Function ncsubproc_createv 

pub unsafe extern "C" fn ncsubproc_createv(
    n: *mut ncplane,
    opts: *const ncsubproc_options,
    bin: *const c_char,
    arg: *const *const c_char,
    cbfxn: ncfdplane_callback,
    donecbfxn: ncfdplane_done_cb,
) -> *mut ncsubproc

see exec(2). p-types use $PATH. e-type passes environment vars.