The documented C interface works by generating a C code out of the declarations.
In particular, for every Forth word declared with c-function,
it generates a wrapper function in C that takes the Forth data from
the Forth stacks, and calls the target C function with these data as
arguments. The C compiler then performs an implicit conversion
between the Forth type from the stack, and the C type for the
parameter, which is given by the C function prototype. After the C
function returns, the return value is likewise implicitly converted to
a Forth type and written back on the stack.
The \c lines are literally included in the C code (but without
the \c), and provide the necessary declarations so that the C
compiler knows the C types and has enough information to perform the
conversion.
These wrapper functions are eventually compiled and dynamically linked into Gforth, and then they can be called.
The libraries added with add-lib are used in the compile
command line to specify dependent libraries with -llib,
causing these libraries to be dynamically linked when the wrapper
function is linked.