轉自segment fault
TCP server 可以，TCP client 也可以。一個套接字只能建立一個連線，無論對於 server 還是 client。

注意報錯訊息是：

[Errno 106] (EISCONN) Transport endpoint is already connected

man 2 connect 說得很清楚了：

Generally, connection-based protocol sockets may successfully connect() only once; connectionless protocol sockets may use connect() multiple times to change their association.

就是說，TCP 套接字最多隻能呼叫 connect 一次。那麼，你的監聽套接字呼叫 connect 了幾次？

來點有意思的。

一個套接字不能連線兩次，並不代表一個本地地址不能用兩次，看！加粗文字加粗文字

>>> import socket
>>> s = socket.socket()
# since Linux 3.9, 見 man 7 socket
>>> s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)
>>> s2 = socket.socket
 
()
>>> s2.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)
>>> s.bind(('127.0.0.1', 12345))
>>> s2.bind(('127.0.0.1', 12345))
# 都可以使用同一本地地址來連線哦
>>> s.connect(('127.0.0.1', 80))
>>> s2.connect(('127.0.0.1', 4321))

>>> netstat -npt | grep 12345
(Not all processes could be identified, non-owned process info
 will not
 
 be shown, you would have to be root to see it all.)
tcp        0      0 127.0.0.1:4321          127.0.0.1:12345         ESTABLISHED 18284/python3
tcp        0      0 127.0.0.1:12345         127.0.0.1:4321          ESTABLISHED 4568/python3
tcp        0      0 127.0.0.1:80            127.0.0.1:12345         ESTABLISHED -
tcp        0      0 127.0.0.1:12345         127.0.0.1:80            ESTABLISHED 45