RE: [Caml-list] Writing to a blocked socket

[David Allsopp <dra-news@metastack.com>]

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Gerd Stolpmann wrote:
> Am Dienstag, den 02.08.2011, 18:01 +0100 schrieb David Allsopp:
> > I don't seem to be able to ask Google this in a way which will give me
> > a reasonable answer!
> >
> > In the same process, if you have one thread blocked on a [recv]
> > operation on a socket, under Unix another thread can still write to
> > the socket. Under Windows, however, the call to [send] blocks because
> > there's another thread blocked on a [recv] to the same socket. Are
> > there any options that can be set to change that behaviour or is that
> > just "the way it is" and the application has to be coded using [select]
> instead?
> 
> Really? This does not make sense at all. It's quite normal that one
> direction is blocked, and the other not. Are you sure about your
> observation?

Seems to be, from the attached - important bit is on the last 10 lines - the function readThread is spawned in a thread of its own and then the loop below reads input and sends each line down the socket.

Compiled with:

ocamlfind ocamlopt -o foo -thread -package unix,threads -linkpkg Foo.ml

and then executed as:

./foo www.google.com

I enter:

GET / HTTP/1.0

followed by two new lines... on Linux I get a response from Google, on Windows it hangs after the first line. 3.12.0 (and 3.10.1 on an old machine) all behaving the same way.

David

[-- Attachment #2: Foo.ml --]
[-- Type: application/octet-stream, Size: 1723 bytes --]

let host = ref ""
let port = ref 80

let usage = "usage: foo host [-p port]"

let _ =
  Arg.parse
    [
      "-p", Arg.Int (fun i -> port := i), "\tPort";
    ]
    (fun s -> host := s) usage;
  if !host = ""
  then Printf.printf "%s\n\n" usage
  else let sockaddr =
         let host =
           try
             Unix.gethostbyname !host
           with Not_found -> failwith "Host not found"
         in
           Unix.ADDR_INET (host.Unix.h_addr_list.(0), !port)
       in
         let socket =
           let s = Unix.socket Unix.PF_INET Unix.SOCK_STREAM 0
           in
             Unix.connect s sockaddr;
             s
         in
           let bufsize = 1024
           in
             let buf = String.create bufsize
             in
               let loop = ref true
               in
                 Printf.printf "Connection ok.\nType 'exit' to quit.\n\n%!";
                 let readThread () =
                   let buf = String.create bufsize in
                     while !loop
                     do
                       let r = Unix.recv socket buf 0 bufsize []
                       in
                         Printf.printf "%s%!" (String.sub buf 0 r)
                     done
                 in
                   ignore (Thread.create readThread ());
                   while !loop
                   do
                     Printf.printf "Ready> %!";
                     let r = Unix.read Unix.stdin buf 0 bufsize
                     in
                       if r = 4 && String.sub buf 0 4 = "exit"
                       then loop := false
                       else ignore (Unix.send socket buf 0 r [])
                   done