python – socket.shutdown vs socket.close

python – socket.shutdown vs socket.close

Calling close and shutdown have two different effects on the underlying socket.

The first thing to point out is that the socket is a resource in the underlying OS and multiple processes can have a handle for the same underlying socket.

When you call close it decrements the handle count by one and if the handle count has reached zero then the socket and associated connection goes through the normal close procedure (effectively sending a FIN / EOF to the peer) and the socket is deallocated.

The thing to pay attention to here is that if the handle count does not reach zero because another process still has a handle to the socket then the connection is not closed and the socket is not deallocated.

On the other hand calling shutdown for reading and writing closes the underlying connection and sends a FIN / EOF to the peer regardless of how many processes have handles to the socket. However, it does not deallocate the socket and you still need to call close afterward.

Heres one explanation:

Once a socket is no longer required,
the calling program can discard the
socket by applying a close subroutine
to the socket descriptor. If a
reliable delivery socket has data
associated with it when a close takes
place, the system continues to attempt
data transfer. However, if the data is
still undelivered, the system discards
the data. Should the application
program have no use for any pending
data, it can use the shutdown
subroutine on the socket prior to
closing it.

python – socket.shutdown vs socket.close

Explanation of shutdown and close: Graceful shutdown (msdn)

Shutdown (in your case) indicates to the other end of the connection there is no further intention to read from or write to the socket. Then close frees up any memory associated with the socket.

Omitting shutdown may cause the socket to linger in the OSs stack until the connection has been closed gracefully.

IMO the names shutdown and close are misleading, close and destroy would emphasise their differences.

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