Canceling WaitCommEvent() with SetCommMask()

ID: Q105302

3.10 3.50 WINDOWS NT kbprg

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

Keywords          : kbAPI kbKernBase kbGrpKernBase 
Version           : 3.10 3.50
Platform          : NT WINDOWS

Last Reviewed: December 13, 1996