Atoms and Shared Memory Objects - Win32 Programmer's Reference

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Atoms and Shared Memory Objects

Certain arguments of DDE messages are global atoms or shared memory objects. Applications using these arguments must follow explicit rules about when to allocate and delete them. In all cases, the message sender must delete any atom or shared memory object that the intended receiver will not receive because of an error condition, such as failure of the PostMessage function.

DDE uses shared memory objects for three purposes:

To carry a data-item value to be exchanged. This is an item referenced by the hData parameter in the WM_DDE_DATA and WM_DDE_POKE messages.
To carry options in a message. This is an item referenced by the hOptions parameter in a WM_DDE_ADVISE message.
To carry a command execution string. This is an item referenced by the hCommands parameter in the WM_DDE_EXECUTE message and its corresponding WM_DDE_ACK message.

An application that receives a DDE shared memory object must treat it as read only. The application must not use the object as a mutual read-write area for the free exchange of data.

As it does with a DDE atom, an application should free a shared memory object to manage memory effectively. The application should also lock and unlock memory objects.

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