[PD] preventing comport freezes

[Hans-Christoph Steiner]

On May 9, 2009, at 2:43 PM, Martin Peach wrote:

> Roman Haefeli wrote:
>> thanks for the info.
>> On Fri, 2009-05-08 at 19:27 +0000, martin.peach at sympatico.ca wrote:
>>>> just out of curiosity: if there is a solution, that works well for
>>>> [tcpserver], couldn't it be applied also to [comport]?
>>>>
>>> Maybe. But I think you should be able to use [comport] with no
>>> hardware handshaking enabled and send data even if no cable is
>>> attached. Sometimes the absence of one of the two input handshake
>>> signals prevents the serial hardware from sending. Also if an error
>>> occurred in the reception of serial data it may not be handled
>>> properly in [comport]. So I'm not sure what is causing this  
>>> particular
>>> crash, since HC said that it happened when the remote device was
>>> disconnected but gave no further detail.
>> iirc, on linux at least, [comport] makes pd hang, _whenever_ the  
>> other
>> end disappears. i.e.:
>> - pulling out the usb-cable, while the arduino is connected
>> - turning off an rfcomm device
>
>
> This looks like something related to the usb interface. I think  
> pulling out an RS-232 cable has no effect, as the serial driver can  
> only be closed by [comport]. With a usb adapter the usb driver can  
> close the port.
> I suspect that the comport_tick routine, which is called  
> periodically to check for received characters, tries to access the  
> serial port after the usb driver has closed it.
> The non-Windows code in comport_tick looks like this:
>
> unsigned char   serial_byte;
> fd_set          com_rfds;
> int             count = 0;
>
> FD_ZERO(&com_rfds);
> FD_SET(fd,&com_rfds);
>
> while((err=select(fd+1,&com_rfds,NULL,NULL,&null_tv)) > 0)
> {
>    err = read(fd,(char *) &serial_byte,1);
>    outlet_float(x->x_data_outlet, (t_float) serial_byte);
>    ++count;
> }
>
> As you can see the select call only checks for the presence of  
> received characters with com_rfds, and doesn't check the write or  
> exception status. I suppose the select call should also check the  
> exception fd_set, as the usb driver has no other way of informing  
> [comport] that it has closed the port, it should have flagged it  
> there.
> (Although if the fd itself is no longer valid I don't know what to  
> do...using non-existent file descriptors is a good way to crash Pd)
>
> ATM I only have 'legacy' RS-232 ports on my hardware so I can't test  
> it, but I can change the code.
>
> Martin


Ok, quick test shows a couple things:

- just using [comport 1 115200] and then yanking the USB out causes  
the crash, so no data needs to be sent to cause this.  This is with a  
standard Arduino USB.

- in the select() that you highlight, it is just testing before  
reading, so I am guessing it would not be so useful to do a write test  
if it is only going to read().

.hc



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