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 below).

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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[PT] 15-Sep-2003: Tk implements the following Xlib functions:

 xbytes.c:
 
 xcolors.c:
   XParseColor
 
 xdraw.c:
   XDrawLine
   XFillRectangle
 
 xgc:
   XCreateGC
   XChangeGC
   XFreeGC
   XSetForeground
   XSetBackground
   XSetDashes
   XSetFunction
   XSetFillRule
   XSetFillStyle
   XSetTSOrigin
   XSetFont
   XSetArcMode
   XSetStipple
   XSetLineAttributes
   XSetClipOrgin
   TkSetRegion
   XSetClipMask
   XCreateFontCursor /* not implemented */
   XDrawImageString  /* not implemented */
   XDrawPoint
   XDrawSegments /* not implemented */
   XFetchBuffer /* not implemented */
   XFetchName /* not implemented */
   XListProperties /* not implemented */
   XMapRaised /* not implemented */
   XPutImage /* not implemented */
   XQueryTextExtents /* not implemented */
   XReparentWindow /* not implemented */
   XRotateBuffers /* not implemented */
   XStoreBuffer /* not implemented */
   XUndefineCursor /* not implemented */
 
 ximage.c:
   XCreateBitmapFromData
 
 xutil.c:
   XInternAtom
   XGetVisualInfo

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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