problem with reading from rfcomm socket

problem with reading from rfcomm socket

[Eiko Oltmanns]

Hi!


I have a problem, with reading from rfcomm sockets. I wrote a little client application, that connects to an other device and just prints all received data. I want to receive data as soon as it is sent from the remove device. The problem is, that the read function blocks, until 4k of data are sent on the remote side, although the number of requested bytes passed to read is much lower. Then read starts to read out the received data. When all data has been read, the next call again blocks until the next 4 kb are available.

Distribution: Debian, "Lenny"
Kernelversion: 2.6.26-13
Bluez-libs: 3.36-1
Bluez-utils: 3.36-3


Here is the minimal code, which reproduces my problem:


int main(int argc, char **argv)
{
	int sock = hci_open_dev( devId );

	if ( sock < 0 ) return -1;

	int rfcommSocket = socket(AF_BLUETOOTH, SOCK_STREAM, BTPROTO_RFCOMM);
	if ( rfcommSocket < 0 ) return -1;

	struct sockaddr_rc addr;
	addr.rc_family = AF_BLUETOOTH;
	addr.rc_channel = channel;
	str2ba("00:09:DD:50:72:0F", &addr.rc_bdaddr );
	
	// connect to server
	int status = connect(rfcommSocket, (struct sockaddr *)&addr, sizeof(addr));

	if (  status < 0 ) rerurn - 1;

	int bytes = write(rfcommSocket,"Test",4);	/* works as expected. Data is received immediately by the remote device */

	if ( bytes != 4 ) return - 1; //tested only for 4 for simplicity

	while (1) 
	{
		unsigned char buf[10];		/* the buf size seems to have no influence on my problem */
		memset( buf, 0, sizeof(buf));
		int bytes = read(rfcommSocket, buf, sizeof(buf)-1); /* blocks until 4k of data are available */

		if ( bytes > 0 )
			cout << buf;
		if ( bytes < 0 )
		{
			return -1;
		}

	}
	

	return 0;
}

Greetings,
Eiko

Re: problem with reading from rfcomm socket

[Eiko Oltmanns]

Eiko Oltmanns schrieb:
> Hi!
>
>
> I have a problem, with reading from rfcomm sockets. I wrote a little 
> client application, that connects to an other device and just prints 
> all received data. I want to receive data as soon as it is sent from 
> the remove device. The problem is, that the read function blocks, 
> until 4k of data are sent on the remote side, although the number of 
> requested bytes passed to read is much lower. Then read starts to read 
> out the received data. When all data has been read, the next call 
> again blocks until the next 4 kb are available.
>
> Distribution: Debian, "Lenny"
> Kernelversion: 2.6.26-13
> Bluez-libs: 3.36-1
> Bluez-utils: 3.36-3
>
>
> Here is the minimal code, which reproduces my problem:
>
>
> int main(int argc, char **argv)
> {
>     int sock = hci_open_dev( devId );
>
>     if ( sock < 0 ) return -1;
>
>     int rfcommSocket = socket(AF_BLUETOOTH, SOCK_STREAM, BTPROTO_RFCOMM);
>     if ( rfcommSocket < 0 ) return -1;
>
>     struct sockaddr_rc addr;
>     addr.rc_family = AF_BLUETOOTH;
>     addr.rc_channel = channel;
>     str2ba("00:09:DD:50:72:0F", &addr.rc_bdaddr );
>     
>     // connect to server
>     int status = connect(rfcommSocket, (struct sockaddr *)&addr, 
> sizeof(addr));
>
>     if (  status < 0 ) rerurn - 1;
>
>     int bytes = write(rfcommSocket,"Test",4);    /* works as expected. 
> Data is received immediately by the remote device */
>
>     if ( bytes != 4 ) return - 1; //tested only for 4 for simplicity
>
>     while (1)     {
>         unsigned char buf[10];        /* the buf size seems to have no 
> influence on my problem */
>         memset( buf, 0, sizeof(buf));
>         int bytes = read(rfcommSocket, buf, sizeof(buf)-1); /* blocks 
> until 4k of data are available */
>
>         if ( bytes > 0 )
>             cout << buf;
>         if ( bytes < 0 )
>         {
>             return -1;
>         }
>
>     }
>     return 0;
> }
>
> Greetings,
> Eiko

I think my code should be correct, isn't it? Is there maybe an option in 
the kernel config, that could cause such a behaviour of the socket? I am 
running a self configured kernel on an embedded hardware. Any other 
suggestions?

Re: problem with reading from rfcomm socket

[Johan Hedberg]

Hi Eiko,

On Mon, Apr 06, 2009, Eiko Oltmanns wrote:
> I think my code should be correct, isn't it? Is there maybe an option in the
> kernel config, that could cause such a behaviour of the socket?

Take a look at existing examples in the bluez source, e.g. tools/rctest.c.
You're missing a call to bind after the call to socket and before the call to
connect.

Joah

Re: problem with reading from rfcomm socket

[Johan Hedberg]

On Mon, Apr 06, 2009, Johan Hedberg wrote:
> Take a look at existing examples in the bluez source, e.g. tools/rctest.c.
> You're missing a call to bind after the call to socket and before the call to
> connect.
> 
> Joah

And despite my signature my name is actually Johan. Guess I should go get some
sleep :P

Johan