James Gettys, Robert W. Scheifler, et al.

“ 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:
Display macros Image format functions and macros Screen information macros 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. ”

“ 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 . 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) . ”

“ 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. ”

“ Both BlackPixel and WhitePixel can be used in implementing a monochrome application. These pixel values are for permanently allocated entries in the default colormap. The actual RGB (red, green, and blue) values are settable on some screens and, in any case, may not actually be black or white. The names are intended to convey the expected relative intensity of the colors.
Both return the black pixel value for the specified screen.
Both return the white pixel value for the specified screen.
Both return a connection number for the specified display. ”

“ Once you have established a connection, you then can use the Xlib macros and functions discussed in this chapter to return information about the display. This chapter discusses how to:
Open (connect to) the display Obtain information about the display, image formats, or screens Generate a NoOperation protocol request Free client-created data Close (disconnect from) a display Use X Server connection close operations Use Xlib with threads Use internal connections 2.1. Opening the Display
To open a connection to the X server that controls a display, use XOpenDisplay. ”

“ If the specified screen_number is valid and sufficient memory for the array can be allocated, XListDepths sets count_return to the number of available depths. Otherwise, it does not set count_return and returns NULL. To release the memory allocated for the array of depths, use XFree. ”