CBaseAllocator Class (Windows Embedded CE 6.0)

1/6/2010

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

Windows Embedded CE	Windows CE 2.12 and later
Note	Microsoft DirectShow applications and DirectShow filters have different include file and Library requirements For more information, see Setting Up the Build Environment

See Also

Reference

DirectShow Classes