python socket always receiving 0 from java socket writeInt

Question

I am trying to send integer from java to python through socket.Here Python is server and java is client.

Here is the code Python server:

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind(("localhost",999))
s.listen(5)
con,addr=s.accept()
num=con.recv(4)
print(int.from_bytes(num,byteorder='big'))#always zero

Java Client:

 Socket s=new Socket("localhost",999);
 dos=new DataOutputStream(s.getOutputStream());
 dos.writeInt(2);

In python server I always receive 0 value no matter what I send in java client. I checked num value it is always single 0 byte. I also tried changing byteorder in python server but no use. What is reason of this behavior?

Edit: I looks like I have to use 4 recv functions to read 4 bytes. I'm still confused why this happens with writeInt. With writeUTF I was able to receive 1024 bytes with single recv.

• Serge Ballesta: This is weird... unless for any reason the read Python side did not return 4 bytes. As Java writeInt explicitely uses big endian order, if the read returned less that 4 bytes it will be converted into a 0. Could you close the socket from the Java side immediately after sending the int value? Race conditions could cause the socket to be closed before the packet has been sent to the network. Then Python will read 0 bytes (end of stream) which gives 0...
• LOLs: @SergeBallesta I tried giving sleep before closing socket still no use.

Answer 1

I think what's going on is you're reading bytes on the Python side, but on the Java side you're sending an int, which is a 4 byte quantity. 4 bytes will be sent as 0 0 0 2 so you're likely reading the first 0.

Two things to try, on the Python side, read four bytes and print the result of each.

Then on the Java side, instead of sending a 2, send 0x01020304 (yes that's a number). This should set each byte sent by the int to 1 2 3 4, so it's easy to read and see which piece you get.

From that you should be able to figure out how to read ints from Java. It would be handy though if Python had a facility for reading four bytes at a time from a byte array or a socket. Sorry I don't know enough Python to say what that would be.

• LOLs: I tried your solution it works. But why python is reading only 1 byte when I have specified 4 bytes. This does doesn't happen in writeUTF. I can read whole string with one recv
• markspace: It common in networking. There's no meta data in low level APIs like TCP/IP. There's only a stream of bytes. You've told it to receive 4 bytes but that's all it knows. So you get bytes. You need to tell it to interpret the bytes as something else, like an integer. Sorry I don't know enough Python to say what that is. (In Java one good solution is to use DataInputStream, the complement to the DataOutputStream you are already using.)