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CBaseAllocatoris an abstract base class that implements the basic mechanisms for an allocator with a fixed number of fixed-size buffers. The number of buffers and their size can be changed using the CBaseAllocator::SetPropertiesmember function when an input pin and an output pin negotiate the allocator between them.
The class provides the basic functionality for a memory allocator by implementing the IMemAllocatorinterface. It provides support for managing a list of CMediaSampleobjects (or objects derived from this class), including support for the IMemAllocator::Commitand IMemAllocator::Decommitmethods, and blocking the IMemAllocator::GetBuffermethod.
Any class derived from this class (such as CMemAllocator) must create CMediaSampleobjects, which this base class does not.
A signaling mechanism employing a semaphore is used so that if there are no samples, a thread can wait until there are samples or until the allocator is decommitted. The m_lFreeand m_hSemmember variables are used to implement this simple signaling mechanism as follows.
When a thread calls CBaseAllocator::GetBufferand there are no samples available, m_lWaitingis incremented and the thread calls the Microsoft Win32 WaitForSingleObjectfunction on the semaphore indicated by m_hSem.
When a sample is freed (by the IUnknown::Releasemethod returning the reference count to zero) or CBaseAllocator::Decommitis called and m_lWaitingis nonzero, the Win32 ReleaseSemaphorefunction is called on m_hSemwith a release count of m_lWaiting, and m_lWaitingis reset to zero.
All member functions in this class that return HRESULTand accept a pointer as a parameter return E_POINTER when passed a null pointer.
Protected Data Members
| m_bChanged | TRUE if the buffers have changed; otherwise, FALSE. |
| m_bCommitted | If TRUE, the allocator is in a state in which all IMemAllocator::GetBuffermethods fail. The IMemAllocator::SetPropertiesmethod is the only member function permitted to operate in this state. |
| m_bDecommitInProgress | If TRUE, the decommit process completes upon the return of all media samples. Until the decommit process has completed, any calls to IMemAllocator::GetBufferreturn E_OUTOFMEMORY. |
| m_hSem | Semaphore for signaling. |
| m_lAlignment | Agreed alignment of the buffer. |
| m_lAllocated | Number of buffers actually allocated. |
| m_lCount | Established number of buffers to provide. |
| m_lFree | List of CMediaSampleobjects that are currently free (free list). |
| m_lPrefix | Agreed prefix of the buffer (precedes value returned by IMediaSample::GetPointer). |
| m_lSize | Size of each buffer. |
| m_lWaiting | Count of threads waiting for samples. |
Member Functions
| CBaseAllocator | Constructs a CBaseAllocatorobject. |
| NotifySample | Notifies a waiting thread that a sample is available on the free list. |
| SetWaiting | Increments the m_lWaitingdata member to indicate that a thread is waiting for a sample. |
Overridable Member Functions
| Alloc | Allocates memory, instantiates CMediaSampleobjects, and adds them to the m_lAllocatedand m_lFreedata members. |
| Free | Decommits memory when the last buffer is freed. |
Implemented IMemAllocator Methods
| Commit | Allocates memory by calling the CBaseAllocator::Allocmember function, which you must override. |
| Decommit | Releases any resources and enters the inactive state. Any blocking calls to IMemAllocator::GetBuffershould return with an error value, and all further calls to GetBufferfail when in the inactive state. |
| GetBuffer | Retrieves a container for a sample. |
| GetProperties | Determines the size, number, and alignment of blocks. |
| ReleaseBuffer | Releases the CMediaSampleobject. |
| SetProperties | Specifies a desired number of blocks, size of the blocks, and block alignment figure. This method returns the actual values for the same. |
Implemented INonDelegatingUnknown Methods
| NonDelegatingQueryInterface | Passes out pointers to any interfaces added to the derived filter class. |