[ros-users] Debuggin ROS_COMM deserializer

[Cedric Pradalier]

The easiest would be to explicitly serialise the bytes. For instance:
For writing:
uint32_t t = value;
buffer[0] = (unsigned char)t; t = t >> 8;
buffer[1] = (unsigned char)t; t = t >> 8;
buffer[2] = (unsigned char)t; t = t >> 8;
buffer[3] = (unsigned char)t;

For reading:
uint32_t t = buffer[3]; t = t << 8;
t = t | buffer[2]; t = t << 8;
t = t | buffer[1]; t = t << 8;
t = t | buffer[0];
value = t;

This would be the C way, but boost probably already provide endianness-safe
serialization.

Before changing the code, just put some cout/printf after the length
decoding and check if the length makes sense. And remember that this is
just my intuition of where the error could come from because there is
definitely a bug there. It might be completely unrelated to your problem,
but the symptoms would fit...

Regards


On Wed, Nov 28, 2012 at 6:22 PM, Claudio Carbone <cla_carbone at tiscali.it>wrote:

>  Thanks for the hint Cedric.
>
> It's the first time I encounter an endianness problem.
> At this point we are not even sure this is an endianness problem.
>
> I filed a ticket to propose a more descriptive error message in case of
> buffer overrun, as the way it is now it's really useless (just buffer
> overrun, but no indication of which buffer, what was the dimension, what
> was the overrun and how was the dimension specified).
>
> By the way: how should I modify the snippet you posted to avoid endianness
> problems?
> Wouldn't compiling with -mlittle-endian solve such problems? (trying now,
> but it will take about 8hrs to compile)
>
> Claudio
>
>
>
> On 28/11/12 17:46, Cedric Pradalier wrote:
>
> Here it is, in roscpp/src/libros, writeHeader
>   uint32_t msg_len = len + 4;
>    boost::shared_array<uint8_t> full_msg(new uint8_t[msg_len]);
>   memcpy(full_msg.get() + 4, buffer.get(), len);
>   *((uint32_t*)full_msg.get()) = len;
>
>  and onHeaderLengthRead:
>    uint32_t len = *((uint32_t*)buffer.get());
>
>  This type of code might create some trouble when run between x86 and arm
> cpu. However, I could only track this bit of code in the UDP and service
> loop, and I only checked that this code was there up to fuerte.
>
>  In my experience some time ago, I could listen to packet from a gumstix
> (cturtle, compressed or raw video) from a x86 laptop (diamondback I think).
> So this might actually be two different bugs...
>
>  Now, for people who wonder why I did not report this earlier:
> contributing to ros_comm has not been very successful so far on my side, so
> I just keep track of my patches until someone really needs them... (shared
> memory and compressed transports out-of-the-box for all topics?)
>
>  Regards
>
>
> On Wed, Nov 28, 2012 at 5:14 PM, Claudio Carbone <cla_carbone at tiscali.it>wrote:
>
>>  Thank you Cedric,
>>
>> I'll wait for your info.
>>
>> Regards
>> --
>>
>> *Eng. Claudio Carbone
>> Embedded Systems Design*
>>
>> P.IVA: 11688471009
>> tel: +393809017424
>> email: Send email
>>  <cla_carbone at tiscali.it>[image: My linkedin profile]<http://it.linkedin.com/in/embeddedesign/en>
>>
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>> [image: My portfolio site] <http://www.fusioncoredesign.it/>
>>
>>
>>
>>
>> On 28/11/12 17:09, Cedric Pradalier wrote:
>>
>> Hi,
>>
>>  last time I checked, some of the deserialization code was actually
>> endianness specific (if I'm not mistaken), in particular the length of the
>> message... This could lead easily to a buffer overrun.
>>
>>  I'll try to point out the little bit of code later today.
>>
>>  Regards
>>
>>
>>
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>>
>>
>
>
>  --
> Cedric Pradalier
>
>
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-- 
Cedric Pradalier
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