hpux man page for xconnectionnumber

XConnectionNumber()													       XConnectionNumber()

Name
  XConnectionNumber, ConnectionNumber - get connection number or file descriptor.

Synopsis
  int XConnectionNumber(display)
	   Display *display;

Arguments
  display   Specifies a connection to an X server; returned from XOpenDisplay().

Returns
  The connection number.

Description
  XConnectionNumber()  returns the connection number for the specified display.  On a POSIX-conformant system, this is the file descriptor of
  the connection.

  The C language macro ConnectionNumber() is equivalent and slightly more efficient.

Xlib - Macro Equivalents												       XConnectionNumber()