flatbuffer from TS to C++ not working - c++

Question

I have a flatbuffer schema for a message:
table NodeConstructionInfo {
type:string (id: 0, required);
name:string (id: 1, required);
}
table AddNodeRequest {
graphId:string (id:0, required);
node:NodeConstructionInfo (id:1, required);
}
which I construct (write) in TypeScript and receive (read) in C++:
let builder = new flatbuffers.Builder(356);
let offGraphId = builder.createString("2992ebff-c950-4184-8876-5fe6ac029aa5");
let offType = builder.createString("MySuperDuperNode");
let offName = builder.createString("DummyNode");
sz.NodeConstructionInfo.startNodeConstructionInfo(builder);
sz.NodeConstructionInfo.addName(builder, offName);
sz.NodeConstructionInfo.addType(builder, offType);
let off = sz.NodeConstructionInfo.endNodeConstructionInfo(builder);
sz.AddNodeRequest.startAddNodeRequest(builder);
sz.AddNodeRequest.addGraphId(builder, offGraphId);
sz.AddNodeRequest.addNode(builder, off);
off = sz.AddNodeRequest.endAddNodeRequest(builder);
builder.finish(off);
let requestPayload = builder.asUint8Array();
In C++ I receive the 356 bytes (requestPayload) and try to verify it by doing
flatbuffers::Verifier v(buffer.getData(), buffer.getSize());
v.VerifyBuffer<AddNodeRequest>();
which always fails in <flatbuffers/flatbuffers.h> at:
template<typename T>
bool VerifyBufferFromStart(const char *identifier, const uint8_t *start)
{
...
// Call T::Verify, which must be in the generated code for this type.
auto o = VerifyOffset(start); <--- HERE (the first read flatbuffers::uoffset_t should not be 0 (don't know why)?)
...
}
Am I missing some important detail?
The buffer looks like
PostData received:
'\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,
\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,
\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,
\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,
\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,
\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,
\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,
\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,
\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,
\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,
\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,\x00,
\x00,\f,\x00,\x00,\x00,\b,\x00,\f,\x00,\b,\x00,\x04,\x00,\b,\x00,\x00,\x00,\x10,\x00,\x00,\x00,#,\x00,\x00,
\x00,\b,\x00,\f,\x00,\x04,\x00,\b,\x00,\b,\x00,\x00,\x00, ,\x00,\x00,\x00,\x04,\x00,\x00,\x00,\x10,\x00,\x00,
\x00,M,y,S,u,p,e,r,D,u,p,e,r,N,o,d,e,\x00,\x00,\x00,\x00,\t,\x00,\x00,\x00,D,u,m,m,y,N,o,d,e,
\x00,\x00,\x00,$,\x00,\x00,\x00,2,9,9,2,e,b,f,f,-,c,9,5,0,-,4,1,8,4,-,8,8,7,6,-,5,f,e,6,a,c,0,2,9,a,a,5,
\x00,\x00,\x00,\x00
Reading messages in TypeScript written from C++ works... (?)
flatbuffer version 1.9.0

Answer 1

As your buffer dump shows, the problem is that it contains a lot of leading zeros. A FlatBuffer while being constructed actually may contain leading zeroes (since it is being constructed in a larger buffer backwards), but asUint8Array normally takes care of trimming that down to just the array you need. So either you're not actually using asUint8Array in your actual code, or the zeroes are being pre-pended by some other code.

Answer 2

The problem is the position and how data is sent in the post. The buffer must be sliced to the position specified in ByteBuffer and the post must be send like a Blob.
builder.finish(end);
var buffer: flatbuffers.ByteBuffer = builder.dataBuffer();
var data: Uint8Array = buffer.bytes().slice(buffer.position());
this.http.post(environment.apiRoot + "hello", new Blob([data])).subscribe(
() => {},
(error) => {}
);