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Linux Networking & TCP/IP for Developers · レッスン

Socket API入門

ソケットの種類やアドレス、一般的なAPI呼び出しを含む、ソケットプログラミングの基本概念を学びます。

「Socket API入門」はCoddyKit上の無料Linux Networking & TCP/IP for Developersレッスンです。 これはレッスン1/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはLinux Networking & TCP/IP for Developers学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Linux Networking & TCP/IP for Developersコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

What are Network Sockets?

Welcome to socket programming! A socket is a crucial concept in network communication. Think of it as one endpoint of a two-way communication link between two programs running on the network.

It's like a phone jack on a wall. You plug a phone into it to connect to the phone network. Similarly, programs use sockets to connect and talk over a computer network.

Python's Socket Module

Python makes socket programming accessible through its built-in socket module. This module provides all the necessary functions and classes to create and manage sockets.

You'll typically import it at the beginning of your script. Let's see how to get started:

import socket

# Now you can use socket functions!

Socket Address Families

Before creating a socket, we need to specify its address family. This determines the type of addresses the socket can work with.

  • socket.AF_INET: Used for IPv4 addresses (e.g., 192.168.1.1). This is the most common.
  • socket.AF_INET6: Used for IPv6 addresses.
  • socket.AF_UNIX: Used for communication between processes on the same machine.

We'll primarily use AF_INET for standard internet communication.

Socket Types: Stream vs. Datagram

Next, we choose the socket type, which defines the communication style.

  • socket.SOCK_STREAM: This type uses TCP (Transmission Control Protocol). It's reliable, ordered, and connection-oriented, like a phone call. Data arrives exactly as sent.
  • socket.SOCK_DGRAM: This type uses UDP (User Datagram Protocol). It's connectionless and less reliable, like sending postcards. Data might arrive out of order or not at all.

Most common applications, especially web browsing, use SOCK_STREAM.

Creating Your First Socket

Now let's put it together and create a basic socket object. We'll specify the address family (IPv4) and the socket type (TCP stream).

This line creates an inactive socket that's ready to be configured for either client or server roles.

import socket

# Create a TCP/IP socket
my_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

print(f"Socket created: {my_socket}")
# Don't forget to close sockets when done!
my_socket.close()

Binding a Socket (Server Side)

For a server, the socket needs an address to listen on. This is called binding. You assign an IP address and a port number to your socket.

Clients will use this address and port to connect to your server. A port is a number that identifies a specific application or service on a host.

import socket

HOST = '127.0.0.1'  # Localhost
PORT = 65432        # Port to listen on (> 1023 for non-privileged)

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    print(f"Socket bound to {HOST}:{PORT}")
    # The socket is now bound, but not yet listening or accepting.

Listening for Connections

After binding, a server socket must listen for incoming client connections. The listen() method puts the socket into server mode.

The argument to listen() is the backlog, which is the maximum number of unaccepted connections that the system will allow before refusing new connections.

import socket

HOST = '127.0.0.1'
PORT = 65432

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    s.listen(1) # Allow 1 pending connection
    print(f"Server listening on {HOST}:{PORT}")
    # Now the server is ready to accept a connection.

Accepting a Connection

When a client tries to connect, the server uses the accept() method. This method blocks (waits) until a client connects.

It then returns a new socket object (conn) representing the connection to the client, and the client's address (addr). All communication with the client happens via this new socket.

import socket

HOST = '127.0.0.1'
PORT = 65432

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    s.listen(1)
    print("Waiting for a client connection...")
    # This part would typically be in a loop in a real server
    conn, addr = s.accept()
    with conn:
        print(f"Connected by {addr}")
        # Now 'conn' can be used to send/receive data to/from the client.

Connecting to a Server (Client Side)

A client socket doesn't bind; instead, it uses the connect() method to establish a connection with a server that is listening on a specific IP address and port.

Once connected, the client can immediately start sending and receiving data. Data is always exchanged as bytes.

import socket

HOST = '127.0.0.1'
PORT = 65432

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    try:
        s.connect((HOST, PORT))
        print(f"Connected to server at {HOST}:{PORT}")
        s.sendall(b'Hello from client!')
        data = s.recv(1024) # Receive up to 1024 bytes
        print(f"Received: {data.decode()}")
    except ConnectionRefusedError:
        print("Error: Server not running or not listening.")

Sending and Receiving Data

Once a connection is established, data can be exchanged using sendall() and recv().

  • socket.sendall(data): Sends all of the byte-like data to the connected socket. It keeps sending until all data is sent or an error occurs.
  • socket.recv(bufsize): Receives data from the socket. bufsize is the maximum number of bytes to be received at once. It returns an empty bytes object (b'') when the connection is closed.

Socket API Quick Check

Which of the following statements about Python's socket module and socket types is TRUE?

Recap: Intro to Sockets

Great job! In this lesson, you've taken your first steps into socket programming with Python. You learned:

  • What a network socket is and its role in communication.
  • About Python's socket module.
  • The difference between address families (AF_INET, AF_INET6) and socket types (SOCK_STREAM for TCP, SOCK_DGRAM for UDP).
  • The basic API calls: socket() for creation, bind() and listen() for servers, and connect() for clients.
  • How accept() handles incoming connections and how sendall()/recv() work.

Next, we'll dive deeper into building full TCP client-server applications!

よくある質問

「Socket API入門」レッスンは無料ですか？

はい。「Socket API入門」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Linux Networking & TCP/IP for Developersコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Linux Networking & TCP/IP for Developersコースには全4レッスンが含まれています。

「Socket API入門」で何を学びますか？

ソケットの種類やアドレス、一般的なAPI呼び出しを含む、ソケットプログラミングの基本概念を学びます。 ブラウザで直接実行するハンズオンコードでLinux Networking & TCP/IP for Developersを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Linux Networking & TCP/IP for Developersを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのLinux Networking & TCP/IP for Developersは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン1/4です。

「Socket API入門」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このLinux Networking & TCP/IP for Developersレッスンでコードを書いて実行できますか？

はい。すべてのLinux Networking & TCP/IP for Developersレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

このコースのすべてのレッスン

  1. Socket API入門
  2. TCPクライアント・サーバーソケット
  3. UDPクライアント・サーバーソケット
  4. ノンブロッキングソケットとselect()
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