Clarification of COMMPROP dwMax?xQueue Members

• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.1, 3.5, and 3.51
      - Microsoft Windows 95 version 4.0
  

SUMMARY

The entry for the COMMPROP structure in the Win32 Programmer's Reference states that for the dwMaxTxQueue and dwMaxRxQueue members, "a value of 0 means that this field is not used".

MORE INFORMATION

This statement means that the provider does not restrict you to maximum Rx and Tx queue values, and these members [returned by GetCommProperties()] should not be used to determine the size of your transmit and receive buffers when calling SetupComm().

Based on the memory present in the system, the Windows NT serial driver determines a default Rx queue size (currently 128 bytes on low memory systems and 4K on high memory systems). The current Rx and Tx queue sizes are located in the dwCurrentTxQueue and dwCurrentRxQueue members.

SetupComm() allows you to change these default queue sizes. However, you should not assume that the given serial driver will allocate any memory. The queue size allocated is stored in the dwCurrentRxQueue member of the COMMPROP structure. You may use this information to set the XonLim and XoffLim members of the device control block (DCB) structure.

The Microsoft-supplied serial driver attempts to allocate at least the amount requested for the RXQUEUE and, failing this, the request will also fail. The driver never attempts to allocate memory for the TXQUEUE.