how to get table's size

Question

I'm receiving flat buffers objects from network, i'm using size prefixed buffers as suggested by the documentation. I'm doing verification before actually unmashalling the data. The problem is how do I know what size of the buffer should I verify?

buff = recv(sock);
Verifier v(buff.data(), ???); // buffer could be half of a buffer or two buffers, what should I supply to the 2nd argument to Verifier?
bool f = VerifySizePrefixedMessageBuffer(v);

Since the buffer received from network could be any size, it could be half of a flat buffer or two buffers, but I will need to give Verifier the size of one message buffer. Is there anyway to do this?

Answer 1

You have to notify the receiver somehow, that the buffer sending is finished, and you should do that manualy.

If you use size-prefixed buffer approach, then the size of the buffer must be sent to the data receiver in the first bytes of the buffer. When the first buffer part is received, the receiver should read the data size from the first bytes of the received buffer and do recv() until all data is transfered (until sum of all received buffer parts' sizes equals total size of the sent buffer, which you already know from the very first received buffer part).

On receiver side it should look like this:

buff = recv(sock);
uint32_t received = buff.size();
auto totalBuff = buff;
// I assume you have 4 bytes for unsigned integer buffer size prefix
while (received < 4) { // receive whole buffer size prefix data
    buff = recv(sock);
    received += buff.size();
    // append new data to total data buffer
    totalBuff.insert(totalBuff.end(), buff.begin(), buff.end());
}
uint32_t totalSize = *(reinterpret_cast<uint32_t>(totalBuff.data()); 
while (received < totalSize) {
    buff = recv(sock);
    received += buff.size();
    // append new data to total data buffer
    totalBuff.insert(totalBuff.end(), buff.begin(), buff.end());
}

// do whole buffer data processing 
ProcessSizePrefixedBuffer(totalBuff);

In this totalBuff data size will be in the first 4 bytes of the buffer. This is more pseudocode than working code, as I don't know your actual buff type and other implementation details.