Communications Resource Handles

A process uses the CreateFile function to open a handle to a communications resource. For example, specifying COM1 opens a handle to a serial port, and LPT1 opens a handle to a parallel port. If the specified resource is currently being used by another process, CreateFile fails. Any thread of the process can use the handle returned by CreateFile to identify the resource in any of the functions that access the resource.

When the process calls CreateFile to open a communications resource, it specifies the following attributes:

• What type of read/write access exists for the specified resource.
• Whether the handle can be inherited by child processes.
• Whether the handle can be used in overlapped (asynchronous) I/O operations. (For a description of overlapped operations, see Synchronization.)

When the process uses CreateFile to open a communications resource, it must specify certain values for the following parameters:

• The fdwShareMode parameter must be zero, opening the resource for exclusive access.
• The fdwCreate parameter must specify the OPEN_EXISTING flag.
• The hTemplateFile parameter must be NULL.

When using CreateFile to open a handle directly to a device, an application must use the special characters " \\ .\" to identify the device. For example, to open a handle to drive A, specify \\ .\a: for the lpszName parameter of CreateFile. The calling process can use the handle in the DeviceIoControl function to send control codes to the device.