'''`tclPkgUnknown`''', defined in `package.tcl`,  is the default handler for
`[package]` when it doesn't know how to load a requested package.



** Description **

When `[package require]` fails, `[tclPkgUnknown]`, the default handler for
unknown packages, scans directories and their immediate subdirectories in
`[auto_path%|%$auto_path]`, and sources any `pkgIndex.tcl` files found.  During
this process, it recognizes and keeps up with changes to `$auto_path`, as for
example when the `pkgIndex.tcl` files for [tcllib] and [tklib] each adds its
own directory in order to get its immediate subdirectories scanned.

The design feature of keeping up dynamically with changes made to `$auto_path`,
along with the requirements that each `pkgIndex.tcl` is read at least once and
that items occurring earlier in `$auto_path` take precedence over later items,
lead to an implementation strategy in which the contents of `$auto_path` are
processed from the end to the beginning.  Because of this, when a particular
value occurs multiple times in `$auto_path` the position of the ''last''
occurrence of the value determines its precedence.  Thus, to make sure that a
value added to the beginning of `$auto_path` really takes precedence, first
remove any occurrences of that value:

======
while {[set i [lsearch -exact $auto_path $mydir]] >= 0} {
    set auto_path [lreplace $auto_path $i $i]
}
set auto_path [linsert $auto_path 0 $mydir]
======



<<discussion>> Obsolete bugs
NB a recent (2006-2007) bug for users of [OpenCascade] on Windows (which uses Tcl to drive its test harness and other uses) is that if you have 8.4.14 installed and OpenCascade 6.1 then the command:

======
package require BWidgets
======

replies:

======
tclPkgUnknown name version ?exact?
======

This can be cured by renaming the directory `C:\OpenCASCADE6.1.0\3rdparty\win32\tcltk` to `C:\OpenCASCADE6.1.0\3rdparty\win32\tcltkz` (or any other name that is not in the path command).

Suggests that there are 2 versions of tclPkgUnknown in the system with different calling conventions.

----

[MHo]: What's this?:

======
D:\Home\Arbeit1\pgm\tcl\usr\Tst\execx2\test>tclkitsh execx2test.tcl
invalid command name "tclPkgUnknown"
    while executing
"tclPkgUnknown execx2 0.1"
    ("package unknown" script)
    invoked from within
"package require execx2 0.1"
    invoked from within
"puts [package require execx2 0.1]"
    (file "execx2test.tcl" line 8)

D:\Home\Arbeit1\pgm\tcl\usr\Tst\execx2\test>tclkitsh
% info patch
8.4.19
%
======
This is ''not'' the case if I call tclsh 8.5.7 from ActiveState…
<<discussion>>

<<categories>> Package | Command