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.

ValueMeaning
EV_BREAKA break was detected on input.
EV_CTSThe CTS (clear-to-send) signal changed state.
EV_DSRThe DSR (data-set-ready) signal changed state.
EV_ERRA line-status error occurred. Line-status errors are CE_FRAME, CE_OVERRUN, and CE_RXPARITY.
EV_RINGA ring indicator was detected.
EV_RLSDThe RLSD (receive-line-signal-detect) signal changed state.
EV_RXCHARA character was received and placed in the input buffer.
EV_RXFLAGThe 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_TXEMPTYThe 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) 
{ 
    WaitCommEvent(args)
 
T1: // Read bytes 
    // Process bytes 

T2: 
}

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.

 

 

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