A [Tcl] '''extension''' is a new command written in the implementation language
of Tcl and provided via `[load]`. Because it has access to the [Tcl C API] and
the structures that implement objects visible at the script level, an extension
can extend the feature set of the Tcl interpreter. A [proc%|%procedure], the
body of which is simply a script to be evaluated by the interpreter, lacks this
capability.  An extension is often included in a [package], where the
`[pkgIndex.tcl]` script uses `[load]` to load the extension into the
interpreter process. An extension may define new actions, new [widget]s, new
processing types, etc.



** See Also **

   [Tcl Extension: User Guide]:   

   [Tcl Extension: Developer Guide]:   

   [Extensions for Tcl and Tk]:   

   [Extending Tcl]:   How to write software that will be implemented as an extension, perhaps in C (or C++ code with `extern "C" {}` around it).

   [Sampleextension]:   

   [package]:   

   [extension language]:   


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