Xlib - C Language X Interface

Some of the terms used in this book are unique to X, and other terms that are common to other window systems have different meanings in X. You may find it helpful to refer to the glossary, which is located at the end of the book.

The X Window System supports one or more screens containing overlapping windows or subwindows. A screen is a physical monitor and hardware that can be color, grayscale, or monochrome. There can be multiple screens for each display or workstation. A single X server can provide display services for any number of screens. A set of screens for a single user with one keyboard and one pointer (usually a mouse) is called a display.

All the windows in an X server are arranged in strict hierarchies. At the top of each hierarchy is a root window, which covers each of the display screens. Each root window is partially or completely covered by child windows. All windows, except for root windows, have parents. There is usually at least one window for each application program. Child windows may in turn have their own children. In this way, an application program can create an arbitrarily deep tree on each screen. X provides graphics, text, and raster operations for windows.

A child window can be larger than its parent. That is, part or all of the child window can extend beyond the boundaries of the parent, but all output to a window is clipped by its parent. If several children of a window have overlapping locations, one of the children is considered to be on top of or raised over the others, thus obscuring them. Output to areas covered by other windows is suppressed by the window system unless the window has backing store. If a window is obscured by a second window, the second window obscures only those ancestors of the second window that are also ancestors of the first window.

A window has a border zero or more pixels in width, which can be any pattern (pixmap) or solid color you like. A window usually but not always has a background pattern, which will be repainted by the window system when uncovered. Child windows obscure their parents, and graphic operations in the parent window usually are clipped by the children.

Each window and pixmap has its own coordinate system. The coordinate system has the X axis horizontal and the Y axis vertical with the origin [0, 0] at the upper-left corner. Coordinates are integral, in terms of pixels, and coincide with pixel centers. For a window, the origin is inside the border at the inside, upper-left corner.

X does not guarantee to preserve the contents of windows. When part or all of a window is hidden and then brought back onto the screen, its contents may be lost. The server then sends the client program an Expose event to notify it that part or all of the window needs to be repainted. Programs must be prepared to regenerate the contents of windows on demand.

X also provides off-screen storage of graphics objects, called pixmaps. Single plane (depth 1) pixmaps are sometimes referred to as bitmaps. Pixmaps can be used in most graphics functions interchangeably with windows and are used in various graphics operations to define patterns or tiles. Windows and pixmaps together are referred to as drawables.

Most of the functions in Xlib just add requests to an output buffer. These requests later execute asynchronously on the X server. Functions that return values of information stored in the server do not return (that is, they block) until an explicit reply is received or an error occurs. You can provide an error handler, which will be called when the error is reported.

If a client does not want a request to execute asynchronously, it can follow the request with a call to XSync, which blocks until all previously buffered asynchronous events have been sent and acted on. As an important side effect, the output buffer in Xlib is always flushed by a call to any function that returns a value from the server or waits for input.

Many Xlib functions will return an integer resource ID, which allows you to refer to objects stored on the X server. These can be of type Window, Font, Pixmap, Colormap, Cursor, and GContext, as defined in the file <X11/X.h>. These resources are created by requests and are destroyed (or freed) by requests or when connections are closed. Most of these resources are potentially sharable between applications, and in fact, windows are manipulated explicitly by window manager programs. Fonts and cursors are shared automatically across multiple screens. Fonts are loaded and unloaded as needed and are shared by multiple clients. Fonts are often cached in the server. Xlib provides no support for sharing graphics contexts between applications.

Client programs are informed of events. Events may either be side effects of a request (for example, restacking windows generates Expose events) or completely asynchronous (for example, from the keyboard). A client program asks to be informed of events. Because other applications can send events to your application, programs must be prepared to handle (or ignore) events of all types.

Input events (for example, a key pressed or the pointer moved) arrive asynchronously from the server and are queued until they are requested by an explicit call (for example, XNextEvent or XWindowEvent). In addition, some library functions (for example, XRaiseWindow) generate Expose and ConfigureRequest events. These events also arrive asynchronously, but the client may wish to explicitly wait for them by calling XSync after calling a function that can cause the server to generate events.

Some functions return Status, an integer error indication. If the function fails, it returns a zero. If the function returns a status of zero, it has not updated the return arguments. Because C does not provide multiple return values, many functions must return their results by writing into client-passed storage. By default, errors are handled either by a standard library function or by one that you provide. Functions that return pointers to strings return NULL pointers if the string does not exist.

The X server reports protocol errors at the time that it detects them. If more than one error could be generated for a given request, the server can report any of them.

Because Xlib usually does not transmit requests to the server immediately (that is, it buffers them), errors can be reported much later than they actually occur. For debugging purposes, however, Xlib provides a mechanism for forcing synchronous behavior (see section 11.8.1). When synchronization is enabled, errors are reported as they are generated.

When Xlib detects an error, it calls an error handler, which your program can provide. If you do not provide an error handler, the error is printed, and your program terminates.

The following include files are part of the Xlib standard:

The following symbols are defined by Xlib and used throughout the manual:

Xlib follows a number of conventions for the naming and syntax of the functions. Given that you remember what information the function requires, these conventions are intended to make the syntax of the functions more predictable.

The major naming conventions are:

The major programming considerations are:

Some of the Xlib functions make reference to specific character sets and character encodings. The following are the most common:

Xlib − C Language X Interface uses the following conventions:

To open a connection to the X server that controls a display, use XOpenDisplay.

The encoding and interpretation of the display name are implementation-dependent. Strings in the Host Portable Character Encoding are supported; support for other characters is implementation-dependent. On POSIX-conformant systems, the display name or DISPLAY environment variable can be a string in the format:

	protocol/hostname:number.screen_number

For example, the following would specify screen 1 of display 0 on the machine named “dual-headed”:

dual-headed:0.1

The XOpenDisplay function returns a Display structure that serves as the connection to the X server and that contains all the information about that X server. XOpenDisplay connects your application to the X server through TCP or DECnet communications protocols, or through some local inter-process communication protocol. If the protocol is specified as "tcp", "inet", or "inet6", or if no protocol is specified and the hostname is a host machine name and a single colon (:) separates the hostname and display number, XOpenDisplay connects using TCP streams. (If the protocol is specified as "inet", TCP over IPv4 is used. If the protocol is specified as "inet6", TCP over IPv6 is used. Otherwise, the implementation determines which IP version is used.) If the hostname and protocol are both not specified, Xlib uses whatever it believes is the fastest transport. If the hostname is a host machine name and a double colon (::) separates the hostname and display number, XOpenDisplay connects using DECnet. A single X server can support any or all of these transport mechanisms simultaneously. A particular Xlib implementation can support many more of these transport mechanisms.

If successful, XOpenDisplay returns a pointer to a Display structure, which is defined in <X11/Xlib.h>. If XOpenDisplay does not succeed, it returns NULL. After a successful call to XOpenDisplay, all of the screens in the display can be used by the client. The screen number specified in the display_name argument is returned by the DefaultScreen macro (or the XDefaultScreen function). You can access elements of the Display and Screen structures only by using the information macros or functions. For information about using macros and functions to obtain information from the Display structure, see section 2.2.1.

X servers may implement various types of access control mechanisms (see section 9.8).

The Xlib library provides a number of useful macros and corresponding functions that return data from the Display structure. The macros are used for C programming, and their corresponding function equivalents are for other language bindings. This section discusses the:

All other members of the Display structure (that is, those for which no macros are defined) are private to Xlib and must not be used. Applications must never directly modify or inspect these private members of the Display structure. The XDisplayWidth, XDisplayHeight, XDisplayCells, XDisplayPlanes, XDisplayWidthMM, and XDisplayHeightMM functions in the next sections are misnamed. These functions really should be named Screenwhatever and XScreenwhatever, not Displaywhatever or XDisplaywhatever. Our apologies for the resulting confusion.

typedef struct {
	int depth;
	int bits_per_pixel;
	int scanline_pad;
} XPixmapFormatValues;

To execute a NoOperation protocol request, use XNoOp.

The XNoOp function sends a NoOperation protocol request to the X server, thereby exercising the connection.

To free in-memory data that was created by an Xlib function, use .

The function is a general-purpose Xlib routine that frees the specified data. You must use it to free any objects that were allocated by Xlib, unless an alternate function is explicitly specified for the object. A NULL pointer cannot be passed to this function.

To close a display or disconnect from the X server, use XCloseDisplay.

The XCloseDisplay function closes the connection to the X server for the display specified in the Display structure and destroys all windows, resource IDs (Window, Font, Pixmap, Colormap, Cursor, and GContext), or other resources that the client has created on this display, unless the close-down mode of the resource has been changed (see ). Therefore, these windows, resource IDs, and other resources should never be referenced again or an error will be generated. Before exiting, you should call XCloseDisplay explicitly so that any pending errors are reported as XCloseDisplay performs a final XSync operation.

XCloseDisplay can generate a BadGC error.

Xlib provides a function to permit the resources owned by a client to survive after the client's connection is closed. To change a client's close-down mode, use .

The defines what will happen to the client's resources at connection close. A connection starts in DestroyAll mode. For information on what happens to the client's resources when the close_mode argument is RetainPermanent or RetainTemporary, see section 2.6.

can generate a BadValue error.

When the X server's connection to a client is closed either by an explicit call to XCloseDisplay or by a process that exits, the X server performs the following automatic operations:

When the close-down mode is DestroyAll, the X server destroys all of a client's resources as follows:

Additional processing occurs when the last connection to the X server closes. An X server goes through a cycle of having no connections and having some connections. When the last connection to the X server closes as a result of a connection closing with the close_mode of DestroyAll, the X server does the following:

However, the X server does not reset if you close a connection with a close-down mode set to RetainPermanent or RetainTemporary.

On systems that have threads, support may be provided to permit multiple threads to use Xlib concurrently.

To initialize support for concurrent threads, use XInitThreads.

Status XInitThreads();

The XInitThreads function initializes Xlib support for concurrent threads. This function must be the first Xlib function a multi-threaded program calls, and it must complete before any other Xlib call is made. This function returns a nonzero status if initialization was successful; otherwise, it returns zero. On systems that do not support threads, this function always returns zero.

It is only necessary to call this function if multiple threads might use Xlib concurrently. If all calls to Xlib functions are protected by some other access mechanism (for example, a mutual exclusion lock in a toolkit or through explicit client programming), Xlib thread initialization is not required. It is recommended that single-threaded programs not call this function.

To lock a display across several Xlib calls, use XLockDisplay.

The XLockDisplay function locks out all other threads from using the specified display. Other threads attempting to use the display will block until the display is unlocked by this thread. Nested calls to XLockDisplay work correctly; the display will not actually be unlocked until has been called the same number of times as XLockDisplay. This function has no effect unless Xlib was successfully initialized for threads using XInitThreads.

To unlock a display, use .

The function allows other threads to use the specified display again. Any threads that have blocked on the display are allowed to continue. Nested locking works correctly; if XLockDisplay has been called multiple times by a thread, then must be called an equal number of times before the display is actually unlocked. This function has no effect unless Xlib was successfully initialized for threads using XInitThreads.

In addition to the connection to the X server, an Xlib implementation may require connections to other kinds of servers (for example, to input method servers as described in chapter 13). Toolkits and clients that use multiple displays, or that use displays in combination with other inputs, need to obtain these additional connections to correctly block until input is available and need to process that input when it is available. Simple clients that use a single display and block for input in an Xlib event function do not need to use these facilities.

To track internal connections for a display, use .

The function registers a procedure to be called each time Xlib opens or closes an internal connection for the specified display. The procedure is passed the display, the specified client_data, the file descriptor for the connection, a Boolean indicating whether the connection is being opened or closed, and a pointer to a location for private watch data. If opening is True, the procedure can store a pointer to private data in the location pointed to by watch_data; when the procedure is later called for this same connection and opening is False, the location pointed to by watch_data will hold this same private data pointer.

This function can be called at any time after a display is opened. If internal connections already exist, the registered procedure will immediately be called for each of them, before returns. returns a nonzero status if the procedure is successfully registered; otherwise, it returns zero.

The registered procedure should not call any Xlib functions. If the procedure directly or indirectly causes the state of internal connections or watch procedures to change, the result is not defined. If Xlib has been initialized for threads, the procedure is called with the display locked and the result of a call by the procedure to any Xlib function that locks the display is not defined unless the executing thread has externally locked the display using XLockDisplay.

To stop tracking internal connections for a display, use XRemoveConnectionWatch.

()

The XRemoveConnectionWatch function removes a previously registered connection watch procedure. The client_data must match the client_data used when the procedure was initially registered.

To process input on an internal connection, use XProcessInternalConnection.

()

The XProcessInternalConnection function processes input available on an internal connection. This function should be called for an internal connection only after an operating system facility (for example, select or poll) has indicated that input is available; otherwise, the effect is not defined.

To obtain all of the current internal connections for a display, use XInternalConnectionNumbers.

()

The XInternalConnectionNumbers function returns a list of the file descriptors for all internal connections currently open for the specified display. When the allocated list is no longer needed, free it by using . This functions returns a nonzero status if the list is successfully allocated; otherwise, it returns zero.

On some display hardware, it may be possible to deal with color resources in more than one way. For example, you may be able to deal with a screen of either 12-bit depth with arbitrary mapping of pixel to color (pseudo-color) or 24-bit depth with 8 bits of the pixel dedicated to each of red, green, and blue. These different ways of dealing with the visual aspects of the screen are called visuals. For each screen of the display, there may be a list of valid visual types supported at different depths of the screen. Because default windows and visual types are defined for each screen, most simple applications need not deal with this complexity. Xlib provides macros and functions that return the default root window, the default depth of the default root window, and the default visual type (see sections 2.2.1 and 16.7).

Xlib uses an opaque Visual structure that contains information about the possible color mapping. The visual utility functions (see section 16.7) use an XVisualInfo structure to return this information to an application. The members of this structure pertinent to this discussion are class, red_mask, green_mask, blue_mask, bits_per_rgb, and colormap_size. The class member specifies one of the possible visual classes of the screen and can be StaticGray, StaticColor, TrueColor, GrayScale, PseudoColor, or DirectColor.

The following concepts may serve to make the explanation of visual types clearer. The screen can be color or grayscale, can have a colormap that is writable or read-only, and can also have a colormap whose indices are decomposed into separate RGB pieces, provided one is not on a grayscale screen. This leads to the following diagram:

                      Color        Gray-Scale
                   R/O    R/W      R/O   R/W
----------------------------------------------
 Undecomposed    Static  Pseudo   Static  Gray
   Colormap      Color   Color    Gray    Scale

 Decomposed       True   Direct
   Colormap       Color  Color
----------------------------------------------

Conceptually, as each pixel is read out of video memory for display on the screen, it goes through a look-up stage by indexing into a colormap. Colormaps can be manipulated arbitrarily on some hardware, in limited ways on other hardware, and not at all on other hardware. The visual types affect the colormap and the RGB values in the following ways:

The red_mask, green_mask, and blue_mask members are only defined for DirectColor and TrueColor. Each has one contiguous set of bits with no intersections. The bits_per_rgb member specifies the log base 2 of the number of distinct color values (individually) of red, green, and blue. Actual RGB values are unsigned 16-bit numbers. The colormap_size member defines the number of available colormap entries in a newly created colormap. For DirectColor and TrueColor, this is the size of an individual pixel subfield.

To obtain the visual ID from a Visual, use XVisualIDFromVisual.

The XVisualIDFromVisual function returns the visual ID for the specified visual type.

All InputOutput windows have a border width of zero or more pixels, an optional background, an event suppression mask (which suppresses propagation of events from children), and a property list (see section 4.3). The window border and background can be a solid color or a pattern, called a tile. All windows except the root have a parent and are clipped by their parent. If a window is stacked on top of another window, it obscures that other window for the purpose of input. If a window has a background (almost all do), it obscures the other window for purposes of output. Attempts to output to the obscured area do nothing, and no input events (for example, pointer motion) are generated for the obscured area.

Windows also have associated property lists (see section 4.3).

Both InputOutput and InputOnly windows have the following common attributes, which are the only attributes of an InputOnly window:

If you specify any other attributes for an InputOnly window, a BadMatch error results.

InputOnly windows are used for controlling input events in situations where InputOutput windows are unnecessary. InputOnly windows are invisible; can only be used to control such things as cursors, input event generation, and grabbing; and cannot be used in any graphics requests. Note that InputOnly windows cannot have InputOutput windows as inferiors.

Windows have borders of a programmable width and pattern as well as a background pattern or tile. Pixel values can be used for solid colors. The background and border pixmaps can be destroyed immediately after creating the window if no further explicit references to them are to be made. The pattern can either be relative to the parent or absolute. If ParentRelative, the parent's background is used.

When windows are first created, they are not visible (not mapped) on the screen. Any output to a window that is not visible on the screen and that does not have backing store will be discarded. An application may wish to create a window long before it is mapped to the screen. When a window is eventually mapped to the screen (using XMapWindow), the X server generates an Expose event for the window if backing store has not been maintained.

A window manager can override your choice of size, border width, and position for a top-level window. Your program must be prepared to use the actual size and position of the top window. It is not acceptable for a client application to resize itself unless in direct response to a human command to do so. Instead, either your program should use the space given to it, or if the space is too small for any useful work, your program might ask the user to resize the window. The border of your top-level window is considered fair game for window managers.

To set an attribute of a window, set the appropriate member of the XSetWindowAttributes structure and OR in the corresponding value bitmask in your subsequent calls to XCreateWindow and XChangeWindowAttributes, or use one of the other convenience functions that set the appropriate attribute. The symbols for the value mask bits and the XSetWindowAttributes structure are:

/* Window attribute value mask bits */

/* Window attribute value mask bits */
#define    CWBackPixmap                    (1L<<0)
#define    CWBackPixel                     (1L<<1)
#define    CWBorderPixmap                  (1L<<2)
#define    CWBorderPixel                   (1L<<3)
#define    CWBitGravity                    (1L<<4)
#define    CWWinGravity                    (1L<<5)
#define    CWBackingStore                  (1L<<6)
#define    CWBackingPlanes                 (1L<<7)
#define    CWBackingPixel                  (1L<<8)
#define    CWOverrideRedirect              (1L<<9)
#define    CWSaveUnder                     (1L<<10)
#define    CWEventMask                     (1L<<11)
#define    CWDontPropagate                 (1L<<12)
#define    CWColormap                      (1L<<13)
#define    CWCursor                        (1L<<14)

/* Values */

typedef struct {
     Pixmap background_pixmap;     /* background, None, or ParentRelative */
     unsigned long background_pixel;     /* background pixel */
     Pixmap border_pixmap;          /* border of the window or CopyFromParent */
     unsigned long border_pixel;     /* border pixel value */
     int bit_gravity;     /* one of bit gravity values */
     int win_gravity;     /* one of the window gravity values */
     int backing_store;     /* NotUseful, WhenMapped, Always */
     unsigned long backing_planes;     /* planes to be preserved if possible */
     unsigned long backing_pixel;     /* value to use in restoring planes */
     Bool save_under;     /* should bits under be saved? (popups) */
     long event_mask;     /* set of events that should be saved */
     long do_not_propagate_mask;     /* set of events that should not propagate */
     Bool override_redirect;     /* boolean value for override_redirect */
     Colormap colormap;     /* color map to be associated with window */
     Cursor cursor;          /* cursor to be displayed (or None) */
} XSetWindowAttributes;

The following lists the defaults for each window attribute and indicates whether the attribute is applicable to InputOutput and InputOnly windows:

Xlib provides basic ways for creating windows, and toolkits often supply higher-level functions specifically for creating and placing top-level windows, which are discussed in the appropriate toolkit documentation. If you do not use a toolkit, however, you must provide some standard information or hints for the window manager by using the Xlib inter-client communication functions (see chapter 14).

If you use Xlib to create your own top-level windows (direct children of the root window), you must observe the following rules so that all applications interact reasonably across the different styles of window management:

For further information, see chapter 14 and the Inter-Client Communication Conventions Manual.

XCreateWindow is the more general function that allows you to set specific window attributes when you create a window. XCreateSimpleWindow creates a window that inherits its attributes from its parent window.

The X server acts as if InputOnly windows do not exist for the purposes of graphics requests, exposure processing, and VisibilityNotify events. An InputOnly window cannot be used as a drawable (that is, as a source or destination for graphics requests). InputOnly and InputOutput windows act identically in other respects (properties, grabs, input control, and so on). Extension packages can define other classes of windows.

To create an unmapped window and set its window attributes, use XCreateWindow.

The XCreateWindow function creates an unmapped subwindow for a specified parent window, returns the window ID of the created window, and causes the X server to generate a CreateNotify event. The created window is placed on top in the stacking order with respect to siblings.

The coordinate system has the X axis horizontal and the Y axis vertical with the origin [0, 0] at the upper-left corner. Coordinates are integral, in terms of pixels, and coincide with pixel centers. Each window and pixmap has its own coordinate system. For a window, the origin is inside the border at the inside, upper-left corner.

The border_width for an InputOnly window must be zero, or a BadMatch error results. For class InputOutput, the visual type and depth must be a combination supported for the screen, or a BadMatch error results. The depth need not be the same as the parent, but the parent must not be a window of class InputOnly, or a BadMatch error results. For an InputOnly window, the depth must be zero, and the visual must be one supported by the screen. If either condition is not met, a BadMatch error results. The parent window, however, may have any depth and class. If you specify any invalid window attribute for a window, a BadMatch error results.

The created window is not yet displayed (mapped) on the user's display. To display the window, call XMapWindow. The new window initially uses the same cursor as its parent. A new cursor can be defined for the new window by calling XDefineCursor. The window will not be visible on the screen unless it and all of its ancestors are mapped and it is not obscured by any of its ancestors.

XCreateWindow can generate BadAlloc, BadColor, BadCursor, BadMatch, BadPixmap, BadValue, and BadWindow errors.

To create an unmapped InputOutput subwindow of a given parent window, use XCreateSimpleWindow.

The XCreateSimpleWindow function creates an unmapped InputOutput subwindow for a specified parent window, returns the window ID of the created window, and causes the X server to generate a CreateNotify event. The created window is placed on top in the stacking order with respect to siblings. Any part of the window that extends outside its parent window is clipped. The border_width for an InputOnly window must be zero, or a BadMatch error results. XCreateSimpleWindow inherits its depth, class, and visual from its parent. All other window attributes, except background and border, have their default values.

XCreateSimpleWindow can generate BadAlloc, BadMatch, BadValue, and BadWindow errors.

Xlib provides functions that you can use to destroy a window or destroy all subwindows of a window.

To destroy a window and all of its subwindows, use XDestroyWindow.

The XDestroyWindow function destroys the specified window as well as all of its subwindows and causes the X server to generate a DestroyNotify event for each window. The window should never be referenced again. If the window specified by the w argument is mapped, it is unmapped automatically. The ordering of the DestroyNotify events is such that for any given window being destroyed, DestroyNotify is generated on any inferiors of the window before being generated on the window itself. The ordering among siblings and across subhierarchies is not otherwise constrained. If the window you specified is a root window, no windows are destroyed. Destroying a mapped window will generate Expose events on other windows that were obscured by the window being destroyed.

XDestroyWindow can generate a BadWindow error.

To destroy all subwindows of a specified window, use XDestroySubwindows.

The XDestroySubwindows function destroys all inferior windows of the specified window, in bottom-to-top stacking order. It causes the X server to generate a DestroyNotify event for each window. If any mapped subwindows were actually destroyed, XDestroySubwindows causes the X server to generate Expose events on the specified window. This is much more efficient than deleting many windows one at a time because much of the work need be performed only once for all of the windows, rather than for each window. The subwindows should never be referenced again.

XDestroySubwindows can generate a BadWindow error.

A window is considered mapped if an XMapWindow call has been made on it. It may not be visible on the screen for one of the following reasons:

Expose events are generated for the window when part or all of it becomes visible on the screen. A client receives the Expose events only if it has asked for them. Windows retain their position in the stacking order when they are unmapped.

A window manager may want to control the placement of subwindows. If SubstructureRedirectMask has been selected by a window manager on a parent window (usually a root window), a map request initiated by other clients on a child window is not performed, and the window manager is sent a MapRequest event. However, if the override-redirect flag on the child had been set to True (usually only on pop-up menus), the map request is performed.

A tiling window manager might decide to reposition and resize other clients' windows and then decide to map the window to its final location. A window manager that wants to provide decoration might reparent the child into a frame first. For further information, see sections 3.2.8 and 10.10. Only a single client at a time can select for SubstructureRedirectMask.

Similarly, a single client can select for ResizeRedirectMask on a parent window. Then, any attempt to resize the window by another client is suppressed, and the client receives a ResizeRequest event.

To map a given window, use XMapWindow.

The XMapWindow function maps the window and all of its subwindows that have had map requests. Mapping a window that has an unmapped ancestor does not display the window but marks it as eligible for display when the ancestor becomes mapped. Such a window is called unviewable. When all its ancestors are mapped, the window becomes viewable and will be visible on the screen if it is not obscured by another window. This function has no effect if the window is already mapped.

If the override-redirect of the window is False and if some other client has selected SubstructureRedirectMask on the parent window, then the X server generates a MapRequest event, and the XMapWindow function does not map the window. Otherwise, the window is mapped, and the X server generates a MapNotify event.

If the window becomes viewable and no earlier contents for it are remembered, the X server tiles the window with its background. If the window's background is undefined, the existing screen contents are not altered, and the X server generates zero or more Expose events. If backing-store was maintained while the window was unmapped, no Expose events are generated. If backing-store will now be maintained, a full-window exposure is always generated. Otherwise, only visible regions may be reported. Similar tiling and exposure take place for any newly viewable inferiors.

If the window is an InputOutput window, XMapWindow generates Expose events on each InputOutput window that it causes to be displayed. If the client maps and paints the window and if the client begins processing events, the window is painted twice. To avoid this, first ask for Expose events and then map the window, so the client processes input events as usual. The event list will include Expose for each window that has appeared on the screen. The client's normal response to an Expose event should be to repaint the window. This method usually leads to simpler programs and to proper interaction with window managers.

XMapWindow can generate a BadWindow error.

To map and raise a window, use XMapRaised.

The XMapRaised function essentially is similar to XMapWindow in that it maps the window and all of its subwindows that have had map requests. However, it also raises the specified window to the top of the stack. For additional information, see XMapWindow.

XMapRaised can generate multiple BadWindow errors.

To map all subwindows for a specified window, use XMapSubwindows.

The XMapSubwindows function maps all subwindows for a specified window in top-to-bottom stacking order. The X server generates Expose events on each newly displayed window. This may be much more efficient than mapping many windows one at a time because the server needs to perform much of the work only once, for all of the windows, rather than for each window.

XMapSubwindows can generate a BadWindow error.

Xlib provides functions that you can use to unmap a window or all subwindows.

To unmap a window, use XUnmapWindow.

The XUnmapWindow function unmaps the specified window and causes the X server to generate an UnmapNotify event. If the specified window is already unmapped, XUnmapWindow has no effect. Normal exposure processing on formerly obscured windows is performed. Any child window will no longer be visible until another map call is made on the parent. In other words, the subwindows are still mapped but are not visible until the parent is mapped. Unmapping a window will generate Expose events on windows that were formerly obscured by it.

XUnmapWindow can generate a BadWindow error.

To unmap all subwindows for a specified window, use XUnmapSubwindows.

The XUnmapSubwindows function unmaps all subwindows for the specified window in bottom-to-top stacking order. It causes the X server to generate an UnmapNotify event on each subwindow and Expose events on formerly obscured windows. Using this function is much more efficient than unmapping multiple windows one at a time because the server needs to perform much of the work only once, for all of the windows, rather than for each window.

XUnmapSubwindows can generate a BadWindow error.

Xlib provides functions that you can use to move a window, resize a window, move and resize a window, or change a window's border width. To change one of these parameters, set the appropriate member of the XWindowChanges structure and OR in the corresponding value mask in subsequent calls to XConfigureWindow. The symbols for the value mask bits and the XWindowChanges structure are:

/* Configure window value mask bits */
#define      CWX              (1<<0)
#define      CWY              (1<<1)
#define      CWWidth          (1<<2)
#define      CWHeight         (1<<3)
#define      CWBorderWidth    (1<<4)
#define      CWSibling        (1<<5)
#define      CWStackMode      (1<<6)

/* Values */

typedef struct {
     int x, y;
     int width, height;
     int border_width;
     Window sibling;
     int stack_mode;
} XWindowChanges;

The x and y members are used to set the window's x and y coordinates, which are relative to the parent's origin and indicate the position of the upper-left outer corner of the window. The width and height members are used to set the inside size of the window, not including the border, and must be nonzero, or a BadValue error results. Attempts to configure a root window have no effect.

The border_width member is used to set the width of the border in pixels. Note that setting just the border width leaves the outer-left corner of the window in a fixed position but moves the absolute position of the window's origin. If you attempt to set the border-width attribute of an InputOnly window nonzero, a BadMatch error results.

The sibling member is used to set the sibling window for stacking operations. The stack_mode member is used to set how the window is to be restacked and can be set to Above, Below, TopIf, BottomIf, or Opposite.

If the override-redirect flag of the window is False and if some other client has selected SubstructureRedirectMask on the parent, the X server generates a ConfigureRequest event, and no further processing is performed. Otherwise, if some other client has selected ResizeRedirectMask on the window and the inside width or height of the window is being changed, a ResizeRequest event is generated, and the current inside width and height are used instead. Note that the override-redirect flag of the window has no effect on ResizeRedirectMask and that SubstructureRedirectMask on the parent has precedence over ResizeRedirectMask on the window.

When the geometry of the window is changed as specified, the window is restacked among siblings, and a ConfigureNotify event is generated if the state of the window actually changes. GravityNotify events are generated after ConfigureNotify events. If the inside width or height of the window has actually changed, children of the window are affected as specified.

If a window's size actually changes, the window's subwindows move according to their window gravity. Depending on the window's bit gravity, the contents of the window also may be moved (see section 3.2.3).

If regions of the window were obscured but now are not, exposure processing is performed on these formerly obscured windows, including the window itself and its inferiors. As a result of increasing the width or height, exposure processing is also performed on any new regions of the window and any regions where window contents are lost.

The restack check (specifically, the computation for BottomIf, TopIf, and Opposite) is performed with respect to the window's final size and position (as controlled by the other arguments of the request), not its initial position. If a sibling is specified without a stack_mode, a BadMatch error results.

If a sibling and a stack_mode are specified, the window is restacked as follows:

If a stack_mode is specified but no sibling is specified, the window is restacked as follows:

Attempts to configure a root window have no effect.

To configure a window's size, location, stacking, or border, use XConfigureWindow.

The XConfigureWindow function uses the values specified in the XWindowChanges structure to reconfigure a window's size, position, border, and stacking order. Values not specified are taken from the existing geometry of the window.

If a sibling is specified without a stack_mode or if the window is not actually a sibling, a BadMatch error results. Note that the computations for BottomIf, TopIf, and Opposite are performed with respect to the window's final geometry (as controlled by the other arguments passed to XConfigureWindow), not its initial geometry. Any backing store contents of the window, its inferiors, and other newly visible windows are either discarded or changed to reflect the current screen contents (depending on the implementation).

XConfigureWindow can generate BadMatch, BadValue, and BadWindow errors.

To move a window without changing its size, use XMoveWindow.

The XMoveWindow function moves the specified window to the specified x and y coordinates, but it does not change the window's size, raise the window, or change the mapping state of the window. Moving a mapped window may or may not lose the window's contents depending on if the window is obscured by nonchildren and if no backing store exists. If the contents of the window are lost, the X server generates Expose events. Moving a mapped window generates Expose events on any formerly obscured windows.

If the override-redirect flag of the window is False and some other client has selected SubstructureRedirectMask on the parent, the X server generates a ConfigureRequest event, and no further processing is performed. Otherwise, the window is moved.

XMoveWindow can generate a BadWindow error.

To change a window's size without changing the upper-left coordinate, use XResizeWindow.

The XResizeWindow function changes the inside dimensions of the specified window, not including its borders. This function does not change the window's upper-left coordinate or the origin and does not restack the window. Changing the size of a mapped window may lose its contents and generate Expose events. If a mapped window is made smaller, changing its size generates Expose events on windows that the mapped window formerly obscured.

If the override-redirect flag of the window is False and some other client has selected SubstructureRedirectMask on the parent, the X server generates a ConfigureRequest event, and no further processing is performed. If either width or height is zero, a BadValue error results.

XResizeWindow can generate BadValue and BadWindow errors.

To change the size and location of a window, use XMoveResizeWindow.

The XMoveResizeWindow function changes the size and location of the specified window without raising it. Moving and resizing a mapped window may generate an Expose event on the window. Depending on the new size and location parameters, moving and resizing a window may generate Expose events on windows that the window formerly obscured.

If the override-redirect flag of the window is False and some other client has selected SubstructureRedirectMask on the parent, the X server generates a ConfigureRequest event, and no further processing is performed. Otherwise, the window size and location are changed.

XMoveResizeWindow can generate BadValue and BadWindow errors.

To change the border width of a given window, use XSetWindowBorderWidth.

The XSetWindowBorderWidth function sets the specified window's border width to the specified width.

XSetWindowBorderWidth can generate a BadWindow error.

Xlib provides functions that you can use to raise, lower, circulate, or restack windows.

To raise a window so that no sibling window obscures it, use XRaiseWindow.

The XRaiseWindow function raises the specified window to the top of the stack so that no sibling window obscures it. If the windows are regarded as overlapping sheets of paper stacked on a desk, then raising a window is analogous to moving the sheet to the top of the stack but leaving its x and y location on the desk constant. Raising a mapped window may generate Expose events for the window and any mapped subwindows that were formerly obscured.

If the override-redirect attribute of the window is False and some other client has selected SubstructureRedirectMask on the parent, the X server generates a ConfigureRequest event, and no processing is performed. Otherwise, the window is raised.

XRaiseWindow can generate a BadWindow error.

To lower a window so that it does not obscure any sibling windows, use XLowerWindow.

The XLowerWindow function lowers the specified window to the bottom of the stack so that it does not obscure any sibling windows. If the windows are regarded as overlapping sheets of paper stacked on a desk, then lowering a window is analogous to moving the sheet to the bottom of the stack but leaving its x and y location on the desk constant. Lowering a mapped window will generate Expose events on any windows it formerly obscured.

If the override-redirect attribute of the window is False and some other client has selected SubstructureRedirectMask on the parent, the X server generates a ConfigureRequest event, and no processing is performed. Otherwise, the window is lowered to the bottom of the stack.

XLowerWindow can generate a BadWindow error.

To circulate a subwindow up or down, use XCirculateSubwindows.

The XCirculateSubwindows function circulates children of the specified window in the specified direction. If you specify RaiseLowest, XCirculateSubwindows raises the lowest mapped child (if any) that is occluded by another child to the top of the stack. If you specify LowerHighest, XCirculateSubwindows lowers the highest mapped child (if any) that occludes another child to the bottom of the stack. Exposure processing is then performed on formerly obscured windows. If some other client has selected SubstructureRedirectMask on the window, the X server generates a CirculateRequest event, and no further processing is performed. If a child is actually restacked, the X server generates a CirculateNotify event.

XCirculateSubwindows can generate BadValue and BadWindow errors.

To raise the lowest mapped child of a window that is partially or completely occluded by another child, use XCirculateSubwindowsUp.

The XCirculateSubwindowsUp function raises the lowest mapped child of the specified window that is partially or completely occluded by another child. Completely unobscured children are not affected. This is a convenience function equivalent to XCirculateSubwindows with RaiseLowest specified.

XCirculateSubwindowsUp can generate a BadWindow error.

To lower the highest mapped child of a window that partially or completely occludes another child, use XCirculateSubwindowsDown.

The XCirculateSubwindowsDown function lowers the highest mapped child of the specified window that partially or completely occludes another child. Completely unobscured children are not affected. This is a convenience function equivalent to XCirculateSubwindows with LowerHighest specified.

XCirculateSubwindowsDown can generate a BadWindow error.

To restack a set of windows from top to bottom, use XRestackWindows.

The XRestackWindows function restacks the windows in the order specified, from top to bottom. The stacking order of the first window in the windows array is unaffected, but the other windows in the array are stacked underneath the first window, in the order of the array. The stacking order of the other windows is not affected. For each window in the window array that is not a child of the specified window, a BadMatch error results.

If the override-redirect attribute of a window is False and some other client has selected SubstructureRedirectMask on the parent, the X server generates ConfigureRequest events for each window whose override-redirect flag is not set, and no further processing is performed. Otherwise, the windows will be restacked in top-to-bottom order.

XRestackWindows can generate a BadWindow error.

Xlib provides functions that you can use to set window attributes. XChangeWindowAttributes is the more general function that allows you to set one or more window attributes provided by the XSetWindowAttributes structure. The other functions described in this section allow you to set one specific window attribute, such as a window's background.

To change one or more attributes for a given window, use XChangeWindowAttributes.

Depending on the valuemask, the XChangeWindowAttributes function uses the window attributes in the XSetWindowAttributes structure to change the specified window attributes. Changing the background does not cause the window contents to be changed. To repaint the window and its background, use XClearWindow. Setting the border or changing the background such that the border tile origin changes causes the border to be repainted. Changing the background of a root window to None or ParentRelative restores the default background pixmap. Changing the border of a root window to CopyFromParent restores the default border pixmap. Changing the win-gravity does not affect the current position of the window. Changing the backing-store of an obscured window to WhenMapped or Always, or changing the backing-planes, backing-pixel, or save-under of a mapped window may have no immediate effect. Changing the colormap of a window (that is, defining a new map, not changing the contents of the existing map) generates a ColormapNotify event. Changing the colormap of a visible window may have no immediate effect on the screen because the map may not be installed (see XInstallColormap). Changing the cursor of a root window to None restores the default cursor. Whenever possible, you are encouraged to share colormaps.

Multiple clients can select input on the same window. Their event masks are maintained separately. When an event is generated, it is reported to all interested clients. However, only one client at a time can select for SubstructureRedirectMask, ResizeRedirectMask, and ButtonPressMask. If a client attempts to select any of these event masks and some other client has already selected one, a BadAccess error results. There is only one do-not-propagate-mask for a window, not one per client.

XChangeWindowAttributes can generate BadAccess, BadColor, BadCursor, BadMatch, BadPixmap, BadValue, and BadWindow errors.

To set the background of a window to a given pixel, use XSetWindowBackground.

The XSetWindowBackground function sets the background of the window to the specified pixel value. Changing the background does not cause the window contents to be changed. XSetWindowBackground uses a pixmap of undefined size filled with the pixel value you passed. If you try to change the background of an InputOnly window, a BadMatch error results.

XSetWindowBackground can generate BadMatch and BadWindow errors.

To set the background of a window to a given pixmap, use XSetWindowBackgroundPixmap.

The XSetWindowBackgroundPixmap function sets the background pixmap of the window to the specified pixmap. The background pixmap can immediately be freed if no further explicit references to it are to be made. If ParentRelative is specified, the background pixmap of the window's parent is used, or on the root window, the default background is restored. If you try to change the background of an InputOnly window, a BadMatch error results. If the background is set to None, the window has no defined background.

XSetWindowBackgroundPixmap can generate BadMatch, BadPixmap, and BadWindow errors. XSetWindowBackground and XSetWindowBackgroundPixmap do not change the current contents of the window.

To change and repaint a window's border to a given pixel, use XSetWindowBorder.

The XSetWindowBorder function sets the border of the window to the pixel value you specify. If you attempt to perform this on an InputOnly window, a BadMatch error results.

XSetWindowBorder can generate BadMatch and BadWindow errors.

To change and repaint the border tile of a given window, use XSetWindowBorderPixmap.

The XSetWindowBorderPixmap function sets the border pixmap of the window to the pixmap you specify. The border pixmap can be freed immediately if no further explicit references to it are to be made. If you specify CopyFromParent, a copy of the parent window's border pixmap is used. If you attempt to perform this on an InputOnly window, a BadMatch error results.

XSetWindowBorderPixmap can generate BadMatch, BadPixmap, and BadWindow errors.

To set the colormap of a given window, use XSetWindowColormap.

The XSetWindowColormap function sets the specified colormap of the specified window. The colormap must have the same visual type as the window, or a BadMatch error results.

XSetWindowColormap can generate BadColor, BadMatch, and BadWindow errors.

To define which cursor will be used in a window, use XDefineCursor.

If a cursor is set, it will be used when the pointer is in the window. If the cursor is None, it is equivalent to XUndefineCursor.

XDefineCursor can generate BadCursor and BadWindow errors.

To undefine the cursor in a given window, use XUndefineCursor.

The XUndefineCursor function undoes the effect of a previous XDefineCursor for this window. When the pointer is in the window, the parent's cursor will now be used. On the root window, the default cursor is restored.

XUndefineCursor can generate a BadWindow error.

Xlib provides functions that you can use to obtain information about the window tree, the window's current attributes, the window's current geometry, or the current pointer coordinates. Because they are most frequently used by window managers, these functions all return a status to indicate whether the window still exists.

To obtain the parent, a list of children, and number of children for a given window, use XQueryTree.

The XQueryTree function returns the root ID, the parent window ID, a pointer to the list of children windows (NULL when there are no children), and the number of children in the list for the specified window. The children are listed in current stacking order, from bottom-most (first) to top-most (last). XQueryTree returns zero if it fails and nonzero if it succeeds. To free a non-NULL children list when it is no longer needed, use .

XQueryTree can generate a BadWindow error.

To obtain the current attributes of a given window, use XGetWindowAttributes.

The XGetWindowAttributes function returns the current attributes for the specified window to an XWindowAttributes structure.

typedef struct {
     int x, y;                     /* location of window */
     int width, height;            /* width and height of window */
     int border_width;             /* border width of window */
     int depth;                    /* depth of window */
     Visual *visual;               /* the associated visual structure */
     Window root;                  /* root of screen containing window */
     int class;                    /* InputOutput, InputOnly*/
     int bit_gravity;              /* one of the bit gravity values */
     int win_gravity;              /* one of the window gravity values */
     int backing_store;            /* NotUseful, WhenMapped, Always */
     unsigned long backing_planes; /* planes to be preserved if possible */
     unsigned long backing_pixel;  /* value to be used when restoring planes */
     Bool save_under;              /* boolean, should bits under be saved? */
     Colormap colormap;            /* color map to be associated with window */
     Bool map_installed;           /* boolean, is color map currently installed*/
     int map_state;                /* IsUnmapped, IsUnviewable, IsViewable */
     long all_event_masks;         /* set of events all people have interest in*/
     long your_event_mask;         /* my event mask */
     long do_not_propagate_mask;   /* set of events that should not propagate */
     Bool override_redirect;       /* boolean value for override-redirect */
     Screen *screen;               /* back pointer to correct screen */
} XWindowAttributes;

The x and y members are set to the upper-left outer corner relative to the parent window's origin. The width and height members are set to the inside size of the window, not including the border. The border_width member is set to the window's border width in pixels. The depth member is set to the depth of the window (that is, bits per pixel for the object). The visual member is a pointer to the screen's associated Visual structure. The root member is set to the root window of the screen containing the window. The class member is set to the window's class and can be either InputOutput or InputOnly.

The bit_gravity member is set to the window's bit gravity and can be one of the following:

The win_gravity member is set to the window's window gravity and can be one of the following:

For additional information on gravity, see section 3.2.3.

The backing_store member is set to indicate how the X server should maintain the contents of a window and can be WhenMapped, Always, or NotUseful. The backing_planes member is set to indicate (with bits set to 1) which bit planes of the window hold dynamic data that must be preserved in backing_stores and during save_unders. The backing_pixel member is set to indicate what values to use for planes not set in backing_planes.

The save_under member is set to True or False. The colormap member is set to the colormap for the specified window and can be a colormap ID or None. The map_installed member is set to indicate whether the colormap is currently installed and can be True or False. The map_state member is set to indicate the state of the window and can be IsUnmapped, IsUnviewable, or IsViewable. IsUnviewable is used if the window is mapped but some ancestor is unmapped.

The all_event_masks member is set to the bitwise inclusive OR of all event masks selected on the window by all clients. The your_event_mask member is set to the bitwise inclusive OR of all event masks selected by the querying client. The do_not_propagate_mask member is set to the bitwise inclusive OR of the set of events that should not propagate.

The override_redirect member is set to indicate whether this window overrides structure control facilities and can be True or False. Window manager clients should ignore the window if this member is True.

The screen member is set to a screen pointer that gives you a back pointer to the correct screen. This makes it easier to obtain the screen information without having to loop over the root window fields to see which field matches.

XGetWindowAttributes can generate BadDrawable and BadWindow errors.

To obtain the current geometry of a given drawable, use XGetGeometry.

The XGetGeometry function returns the root window and the current geometry of the drawable. The geometry of the drawable includes the x and y coordinates, width and height, border width, and depth. These are described in the argument list. It is legal to pass to this function a window whose class is InputOnly.

XGetGeometry can generate a BadDrawable error.

Applications sometimes need to perform a coordinate transformation from the coordinate space of one window to another window or need to determine which window the pointing device is in. XTranslateCoordinates and XQueryPointer fulfill these needs (and avoid any race conditions) by asking the X server to perform these operations.

To translate a coordinate in one window to the coordinate space of another window, use XTranslateCoordinates.

If XTranslateCoordinates returns True, it takes the src_x and src_y coordinates relative to the source window's origin and returns these coordinates to dest_x_return and dest_y_return relative to the destination window's origin. If XTranslateCoordinates returns False, src_w and dest_w are on different screens, and dest_x_return and dest_y_return are zero. If the coordinates are contained in a mapped child of dest_w, that child is returned to child_return. Otherwise, child_return is set to None.

XTranslateCoordinates can generate a BadWindow error.

To obtain the screen coordinates of the pointer or to determine the pointer coordinates relative to a specified window, use XQueryPointer.

The XQueryPointer function returns the root window the pointer is logically on and the pointer coordinates relative to the root window's origin. If XQueryPointer returns False, the pointer is not on the same screen as the specified window, and XQueryPointer returns None to child_return and zero to win_x_return and win_y_return. If XQueryPointer returns True, the pointer coordinates returned to win_x_return and win_y_return are relative to the origin of the specified window. In this case, XQueryPointer returns the child that contains the pointer, if any, or else None to child_return.

XQueryPointer returns the current logical state of the keyboard buttons and the modifier keys in mask_return. It sets mask_return to the bitwise inclusive OR of one or more of the button or modifier key bitmasks to match the current state of the mouse buttons and the modifier keys.

Note that the logical state of a device (as seen through Xlib) may lag the physical state if device event processing is frozen (see section 12.1).

XQueryPointer can generate a BadWindow error.

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. 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 appendix C. 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 an explanation of the functions that let you get and set much of the information stored in these predefined properties, see chapter 14.

The core protocol imposes no semantics on these property names, but semantics are specified in other X Consortium standards, such as the Inter-Client Communication Conventions Manual and the X Logical Font Description Conventions.

You can use properties to communicate other information between applications. The functions described in this section let you define new properties and get the unique atom IDs in your applications.

Although any particular atom can have some client interpretation within each of the name spaces, atoms occur in five distinct name spaces within the protocol:

The built-in selection property names are:

The built-in property names are:

The built-in property types are:

The built-in font property names are:

For further information about font properties, see section 8.5.

To return an atom for a given name, use XInternAtom.

The XInternAtom function returns the atom identifier associated with the specified atom_name string. If only_if_exists is False, the atom is created if it does not exist. Therefore, XInternAtom can return None. If the atom name is not in the Host Portable Character Encoding, the result is implementation-dependent. Uppercase and lowercase matter; the strings “thing”, “Thing”, and “thinG” all designate different atoms. The atom will remain defined even after the client's connection closes. It will become undefined only when the last connection to the X server closes.

XInternAtom can generate BadAlloc and BadValue errors.

To return atoms for an array of names, use XInternAtoms.

The XInternAtoms function returns the atom identifiers associated with the specified names. The atoms are stored in the atoms_return array supplied by the caller. Calling this function is equivalent to calling XInternAtom for each of the names in turn with the specified value of only_if_exists, but this function minimizes the number of round-trip protocol exchanges between the client and the X server.

This function returns a nonzero status if atoms are returned for all of the names; otherwise, it returns zero.

XInternAtoms can generate BadAlloc and BadValue errors.

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

The XGetAtomName function returns the name associated with the specified atom. If the data returned by the server is in the Latin Portable Character Encoding, then the returned string is in the Host Portable Character Encoding. Otherwise, the result is implementation-dependent. To free the resulting string, call .

XGetAtomName can generate a BadAtom error.

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

The XGetAtomNames function returns the names associated with the specified atoms. The names are stored in the names_return array supplied by the caller. Calling this function is equivalent to calling XGetAtomName for each of the atoms in turn, but this function minimizes the number of round-trip protocol exchanges between the client and the X server.

This function returns a nonzero status if names are returned for all of the atoms; otherwise, it returns zero.

XGetAtomNames can generate a BadAtom error.

You can attach a property list to every window. Each property has a name, a type, and a value (see section 4.3). The value is an array of 8-bit, 16-bit, or 32-bit quantities, whose interpretation is left to the clients. The type char is used to represent 8-bit quantities, the type short is used to represent 16-bit quantities, and the type long is used to represent 32-bit quantities.

Xlib provides functions that you can use to obtain, change, update, or interchange window properties. In addition, Xlib provides other utility functions for inter-client communication (see chapter 14).

To obtain the type, format, and value of a property of a given window, use XGetWindowProperty.

The XGetWindowProperty function returns the actual type of the property; the actual format of the property; the number of 8-bit, 16-bit, or 32-bit items transferred; the number of bytes remaining to be read in the property; and a pointer to the data actually returned. XGetWindowProperty sets the return arguments as follows:

If the returned format is 8, the returned data is represented as a char array. If the returned format is 16, the returned data is represented as a short array and should be cast to that type to obtain the elements. If the returned format is 32, the returned data is represented as a long array and should be cast to that type to obtain the elements.

XGetWindowProperty always allocates one extra byte in prop_return (even if the property is zero length) and sets it to zero so that simple properties consisting of characters do not have to be copied into yet another string before use.

If delete is True and bytes_after_return is zero, XGetWindowProperty deletes the property from the window and generates a PropertyNotify event on the window.

The function returns Success if it executes successfully. To free the resulting data, use .

XGetWindowProperty can generate BadAtom, BadValue, and BadWindow errors.

To obtain a given window's property list, use XListProperties.