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This is 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::SetProperties member 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 IMemAllocator interface.
It provides support for managing a list of CMediaSample objects (or objects derived from this class), including support for the IMemAllocator::Commit and IMemAllocator::Decommit methods, and blocking the IMemAllocator::GetBuffer method.
Any class derived from this class (such as CMemAllocator) must create CMediaSample objects, 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_lFree and m_hSem member variables are used to implement this simple signaling mechanism as follows.
When a thread calls CBaseAllocator::GetBuffer and there are no samples available, m_lWaiting is incremented and the thread calls the Win32 WaitForSingleObject function on the semaphore indicated by m_hSem.
When a sample is freed (by the IUnknown::Release method returning the reference count to zero) or CBaseAllocator::Decommit is called and m_lWaiting is nonzero, the Win32 ReleaseSemaphore function is called on m_hSem with a release count of m_lWaiting, and m_lWaiting is reset to zero.
All member functions in this class that return HRESULT and accept a pointer as a parameter return E_POINTER when passed a null pointer.
Protected Data Members
| Member | Description |
|---|---|
| m_bChanged | TRUE if the buffers have changed; otherwise, FALSE. |
| m_bCommitted | If TRUE, the allocator is in a state in which all IMemAllocator::GetBuffer methods fail.
The IMemAllocator::SetProperties method 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::GetBuffer return 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 CMediaSample objects 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
| Member function | Description |
|---|---|
| CBaseAllocator | Constructs a CBaseAllocator object. |
| NotifySample | Notifies a waiting thread that a sample is available on the free list. |
| SetWaiting | Increments the m_lWaiting data member to indicate that a thread is waiting for a sample. |
Overrideable Member Functions
| Member function | Description |
|---|---|
| Alloc | Allocates memory, instantiates CMediaSample objects, and adds them to the m_lAllocated and m_lFree data members. |
| Free | Decommits memory when the last buffer is freed. |
Implemented IMemAllocator Methods
| Method | Description |
|---|---|
| Commit | Allocates memory by calling the CBaseAllocator::Alloc member function, which you must override. |
| Decommit | Releases any resources and enters the inactive state.
Blocking calls to IMemAllocator::GetBuffer should return with an error value, and all further calls to GetBuffer fail when in the inactive state. |
| GetBuffer | Retrieves a container for a sample. |
| GetProperties | Determines the size, number, and alignment of blocks. |
| ReleaseBuffer | Releases the CMediaSample object. |
| 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
| Method | Description |
|---|---|
| NonDelegatingQueryInterface | Passes out pointers to any interfaces added to the derived filter class. |
Requirements
DirectShow applications and DirectShow filters have different include file and link library requirements.
For more information, see Setting Up the Build Environment.
OS Versions: Windows CE 2.12 and later. Version 2.12 requires DXPAK 1.0 or later.
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