Avoid Calling SendMessage() Inside a Hook Filter Function

 Last reviewed: November 2, 1995
Article ID: Q74857
The information in this article applies to:
  • Microsoft Windows Software Development Kit (SDK) versions 3.1
  • Microsoft Win32 Application Programming Interface (API) included with: 

        - Microsoft Windows NT versions 3.5 and 3.51
    - Microsoft Windows 95 version 4.0

A hook filter function should not call SendMessage() to pass intertask messages because this behavior can create a deadlock condition in Windows. If a hook filter function is called as a result of an intertask SendMessage(), and if the hook function then yields control with an intertask SendMessage(), a message deadlock condition may occur. For this reason, hook filters should use PostMessage() rather than SendMessage() to pass messages to other applications.

NOTE: A hook filter can use SendMessage() to pass a message to the current task because this will not yield the control.

Section 1.1.5 of the "Microsoft Windows Software Development Kit Reference Volume 1" from the Windows SDK version 3.0 documentation has more information on message deadlocks.

Additional reference words: 3.00 3.10 3.50 3.51 4.00 95
KBCategory: kbui
KBSubcategory: UsrHks

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Last reviewed: November 2, 1995
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