July 01, 2015

by Ryan Phillips

In Tim’s previous blog post titled Hardware Control he introduced LuaJit’s FFI. FFI is short for ‘Foreign Function Interface.’ In Luvit’s context the FFI allows Lua to load and run code from shared libraries without writing C bindings. In most cases, the FFI layer is faster at calling the function than the standard C Lua API.

To show off more on how Luvit and the FFI can work together, I have taken libcurl and bound the libcurl event loop to Luvit’s libuv event loop. The advantage is that both libraries work in harmony with each other.

libuv

libcurl handles the socket management for us and exposes a smart API to expose the file descriptor needed to add them to the libuv event loop.

libuv contains a uv_poll_t data structure that watches a file descriptor for read and write events (External I/O).

We are going to use the following functions from the LUV API:

local function perform(err, events) end
local poll = uv.new_poll(sockfd)
uv.poll_start(poll, 'r', perform)
uv.poll_start(poll, 'w', perform)
uv.poll_stop(poll)
uv.close(poll)

The binding

The latest version of the example is located within Luvit repository poll-libcurl.lua.

To start out the binding we:

1. Create a libcurl multi handle
2. hook libcurl’s socket and timer callback within the constructor initialize of the Multi class.

self.socketWrapper = ffi.cast('curlm_socketfunction_ptr_t', utils.bind(Multi._onSocket, self))
self.timeoutWrapper = ffi.cast('curlm_timeoutfunction_ptr_t', utils.bind(Multi._onTimeout, self))
self.multi = libcurl.curl_multi_init()
libcurl.curl_multi_setopt(self.multi, ffi.C.CURLMOPT_SOCKETFUNCTION, self.socketWrapper)
libcurl.curl_multi_setopt(self.multi, ffi.C.CURLMOPT_TIMERFUNCTION, self.timeoutWrapper)

When libcurl wants the event loop to trigger an action, it will call the _onSocket callback. This callback receives three parameters:

1. the easy handle
2. the socket file descriptor
3. the action (in, out, or remove).

Depending on the type of the action, we will read, write, or remove the event.

if action == ffi.C.CURL_POLL_IN then
  uv.poll_start(poll, 'r', perform)
elseif action == ffi.C.CURL_POLL_OUT then
  uv.poll_start(poll, 'w', perform)
elseif action == ffi.C.CURL_POLL_REMOVE then
  uv.poll_stop(poll)
  uv.close(poll)
  self.polls[sockfd] = nil -- remove from map
end

Dig it

Luajit’s FFI interface is extremely powerful and easy to use. Many Luajit Bindings have been created and are in use today. Remember to keep in mind that some bindings may use blocking APIs and block the libuv event loop.