National Instruments 320682C Property Handles and Types, Communicating with Local Applications, The Hidden Window

Chapter 9 X Property Library

With the LabWindows/CVI X Property Library functions, you can connect to X displays and

obtain the root window ID, read and write properties on windows, and monitor when specific

properties change.

Property Handles and Types

Before you can read or write properties on windows, you must create the property and its type.

The function CreateXProperty takes a property name and a property type and returns a

property handle you can use to access properties on windows. The property type, created by the

function CreateXPropType, contains the attributes that determine how data for the property

are stored and retrieved. More specifically, these attributes are the size and unit. The size is the

number of bytes in a single property item. The unit is the number of bytes in the basic entities

that make up a property item. See the description of CreateXPropType for more information

on the meanings of the size and unit attributes.

Table 9-2 lists the three predefined property types that you do not have to create. These types are

useful for defining properties to store X window IDs, integers, and strings.

Table 9-2. Predefined Property Types

Property Type Name Size/Unit

WINDOW_X_PROP_TYPE "WINDOW" sizeof(WindowX)

INTEGER_X_PROP_TYPE "INTEGER" sizeof(int)

STRING_X_PROP_TYPE "STRING" sizeof(char)

Communicating with Local Applications

You can use the function ConnectToXDisplay to connect to any X server on a network.

However, if your program communicates only with other applications connected to the same

display as LabWindows/CVI, you do not need to connect to the display using

ConnectToXDisplay. Instead, use the global variable CVIXDisplay, which is a pointer to

the X display that LabWindows/CVI uses. The variable CVIXRootWindow contains the

X window ID of the root window of the display that LabWindows/CVI uses.

The Hidden Window

Before you can read or write property data, you need the X window IDs of the windows that will

have the properties associated with them.

One option is to always use the root window ID for attaching properties. You could get the root

window ID for the local display from the variable CVIXRootWindow. To get the root window

ID for a remote display you could use the value returned by ConnectToXDisplay. This

approach has disadvantages. First, if your program adds a property to the root window and does