ffi.cdef("""
    struct ev_io { ...; }

    struct my_io {
        ev_io io;
        void *handle;
        ...;
    };
""")

ffi.set_source("_example", """

static int (*python_callback)(void *handle, int revents);
static void (*python_handle_error)(void *handle);

static void my_io_callback(struct ev_loop *loop, ev_io *io, int revents)
{
    /* invoke 'self.callback()' */
    void *handle = ((struct my_io *)io)->handle;
    if (python_callback(handle, revents) < 0) {
        /* in case of exception, call 'self.loop.handle_error()' */
        python_handle_error(handle);
    }
    ... more code to stop the event, like _run_callback() does ...
}

""")


# this is done globally:

@ffi.callback("int(void *handle, int revents)", error=-1)
def python_callback(handle, revents):
    watcher = ffi.from_handle(handle)
    watcher.callback(*watcher.args)
    return 0

lib.python_callback = python_callback


# then this is done in class io's __init__:

class io(watcher):
    def __init__(self, ...):
        self._handle = ffi.new_handle(self)
        self._watcher = ffi.new("my_io *")
        self._watcher.handle = self._handle
        # give the same C-level callback function to all of them
        self._watcher_init(self._watcher, lib.my_io_callback)