一执桌、select原理

網(wǎng)絡(luò)通信被Unix系統(tǒng)抽象為文件的讀寫，通常是一個(gè)設(shè)備芜赌，由設(shè)備驅(qū)動(dòng)程序提供仰挣，驅(qū)動(dòng)可以知道自身的數(shù)據(jù)是否可用。支持阻塞操作的設(shè)備驅(qū)動(dòng)通常會實(shí)現(xiàn)一組自身的等待隊(duì)列较鼓，如讀/寫等待隊(duì)列用于支持上層(用戶層)所需的block或non-block操作椎木。設(shè)備的文件的資源如果可用（可讀或者可寫）則會通知進(jìn)程，反之則會讓進(jìn)程睡眠博烂，等到數(shù)據(jù)到來可用的時(shí)候，再喚醒進(jìn)程漱竖。

這些設(shè)備的文件描述符被放在一個(gè)數(shù)組中禽篱，然后select調(diào)用的時(shí)候遍歷這個(gè)數(shù)組，如果對于的文件描述符可讀則會返回改文件描述符馍惹。當(dāng)遍歷結(jié)束之后躺率，如果仍然沒有一個(gè)可用設(shè)備文件描述符玛界，select讓用戶進(jìn)程則會睡眠，直到等待資源可用的時(shí)候在喚醒悼吱，遍歷之前那個(gè)監(jiān)視的數(shù)組慎框。每次遍歷都是線性的。

二后添、select方法

Python的select()方法直接調(diào)用操作系統(tǒng)的IO接口笨枯，它監(jiān)控sockets,open files, and pipes(所有帶fileno()方法的文件句柄)何時(shí)變成readable 和writeable, 或者通信錯(cuò)誤，select()使得同時(shí)監(jiān)控多個(gè)連接變的簡單遇西，并且這比寫一個(gè)長循環(huán)來等待和監(jiān)控多客戶端連接要高效馅精，因?yàn)閟elect直接通過操作系統(tǒng)提供的C的網(wǎng)絡(luò)接口進(jìn)行操作，而不是通過Python的解釋器粱檀。

示例(只支持Unix洲敢，不支持Windows)：
服務(wù)端：

import select
import socket
import sys
import Queue

# Create a TCP/IP socket
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.setblocking(0)
 
# Bind the socket to the port
server_address = ('localhost', 10000)
print >>sys.stderr, 'starting up on %s port %s' % server_address
server.bind(server_address)
 
# Listen for incoming connections
server.listen(5)

select()方法接收并監(jiān)控3個(gè)通信列表， 第一個(gè)是所有的輸入的data,就是指外部發(fā)過來的數(shù)據(jù)茄蚯，第2個(gè)是監(jiān)控和接收所有要發(fā)出去的data(outgoing data),第3個(gè)監(jiān)控錯(cuò)誤信息压彭，接下來我們需要?jiǎng)?chuàng)建2個(gè)列表來包含輸入和輸出信息來傳給select().

readable , writable , exceptional = select.select(inputs, outputs, inputs, timeout)

所有客戶端的進(jìn)來的連接和數(shù)據(jù)將會被server的主循環(huán)程序放在上面的list中處理，我們現(xiàn)在的server端需要等待連接可寫（writable)之后才能過來渗常，然后接收數(shù)據(jù)并返回(因此不是在接收到數(shù)據(jù)之后就立刻返回)哮塞，因?yàn)槊總€(gè)連接要把輸入或輸出的數(shù)據(jù)先緩存到queue里，然后再由select取出來再發(fā)出去

當(dāng)你把inputs,outputs,exceptional(這里跟inputs共用)傳給select()后凳谦，它返回3個(gè)新的list忆畅，我們上面將他們分別賦值為readable,writable,exceptional, 所有在readable list中的socket連接代表有數(shù)據(jù)可接收(recv),所有在writable list中的存放著你可以對其進(jìn)行發(fā)送(send)操作的socket連接，當(dāng)連接通信出現(xiàn)error時(shí)會把error寫到exceptional列表中尸执。

Readable list 中的socket 可以有3種可能狀態(tài)家凯，第一種是如果這個(gè)socket是main "server" socket,它負(fù)責(zé)監(jiān)聽客戶端的連接，如果這個(gè)main server socket出現(xiàn)在readable里如失，那代表這是server端已經(jīng)ready來接收一個(gè)新的連接進(jìn)來了绊诲，為了讓這個(gè)main server能同時(shí)處理多個(gè)連接，在下面的代碼里褪贵，我們把這個(gè)main server的socket設(shè)置為非阻塞模式掂之。

# Handle inputs
for s in readable:
 
    if s is server:
        # A "readable" server socket is ready to accept a connection
        connection, client_address = s.accept()
        print >>sys.stderr, 'new connection from', client_address
        connection.setblocking(0)
        inputs.append(connection)
 
        # Give the connection a queue for data we want to send
        message_queues[connection] = Queue.Queue()

第二種情況是這個(gè)socket是已經(jīng)建立了的連接，它把數(shù)據(jù)發(fā)了過來脆丁，這個(gè)時(shí)候你就可以通過recv()來接收它發(fā)過來的數(shù)據(jù)世舰，然后把接收到的數(shù)據(jù)放到queue里，這樣你就可以把接收到的數(shù)據(jù)再傳回給客戶端了槽卫。

else:
     data = s.recv(1024)
     if data:
         # A readable client socket has data
         print >>sys.stderr, 'received "%s" from %s' % (data, s.getpeername())
         message_queues[s].put(data)
         # Add output channel for response
         if s not in outputs:
             outputs.append(s)

第三種情況就是這個(gè)客戶端已經(jīng)斷開了跟压，所以你再通過recv()接收到的數(shù)據(jù)就為空了，所以這個(gè)時(shí)候你就可以把這個(gè)跟客戶端的連接關(guān)閉了歼培。

else:
    # Interpret empty result as closed connection
    print >>sys.stderr, 'closing', client_address, 'after reading no data'
    # Stop listening for input on the connection
    if s in outputs:
        outputs.remove(s)  #既然客戶端都斷開了震蒋，我就不用再給它返回?cái)?shù)據(jù)了茸塞，所以這時(shí)候如果這個(gè)客戶端的連接對象還在outputs列表中，就把它刪掉
    inputs.remove(s)    #inputs中也刪除掉
    s.close()           #把這個(gè)連接關(guān)閉掉
 
    # Remove message queue
    del message_queues[s] 


else:
    # Interpret empty result as closed connection
    print >>sys.stderr, 'closing', client_address, 'after reading no data'
    # Stop listening for input on the connection
    if s in outputs:
        outputs.remove(s)  #既然客戶端都斷開了查剖，我就不用再給它返回?cái)?shù)據(jù)了钾虐，所以這時(shí)候如果這個(gè)客戶端的連接對象還在outputs列表中，就把它刪掉
    inputs.remove(s)    #inputs中也刪除掉
    s.close()           #把這個(gè)連接關(guān)閉掉
 
    # Remove message queue
    del message_queues[s]  

對于writable list中的socket笋庄，也有幾種狀態(tài)效扫，如果這個(gè)客戶端連接在跟它對應(yīng)的queue里有數(shù)據(jù)，就把這個(gè)數(shù)據(jù)取出來再發(fā)回給這個(gè)客戶端无切，否則就把這個(gè)連接從output list中移除荡短，這樣下一次循環(huán)select()調(diào)用時(shí)檢測到outputs list中沒有這個(gè)連接，那就會認(rèn)為這個(gè)連接還處于非活動(dòng)狀態(tài)

# Handle outputs
for s in writable:
    try:
        next_msg = message_queues[s].get_nowait()
    except Queue.Empty:
        # No messages waiting so stop checking for writability.
        print >>sys.stderr, 'output queue for', s.getpeername(), 'is empty'
        outputs.remove(s)
    else:
        print >>sys.stderr, 'sending "%s" to %s' % (next_msg, s.getpeername())
        s.send(next_msg)

最后哆键，如果在跟某個(gè)socket連接通信過程中出了錯(cuò)誤掘托，就把這個(gè)連接對象在inputs\outputs\message_queue中都刪除，再把連接關(guān)閉掉

# Handle "exceptional conditions"
for s in exceptional:
    print >>sys.stderr, 'handling exceptional condition for', s.getpeername()
    # Stop listening for input on the connection
    inputs.remove(s)
    if s in outputs:
        outputs.remove(s)
    s.close()
 
    # Remove message queue
    del message_queues[s]

客戶端

下面的這個(gè)是客戶端程序展示了如何通過select()對socket進(jìn)行管理并與多個(gè)連接同時(shí)進(jìn)行交互

import socket
import sys
 
messages = [ 'This is the message. ',
             'It will be sent ',
             'in parts.',
             ]
server_address = ('localhost', 10000)
 
# Create a TCP/IP socket
socks = [ socket.socket(socket.AF_INET, socket.SOCK_STREAM),
          socket.socket(socket.AF_INET, socket.SOCK_STREAM),
          ]

# Connect the socket to the port where the server is listening
print >>sys.stderr, 'connecting to %s port %s' % server_address
for s in socks:
    s.connect(server_address)

接下來通過循環(huán)通過每個(gè)socket連接給server發(fā)送和接收數(shù)據(jù)

for message in messages:
 
    # Send messages on both sockets
    for s in socks:
        print >>sys.stderr, '%s: sending "%s"' % (s.getsockname(), message)
        s.send(message)
 
    # Read responses on both sockets
    for s in socks:
        data = s.recv(1024)
        print >>sys.stderr, '%s: received "%s"' % (s.getsockname(), data)
        if not data:
            print >>sys.stderr, 'closing socket', s.getsockname()

服務(wù)端完整代碼

#_*_coding:utf-8_*_
 
import select
import socket
import sys
import queue
 
# Create a TCP/IP socket
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.setblocking(False)
 
# Bind the socket to the port
server_address = ('localhost', 10000)
print(sys.stderr, 'starting up on %s port %s' % server_address)
server.bind(server_address)
 
# Listen for incoming connections
server.listen(5)
 
# Sockets from which we expect to read
inputs = [ server ]
 
# Sockets to which we expect to write
outputs = [ ]
 
message_queues = {}
while inputs:
 
    # Wait for at least one of the sockets to be ready for processing
    print( '\nwaiting for the next event')
    readable, writable, exceptional = select.select(inputs, outputs, inputs)
    # Handle inputs
    for s in readable:
 
        if s is server:
            # A "readable" server socket is ready to accept a connection
            connection, client_address = s.accept()
            print('new connection from', client_address)
            connection.setblocking(False)
            inputs.append(connection)
 
            # Give the connection a queue for data we want to send
            message_queues[connection] = queue.Queue()
        else:
            data = s.recv(1024)
            if data:
                # A readable client socket has data
                print(sys.stderr, 'received "%s" from %s' % (data, s.getpeername()) )
                message_queues[s].put(data)
                # Add output channel for response
                if s not in outputs:
                    outputs.append(s)
            else:
                # Interpret empty result as closed connection
                print('closing', client_address, 'after reading no data')
                # Stop listening for input on the connection
                if s in outputs:
                    outputs.remove(s)  #既然客戶端都斷開了籍嘹，我就不用再給它返回?cái)?shù)據(jù)了闪盔，所以這時(shí)候如果這個(gè)客戶端的連接對象還在outputs列表中，就把它刪掉
                inputs.remove(s)    #inputs中也刪除掉
                s.close()           #把這個(gè)連接關(guān)閉掉
 
                # Remove message queue
                del message_queues[s]
    # Handle outputs
    for s in writable:
        try:
            next_msg = message_queues[s].get_nowait()
        except queue.Empty:
            # No messages waiting so stop checking for writability.
            print('output queue for', s.getpeername(), 'is empty')
            outputs.remove(s)
        else:
            print( 'sending "%s" to %s' % (next_msg, s.getpeername()))
            s.send(next_msg)
    # Handle "exceptional conditions"
    for s in exceptional:
        print('handling exceptional condition for', s.getpeername() )
        # Stop listening for input on the connection
        inputs.remove(s)
        if s in outputs:
            outputs.remove(s)
        s.close()
 
        # Remove message queue
        del message_queues[s]

客戶端完整代碼

import socket
import sys
 
messages = [ 'This is the message. ',
             'It will be sent ',
             'in parts.',
             ]
server_address = ('localhost', 10000)
 
# Create a TCP/IP socket
socks = [ socket.socket(socket.AF_INET, socket.SOCK_STREAM),
          socket.socket(socket.AF_INET, socket.SOCK_STREAM),
          ]
 
# Connect the socket to the port where the server is listening
print >>sys.stderr, 'connecting to %s port %s' % server_address
for s in socks:
    s.connect(server_address)
 
for message in messages:
 
    # Send messages on both sockets
    for s in socks:
        print >>sys.stderr, '%s: sending "%s"' % (s.getsockname(), message)
        s.send(message)
 
    # Read responses on both sockets
    for s in socks:
        data = s.recv(1024)
        print >>sys.stderr, '%s: received "%s"' % (s.getsockname(), data)
        if not data:
            print >>sys.stderr, 'closing socket', s.getsockname()
            s.close()

[本文轉(zhuǎn)自:http://www.cnblogs.com/alex3714/p/4372426.html#top]