Python和UDP监听

Question

我有一个应用程序,软件定义无线电,在一个端口上广播UDP数据包,告诉听众已经设置了什么频率和解调模式(等等).

我编写了一个演示python客户端(下面的代码)来监听端口,并将相应数据包中的信息转储到控制台.

这些都在OSX 10.6,Snow Leopard下运行.他们在那里工作.

我的问题是:Python应用程序必须在无线电应用程序之前启动,或者在绑定期间声称端口已在使用(ERRNO 47),我不明白为什么.无线电应用正在广播UDP; 当然,我想容纳多个听众 - 这就是广播的想法,或者至少,所以我想.

所以这里是Python代码(由于堆栈溢出真的很愚蠢"make-it-code"缩进,缩进有点混乱,但我保证你没关系):

#!/usr/bin/python
import select, socket 

# AA7AS - for SdrDx UDP broadcast

# this is a sample python script that captures the UDP messages
# that are coming from SdrDx. SdrDx tells you what frequency and
# mode it has been set to. This, in turn, would be used to tell
# another radio to tune to that frequency and mode.

# UDP packet from SdrDx is zero terminated, but receiving the
# packet makes it seem like the string contains zeros all the
# way out to the 1024th character. This function extracts a
# python string up to the point where it hits the first zero,
# then returns that string.
# -----------------------------------------------------------
def zeroterm(msg):
    counter = 0;
    for c in msg:
        if ord(c) != 0:
            counter += 1
    strn = msg[:counter]
    return strn

port = 58083        # port where we expect to get a msg
bufferSize = 1024   # room for message

# Create port to listen upon
# --------------------------
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
try:
    s.bind(('', port))
except:
    print 'failure to bind'
    s.close()
    raise
    s.setblocking(0)

# Listen for messages
# -------------------
looping = True

while looping:
try:
    result = select.select([s],[],[])
except: # you can kill the client here with control-c in its shell
    s.close()   # must close socket
    print 'Closing, exception encountered during select' # warn
    raise SystemExit    # and quit
msg = result[0][0].recv(bufferSize) # actually fetch the UDP data
msg = zeroterm(msg) # convert content into python string

# in next line, [] contain optional repeats
# message format is keyword:data[|keyword:data]
# where from 1...n keyword:data pairs may appear, up to 1024 bytes
# ----------------------------------------------------------------

try:
    msgs = msg.split('|')       # can be more than one message in packet
except: # failed to split
    msgs = []                   # on the other hand, can just be one. :)
    msgs.append(msg)            # so build array with that one.
for m in msgs:                  # now, for every message we have
    keyw,data = m.split(':')    # break into keyword and data
    print keyw + "-->" + data   # you'd do something with this
    if keyw == "closing":       # Our client terminates when SdrDx does
        looping = False         # loop stops

 s.close()                          # must close socket
print 'Normal termination'

作为参考,这是发送UDP消息的Qt代码:

建立:

bcast = new QHostAddress("192.168.1.255");
if (bcast)
{
    udpSocketSend = new QUdpSocket(0);
    if (udpSocketSend)
    {
        udpSocketSend->bind(*bcast, txudp);
    }
}

广播:

if (udpSocketSend)
{
    QByteArray *datagram = new QByteArray(1024,0);  // datagram is full of zeroes
    strcpy(datagram->data(),msg);                   // except where text message is in it at beginning
    udpSocketSend->writeDatagram(*datagram, QHostAddress::Broadcast,txudp); // send
}

Answer 1

您正在尝试绑定同一个端口,两次.

您在发件人中绑定一次:

if (udpSocketSend)
{
    udpSocketSend->bind(*bcast, txudp);
}

并再次在接收器

s.bind(('', port))

由于它们在同一台机器上运行,因此出现错误.

除非您关心源端口是什么,否则您不需要bind()在发送方上,只需发送它,堆栈将选择适当的输出源端口号.对于发送方,当您传输UDP数据报时,您指定destination(udpSocketSend->writeDatagram(...)),并bind实际确定传出数据报的来源.如果你不这样做bind,那很好,堆栈将为你分配一个端口.

如果您确实关心源端口是什么,那么我建议您为传出源端口和传入目标端口使用不同的端口号.然后你就可以毫无问题地绑定发送者和接收者.

最后,可以选择设置SO_REUSEADDR套接字选项(使用您正在使用的任何语言).如果要在同一台计算机上运行多个客户端,则必须这样做,因为所有客户端都必须绑定到同一个地址.但是,我不确定这个套接字选项是否是跨平台的(*nix工作正常),我认为上述建议更好.