why sys socket recv function don't fill data but return bytes length?

Question

I'm writing a c++ client. The client connects to the server with TCP protocol successfully and sends data too. I wrote the code below to receive data :

char data[9];
int received_size = recv(fd, data, 9, flags);
std::string str{ data };
// str.empty() is true

Which flags is MSG_NOSIGNAL.

The problem is that after execution of this line the received_size is 9 but the data length is zero.

Answer 1

If recv is returning a value, then that is the number of bytes received.

The issue is that you're using the wrong functions to determine the data that you're receiving. The std::string constructor that you're using will stop at the first null byte, irrespective of the number of actual bytes recv returned.

Instead, use the other version of the std::string constructor that takes a byte length:

char data[9];
int received_size = recv(fd, data, 9, flags);
std::string str(data, received_size);

Answer 2

You are expecting the received data to be zero-terminated. Which is a wrong expectation because it can receive incomplete data. The correct way is to use received_size (after error checking has been done):

std::string str(data, received_size);

That still is not enough in real-world applications. You need to delimit messages somehow.