Sharing data between C/C++ and python

[mania...@gmail.com]

Hi all,

Basically, I want to build a message in python then share it ( via serializing/deserializing ) with in C++ and do stuff, then again share the results 

back to python. Apparently there is a way to deserialize the object/message back from byte array in python, but I dont know how to deserialize it in C++.

I just want to emphasis that I am looking in share memory approach No files based method appreciated.

Anybody have any suggestion ?

Regards

Mani

[Kenton Varda]

Hi Mani,

Not sure I fully understand your question. To deserialize a byte array in C++, use capnp::FlatArrayMessageReader. Use capnp::messageToFlatArray() to serialize it again. Both are defined in capnp/serialize.h.

-Kenton

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[mania...@gmail.com]

Hi Kenton

Thanks for the reply,

Let me explain more about my question,

In python I use pyCapnp to serialize a message by using messgae.to_bytes()

the serialize is a string format, so I can simply read string ( via ctype ) in c++ DLL

but  FlatArrayMessageReader get a array of word as input, 

how can I conver a String to array of Word ?

And vice versa ?

Appreciate your help in advance

Mani 

[Kenton Varda]

If the data is aligned, you can use a reinterpret_cast, like so:

kj::ArrayPtr<const capnp::word> toWordsAlias(const char* data, size_t size) {

  return kj::arrayPtr(reinterpret_cast<const capnp::word*>(data), size / sizeof(word));

}

If you don't know for sure whether the data is aligned, you will need to make a copy instead:

kj::Array<word> toWordsCopy(const char* data, size_t size) {

  auto result = kj::heapArray<word>(size / sizeof(word));

  memcpy(result.begin(), data, size);

  return result;

}

-Kenton

[Henry Harrison]

I have a similar problem. I am trying to convert my buffer to a char* and then back to a FlatArray for reading.

I am getting an error stating that: Caught exception in main: src/capnp/serialize.c++:43: failed: expected array.size() >= offset; Message ends prematurely in segment table.

My code looks like this:

auto msg = capnp::messageToFlatArray(messagebuilder);

memcpy(target_, msg.begin(), (msg.size()*sizeof(capnp::word)));

auto words = kj::heapArray<capnp::word>(sizeof(target_)*sizeof(char)/ sizeof(capnp::word));

memcpy(words.begin(), &target_, sizeof(target_));

capnp::FlatArrayMessageReader message(words);

Message::Reader messageReader = message.getRoot<Message>();

thanks

[Kenton Varda]

Hi Henry,

On Tue, Feb 2, 2016 at 8:31 AM, Henry Harrison <hdh...@york.ac.uk> wrote:

auto words = kj::heapArray<capnp::word>(sizeof(target_)*sizeof(char)/ sizeof(capnp::word));

memcpy(words.begin(), &target_, sizeof(target_));

sizeof(target_) looks suspicious here. How is target_ defined? If it has a fixed width, perhaps that width is too small for the message. If it is dynamically-allocated, then sizeof(target_) gives you the size of the pointer.

(Also note that multiplying by sizeof(char) is logically incorrect since sizeof(target_) returns bytes, not an element count -- but since sizeof(char) is 1 anyway this probably isn't affecting anything.)

-Kenton