[PD] preventing comport freezes

[Hans-Christoph Steiner]

On May 15, 2009, at 4:09 AM, Roman Haefeli wrote:

>
> Am 14.05.2009 um 22:14 schrieb 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().
>
>
> i can confirm this. when a [print] is connected to [comport] and  
> after unplugging the usb cable, [comport] starts sending '0'  
> messages with maximum rate.  so it seems, that it is bogusly  
> receiving data.
>
> roman

Does it crash?  I just hooked up a print to my Arduino Stamp USB- 
serial adapter and got nothing, and it doesn't crash either.  I find  
that when disconnecting using an arduino, Pd usually crashes.

.hc



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