hololens socket can't receive full packet - c++

Question

A simple summary:
boost asio server, send a video frame 720x768x3 with simple compression
packet size is 186476, not really to much
nothing to complicated, anyway, if i test it in the hololens emulator or on the physical device
// uint32_t data_length == size of frame 'data_ptr'
enum max_length = sizeof(uint32_t);
memcpy(data_, &data_length, max_length);
auto length = boost::asio::write(*socket_, boost::asio::buffer(data_, max_length), e);
length = boost::asio::write(*socket_, boost::asio::buffer(data_ptr, data_length), e);
// receive
char data_[max_length] = { 0 };
fd_set readSet;
FD_ZERO(&readSet);
FD_SET(_socket, &readSet);
timeval timeout;
timeout.tv_sec = 0; // Zero timeout (poll)
timeout.tv_usec = 0;
auto result = select(_socket, &readSet, nullptr, nullptr, &timeout);
if (result == 0)
continue;
result = recv(_socket, data_, max_length, 0);
if (result == SOCKET_ERROR) {
closesocket(_socket);
_socket = INVALID_SOCKET;
break;
}
uint32_t msg_size(0);
memcpy(&msg_size, data_, max_length);
std::vector<char> vec(msg_size);
result = recv(_socket, &vec[0], msg_size, 0);
while (result < msg_size) {
result += recv(_socket, &vec[result], msg_size - result, 0);
}
but the hololens can't receive the full packet, i try it also with the .net streamsockets, same result. it tried a few times and then recv blocks in the while loop and doesn't receive anymore.
anyone, any idea? is it an uwp app problem, that i can't receive 'bigger' packets, or get it killed because it takes too long?

Answer 1

You have two main problems:
First, you need to check the return value of recv for errors. If it returns 0 or -1, you need to handle that.
Second, you ignore all the data you received from your first call to recv. You set msg_size to zero when it should be result minus however many bytes the length took.
I would suggest writing a function that reads exactly the specified number of bytes, checking for errors. Call it first to receive four bytes and check if it returned an error. Then call it to receive the number of bytes indicated by the length data you received.
Smaller problems include:
What if the first recv only returns one byte?
What if the way your platform stores 32-bit integers isn't the same as the way the emulator sends it?