4.3 Properties and Atoms

A property is a collection of named, typed data. The window system has a set of predefined properties (for example, the name of a window, size hints, and so on), and users can define any other arbitrary information and associate it with windows. Each property has a name, which is an ISO Latin-1 string. For each named property, a unique identifier (atom) is associated with it. A property also has a type, for example, string or integer. These types are also indicated using atoms, so arbitrary new types can be defined. Data of only one type may be associated with a single property name. Clients can store and retrieve properties associated with windows. For efficiency reasons, an atom is used rather than a character string. XInternAtom() can be used to obtain the atom for property names.

A property is also stored in one of several possible formats. The X server can store the information as 8-bit quantities, 16-bit quantities, or 32-bit quantities. This permits the X server to present the data in the byte order that the client expects.

Note

If you define further properties of complex type, you must encode and decode them yourself. These functions must be carefully written if they are to be portable. For further information about how to write a library extension, see "Extensions".
The type of a property is defined by an atom, which allows for arbitrary extension in this type scheme.

Certain property names are predefined in the server for commonly used functions. The atoms for these properties are defined in X11/Xatom.h. To avoid name clashes with user symbols, the #define name for each atom has the XA_ prefix. For definitions of these properties, see below. For an explanation of the functions that let you get and set much of the information stored in these predefined properties, see "Inter-Client Communication Functions".

The core protocol imposes no semantics on these property names, but semantics are specified in other X Consortium standards, such as the Selections

  • Property names

  • Property types

  • Font properties

  • Type of a ClientMessage event (none are built into the X server)

    The built-in selection property names are:

    PRIMARY
    SECONDARY

    The built-in property names are:
    CUT_BUFFER0 RESOURCE_MANAGER
    CUT_BUFFER1 WM_CLASS
    CUT_BUFFER2 WM_CLIENT_MACHINE
    CUT_BUFFER3 WM_COLORMAP_WINDOWS
    CUT_BUFFER4 WM_COMMAND
    CUT_BUFFER5 WM_HINTS
    CUT_BUFFER6 WM_ICON_NAME
    CUT_BUFFER7 WM_ICON_SIZE
    RGB_BEST_MAP WM_NAME
    RGB_BLUE_MAP WM_NORMAL_HINTS
    RGB_DEFAULT_MAP WM_PROTOCOLS
    RGB_GRAY_MAP WM_STATE
    RGB_GREEN_MAP WM_TRANSIENT_FOR
    RGB_RED_MAP WM_ZOOM_HINTS

    The built-in property types are:

    ARC POINT
    ATOM RGB_COLOR_MAP
    BITMAP RECTANGLE
    CARDINAL STRING
    COLORMAP VISUALID
    CURSOR WINDOW
    DRAWABLE WM_HINTS
    FONT WM_SIZE_HINTS
    INTEGER
    PIXMAP

    The built-in font property names are:
    MIN_SPACE STRIKEOUT_DESCENT
    NORM_SPACE STRIKEOUT_ASCENT
    MAX_SPACE ITALIC_ANGLE
    END_SPACE X_HEIGHT
    SUPERSCRIPT_X QUAD_WIDTH
    SUPERSCRIPT_Y WEIGHT
    SUBSCRIPT_X POINT_SIZE
    SUBSCRIPT_Y RESOLUTION
    UNDERLINE_POSITION COPYRIGHT
    UNDERLINE_THICKNESS NOTICE
    FONT_NAME FAMILY_NAME
    FULL_NAME CAP_HEIGHT

    For further information about font properties, see "Font Metrics".

    To return an atom for a given name, use XInternAtom(). To return atoms for an array of names, use XInternAtoms().

    To return a name for a given atom identifier, use XGetAtomName().

    To return the names for an array of atom identifiers, use XGetAtomNames().

    Next: Obtaining and Changing Window Properties
    Christophe Tronche, ch.tronche@computer.org