Using byte array (cpp) in GRPC - c++

Question

I am having a lot of trouble with GRPC when using byte array. This is by .proto
message myType {
int32 format = 1;
bytes data = 2;
}
I am using CPP for Server implementation and Java for Client. Using ByteString in Java is a breeze but cannot deserialize in CPP (byte[] being changed from what was being sent from Java).
buffer is a byte[] byte buffer[<large_size>] And I'm converting the byte array (it's an image) into a smaller byte array, and it's crashing when trying to convert the byte[] received from grpc. The conversion function in CPP is good as I used it with the same image before using GRPC
This is the deserialization code for CPP. Here "req" is a myType object, and buffer is a byte[]
myFormat = req->format();
dataLen = req->data().length();
memcpy(buffer, req->data().c_str(), dataLen);
From what I understand, req->data() is in cpp std::string format

Answer 1

On the client side, you should pass both the parameter and its length.
parameter.set_framemat(mat, 12);

Answer 2

Do check if the length of the array at the server side is not zero. Note that bytes is char array and grpc is de marshalling it as string. So if say the array is filled with null characters the data length comes as zero.
I was trying a similar use case
Proto file
message Parameters {
bytes framemat = 16;
};
Client Snippet
const char mat[12] = {0}
parameter.set_framemat(mat);
stream->Write(parameter);
Server Snippet
std::thread reader([&]() {
::nokia::nas::Parameters request;
while (stream->Read(&request))
{
//get the params
grpc::string mat = request.framemat();
logger->info(" Length of Bytes= {} , mat.length()}
Output was zero!
So I changed the client input to check with some char strings and sure enough the data length was coming as 4 at the server side; because this time it was a valid string and string length matched.
const char mat[12] = "sdsd";
Better way is to specify in the proto the data as well as the data length
message StreamBytes {
bytes data_bytes =1;
int32 data_length = 2;
};