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pthread_getaffinity_np, pthread_setaffinity_np – manage CPU affinity



POSIX Threads Library (libpthread, -lpthread)


#include <pthread_np.h>

pthread_getaffinity_np(pthread_t td, size_t cpusetsize, cpuset_t *cpusetp);

pthread_setaffinity_np(pthread_t td, size_t cpusetsize, const cpuset_t *cpusetp);


pthread_getaffinity_np() and pthread_setaffinity_np() allow the manipulation of sets of CPUs available to the specified thread.

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 pthread_getaffinity_np() even if the result set would fit within the user supplied set. Calls to pthread_setaffinity_np() tolerate small sets with no restrictions.

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

pthread_getaffinity_np() retrieves the mask from the thread specified by td, and stores it in the space provided by cpusetp.

pthread_setaffinity_np() attempts to set the mask for the thread specified by td to the value in cpusetp.


If successful, the pthread_getaffinity_np() and pthread_setaffinity_np() functions will return zero. Otherwise an error number will be returned to indicate the error.


The pthread_getaffinity_np() and pthread_setaffinity_np() functions may fail if:
  The pthread_setaffinity_np() 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 cpusetp pointer passed was invalid.
  The thread specified by the td argument could not be found.
  The cpusetsize was either preposterously large or smaller than the kernel set size.
  The calling thread did not have the credentials required to complete the operation.


cpuset(1), cpuset(2), cpuset_getid(2), cpuset_setid(2), pthread(3), pthread_attr_getaffinity_np(3), pthread_attr_setaffinity_np(3)


The pthread_getaffinity_np and pthread_setaffinity_np functions are non-standard FreeBSD extensions and may be not available on other operating systems.


The pthread_getaffinity_np and pthread_setaffinity_np function first appeared in FreeBSD 7.2 .


The pthread_getaffinity_np and pthread_setaffinity_np functions were written by David Xu <Mt davidxu@FreeBSD.org>, and this manpage was written by Xin LI <Mt delphij@FreeBSD.org>.

PTHREAD_AFFINITY_NP (3) March 23, 2010

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