Question

Hi I am reading TLPI (The Linux Programming Interface), I have a question about connect().

As I understand, connect() will immediately return if the pending connection numbers of listen() doesn’t reach “backlog”.
And it will blocks otherwise. (according to figure 56-2)

But for TCP socket, it will always block until accept() on server side is called (according to figure 61-5).

Am I correct?
Because I saw that in the example code (p.1265), it calls listen() to listen to a specific port and then calls connect() to that port BEFORE calling accept().

So connect() blocks forever in this case, doesn’t it?

Thanks!!

Answer 1

There’s hardly any “immediately” regarding networking, stuff can be lost on the way, and an operation that should be performed immediately in theory might not do so in practice, and in any case there’s the end to end transmission time.

However

• connect() on a TCP socket is a blocking operation unless the socket descriptor is put into non-blocking mode.

• The OS takes care of the TCP handshake, when the handshake is finished, connect() returns. (that is,
  connect() does not block until the other end calls accept())

• A successful TCP handshake will be queued to the server application, and can be accept()’ed any time later.