Xlib is the library that [X] uses for applications to communicate with the X server.  It's a higher level library built upon the X protocol.  It hides the details of the protocol.  It's often used for writing widgets, and Tk emulates many Xlib functions on platforms that don't have [X], such as [Win32] and the Mac (see [XLEL]).

There are two bindings for Xlib.  The first was written in [Lisp] many years ago, and the newer version is written in C.  X.org has archives that contain the Lisp version.  Tk AFAIK has always used the C version.

If you want to learn how to program in Xlib I (GPS) suggest you look for the Xlib Programming Manual by Adrian Nye.  The postscript papers included in the X documentation are quite good as well, but tend to be verbose.

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'''References'''

   * [X]
   * X Protocol: http://www.faqs.org/rfcs/rfc1013.html
   * FYI (for starting out): http://www.faqs.org/rfcs/fyi/fyi6.html
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[Category Tk Library]