When does socket.recv(recv_size) return?

Question

From test, I concluded that in following three cases the socket.recv(recv_size) will return.

1. After the connection was closed. For example, the client side called socket.close() or any socket error occurred, it would return empty string.

2. Some data come, the size of data is more than recv_size.

3. Some data come, the size of data is less than recv_size and no more data come after a short time (I found 0.1s would work).

More details about #3:

#server.py  while True:     data = sock.recv(10)     print data, 'EOF'  #client1.py  sock.sendall("12345") sock.sendall("a" * 50)  #client2.py  sock.sendall("12345") time.sleep(0.1) sock.sendall("a" * 50) 

When I run client1.py, the server.py echos:

12345aaaaa EOF aaaaaaaaaa EOF aaaaaaaaaa EOF aaaaaaaaaa EOF aaaaaaaaaa EOF aaaaa EOF 

When I run client2.py, the server.py echos:

12345 EOF aaaaaaaaaa EOF aaaaaaaaaa EOF aaaaaaaaaa EOF aaaaaaaaaa EOF aaaaaaaaaa EOF 

Are my conclusions correct? Where can I see the official description about #3?

Answer 1

Yes, your conclusion is correct. socket.recv is a blocking call.

socket.recv(1024) will read at most 1024 bytes, blocking if no data is waiting to be read. If you don't read all data, an other call to socket.recv won't block.

socket.recv will also end with an empty string if the connection is closed or there is an error.

If you want a non-blocking socket, you can use the select module (a bit more complicated than just using sockets) or you can use socket.setblocking.

I had issues with socket.setblocking in the past, but feel free to try it if you want.

Answer 2

It'll have the same behavior as the underlying recv libc call see the man page for an official description of behavior (or read a more general description of the sockets api).