[Bluez-devel] select(2) or read(2) on bluetooth sockets not working correctly?

[Bluez-devel] select(2) or read(2) on bluetooth sockets not working correctly?

[Pawel Kot]

Hi,

In gnokii project, we use Bluetooth sockets as one of the phone
communication medium. We use it in quite usual way (simplified code):

fd = socket(PF_BLUETOOTH, SOCK_STREAM, BTPROTO_RFCOMM);
[...]
bind(fd, (struct sockaddr *)&laddr, sizeof(laddr));
[...]
connect(fd, (struct sockaddr *)&raddr, sizeof(raddr));

Then we have (simplified code again):

while (1) {
   [...]
   write(fd, write_buf, size);
   [...]
   FD_ZERO(&readfds);
   FD_SET(fd, &readfds);
   if (select(fd + 1, &readfds, NULL, NULL, &timeout) > 0) {
      read(fd, read_buf, size);
      [...]
   }
   [...]
}

The problem is (or rather was) that then bluetooth device gets out of
range (or bluetooth gets disabled in the device) the program hangs on
read(2). So select(2) went OK and told us that there's something to be
read, but actually nothing can be read. According to select(2) manual
page, after the successful select(2), read(2) should not hang (it may
fail though).

It can be workarounded by setting O_NONBLOCK flag on the socket, but
still I consider it as a bug that read(2) hangs after successful
select(2).

take care,
pkot
PS. Please Cc me on replies.
-- 
Pawel Kot

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Re: [Bluez-devel] select(2) or read(2) on bluetooth sockets not working correctly?

[Marcel Holtmann]

Hi Pawel,

> In gnokii project, we use Bluetooth sockets as one of the phone
> communication medium. We use it in quite usual way (simplified code):
> 
> fd = socket(PF_BLUETOOTH, SOCK_STREAM, BTPROTO_RFCOMM);
> [...]
> bind(fd, (struct sockaddr *)&laddr, sizeof(laddr));
> [...]
> connect(fd, (struct sockaddr *)&raddr, sizeof(raddr));
> 
> Then we have (simplified code again):
> 
> while (1) {
>    [...]
>    write(fd, write_buf, size);
>    [...]
>    FD_ZERO(&readfds);
>    FD_SET(fd, &readfds);
>    if (select(fd + 1, &readfds, NULL, NULL, &timeout) > 0) {
>       read(fd, read_buf, size);
>       [...]
>    }
>    [...]
> }
> 
> The problem is (or rather was) that then bluetooth device gets out of
> range (or bluetooth gets disabled in the device) the program hangs on
> read(2). So select(2) went OK and told us that there's something to be
> read, but actually nothing can be read. According to select(2) manual
> page, after the successful select(2), read(2) should not hang (it may
> fail though).
> 
> It can be workarounded by setting O_NONBLOCK flag on the socket, but
> still I consider it as a bug that read(2) hangs after successful
> select(2).

which kernel version? Can you construct a simple demo program that shows
this effect easily with a 2.6.24-rc2 kernel?

Regards

Marcel



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Re: [Bluez-devel] select(2) or read(2) on bluetooth sockets not working correctly?

[Pawel Kot]

Hi,

I have put simple program demonstrating the problem at
http://tfuj.pl/gnokii/bt.c with compilation hints and usage.
The testing I did was on Ubuntu kernels, so that's for sure not the
recent one, but I think it is really easy to reproduce if the problem
still persists.

You may catch me on #bluez if needed.

take care,
pkot
PS. I'm really not subscribed, so please Cc on reply.
-- 
Pawel Kot

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Re: [Bluez-devel] select(2) or read(2) on bluetooth sockets not working correctly?

[Marcel Holtmann]

Hi Pawel,

> I have put simple program demonstrating the problem at
> http://tfuj.pl/gnokii/bt.c with compilation hints and usage.
> The testing I did was on Ubuntu kernels, so that's for sure not the
> recent one, but I think it is really easy to reproduce if the problem
> still persists.

this from the select() manual page:

Those listed in readfds will be watched to see if characters  become
available  for reading  (more  precisely, to see if a read will not
block; in particular, a file descriptor is also ready on end-of-file)

If a device disconnects or gets out of range, it is considered to be an
end of file situation. It might help to also watch the exceptfds for the
hangup signal. The read() should return an error if the connection has
been terminated. So that part is strange.

It means that if you connect, then poweroff and then call read() you
would see the same effect. The read() command will block. Can you
confirm this?

Regards

Marcel



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