IEG 4180 Tutorial 4

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IEG 4180 Tutorial 4

• Prepared by Shing
• (Remark Modified by Zero)

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Outline

• Traditional Blocking I/O
• Overlapped I/O
• Event Object Signaling
• Alertable I/O

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Traditional Blocking I/O
Time Required A B C D
Max Rate Packet Size / Time Required
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Overlapped I/O

• When using overlapped I/O, the I/O operation will
  run in the background
• While the I/O operation runs in the background,
  the application can do some other processing
• When the I/O operation completes, the application
  is notified
• There are multiple mechanisms for notifying the
  application that an I/O operation has been
  completed
• Event Object Signaling
• Alertable I/O

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Overlapped I/O

• Advantages
• Non-blocking
• Use application buffers to receive data directly
• Allow posting multiple receive calls

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Overlapped I/O The Model - Use of Event Object
Need to Figure Out which Buffer is Being Filled
(or Returned)
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Overlapped I/O Create Overlapped Socket

• Use WSASocket() instead of socket()
• Use normal bind(), accept(), connect() etc

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Overlapped I/O Send Receive Data

• For TCP, use
• WSASend()
• WSARecv()
• For UDP, use
• WSASendTo()
• WSARecvFrom()

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Overlapped I/O Receive

• Important parameters for WSARecv and WSARecvFrom
• Socket
• Array of WSABUF structures
• Number of elements in WSABUF array
• WSAOVERLAPPED structure
• Pointer to I/O completion routine (used for
  alertable I/O)

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Overlapped I/O Receive

• The return value
• Does not return the number of bytes received.
• Only tell you it success or error.
• SOCKET_ERROR may be returned even there was no
  error.
• Use WSAGetLastError() to check, if error code is
  WSA_IO_PENDING, it means there is no error!!!

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Overlapped I/O WSABUF

• The definition of buffer for overlapped I/O
• len
• The of buffer
• Have to be filled in in advance
• buf
• The memory space that actually hold the data

typedef struct __WSABUF u_long len char
FAR buf WSABUF, LPWSABUF
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Overlapped I/O WSAOVERLAPPED structure

• A mean for notification

typedef struct _WSAOVERLAPPED DWORD Internal
DWORD InternalHigh DWORD Offset DWORD
OffsetHigh WSAEVENT hEvent WSAOVERLAPPED,
LPWSAOVERLAPPED

• hEvent
• Function call returns immediately, some
  mechanisms are needed to determine the status and
  the completion of the request
• Used in event object notification

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Overlapped I/O Event Object Notification

• Create an event object
• Similar to Mutex and Semaphore, event objects
  also have signaled or nonsignaled state
• Pass this object to hEvent of the WSAOVERLAPPED
  structure
• To know when the I/O operation complete
• WSAWaitForMultipleEvents()
• To retrieve the results of overlapped operations
• WSAGetOverlappedResult()
• Reset the event object to nonsignaled state
• WSAResetEvent()
• To free resources occupied by the event object
• WSACloseEvent()

WSAEVENT WSACreateEvent(void)
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Overlapped I/O Alertable I/O-Introduction

• Instead of using event object notification, make
  the OS calls one of your functions when I/O
  operations complete
• Completion routines
• Functions that will be called when I/O complete
• Specified in the last parameter of WSASend() /
  WSASendTo() / WSARecv() / WSARecvFrom()

int i WSARecvFrom(..., lpOverlapped,
lpCompletionRoutine)
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Overlapped I/O Alertable I/O
Move Data Processing to the Completion Routine
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Overlapped I/O Alertable I/O -Completion Routines
void CALLBACK CompletionRoutine( IN DWORD
dwError, / the error code / IN
DWORD cbTransferred, / in bytes / IN
LPWSAOVERLAPPED lpOverlapped, / the structure of
this I/O / IN DWORD dwFlags )

• cbTransferred
• Number of bytes transferred
• Equals to zero when connection is closed
• lpOverlapped
• hEvent can be freely used by your code, just like
  the LPVOID parameter in thread procedure
• You have to manage the buffer usage yourself!
• For example, you issued 10 WSARecv() with 10
  buffers
• The data will be filled in the buffers according
  to the calling order
• Reissue WSARecv() on processed buffers

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Overlapped I/O Alertable Wait state

• The thread using alertable I/O has to enter
  alertable wait state, so that the completion
  routines can be called
• To enter alertable wait state
• Just like the ordinary Sleep()
• Return when timeout or completion

DWORD SleepEx( DWORD dwMilliseconds, BOOL
bAlertable / set to true / )