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SetThreadAffinityMask function

Sets a processor affinity mask for the specified thread.


DWORD_PTR WINAPI SetThreadAffinityMask(
  _In_ HANDLE    hThread,
  _In_ DWORD_PTR dwThreadAffinityMask


hThread [in]

A handle to the thread whose affinity mask is to be set.

This handle must have the THREAD_SET_INFORMATION or THREAD_SET_LIMITED_INFORMATION access right and the THREAD_QUERY_INFORMATION or THREAD_QUERY_LIMITED_INFORMATION access right. For more information, see Thread Security and Access Rights.

Windows Server 2003 and Windows XP:  The handle must have the THREAD_SET_INFORMATION and THREAD_QUERY_INFORMATION access rights.

dwThreadAffinityMask [in]

The affinity mask for the thread.

On a system with more than 64 processors, the affinity mask must specify processors in the thread's current processor group.

Return value

If the function succeeds, the return value is the thread's previous affinity mask.

If the function fails, the return value is zero. To get extended error information, call GetLastError.

If the thread affinity mask requests a processor that is not selected for the process affinity mask, the last error code is ERROR_INVALID_PARAMETER.


A thread affinity mask is a bit vector in which each bit represents a logical processor that a thread is allowed to run on. A thread affinity mask must be a subset of the process affinity mask for the containing process of a thread. A thread can only run on the processors its process can run on. Therefore, the thread affinity mask cannot specify a 1 bit for a processor when the process affinity mask specifies a 0 bit for that processor.

Setting an affinity mask for a process or thread can result in threads receiving less processor time, as the system is restricted from running the threads on certain processors. In most cases, it is better to let the system select an available processor.

If the new thread affinity mask does not specify the processor that is currently running the thread, the thread is rescheduled on one of the allowable processors.


Minimum supported client

Windows XP [desktop apps only]

Minimum supported server

Windows Server 2003 [desktop apps only]


WinBase.h (include Windows.h)





See also

Multiple Processors
Process and Thread Functions
Processor Groups



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