How Can I Reliably Read Exactly N Bytes From A TCP Socket?
Solution 1:
The solution is to use recv_into
and a memoryview
. Python allows to pre-allocate a modifiable bytearray
that can be passed to recv_into
. But you cannot receive data into a slice of the bytearray, because the slice would be a copy. But a memoryview
allows to recieve multiple fragments into the same bytearray
:
def readnbyte(sock, n):
buff = bytearray(n)
pos = 0
while pos < n:
cr = sock.recv_into(memoryview(buff)[pos:])
if cr == 0:
raise EOFError
pos += cr
return buff
Solution 2:
You can use socket.makefile() to wrap the socket in a file-like object. Then reads will return exactly the amount requested, unless the socket is closed where it can return the remainder. Here's an example:
server.py
from socket import *
sock = socket()
sock.bind(('',5000))
sock.listen(1)
with sock:
client,addr = sock.accept()
with client, client.makefile() as clientfile:
while True:
data = clientfile.read(5)
if not data: break
print(data)
client.py
from socket import *
import time
sock = socket()
sock.connect(('localhost',5000))
with sock:
sock.sendall(b'123')
time.sleep(.5)
sock.sendall(b'451234')
time.sleep(.5)
sock.sendall(b'51234')
Server Output
12345
12345
1234
Solution 3:
A minor addition to @Serge's answer which returns an IncompleteReadError
(which is a subclass of EOFError
). This contains a partial
attribute containing the partially read data.
import socket
from asyncio import IncompleteReadError
def readexactly(sock: socket.socket, num_bytes: int) -> bytes:
buf = bytearray(num_bytes)
pos = 0
while pos < num_bytes:
n = sock.recv_into(memoryview(buf)[pos:])
if n == 0:
raise IncompleteReadError(bytes(buf[:pos]), num_bytes)
pos += n
return bytes(buf)
Usage:
try:
print(readexactly(sock, 26))
except IncompleteReadError as e:
print(f"Only read {len(e.partial)} out of {e.expected} bytes. :(")
print(e.partial)
Example output upon only reading 5 bytes b"ABCDE"
:
Only read 5 out of 26 bytes. :(
b'ABCDE'
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