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c++clinuxsocketsc++11

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

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.

Solution

  • 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);