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FillVolatileMemory function

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The FillVolatileMemory function fills a block of memory with the specified fill value.

Important

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Parameters

Param Destination [out]

A pointer to the starting address of the block of memory to fill.

Param Length [in]

The size of the block of memory to fill, in bytes. This value must be less than the size of the Destination buffer.

Param Fill [in]

The byte value with which to fill the memory block.

Syntax

volatile void*
  FillVolatileMemory (
    _Out_writes_bytes_all_(Length) volatile void* Destination,
    SIZE_T Length,
    INT Fill
  );

Remarks

This API exists to provide FillMemory behavior (i.e., setting the contents of a buffer) in situations where the developer needs to be sure that the setting operation occurs (i.e., is not subject to compiler optimizations). The API has the following properties:

  • The API is not recognized as a compiler intrinsic so the compiler will never optimize away the call (either entirely or replace the call with an "equivalent" sequence of instructions). This differs from FillMemory which is subject to a variety of compiler optimizations.
  • When the call returns, the buffer has been overwritten with the desired value. This functions memory accesses to the Destination will only be performed within the function (i.e. the compiler cannot move memory accesses out of this function).
  • The API may perform unaligned memory accesses if the platform allows for it.
  • The API may access memory locations more than once as part of its operation.

Note

This function works on all versions of Windows, not just the latest. You need to consume the latest SDK to get the function declaration from the winbase.h header. You also need the library (volatileaccessu.lib) from the latest SDK. However, the resulting binary will run fine on older versions of Windows.

Example

UCHAR SensitiveData[100];

// Imagine we temporarily store some sensitive cryptographic
// material in a buffer.

StoreCryptographicKey(&SensitiveData);
DoCryptographicOperation(&SensitiveData);

// Now that we are done using the sensitive data we want to
// erase it from the stack. We cannot call FillMemory because
// if the compiler realizes that "SensitiveData" is not
// referenced again the compiler can remove the call to FillMemory.
// Instead we can call SecureZeroMemory2, ZeroVolatileMemory, or FillVolatileMemory
// (the former two are convenience wrappers around the latter). These
// calls will not be optimized away by the compiler.
// Note that SecureZeroMemory2 performs better than the old
// SecureZeroMemory API.

FillVolatileMemory(&SensitiveData, sizeof(SensitiveData), 0);

Requirements

Minimum supported client: Windows 11 Insider Preview Build TBD

Header: winbase.h (include Winbase.h)

Kernel-mode library: volatileaccessk.lib

User-mode library: volatileaccessu.lib

See also