Communications Events

A process can monitor a set of events that occur in a communications resource. For example, an application can use event monitoring to determine when the CTS (clear-to-send) and DSR (data-set-ready) signals change state.

A process can monitor events on a given communications resource by using the SetCommMask function to create an event mask. To determine the current event mask for a communications resource, a process can use the GetCommMask function. The following values specify events that can be monitored.

Value Meaning
EV_BREAK A break was detected on input.
EV_CTS The CTS (clear-to-send) signal changed state.
EV_DSR The DSR (data-set-ready) signal changed state.
EV_ERR A line-status error occurred. Line-status errors are CE_FRAME, CE_OVERRUN, and CE_RXPARITY.
EV_RING A ring indicator was detected.
EV_RLSD The RLSD (receive-line-signal-detect) signal changed state.
EV_RXCHAR A character was received and placed in the input buffer.
EV_RXFLAG The event character was received and placed in the input buffer. The event character is specified in the device's DCB structure, which is applied to a serial port by using the SetCommState function.
EV_TXEMPTY The last character in the output buffer was sent.


After a set of events is specified, a process uses the WaitCommEvent function to wait for one of the events to occur. WaitCommEvent can be used synchronously or as an overlapped operation. For additional information about executing a function as an overlapped operation, see Synchronization.

When one of the events specified in the event mask occurs, the process completes the wait operation and sets an event mask variable to indicate the type of event detected. If the SetCommMask is called for a communications resource while a wait is pending for that resource, WaitCommEvent returns an error.

The WaitCommEvent function detects events that have occurred since the last call to SetCommMask or WaitCommEvent. For example, if you specify the EV_RXCHAR event as a wait-satisfying event, a call to WaitCommEvent will be satisfied if there are characters in the driver's input buffer that have arrived since the last call to WaitCommEvent or SetCommMask. Thus, given the following pseudocode, any characters received between T1 and T2 will satisfy the next call to WaitCommEvent.

while (!bFinished) 
T1: // Read bytes 
    // Process bytes 


When monitoring an event that occurs when a signal (CTS, DSR, and so on) changes state, WaitCommEvent reports the change, but not the current state. To query the current state of the CTS (clear-to-send), DSR (data-set-ready), RLSD (receive-line-signal-detect), and ring indicator signals, a process can use the GetCommModemStatus function.