Network server programming with OCaml

Some years ago, I experimented with networking in SML/NJ and spent a few hours figuring out how to write a multithreaded TCP/IP server using SML/NJ. Here, I’ve performed the same exercise with OCaml 3.12.1.

The code for this example is on github.

Download source code, building, and running

The following example is comprised of a _tags file for controlling ocamlbuild and the .ml file itself. The complete sources:

Running the following command compiles the project:

ocamlbuild test.native

The ocamlbuild output is a native executable. The executable is placed in the _build directory, and a symbolic link to the executable is placed in the working directory. To run the program:

The build control file

The _tags file contains

true: use_unix
true: thread

which instructs the build system to include the Unix POSIX module, which provides a BSD-sockets API, and to configure the OCaml runtime to support the Thread lightweight-threads module. For more information about ocamlbuild, see here.

The example source code

Turning to now, we first bring the contents of a few modules into scope:

open Unix
open Printf
open Thread

The Unix module, mentioned above, provides a POSIX BSD-sockets API; Printf is for formatted printing; and Thread is for multithreading. We’ll be using a single thread per connection. Other models are possible.

OCaml programs end up being written upside down, in a sense, because function definitions need to precede their use (unless mutually-recursive definitions are used). For this reason, the next chunk is conn_main, the function called in a new lightweight thread when an inbound TCP connection has been accepted. Here, it simply prints out a countdown from 10 over the course of the next five seconds or so, before closing the socket. Multiple connections end up running conn_main in independent threads of control, leading automatically to the natural and obvious interleaving of outputs on concurrent connections.

let conn_main s =
  let cout = out_channel_of_descr s in
  let rec count n =
    match n with
    | 0 ->
        fprintf cout "Bye!\r\n%!"
    | _ ->
        fprintf cout "Hello %d\r\n%!" n;
        Thread.delay 0.5;
        count (n - 1)
  count 10;
  printf "Closing the connection.\n%!";
  close s

Note the mysterious %! format specifiers in the format strings: these translate into calls to flush, forcing buffered output out to the actual socket. The alternative would be to call flush cout directly ourselves.

The function that depends on conn_main is the accept loop, which repeatedly accepts a connection and spawns a connection thread for it.

let rec accept_loop sock =
  let (s, _) = accept sock in
  printf "Accepted a connection.\n%!";
  let _ = Thread.create conn_main s in
  accept_loop sock

Finally, the block of code that starts the whole service running, creating the TCP server socket and entering the accept loop. We set SO_REUSEADDR on the socket, listen on port 8989 with a connection backlog of 5, and enter the accept loop. We catch and ignore SIGPIPE in order to avoid crashing when a client departs unexpectedly.

let _ =
  Sys.set_signal Sys.sigpipe Sys.Signal_ignore;
  let sock = socket PF_INET SOCK_STREAM 0 in
  setsockopt sock SO_REUSEADDR true;
  bind sock (ADDR_INET (inet_addr_of_string "", 8989));
  listen sock 5;
  printf "Entering accept loop...\n%!";
  accept_loop sock