Canceling WaitCommEvent() with SetCommMask()

Last reviewed: December 11, 1996
Article ID: Q105302

The information in this article applies to:

Microsoft Win32 Application Programming Interface (API) included with:
```
    - Microsoft Windows NT versions 3.1 and 3.5
```

If a serial port is in nonoverlapped mode (without FILE_FLAG_OVERLAPPED) and SetCommMask() is called, the call does not return until any pending WaitCommEvent() calls return. This apparently contradicts the following statement from the SetCommMask() Help

   If SetCommMask() is called for a communications resource while a wait
   is pending for that resource, WaitCommEvent() returns an error.

and the following statement from the WaitCommEvent() Help:

   If a process attempts to change the device handle's event mask by
   using the SetCommMask() function while a WaitCommEvent() operation
   is in progress, WaitCommEvent() returns immediately.

However, this is the expected behavior. If you open a serial port in the nonoverlapped mode, then you can do only one thing at a time with the serial port. SetCommMask() must block while the WaitCommEvent() call is blocking.

If the serial port was opened with FILE_FLAG_OVERLAPPED, WaitCommEvent() will return after SetCommEvent() has been called.

The SDK 3.51 docs have been corrected.

Additional reference words: 3.10 3.50 com1 com2
KBCategory: kbprg
KBSubcategory: BseCommapi

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Last reviewed: December 11, 1996
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