Enables or disables the placement of line status and modem status values into the regular data stream that an application acquires through the ReadFile function.

When this line-status and modem-status data placement mode is enabled, status values are preceded in the data stream by an escape character. The user-definable escape character is set by the IOCTL_SERIAL_LSRMST_INSERT control code. See the Remarks section for status value details.

To perform this operation, call the DeviceIoControl function with the following parameters.

BOOL DeviceIoControl(
  (HANDLE) hDevice,            // handle to device
  (LPVOID) lpInBuffer,         // input buffer 
  (DWORD) nInBufferSize,       // size of input buffer 
  NULL,                        // lpOutBuffer
  0,                           // nOutBufferSize
  (LPDWORD) lpBytesReturned,   // number of bytes returned
  (LPOVERLAPPED) lpOverlapped  // OVERLAPPED structure



A handle to the device on which the operation is to be performed. To retrieve a device handle, call the CreateFile function.


The control code for the operation. Use IOCTL_SERIAL_LSRMST_INSERT for this operation.


A pointer to a buffer that contains a single BYTE.

If the BYTE is a nonzero value, it is the desired line-status and modem-status value escape character. The operation will turn the LSRMST_INSERT line-status and modem-status data placement mode on.

If the BYTE is zero, the operation will turn the LSRMST_INSERT line-status and modem-status data placement mode off.

The escape character cannot be the XON or XOFF character.


The size of the input buffer, in bytes. For this operation, this value should be 1.


Not used with this operation; set to NULL.


Not used with this operation; set to zero.


A pointer to a variable that receives the size of the data stored in the output buffer, in bytes.

If lpOverlapped is NULL, lpBytesReturned cannot be NULL. Even when an operation returns no output data and lpOutBuffer is NULL, DeviceIoControl makes use of lpBytesReturned. After such an operation, the value of lpBytesReturned is meaningless.

If lpOverlapped is not NULL, lpBytesReturned can be NULL. If this parameter is not NULL and the operation returns data, lpBytesReturned is meaningless until the overlapped operation has completed. To retrieve the number of bytes returned, call GetOverlappedResult. If hDevice is associated with an I/O completion port, you can retrieve the number of bytes returned by calling GetQueuedCompletionStatus.


A pointer to an OVERLAPPED structure.

If hDevice was opened without specifying FILE_FLAG_OVERLAPPED, lpOverlapped is ignored.

If hDevice was opened with the FILE_FLAG_OVERLAPPED flag, the operation is performed as an overlapped (asynchronous) operation. In this case, lpOverlapped must point to a valid OVERLAPPED structure that contains a handle to an event object. Otherwise, the function fails in unpredictable ways.

For overlapped operations, DeviceIoControl returns immediately, and the event object is signaled when the operation has been completed. Otherwise, the function does not return until the operation has been completed or an error occurs.

Return value

If the operation completes successfully, DeviceIoControl returns a nonzero value.

If the operation fails or is pending, DeviceIoControl returns zero. To get extended error information, call GetLastError.


Note  An application that uses this scheme must examine each character in the data stream to determine the presence of modem-status or line-status data.

The following values follow the designated escape character in the data stream if the LSRMST_INSERT mode has been turned on.

SERIAL_LSRMST_ESCAPEIndicates the reception of the escape character itself into the data stream.
SERIAL_LSRMST_LSR_DATAIndicates that a line status change occurred, and data was available in the receive hardware buffer. Following this BYTE is a BYTE value of the line status register is the BYTE present in the receive hardware buffer when the line status change was processed.
SERIAL_LSRMST_LSR_NODATAIndicates that a line status change occurred, but no data was available in the receive hardware buffer.
SERIAL_LSRMST_MSTIndicates that a modem status change occurred. Following this BYTE is a BYTE that is the value of the modem status register when the modem status change was processed.



Minimum supported client

Windows XP

Minimum supported server

Windows Server 2003


WinIoCtl.h (include Windows.h)

See also