> does anyone know a package for doing this (like libffi but the
> reverse way) ?
The `vacall' and `callback' packages of the ffcall package [1][2] do this.
Bruno
[1] http://clisp.cons.org/~haible/packages-ffcall.html
[2] ftp://ftp.santafe.edu/pub/gnu/ffcall-1.6.tar.gz