LocateXStateFeature function

Retrieves a pointer to the processor state for an XState feature within a CONTEXT structure.

The definition of XState feature bits are processor vendor specific. Please refer to the relevant processor reference manuals for additional information on a particular feature.


PVOID WINAPI LocateXStateFeature(
  _In_      PCONTEXT Context,
  _In_      DWORD    FeatureId,
  _Out_opt_ PDWORD   Length


Context [in]

A pointer to a CONTEXT structure containing the state to retrieve or set. This CONTEXT should have been initialized with InitializeContext with the CONTEXT_XSTATE flag set in the ContextFlags parameter.

FeatureId [in]

The number of the feature to locate within the CONTEXT structure.

Length [out, optional]

A pointer to a variable which receives the length of the feature area in bytes. The contents of this variable are undefined if this function returns NULL.

Return value

If the specified feature is supported by the system and the specified CONTEXT structure has been initialized with the CONTEXT_XSTATE flag, this function returns a pointer to the feature area for the specified feature. The contents and layout of this area is processor-specific.

If the CONTEXT_XSTATE flag is not set in the CONTEXT structure or the FeatureID is not supported by the system, the return value is NULL. No additional error information is available.


The LocateXStateFeature function must be used to find an individual XState feature within an extensible CONTEXT structure. Features are not necessarily contiguous in memory and applications should not assume the offset between two consecutive features will remain constant in the future.

The FeatureID parameter of the function corresponds to a bit within the feature mask. For example, FeatureId 2 corresponds to a FeatureMask of 4 in SetXStateFeaturesMask. FeatureID values of 0 and 1 correspond to X87 FPU state and SSE state, respectively.

If you are setting XState on a thread via the SetThreadContext or Wow64SetThreadContext APIs, you must also call SetXStateFeaturesMask on the CONTEXT structure with the mask value of the filled-in feature to mark the feature as active.

Windows 7 with SP1 and Windows Server 2008 R2 with SP1:  The AVX API is first implemented on Windows 7 with SP1 and Windows Server 2008 R2 with SP1 . Since there is no SDK for SP1, that means there are no available headers and library files to work with. In this situation, a caller must declare the needed functions from this documentation and get pointers to them using GetModuleHandle on "Kernel32.dll", followed by calls to GetProcAddress. See Working with XState Context for details.


Minimum supported client

Windows 7 with SP1 [desktop apps | Windows Store apps]

Minimum supported server

Windows Server 2008 R2 with SP1 [desktop apps | Windows Store apps]


WinBase.h (include Windows.h)





See also

Intel AVX
Working with XState Context



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