python socket recv接收网络数据返回空字符串

[AlexChenLei]

在与服务器建立连接后通过
serverSocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
接收来自后台的消息，
通过socket.recv(n)接收消息，有时候会接收到空字符串，但通过抓包发现服务器端发送过来的并非空字符串。
但如果在recv前面先sleep(10)，再接收就没有这个问题， 很奇怪，感觉像是recv太快了... 我没有设置socket选项，应该默认是阻塞io，求解释。

************ 但如果sleep(2)，还是有可能出现，只是不再最开始出现。

client端代码如下：

import socket, sys, time, struct
import pdb

serverSocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

#------------------------------------------------------
# Print the struct.pack Binary content as Hex
#     16 bytes per line
#------------------------------------------------------
def PrintBinHex(s, flag):
	global dbugFlag
	# dbug < 0: accurate output
	# dbug >=0: range output 
	
	if(dbugFlag < 0):
		if(flag != -1*dbugFlag):
			return
	elif(flag < dbugFlag):
		return
			
	bytes = bytearray(s)
	cnt = 0
	for i in bytes:
		cnt=cnt+1
		print "%02x" % i,
		
		if (cnt%16) == 0:
			print "\n"
		elif (cnt%8) == 0:
			print "   ",
	print


#------------------------------------------------------
# InitFrontEnd
#     connect to the target
#------------------------------------------------------
def InitFrontEnd():
	print "Connect to host..."
	try:
		port = 123456
		host = "192.168.1.100"
		serverSocket.connect((host, port))
		
	except socket.error, e:
		print "Connection error: %s" % e
		sys.exit(1)
	else:
		print "Connection succ..."

#------------------------------------------------------
# RecvN
#     recv N bytes to target
#------------------------------------------------------
def RecvN(socket, n):
	return socket.recv(n)


#------------------------------------------------------
# Recv Fake Rsp Info from server
#     get only 1 int16 first for msg len and then msg body from server to test the connection
#------------------------------------------------------
def RecvWholeMsgFromServer():
	global serverSocket
	#print "try to get... "
	#pdb.set_trace()
	lenBuf = ''
	length = 0
	try:
		
		lenBuf = RecvN(serverSocket, 2)		
		length, = struct.unpack('!H', lenBuf)			
		bodyBuf = RecvN(serverSocket, length-2)
		Msg = lenBuf + bodyBuf
	except : 
		print "****** Except Length : %d *******\n" %length
		PrintBinHex(lenBuf, 1)
		raise


#------------------------------------------------------
# Get Msg from  Server
#     RecvLoop
#------------------------------------------------------
def RecvLoop():
	x = 1
	while x <= 100:
		RecvWholeMsgFromServer()
		x+=1
		if x % 10 == 0:
			print "Recv [%d]" % x


#------------------------------------------------------
# connect to server and get msg
#
#------------------------------------------------------
def FrontEndConnAndRecv():
	# 1. client connect BackEnd
	InitFrontEnd()
	#time.sleep(10)  <------------------------ 如果不加这里，RecvN会返回空字符串
	RecvLoop()

=========================================================
执行结果如下：

Connect to host...
Connection succ...
****** Except Length : 0 *******

00 00
Traceback (most recent call last):
File "R:\Project\MyTest1\sources\tools\MyTool1\MyTest1_SendAndRecvTest
\FrontEnd.py", line 317, in <module>
FrontEndConnAndRecv()
File "R:\Project\MyTest1\sources\tools\MyTool1\MyTest1_SendAndRecvTest
\FrontEnd.py", line 268, in FrontEndConnAndRecv
RecvLoop()
File "R:\Project\MyTest1\sources\tools\MyTool1\MyTest1_SendAndRecvTest
\FrontEnd.py", line 252, in RecvLoop
RecvWholeMsgFromServer()
File "R:\Project\MyTest1\sources\tools\MyTool1\MyTest1_SendAndRecvTest
\FrontEnd.py", line 237, in RecvWholeMsgFromServer
bodyBuf = RecvN(serverSocket, length-2)
File "R:\Project\MyTest1\sources\tools\MyTool1\MyTest1_SendAndRecvTest
\FrontEnd.py", line 105, in RecvN
return socket.recv(n)
ValueError: negative buffersize in recv

---------------------------------------------------------------------------
sleep(2)时，在中途出现读取失败：


Connect to host...
Connection succ...
Recv [10]
Recv [20]
Recv [30]
Recv [40]
Recv [50]
Recv [60]
****** Except Length : 0 *******

00 00
Traceback (most recent call last):
File "R:\Project\MyTest1\sources\tools\MyTool1\MyTest1_SendAndRecvTest
\FrontEnd.py", line 317, in <module>
FrontEndConnAndRecv()
File "R:\Project\MyTest1\sources\tools\MyTool1\MyTest1_SendAndRecvTest
\FrontEnd.py", line 268, in FrontEndConnAndRecv
RecvLoop()
File "R:\Project\MyTest1\sources\tools\MyTool1\MyTest1_SendAndRecvTest
\FrontEnd.py", line 252, in RecvLoop
RecvWholeMsgFromServer()
File "R:\Project\MyTest1\sources\tools\MyTool1\MyTest1_SendAndRecvTest
\FrontEnd.py", line 237, in RecvWholeMsgFromServer
bodyBuf = RecvN(serverSocket, length-2)
File "R:\Project\MyTest1\sources\tools\MyTool1\MyTest1_SendAndRecvTest
\FrontEnd.py", line 105, in RecvN
return socket.recv(n)
ValueError: negative buffersize in recv

[panghuhu250]

...
        lenBuf = RecvN(serverSocket, 2)       
        length, = struct.unpack('!H', lenBuf)           
        bodyBuf = RecvN(serverSocket, length-2)
        Msg = lenBuf + bodyBuf

可能的问题: 1. recv(n)有可能接收到少于n个byte的信息, 所以recv(n)后要检查接收到的信息长度, 不够的话再调用recv接收. 1. 数据流格式你理解的对不对?数据流是否正确? (听起来有点冒犯, 但这确实是一个问题, 比如length好象表示信息的长度, 它包括length所用的两个byte吗?)

[jeky_zhang2013]

还没有尝试分开 接收，帮顶下

[The_Third_Wave]

mark，soket玩的少

[angel_su]

空字串unpack肯定错，所以开始确实是收了2个字符，只是解析成整形0，至于什么原因就不知道了...

[AlexChenLei]

引用 1 楼 panghuhu250 的回复:

...
        lenBuf = RecvN(serverSocket, 2)       
        length, = struct.unpack('!H', lenBuf)           
        bodyBuf = RecvN(serverSocket, length-2)
        Msg = lenBuf + bodyBuf

可能的问题: 1. recv(n)有可能接收到少于n个byte的信息, 所以recv(n)后要检查接收到的信息长度, 不够的话再调用recv接收. 1. 数据流格式你理解的对不对?数据流是否正确? (听起来有点冒犯, 但这确实是一个问题, 比如length好象表示信息的长度, 它包括length所用的两个byte吗?)

谢谢回复，问题原因就是：”recv(n)有可能接收到少于n个byte的信息“，对python不是很熟，理解错误，我之前以为python的recv(n)会在收到n个bytes才返回。现象的原因是因为我的每个package都是[msglen:2 | data:N]，并且data区全0x0，由于前一次根据msglen读取data的时候，并没有完整读完所有len的data区，我就直接认为data区已经读完，紧接着读取下一个package的len时，实际上是data区的内容，这时len的内容即为data区中的全0，导致下一个data的len计算为负数。将RecvN调整为：

#------------------------------------------------------
# RecvN
#     recv N bytes to target
#------------------------------------------------------
def RecvN(socket, n):	
	totalContent = ''
	totalRecved = 0
	while totalRecved < n:
		onceContent = socket.recv(n - totalRecved)		
		totalContent += onceContent
		totalRecved = len(totalContent)

	return totalContent