socket shutdown vs socket close

Question

I recently saw a bit of code that looked like this  with sock being a socket object of course    sock shutdown socket SHUT RDWR  sock close     What exactly is the purpose of calling shutdown on the socket and then closing it   If it makes a difference  this socket is being used for non-blocking IO

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there are some flavours of shutdown  http   msdn microsoft com en-us library system net sockets socket shutdown aspx   nix is similar

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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

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Here s 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

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it s mentioned right in the Socket Programming HOWTO  py2 py3      Disconnecting      Strictly speaking  you   re supposed to use shutdown on a socket before you close it    The shutdown is an advisory to the socket at the other end  Depending on the argument you pass it  it can mean    I   m not going to send anymore  but I   ll still listen     or    I   m not listening  good riddance        Most socket libraries  however  are so used to programmers neglecting to use this piece of etiquette that normally a close is the same as shutdown    close      So in most situations  an explicit shutdown is not needed

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Shutdown 1    forces the socket no to send any more data  This is usefull in  1- Buffer flushing  2- Strange error detection  3- Safe guarding  Let me explain more   when you send a data from A to B   it s not guaranteed to be  sent to B   it s only guaranteed to be sent to the A os buffer    which in turn sends it to the B os buffer  So by calling shutdown 1  on A   you flush A s buffer and an error is raised  if the buffer is not empty ie  data has not been sent to the peer yet  Howoever this is irrevesable   so you can do that after you completely  sent all your data and you want to be sure that it s atleast at the peer  os buffer

User · Answer

it s mentioned right in the Socket Programming HOWTO  py2 py3      Disconnecting      Strictly speaking  you   re supposed to use shutdown on a socket before you close it    The shutdown is an advisory to the socket at the other end  Depending on the argument you pass it  it can mean    I   m not going to send anymore  but I   ll still listen     or    I   m not listening  good riddance        Most socket libraries  however  are so used to programmers neglecting to use this piece of etiquette that normally a close is the same as shutdown    close      So in most situations  an explicit shutdown is not needed

User · Answer

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

User · Answer

Here s 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

User · Answer

there are some flavours of shutdown  http   msdn microsoft com en-us library system net sockets socket shutdown aspx   nix is similar

User · Answer

Here s 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

User · Answer

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

User · Answer

there are some flavours of shutdown  http   msdn microsoft com en-us library system net sockets socket shutdown aspx   nix is similar

User · Answer

there are some flavours of shutdown  http   msdn microsoft com en-us library system net sockets socket shutdown aspx   nix is similar

User · Answer

Here s 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

User · Answer

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

User · Answer

Isn t this code above wrong   The close call directly after the shutdown call might make the kernel discard all outgoing buffers anyway   According to http   blog netherlabs nl articles 2009 01 18 the-ultimate-so linger-page-or-why-is-my-tcp-not-reliable one needs to wait between the shutdown and the close until read returns 0

User · Answer

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

User · Answer

Isn t this code above wrong   The close call directly after the shutdown call might make the kernel discard all outgoing buffers anyway   According to http   blog netherlabs nl articles 2009 01 18 the-ultimate-so linger-page-or-why-is-my-tcp-not-reliable one needs to wait between the shutdown and the close until read returns 0

User · Answer

Shutdown 1    forces the socket no to send any more data  This is usefull in  1- Buffer flushing  2- Strange error detection  3- Safe guarding  Let me explain more   when you send a data from A to B   it s not guaranteed to be  sent to B   it s only guaranteed to be sent to the A os buffer    which in turn sends it to the B os buffer  So by calling shutdown 1  on A   you flush A s buffer and an error is raised  if the buffer is not empty ie  data has not been sent to the peer yet  Howoever this is irrevesable   so you can do that after you completely  sent all your data and you want to be sure that it s atleast at the peer  os buffer

User · Answer

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

[python] socket.shutdown vs socket.close

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