Re: [Caml-list] Unix library and serial port

[Anders Fugmann <anders@fugmann.net>]

Hi Edouard,

Setting the nonblock flag on the socket specifies exactly that calls 
will never wait but raise EGAGIN when no data is available. That is - no 
operations on the socket will ever block.

I would advice you to not open using the O_NONBLOCK flag, and instead
read exactly the amount of bytes you expect. If, for example, the motor 
returns data ending with newlines, I would read one char at a time until 
the terminating char is encountered and then parse the message received.

Another solution is to poll for data and then sleep a bit when EAGAIN is 
raised. Or even use select.

/Anders

On 08/13/2014 09:23 AM, Edouard Evangelisti wrote:
> Dear all,
>
> I encountered a problem while trying to exchange informations with a ZStepper
> Motor connected to the computer through a serial/USB adapter. I can send
> commands and move the motor, but I cannot retrieve the informations which are
> sent by the motor directly after completion. For this I am using the Unix
> library as follows.
>
> First of all, I get a read/write file descriptor :
>
> let usb0 = "/dev/ttyUSB0"
> let flags = Unix.([O_RDWR; O_NOCTTY; O_NONBLOCK])
> let perm = 0o666
> let fd = Unix.openfile usb0 flags perm
>
> I then set the parameters using Unix.tcgetattr/Unix.tcsetattr (for instance
> baud and parity check). Most of the default values are fine. Then I am able to
> send commands :
>
> let write_serial () =
>   let cmd = "/2P1000R\r\n" in (* one command as  an example. *)
>   let len = String.length cmd in
>   let ret = Unix.write fd cmd 0 len in
>   assert (ret = len); (* not the real code here for error management *)
>   Unix.tcdrain fd
>
> The call to write_serial triggers ZStepper motion as expected. Then, the motor
> send back some data. I have tried to read them using :
>
> let read_serial () =
>     let len = 10 in
>     let buf = Buffer.create len in
>     let str = String.create len in
>     let rec loop () =
>     let n = Unix.read fd str 0 len in
>     if n > 0 then (
>       Buffer.add_substring buf str 0 n;
>       loop ()
>     ) else Buffer.output_buffer stdout buf in
>     try loop () with Unix.Unix_error (Unix.EAGAIN, _, _) -> ()

>
> I always got EAGAIN error and no data available. When using
> Unix.in_channel_of_descr, I can retrieve some data but asynchronously.
>
> I have tried to launch simultaneously my program and GtkTerm configured with
> the same settings and GtkTerm is able to retrieve the informations sent by the
> motor after completion. The source code of GtkTerm looks very similar to mine.
> I probably do something wrong but I cannot see where is the problem.
>
> Would you have an idea ?
>
> Many thanks in advance for your precious help.
>
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