WaitCommEvent function (winbase.h)

Article
10/13/2021

Waits for an event to occur for a specified communications device. The set of events that are monitored by this function is contained in the event mask associated with the device handle.

Syntax

BOOL WaitCommEvent(
  [in]  HANDLE       hFile,
  [out] LPDWORD      lpEvtMask,
  [in]  LPOVERLAPPED lpOverlapped
);

Parameters

[in] hFile

A handle to the communications device. The CreateFile function returns this handle.

[out] lpEvtMask

A pointer to a variable that receives a mask indicating the type of event that occurred. If an error occurs, the value is zero; otherwise, it is one of the following values.

Value	Meaning
EV_BREAK 0x0040	A break was detected on input.
EV_CTS 0x0008	The CTS (clear-to-send) signal changed state.
EV_DSR 0x0010	The DSR (data-set-ready) signal changed state.
EV_ERR 0x0080	A line-status error occurred. Line-status errors are CE_FRAME, CE_OVERRUN, and CE_RXPARITY.
EV_RING 0x0100	A ring indicator was detected.
EV_RLSD 0x0020	The RLSD (receive-line-signal-detect) signal changed state.
EV_RXCHAR 0x0001	A character was received and placed in the input buffer.
EV_RXFLAG 0x0002	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 0x0004	The last character in the output buffer was sent.

[in] lpOverlapped

A pointer to an OVERLAPPED structure. This structure is required if hFile was opened with FILE_FLAG_OVERLAPPED.

If hFile was opened with FILE_FLAG_OVERLAPPED, the lpOverlapped parameter must not be NULL. It must point to a valid OVERLAPPED structure. If hFile was opened with FILE_FLAG_OVERLAPPED and lpOverlapped is NULL, the function can incorrectly report that the operation is complete.

If hFile was opened with FILE_FLAG_OVERLAPPED and lpOverlapped is not NULL, WaitCommEvent is performed as an overlapped operation. In this case, the OVERLAPPED structure must contain a handle to a manual-reset event object (created by using the CreateEvent function).

If hFile was not opened with FILE_FLAG_OVERLAPPED, WaitCommEvent does not return until one of the specified events or an error occurs.

Return value

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

The WaitCommEvent function monitors a set of events for a specified communications resource. To set and query the current event mask of a communications resource, use the SetCommMask and GetCommMask functions.

If the overlapped operation cannot be completed immediately, the function returns FALSE and the GetLastError function returns ERROR_IO_PENDING, indicating that the operation is executing in the background. When this happens, the system sets the hEvent member of the OVERLAPPED structure to the not-signaled state before WaitCommEvent returns, and then it sets it to the signaled state when one of the specified events or an error occurs. The calling process can use one of the wait functions to determine the event object's state and then use the GetOverlappedResult function to determine the results of the WaitCommEvent operation. GetOverlappedResult reports the success or failure of the operation, and the variable pointed to by the lpEvtMask parameter is set to indicate the event that occurred.

If a process attempts to change the device handle's event mask by using the SetCommMask function while an overlapped WaitCommEvent operation is in progress, WaitCommEvent returns immediately. The variable pointed to by the lpEvtMask parameter is set to zero.

Examples

For an example, see Monitoring Communications Events.

Requirements

Requirement	Value
Minimum supported client	Windows XP [desktop apps \| UWP apps]
Minimum supported server	Windows Server 2003 [desktop apps \| UWP apps]
Target Platform	Windows
Header	winbase.h (include Windows.h)
Library	Kernel32.lib
DLL	Kernel32.dll