# Access Modes

In a world where multiple processes and users can access an object simultaneously, mechanisms for controlling that access are essential. OLE provides these mechanisms by defining access modes for both storage and stream objects. The access mode specified for a parent storage object is inherited by its children, though you can place additional restrictions on the child storage or stream. A nested storage or stream object can be opened in the same mode or in a more restricted mode than that of its parent, but it cannot be opened in a less restricted mode than that of its parent.

You specify access modes by using the values listed in the STGM enumeration. These values serve as flags to be passed as arguments to methods in the IStorage interface and associated API functions. Typically, several flags are combined in the parameter grfMode, using a Boolean OR operation.

The flags fall into six groups: transaction flags, storage creation flags, temporary creation flag, priority flag, access permission flags, and shared access flags. Only one flag from each group can be specified at a time.

## Transaction Flags

An object can be opened in either direct or transacted mode. When an object is opened in direct mode, changes are made immediately and permanently. When an object is opened in transacted mode, changes are buffered so they can be explicitly committed or reverted once editing is complete. Committed changes are saved to the object while reverted changes are discarded. Direct mode is the default access mode.

Transacted mode is not required on a parent storage object in order to use it on a nested element. A transaction for a nested element, however, is nested within the transaction for its parent storage object. Therefore, changes made to a child object cannot be committed until those made to the parent are committed, and both are not committed until the root storage object (the top-level parent) is actually written to disk. In other words, the changes move outward: inner objects publish changes to the transactions of their immediate containers.

## Storage Creation Flags

Storage creation flags specify what OLE should do if an existing storage, stream, or lockbytes object has the same name as a new storage or stream object that you are creating. The default is to return an error message and not create the new object. You can use only one of these flags in a given creation call.

## Temporary Creation Flag

The temporary creation flag indicates that the underlying file is to be automatically destroyed when the root storage object is released. This capability is most useful for creating temporary files.

## Priority Flag

The priority flag opens a storage object in priority mode. When it opens an object, an application usually works from a snapshot copy because other applications may also be using the object at the same time. When opening a storage object in priority mode, however, the application has exclusive rights to commit changes to the object.

Priority mode enables an application to read some streams from storage before opening the object in a mode that would require the system to make a snapshot copy to be made. Since the application has exclusive access, it doesn’t have to make a snapshot copy of the object. When the application subsequently opens the object in a mode where a snapshot copy is required, the application can exclude from the snapshot the streams it has already read, thereby reducing the overhead of opening the object.

Since other applications cannot commit changes to an object while it is open in priority mode, applications should keep it in that mode for as short a time as possible.

## Access Permission Flags

Access permission flags specify the type of access a client application has to an open object: read, write, or read/write. Read permission enables an application to read the contents of a stream but not to write changes to the stream. Write permission enables an application to call a function that commits changes to an object’s storage but not to read the contents of the stream. Read/write permission enables an application to either read the object’s streams or write to the object’s storage.

## Shared Access Flags

The shared access flags specify the degree of access other applications can have to an object that your application is opening. You can deny read access, deny write access, or deny all access. You can also specify explicitly that no type of access be denied.