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IStream interface

The IStream interface lets you read and write data to stream objects. Stream objects contain the data in a structured storage object, where storages provide the structure. Simple data can be written directly to a stream but, most frequently, streams are elements nested within a storage object. They are similar to standard files.

The IStream interface defines methods similar to the MS-DOS FAT file functions. For example, each stream object has its own access rights and a seek pointer. The main difference between a DOS file and a stream object is that in the latter case, streams are opened using an IStream interface pointer rather than a file handle.

The methods in this interface present your object's data as a contiguous sequence of bytes that you can read or write. There are also methods for committing and reverting changes on streams that are open in transacted mode and methods for restricting access to a range of bytes in the stream.

Streams can remain open for long periods of time without consuming file-system resources. The IUnknown::Release method is similar to a close function on a file. Once released, the stream object is no longer valid and cannot be used.

Clients of asynchronous monikers can choose between a data-pull or data-push model for driving an asynchronous IMoniker::BindToStorage operation and for receiving asynchronous notifications. See URL Monikers for more information. The following table compares the behavior of asynchronous ISequentialStream::Read and IStream::Seek calls returned in IBindStatusCallback::OnDataAvailable in these two download models:

IStream method callBehavior in data-pull modelBehavior in data-push model
Read is called to read partial data (that is, not all the available data)Returns S_OK. The client must continue to read all available data before returning from IBindStatusCallback::OnDataAvailable or else the bind operation is blocked. (that is, read until S_FALSE or E_PENDING is returned)Returns S_OK. Even if the client returns from IBindStatusCallback::OnDataAvailable at this point the bind operation continues and IBindStatusCallback::OnDataAvailable will be called again repeatedly until the binding finishes.
Read is called to read all the available dataReturns E_PENDING if the bind operation has not completed, and IBindStatusCallback::OnDataAvailable will be called again when more data is available.Same as data-pull model.
Read is called to read all the available data and the bind operation is over (end of file)Returns S_FALSE. There will be a subsequent call to IBindStatusCallback::OnDataAvailable with the grfBSC flag set to BSCF_LASTDATANOTIFICATION.Same as data-pull model.
Seek is calledSeek does not work in data-pull modelSeek does not work in data-push model.

 

For general information on this topic, see Asynchronous Monikers and Data-Pull-Model versus Data Push-Model for more specific information. Also, see Managing Memory Allocation for details on COM's rules for managing memory.

When to implement

Implement IStream on a container or object application when you require functionality not provided by the COM compound file implementation. The specification of IStream defines more functionality than the COM implementation supports. In addition, if you are creating a stream object that is larger than the heap in your machine's memory and you are using a global memory handle, the compound file implementation calls GlobalReAlloc internally whenever it needs more memory, which can be very inefficient. In this case, the preferred solution is to implement an IStream that uses memory allocated by VirtualAlloc instead of GlobalAlloc. This can reserve a large chunk of virtual address space and then commit memory within that address space as required. No data copying occurs and memory is committed only as it is needed. For more information, refer to IStream - Compound File Implementation.

The IStream interface inherits Read and Write methods from ISequentialStream.

Note  Most containers do not implement ISequentialStream as a separate interface, and you are not required to provide it separately even if you provide an IStream implementation. For example, the compound file implementation of structured storage does not succeed on a QueryInterface method for ISequentialStream but it includes the Read and Write methods through the IStream interface pointer.

When to use

Call the methods of the IStream interface from a container or application to read and write the data for an object. Since stream objects can be marshaled to other processes, applications can share the data in storage objects without having to use global memory.

Members

The IStream interface inherits from the IUnknown interface. IStream also has these types of members:

Methods

The IStream interface has these methods.

MethodDescription
Clone

Creates a new stream object that references the same bytes as the original stream but provides a separate seek pointer to those bytes.

Commit

Ensures that any changes made to a stream object open in transacted mode are reflected in the parent storage object.

CopyTo

Copies a specified number of bytes from the current seek pointer in the stream to the current seek pointer in another stream.

LockRegion

Restricts access to a specified range of bytes in the stream. Supporting this functionality is optional since some file systems do not provide it.

Read

Reads a specified number of bytes from the stream object into memory starting at the current seek pointer.

(Inherited from ISequentialStream)
Revert

Discards all changes that have been made to a transacted stream since the last call to IStream::Commit.

Seek

Changes the seek pointer to a new location relative to the beginning of the stream, the end of the stream, or the current seek pointer.

SetSize

Changes the size of the stream object.

Stat

Retrieves the STATSTG structure for this stream.

UnlockRegion

Removes the access restriction on a range of bytes previously restricted with IStream::LockRegion.

Write

Writes a specified number of bytes into the stream object starting at the current seek pointer.

(Inherited from ISequentialStream)

 

Requirements

Minimum supported client

Windows 2000 Professional [desktop apps | Windows Store apps]

Minimum supported server

Windows 2000 Server [desktop apps | Windows Store apps]

Header

Objidl.h

IDL

Objidl.idl

Library

Uuid.lib

DLL

Ole32.dll

IID

IID_IStream is defined as 0000000C-0000-0000-C000-000000000046

See also

IStream - Compound File Implementation
ISequentialStream

 

 

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