recv() in FTP client

Question

I am trying to create FTP client in C++ using BSD sockets, I can create socket, get welcome message, send user name, but I can not move on. Here is my dubug output:

Socket created
Connected

220 server - welcome message

Msg: USER anonymous

Received bytes: 75
331 Anonymous login ok, send your complete email address as your password

Then I am stucked at this point for 5 minutes, server terminates connection and I finally get response:

Received bytes: 61
331 Anonymous login ok, send your complete email address as your password
421 Login timeout (300 seconds): closing control connection
our password

Received bytes: 0
Msg: PASS password

Received bytes: 0
Msg: SYST

Received bytes: 0
Msg: PASV

Received bytes: 0
Msg: LIST

Received bytes: 0
Msg: QUIT

Received bytes: 0
Program ended with exit code: 0

Here is my function for sending and receiving messages. I put there socket and message for FTP server (for example USER anonymous\\r\\n )

void communication(int sock, const char *msg) {
string response;
char server_reply[2000];
printf("Msg: %s\n", msg);
send(sock, msg, strlen(msg), 0);
//Receive a reply from the server
int rval;
do
{
    rval = (int) recv(sock , server_reply , 2000 , 0);
    printf("Received bytes: %d\n", rval);
    if (rval <= 0)
    {
        break;
    }
    else {
        response.append(server_reply);
        puts(response.c_str());
    }
}
while (true);

My program looks like this:

//Receive a welcome message from the server
if( recv(sock , server_reply , 2000 , 0) < 0) {
    puts("recv failed");
}

puts(server_reply);
memset(&server_reply[0], 0, sizeof(server_reply));

const char *usernameMsg = "USER anonymous\r\n";`
communication(sock, usernameMsg);`
const char *passwdMsg = "PASS email@email.com\r\n";`
communication(sock, passwdMsg);
communication(sock, "SYST\r\n");
communication(sock, "PASV\r\n");
communication(sock, "LIST\r\n");
communication(sock, "QUIT\r\n");

Could you tell me, what's wrong please? Thank you

Answer 1

You obviously expect the recv to return 0, when there's no more response from the server.

But that's wrong for blocking mode. In the blocking mode recv will always wait until there are some data. Note that the socket is a generic interface to a connection. The connection is a just a stream of data. There's no end-of-the-message mark. So the socket cannot magically find out that a complete response was received from the FTP server. It's up to you. It's you task to call recv only until you receive the CRLF sequence, which signals the end-of-the-response in the FTP protocol. And it's actually even more complicated as the FTP response can be multi-line. Read the FTP specification.

---

Your primary problem now, is that your first call to the communication() function never finishes.

Once you read the 331 response to the USER command, you start waiting for the next message (second round [or even later, in the rare case the 331 response is longer than the 2000 characters] of the while loop in the first call of the communication() function). But the server is actually waiting for your next command, which you never send. So the server eventually gives up, sends you the 421 response and disconnects you.