Unpack in /usr/lib, /usr/local/lib, or wherever your tcl resides
Bignum provides the following packages:
Of necessity, licensed under GPL, since libgmp is.
To use, either unpack the Starkit and copy the contents of the lib directory into somewere on your auto_path, or just source it directly
tclkit % source bignum.kit % package require bignum % bigint::add 1111111111111111111111111111 2222222222222222222222222222 3333333333333333333333333333
The binaries on Windows and Linux are statically linked against libgmp (and therefore self-contained) but I couldn't get this working on OSX so you'll need to have gmp installed on your LDPATH to use bignum on OSX. I'll have another go later.
The modified source is available at [L2 ]
::bigint::powm 5 6 0
...and Tcl crashed (pure DOS or Tkcon). It only seems to happen with the 0, but it does happen every time. Maybe I shouldn't be using 0, I admit I do not understand what that function really does (I wanted to calculate a large number raised to a large power). But I don't think that Tcl or bignum should crash on me just because I threw a zero at it. :-)
Lars H: powm is probably "power modulo" (a very common operation in many cryptographical algorithms), so a zero as last argument is likely to lead to "divide by zero" conditions. This could either be a bug in libgmp, or a failure of the wrapper to catch input not supported by that library, but come Tcl 8.5 we'll most likely have equivalent functionality in the core anyway (happy, happy; joy, joy).