The IMemAllocator interface allocates IMediaSample blocks to be used for data transfer between pins. The blocks can be provided by the input pin, output pin, or a third party. Release the IMediaSample object by calling the IUnknown::Release method.
When to Implement
Implement this interface if you are providing a buffer for media samples. This is typically done by filters that originate, copy, or provide a destination for the media stream. For example, a source filter provides an allocator corresponding to the incoming media, and a renderer filter provides an allocator corresponding to the hardware memory (DirectX®, for example). Intermediate transform filters can create their own allocator if copying samples and not writing directly to the renderer's allocator. Use the CBaseAllocator class to implement the IMemAllocator interface.
When to Use
Filters use this interface when retrieving or sending media samples. The allocator used might actually belong to a filter further upstream or downstream than the next filter, because many filters choose to pass the allocators through and simply modify the data in place.
Methods in Vtable Order
| IUnknown methods | Description |
| QueryInterface | Returns pointers to supported interfaces. |
| AddRef | Increments the reference count. |
| Release | Decrements the reference count. |
| IMemAllocator methods | Description |
| SetProperties | Specifies a desired number of blocks, size of the blocks, and block alignment figure. This method returns the actual values for the same. |
| GetProperties | Determines the size, number, and alignment of blocks. |
| Commit | Commits the memory for the specified buffers. |
| Decommit | Releases the memory for the specified buffers. |
| GetBuffer | Retrieves a container for a sample. |
| ReleaseBuffer | Releases a container for a sample. |
Commits the memory for the specified buffers.
HRESULT Commit(void);
Returns an HRESULT value.
The IMemAllocator::SetProperties method must be called before calling this method. The IMemAllocator::GetBuffer method fails if it is called before this method.
Releases the memory for the specified buffers.
HRESULT Decommit(void);
Returns an HRESULT value.
Any threads waiting in the IMemAllocator::GetBuffer method return with an error after this method is called. The IMemAllocator::GetBuffer method fails if it is called before the IMemAllocator::Commit method or after this method.
Retrieves a container for a sample.
HRESULT GetBuffer(
IMediaSample ** ppBuffer,
REFERENCE_TIME * pStartTime,
REFERENCE_TIME * pEndTime,
DWORD dwFlags
);
Returns an HRESULT value.
IMemAllocator::GetBuffer is a locking, synchronous method to get the next free buffer. Upon return, the properties are invalid, but the buffer pointer and size are correct. This method succeeds only if memory has been committed. GetBuffer returns with an error value if it is blocked waiting for a buffer, and the IMemAllocator::Decommit method is called on another thread.
Returns the properties being used on this allocator.
HRESULT GetProperties(
ALLOCATOR_PROPERTIES * pProps
);
Returns an HRESULT value.
Calls to this method need not succeed until the IMemAllocator::Commit method is called.
Releases the CMediaSample object.
HRESULT ReleaseBuffer(
IMediaSample *pBuffer
);
Returns an HRESULT value.
Specifies the size, number, and alignment of blocks.
HRESULT SetProperties(
ALLOCATOR_PROPERTIES * pRequest,
ALLOCATOR_PROPERTIES * pActual
);
Returns an HRESULT value.
The pRequest parameter is filled in by the caller with the requested values for the count, number, and alignment as specified by the ALLOCATOR_PROPERTIES structure. The pActual parameter is filled in by the allocator with the closest values that it can provide for the request. This method cannot be called unless the allocator has been decommitted using the IMemAllocator::Decommit method.
This method assumes that blocks are all the same size.
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