Com Port

ClearCommError

The ClearCommError function retrieves information about a communications error and reports the current status of a communications device. The function is called when a communications error occurs, and it clears the device's error flag to enable additional input and output (I/O) operations.


BOOL ClearCommError(
  HANDLE hFile,
  LPDWORD lpErrors,
  LPCOMSTAT lpStat
);

Parameters:

hFile
[in] Handle to the communications device. The CreateFile function returns this handle.
 
lpErrors
[out] Pointer to a variable to be filled with a mask indicating the type of error. This parameter can be one or more of the following values. Value Meaning
CE_BREAK The hardware detected a break condition.
CE_DNS Windows Me/98/95:  A parallel device is not selected.
CE_FRAME The hardware detected a framing error.
CE_IOE An I/O error occurred during communications with the device.
CE_MODE The requested mode is not supported, or the hFile parameter is invalid. If this value is specified, it is the only valid error.
CE_OOP Windows Me/98/95:  A parallel device signaled that it is out of paper.
CE_OVERRUN A character-buffer overrun has occurred. The next character is lost.
CE_PTO Windows Me/98/95:  A time-out occurred on a parallel device.
CE_RXOVER An input buffer overflow has occurred. There is either no room in the input buffer, or a character was received after the end-of-file (EOF) character.
CE_RXPARITY The hardware detected a parity error.
CE_TXFULL The application tried to transmit a character, but the output buffer was full.

lpStat
[out] Pointer to a COMSTAT structure in which the device's status information is returned. If this parameter is NULL, no status information is returned.
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 communications port has been set up with a TRUE value for the fAbortOnError member of the setup DCB structure, the communications software will terminate all read and write operations on the communications port when a communications error occurs. No new read or write operations will be accepted until the application acknowledges the communications error by calling the ClearCommError function.

The ClearCommError function fills the status buffer pointed to by the lpStat parameter with the current status of the communications device specified by the hFile parameter.

COMSTAT

The COMSTAT structure contains information about a communications device. This structure is filled by the ClearCommError function.


typedef struct _COMSTAT
{
   DWORD fCtsHold  :1;
   DWORD fDsrHold  :1;
   DWORD fRlsdHold :1;
   DWORD fXoffHold :1;
   DWORD fXoffSent :1;
   DWORD fEof      :1;
   DWORD fTxim     :1;
   DWORD fReserved :25;
   DWORD cbInQue;
   DWORD cbOutQue;
} COMSTAT, *LPCOMSTAT;

Members:

fCtsHold
If this member is TRUE, transmission is waiting for the CTS (clear-to-send) signal to be sent.

fDsrHold
If this member is TRUE, transmission is waiting for the DSR (data-set-ready) signal to be sent.

fRlsdHold
If this member is TRUE, transmission is waiting for the RLSD (receive-line-signal-detect) signal to be sent.

fXoffHold
If this member is TRUE, transmission is waiting because the XOFF character was received.

fXoffSent
If this member is TRUE, transmission is waiting because the XOFF character was transmitted. (Transmission halts when the XOFF character is transmitted to a system that takes the next character as XON, regardless of the actual character.)

fEof
If this member is TRUE, the end-of-file (EOF) character has been received.

fTxim
If this member is TRUE, there is a character queued for transmission that has come to the communications device by way of the TransmitCommChar function. The communications device transmits such a character ahead of other characters in the device's output buffer.

fReserved
Reserved; do not use.

cbInQue
Number of bytes received by the serial provider but not yet read by a ReadFile operation.

cbOutQue
Number of bytes of user data remaining to be transmitted for all write operations. This value will be zero for a nonoverlapped write.