Data Types
The main types of data in Xlib are the Display structure and the types of the identifiers.
Informally, a display is a physical or virtual device where graphical operations are done. The Display structure of the Xlib library contains information about the display, but more importantly it contains information relative to the channel between the client and the server. For example, in a Unix-like operating system, the Display structure contains the file handle of the socket of this channel (this can be retrieved using the ConnectionNumber macro.) Most Xlib functions have a Display structure as an argument because they either operate on the channel or are relative to a specific channel. In particular, all Xlib functions that interact with the server need this structure for accessing the channel. Some other functions need this structure, even if they operate locally, because they operate on data relative to a specific channel. Operations of this kind include for example operations on the event queue, which is described below.
Windows, colormaps, etc. are managed by the server, which means that the data about their actual implementation is all stored in the server. The client operates on these objects by using their identifiers. The client cannot directly operate on an object, but can only request the server to perform the operation specifying the identifier of the object.
The types Windows, Pixmap, Font, Colormap, etc. are all identifiers, which are 32-bit integers (just as in the X11 protocol itself). A client “creates” a window by requesting that the server create a window. This is done via a call to an Xlib function that returns an identifier for the window, that is, a number. This identifier can then be used by the client for requesting other operations on the same window to the server.
The identifiers are unique to the server. Most of them can be used by different applications to refer to the same objects. For example, two applications connecting with the same server use the same identifier to refer to the same window. These two applications use two different channels, and therefore have two different Display structures; however, when they request operations on the same identifier, these operations will be done on the same object.
Read more about this topic: Xlib
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