IMFMediaBuffer::Lock method (mfobjects.h)

Article
10/13/2021

Gives the caller access to the memory in the buffer, for reading or writing

Syntax

HRESULT Lock(
  [out] BYTE  **ppbBuffer,
  [out] DWORD *pcbMaxLength,
  [out] DWORD *pcbCurrentLength
);

Parameters

[out] ppbBuffer

Receives a pointer to the start of the buffer.

[out] pcbMaxLength

Receives the maximum amount of data that can be written to the buffer. This parameter can be NULL. The same value is returned by the IMFMediaBuffer::GetMaxLength method.

[out] pcbCurrentLength

Receives the length of the valid data in the buffer, in bytes. This parameter can be NULL. The same value is returned by the IMFMediaBuffer::GetCurrentLength method.

Return value

The method returns an HRESULT. Possible values include, but are not limited to, those in the following table.

Return code	Description
S_OK	The method succeeded.
D3DERR_INVALIDCALL	For Direct3D surface buffers, an error occurred when locking the surface.
MF_E_INVALIDREQUEST	The buffer cannot be locked at this time.

Remarks

This method gives the caller access to the entire buffer, up to the maximum size returned in the pcbMaxLength parameter. The value returned in pcbCurrentLength is the size of any valid data already in the buffer, which might be less than the total buffer size.

The pointer returned in ppbBuffer is guaranteed to be valid, and can safely be accessed across the entire buffer for as long as the lock is held. When you are done accessing the buffer, call IMFMediaBuffer::Unlock to unlock the buffer. You must call Unlock once for each call to Lock. After you unlock the buffer, the pointer returned in ppbBuffer is no longer valid, and should not be used. Generally, it is best to call Lock only when you need to access the buffer memory, and not earlier.

Locking the buffer does not prevent other threads from calling Lock, so you should not rely on this method to synchronize threads.

This method may allocate memory, but does not transfer ownership of the memory to the caller. Do not release or free the memory; the media buffer will free the memory when the media buffer is destroyed.

If you modify the contents of the buffer, update the current length by calling IMFMediaBuffer::SetCurrentLength.

This method may internally allocate some memory, so if the buffer supports the IMF2DBuffer interface, you should use the IMF2DBuffer::Lock2D method to lock the buffer instead. For 2-D buffers, the Lock2DSize method can be more efficient than the Lock method, depending on the MF2DBuffer_LockFlags value you specify. Calling Lock2DSize with MF2DBuffer_LockFlags_Read won’t incur a copy back when the buffer is unlocked and calling it with MF2DBuffer_LockFlags_Write won’t incur a copy from the internal buffer. Calling Lock2DSize with LockFlags_ReadWrite behaves the same as Lock and Lock2D and will incur a both copy from and copy back when unlocked. The general guidance for best performance is to avoid using IMFMediaBuffer and IMF2DBuffer whenever possible and instead use IMF2DBuffer2 with the minimum required lock flags. Note that if the buffer is locked using Lock2D, the Lock method might return MF_E_INVALIDREQUEST.

This interface is available on the following platforms if the Windows Media Format 11 SDK redistributable components are installed:

Windows XP with Service Pack 2 (SP2) and later.
Windows XP Media Center Edition 2005 with KB900325 (Windows XP Media Center Edition 2005) and KB925766 (October 2006 Update Rollup for Windows XP Media Center Edition) installed.

Requirements

Requirement	Value
Minimum supported client	Windows Vista [desktop apps \| UWP apps]
Minimum supported server	Windows Server 2008 [desktop apps \| UWP apps]
Target Platform	Windows
Header	mfobjects.h (include Mfidl.h)
Library	Mfuuid.lib