PRB: Synch OLE Call Fails in Inter-Process/Thread SendMessage

• Microsoft OLE Libraries for Windows and Win32s, version 2.03
• Microsoft OLE libraries included with:

      - Microsoft Windows NT version 3.5
      - Microsoft Windows 95
  

SYMPTOMS

A synchronous OLE call made by the recipient of an inter-process/inter- thread SendMessage fails with RPC_E_CANTCALLOUT_ININPUTSYNCCALL.

CAUSE

The cause is discussed in detail in the "More Information" section of this article.

RESOLUTION

Use PostMessage instead of an inter-process/inter-thread SendMessage.

STATUS

This behavior is by design.

MORE INFORMATION

See the beginning of chapter 13 in the OLE 2 Programmer's Reference Volume 1 for the categories of OLE calls. An understanding of these categories is required for this article.

The majority of OLE calls are synchronous calls. A synchronous call to a different process yields to that process and waits for a reply from that process. In addition, OLE has input-synchronized calls that relate to the inplace-activation interfaces. Input-synchronized calls are implemented using an inter-process/inter-thread SendMessage.

16-bit Windows doesn't allow a task to yield while in an inter- process/inter-thread SendMessage because a system deadlock could occur. The deadlock occurs because a message for the sender could be present at the top of the shared system queue, and this prevents other tasks, including the recipient of the SendMessage, from retrieving their messages from the system queue until the sender does. The sender cannot retrieve its message because it is waiting for the inter-process/inter-thread SendMessage to return.

In 32-bit Windows, each process has its own system queue and this architecture normally prevents deadlock problem from occurring. However, when one process is inplace active in another process's window, the system queues of the two processes are synchronized as in 16-bit windows, so the deadlock could occur. To prevent this, OLE stops synchronous OLE calls from being made while the caller is the recipient of an input-synchronized call.

OLE determines if the caller of the synchronous call is a recipient of an input-synchronized call by using the InSendMessage() API. This broad check prevents a synchronous call from being made if the caller is currently a recipient of any inter-process/inter-thread SendMessage.