Из справки по Visual C
The PurgeComm function discards all characters from the output or input buffer of a specified communications resource. It can also terminate pending read or write operations on the resource.


BOOL PurgeComm(
HANDLE hFile,
DWORD dwFlags
);

Parameters
hFile
[in] Handle to the communications resource. The CreateFile function returns this handle.
dwFlags
[in] This parameter can be one or more of the following values. Value Meaning
PURGE_TXABORT Terminates all outstanding overlapped write operations and returns immediately, even if the write operations have not been completed.
PURGE_RXABORT Terminates all outstanding overlapped read operations and returns immediately, even if the read operations have not been completed.
PURGE_TXCLEAR Clears the output buffer (if the device driver has one).
PURGE_RXCLEAR Clears the input buffer (if the device driver has one).

Return Values
If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero. To get extended error information, call GetLastError.

Remarks
If a thread uses PurgeComm to flush an output buffer, the deleted characters are not transmitted. To empty the output buffer while ensuring that the contents are transmitted, call the FlushFileBuffers function (a synchronous operation).

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Note, however, that FlushFileBuffers is subject to flow control but not to write time-outs, and it will not return until all pending write operations have been transmitted.
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The FlushFileBuffers function flushes the buffers of the specified file and causes all buffered data to be written to the file.
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If hFile is a handle to a communications device, the function only flushes the transmit buffer.