Zerocopy with sequence datatype

[ussefdesouky]

Is there an already existing issue for this?

• I have searched the existing issues

Expected behavior

I am using the zerocopy example, just changed the message struct and added a sequence<octet> img;

Current behavior

The Zerocopy example totally depending on the following line of code, so if it's true it publishes the message.
ReturnCode_t::RETCODE_OK == writer_->loan_sample(sample)

Steps to reproduce

when I change the normal .idl message from
struct LoanableHelloWorld
{
unsigned long index;
unsigned long long timestamp;
char data[256];
};
to the following
struct LoanableHelloWorld { unsigned long index; unsigned long long timestamp; sequence<octet> img; };

this line of code returns false so it does not publish any data again.

Is the sequence datatype can't be used in such transport protocol?

Fast DDS version/commit

master

Platform/Architecture

Other. Please specify in Additional context section.

Transport layer

Zero copy

Additional context

No response

XML configuration file

No response

Relevant log output

Starting 
LoanableHelloWorld DataWriter created.
LoanableHelloWorld DataWriter waiting for DataReaders.
DataWriter matched.

Network traffic capture

No response

[JLBuenoLopez]

Thanks for the report @ussefdesouky

Please, take into account the zero-copy mechanism constraints. The modification introduced in the IDL file prevents both the plain and bounded requirements to be met. Besides, the sequence keyword requires as a non-optional parameter the type of the sequence, so the given IDL is malformed.

I am moving this to the support discussion forum as per Fast DDS CONTRIBUTING guidelines.

[ussefdesouky]

I have updated the idl message content it was written in wrong form. Also, you mean that
struct LoanableHelloWorld { sequence<octet> img; };
this form can work with no issues.

[JLBuenoLopez]

The sequence is not bounded, so no, zero-copy mechanism is not going to work with that sequence. Data-sharing mechanism requires for the type to be bounded. Fast DDS provides API to check if the type is bounded and plain. If those methods return true, then the type might be used with zero-copy mechanism.

[ussefdesouky]

Thanks a lot, quite clear now. If I am going to publish image frame with the least latency which transport protocol you would recommend?

Thanks agian, Bueno.