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Key terms

The X window system has nuanced usage of a number of terms when compared to common usage, particularly "display" and "screen", a subset of which is given here for convenience:

device

A graphics device such as a computer graphics card or a computer motherboard's integrated graphics chipset.

monitor

A physical device such as a CRT or a flat screen computer display.

screen

An area into which graphics may be rendered, either through software alone into system memory as with VNC, or within a graphics device, some of which can render into more than one screen simultaneously, either viewable simultaneously or interchangeably. Interchangeable screens are often set up to be notionally left and right from one another, flipping from one to the next as the mouse pointer reaches the edge of the monitor.

virtual screen

Two different meanings are associated with this term:

A technique allowing panning a monitor around a screen running at a larger resolution than the monitor is currently displaying.

An effect simulated by a window manager by maintaining window position information in a larger coordinate system than the screen and allowing panning by simply moving the windows in response to the user.

display

A collection of screens, often involving multiple monitors, generally configured to allow the mouse to move the pointer to any position within them. Linux-based workstations are usually capable of having multiple displays, among which the user can switch with a special keyboard combination such as control-alt-function-key, simultaneously flipping all the monitors from showing the screens of one display to the screens in another.

The term "display" should not be confused with the more specialized jargon "Zaphod display". The latter is a rare configuration allowing multiple users of a single computer to each have an independent set of display, mouse, and keyboard, as though they were using separate computers, but at a lower per-seat cost.

[링크 : https://en.wikipedia.org/wiki/X_Window_System] 

Display Names

From the user's perspective, every X server has a display name of the form:

hostname:displaynumber.screennumber

This information is used by the application to determine how it should connect to the server and which screen it should use by default (on displays with multiple monitors):

hostname

The hostname specifies the name of the machine to which the display is physically connected. If the hostname is not given, the most efficient way of communicating to a server on the same machine will be used.

displaynumber

The phrase "display" is usually used to refer to collection of monitors that share a common keyboard and pointer (mouse, tablet, etc.). Most workstations tend to only have one keyboard, and therefore, only one display. Larger, multi-user systems, however, frequently have several displays so that more than one person can be doing graphics work at once. To avoid confusion, each display on a machine is assigned a display number (beginning at 0) when the X server for that display is started. The display number must always be given in a display name.

screennumber

Some displays share a single keyboard and pointer among two or more monitors. Since each monitor has its own set of windows, each screen is assigned a screen number (beginning at 0) when the X server for that display is started. If the screen number is not given, screen 0 will be used.

[링크 : http://linux.die.net/man/7/x] 

$ cat test.c

#include <stdio.h>

int main()

{

        int *p;

        free(p);

        return 0;

}