[issue21819] Remaining buffer from socket isn't available anymore after calling socket.recv the first time

[Charles-François Natali]

Charles-François Natali added the comment:

> I'm wondering how would it be possible then to fetch packets of an unknown 
> size without using an extremely big buffer.

IP packets are limited to 64K, so just pass a 64K buffer, that's not
"extremely big".
If you really wanted to avoid this, you could try the FIONREAD ioctl,
but I wouldn't advise it.

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[issue21819] Remaining buffer from socket isn't available anymore after calling socket.recv the first time

[Sworddragon]

Sworddragon added the comment:

> and if you try to receive less bytes than the datagram size, the rest will be 
> discarded, like UDP.

I'm wondering how would it be possible then to fetch packets of an unknown size 
without using an extremely big buffer.

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[issue21819] Remaining buffer from socket isn't available anymore after calling socket.recv the first time

[Charles-François Natali]

Charles-François Natali added the comment:

> If I'm receiving data from a socket (several bytes) and making the
> first call to socket.recv(1) all is fine but the second call won't get
> any further data. But doing this again with socket.recv(2) instead will
> successfully get the 2 bytes. Here is a testcase:

First, note that Python just calles the underlying recv() syscall, so it's not 
at fault here.

And actually, noone's at fault here, because what you're trying to do doesn't 
make sense: ICMP is datagram-oriented, so you should use recvfrom(): and if you 
try to receive less bytes than the datagram size, the rest will be discarded, 
like UDP.

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nosy: +neologix
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue21819] Remaining buffer from socket isn't available anymore after calling socket.recv the first time

[Sworddragon]

New submission from Sworddragon:

If I'm receiving data from a socket (several bytes) and making the first call 
to socket.recv(1) all is fine but the second call won't get any further data. 
But doing this again with socket.recv(2) instead will successfully get the 2 
bytes. Here is a testcase:


Script:

def icmp_packet(type, code, data):
length_data = len(data)
if length_data % 2 == 1:
data += b'\x00'
checksum = code | type << 8
i = 0
while i < length_data:
checksum += data[i + 1] | data[i] << 8
checksum = (checksum & 65535) + (checksum >> 16)
i += 2
return bytes([type]) + bytes([code]) + (checksum ^ 65535).to_bytes(2, 
'big') + data

import socket

connection = socket.socket(proto = socket.IPPROTO_ICMP, type = socket.SOCK_RAW)
connection.settimeout(1)
connection.sendto(icmp_packet(8, 0, b'\x00\x00\x00\x00'), ('8.8.8.8', 0))
print(connection.recv(2))
connection.close()
connection = socket.socket(proto = socket.IPPROTO_ICMP, type = socket.SOCK_RAW)
connection.settimeout(1)
connection.sendto(icmp_packet(8, 0, b'\x00\x00\x00\x00'), ('8.8.8.8', 0))
print(connection.recv(1))
print(connection.recv(1))
connection.close()


Here is the result:

root@ubuntu:/home/sworddragon/tmp# python3 test.py
b'E\x00'
b'E'
Traceback (most recent call last):
  File "test.py", line 24, in 
print(connection.recv(1))
socket.timeout: timed out

--
components: Library (Lib)
messages: 221155
nosy: Sworddragon
priority: normal
severity: normal
status: open
title: Remaining buffer from socket isn't available anymore after calling 
socket.recv the first time
type: behavior
versions: Python 3.4

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