ffi/trampoline

    cfunction
    src/core/ffi.c on line 1408, column 1

    (ffi/trampoline cc)

    Get a native function pointer that can be used as a callback and 
    passed to C libraries. This callback trampoline has the signature 
    `void trampoline(void \*ctx, void \*userdata)` in the given calling 
    convention. This is the only function signature supported. It is up 
    to the programmer to ensure that the `userdata` argument contains a 
    janet function the will be called with one argument, `ctx` which is 
    an opaque pointer. This pointer can be further inspected with 
    `ffi/read`.