When and why socket.send() returns 0 in python?

Question

The python3 socket programming howto presents this code snippet

class MySocket:
    \"\"\"demonstration class only
      - coded for clarity, not efficiency
             


        

Answer 1

Upon seeing the question I was somehow stunned, because a send C call can return 0 bytes and the connection is of course still alive (the socket cannot simply send more bytes at that given moment in time)

• https://github.com/python/cpython/blob/master/Modules/socketmodule.c

I decided to "use the source" and unless I am very wrong (which can always be and often is) this is a bug in the HOWTO.

Chain:

• send is an alias for sock_send
• sock_send calls in turn sock_call
• sock_call calls in turn sock_call_ex
• sock_call calls in turn sock_send_impl (which has been passed down the chain starting with sock_send)

Unwinding:

• sock_send_impl returns true or false (1 or 0) with return (ctx->result >= 0)

• sock_call_ex returns

  • -1 if sock_send_impl returns false
  • 0 if sock_send_impl returns true

• sock_call returns this value transparently.

• sock_send

  • returns NULL for a -1 (because an error has been set and an exception will be raised)

  • returns ctx->result for 0from sock_call

    And ctx->result is the number of bytes written by the C call send in sock_send_impl.

The chain shows that if 0 bytes have been sent, there is no error and this actually is a potential real life socket situation.

If my logic is wrong, someone please let me know.