro descriptions to a
separate CPU_SET(3) page.