How to fix sock.recv() blocking state in python

Question

I am quite newbie in networking programming in python. I use the script I found on the internet that that connects to multicast adress and recieve MPEG-TS packets. I see on wireshark that after sock.setsockopt command, MPEG-TS packets are arriving to my computer.

Screen from wireshark
https://i.stack.imgur.com/fSSlU.jpg

But the problem occurs when I want to print the sock.recv() result. I believe it's because of blocking state if I'm understanding the documentation properly. After uncommenting setblocking(0) I got 10035 error. Do you have any clue what I need to add in order to print recieved data in terminal? Thank you in advance.

I tried changing the buffer_size in sock.recv() to be less, equal and just how it is right now - above 1358 bytes which is amount of single datagram bytes.

    import socket
    import struct
    import time

    MCAST_GRP = '239.0.1.104'
    MCAST_PORT = 12345
    IS_ALL_GROUPS = True


    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
    sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
    if IS_ALL_GROUPS:
        # on this port, receives ALL multicast groups
        sock.bind(('', MCAST_PORT))
    else:
        # on this port, listen ONLY to MCAST_GRP
        sock.bind((MCAST_GRP, MCAST_PORT))

    mreq = struct.pack("4sl", socket.inet_aton(MCAST_GRP), socket.INADDR_ANY)

    sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq)

    #sock.setblocking(0)
    print(f"Entering while loop")
    while True:
        time.sleep(1)
        print(f"I'm in while loop")    
        print(sock.recv(4096))

Answer 1

Try this simple code, you can set less BUFF_SIZE if you want.

#BUFF_SIZE = 4096
BUFF_SIZE = 1024
data = b''
while True:
    #chunk = s.recv(BUFF_SIZE)

    #FOR UDP (@Saeed credits)
    chuck = sock.recvfrom(BUFF_SIZE)

    data += chunk
    if len(chunk) < BUFF_SIZE:
        break

print(data)

Answer 2

Based on the example code from How do you UDP multicast in Python? when I try to run it on my system, it works fine. So, I changed it with your changes, this is the receiver.py code,

import socket
import struct

MCAST_GRP = '239.0.1.104'
MCAST_PORT = 12345
IS_ALL_GROUPS = True

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
if IS_ALL_GROUPS:
    # on this port, receives ALL multicast groups
    sock.bind(('', MCAST_PORT))
else:
    # on this port, listen ONLY to MCAST_GRP
    sock.bind((MCAST_GRP, MCAST_PORT))

mreq = struct.pack("4sl", socket.inet_aton(MCAST_GRP), socket.INADDR_ANY)

sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq)

print("Entering while loop")
while True:
    print("I'm in while loop")
    print(sock.recv(4096))

Notice, I slightly modified print statements and removed f as it was generating an error. And this is the sender.py code,

import socket

MCAST_GRP = '239.0.1.104'
MCAST_PORT = 12345
# regarding socket.IP_MULTICAST_TTL
# ---------------------------------
# for all packets sent, after two hops on the network the packet will not 
# be re-sent/broadcast (see https://www.tldp.org/HOWTO/Multicast-HOWTO-6.html)
MULTICAST_TTL = 2

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, MULTICAST_TTL)
sock.sendto(bytes("robot", 'UTF-8'), (MCAST_GRP, MCAST_PORT))

Notice that I am converting robot string into bytes with UTF-8 encoding, as this is necessary if using python3, it is not necessary in python2. This is my execution result,

As you can see when I invoke the sender, then the receiver does display robot string and loops through the while loop. It works fine.

In the case, if you are already receiving the multicast packets which you showed on your Wireshark, you need to ensure that those packets are being sent to the same port number on which you are listening, i-e 12345 in your case. If they are being sent to different port number [which is probably the case], then please change your receiver to listen on that port number to start receiving those packets.