auto_path is one of the [magic names] that Tcl knows.

It is a [list] of directories where Tcl will look when attempting to resolve
packages or run auto_load.

If you need, with your application, to add a directory to the auto_path
list, be certain to remember it is a list - so don't make the mistake
that [LV] made and treat it as a single value.

Instead use something like

 set auto_path [linsert $auto_path 0 /home/mystuff/someextension/unix] 


Tcl's init.tcl start up code sets auto_path as a global within an
interpreter instance.
As the distribution init.tcl for 8.3 says right at the top,




    # The values on the path come from several locations:
    #
    # The environment variable TCLLIBPATH
    #
    # tcl_library, which is the directory containing this init.tcl script.
    # tclInitScript.h searches around for the directory containing this
    # init.tcl and defines tcl_library to that location before sourcing it.
    #
    # The parent directory of tcl_library. Adding the parent
    # means that packages in peer directories will be found automatically.
    #
    # Also add the directory where the executable is located, plus ../lib
    # relative to that path.
    #
    # tcl_pkgPath, which is set by the platform-specific initialization routines
    #       On UNIX it is compiled in
    #       On Windows, it is not used
    #       On Macintosh it is "Tool Command Language" in the Extensions folder


Two points to emphasize from the above:
   * Notice the rather baroque mixture of platform-dependence, generation-time, and run-time assignments.
   * As is customary in idiomatic Tcl, "the directory where the executable is located" refers to tclsh (and so on), rather than, for example, my_script.tcl.

----
Question: is there a Wiki page that discusses how the whole Tcl start up,
locate extension, load extension thing works?  One of the toughest problems
I encounter with Tcl is figuring out how to fix problems when an extension
I think should load, doesn't load.

For instance, [Tix] is a Tk based extension.  When one installs it, it
places the .so in $[exec_prefix]/lib and its scripting in
$[exec_prefix]/lib/tix8.2/ (or whatever version is being installed).

When this is a normal file system, and one does the
 package require Tk
 package require Tix
the extension is found, loaded, and things work.
When the same directory structure is used within a [starkit], an error about
not finding the Tix script code is generated.

Understanding the process better - and perhaps some debugging tools - would make fixing things like this easier.
----
Word is: it's bad practice to lappend a relative path to ''auto_path''
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[Category Internals]