The Compiled Runtime In Tcl lets you easily embed C code in Tcl.
CriTcl is a system to build C extension packages for Tcl on the fly, from C code embedded within Tcl scripts, for all who wish to make their code go faster.
Version 3 is the actively developed version of Critcl, with several new features. This version has changes to the public API which make it incompatible with packages using Critcl version 2.x, or earlier.
Declaration, export and import of C-APIs through stubs tables.
Declaration, initialization and use of user-specified configuration options. An important use is the declaration and use of custom build configurations, like 'link a 3rd party library dynamically, statically, build it from copy of its sources, etc.', etc.
This is of course not everything. For the details please read the Changes section of the documentation.