Tcl for Embedded Applications
The Command Writer's Manual is useful for people who wish to extend Tcl with application-specific commands written in C. Note that this is currently only available in PDF format.
Tcl 6 & 7 use character strings throughout for all data elements. Tcl 8 and beyond has an object system that is much more complicated to interface with in C but offers substantially higher performance. The command writer's manual referenced above is for Tcl 6 only; a newer version relevant to Tcl 8 ships with TclX.