Clipboard

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Clipboard
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Transferring Data BetweenApplications with the
Clipboard

• The Windows Clipboard
• A global memory block maintained by the Windows
  OS
• Available since the earliest versions of Windows
• Provides a common mechanism for getting data from
  application to application
• Also be used for single-program cut/copy and
  paste
• Many programs have "Cut", "Copy", "Paste" menu
  items
• Cut or Copy transfers data to clipboard
• Paste retrieves data from clipboard
• Clipboard contents are available to any running
  app

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Global memory allocation functions

• 1. GlobalAlloc ()
• hMem GlobalAlloc(allocation_flags,
  numbytes_to_allocate)
• Allocation_falgs GMEM_MOVEABLE GMEM_ZEROINT
• Returns a handle to the block that can be used in
  subsequent calls

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Global memory allocation functions

• 2- pBlock GlobalLock(hMem)
• Fixes the block in memory and returns a pointer
  to it
• Now we can use the returned pointer to read/write
  data from/to the      block of memory.

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Global memory allocation functions

• 3- GlobalUnlock(hMem)
• After we're all done accessing the block of
  memory, we must make it movable again.
• 4- GlobalFree(hMem)
• Release memory

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Clipboard functions

• OpenClipboard(hWnd)
• Opens the Clipboard for access by the program
• EmptyClipboard()
• Empties the clipboard memory block and frees all
  data currently in the clipboard
• GetClipboardData(uFormat)
• Gets a handle to read-only data in the Clipboard.
  The uFormat parameter specifies the data format

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Clipboard functions

• SetClipboardData(uFormat, hData)
• Passes the memory block whose handle is specified
  to the clipboard
• returns a handle to the data
• CloseClipboard()
• Closes the clipboard so that other programs can
  access it

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Sending Data to the Clipboard

• Allocate and lock a global memory block
• Copy the data into the global memory block
• Unlock the block
• Open the clipboard with OpenClipboard()
• Empty the current contents of the clipboard with
  EmptyClipboard()
• Transfer the block to the Clipboard with
  SetClipboardData()  (Windows now controls the
  block.)
• Close the clipboard with CloseClipboard().

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Getting Data from the Clipboard

• Open the clipboard with OpenClipboard()
• Get a handle to the clipboard data with
  GetClipboardData()
• Allocate and lock a memory block to hold the
  clipboard data
• Lock the clipboard data block
• Copy the data from the clipboard data block to
  the program's data block
• Unlock both memory blocks
• Close the clipboard with CloseClipboard().

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Some Precautions

• Close the clipboard as soon as possible after you
  open it
• After getting data from the clipboard, an
  application must not leave the   block locked
• After calling SetClipboardData(), an application
  should not use the block,   since it belongs to
  Windows

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Clipboard Data Formats

• Custom formats can be created using
  RegisterClipboardFormat().
• IsClipboardFormatAvailable(uFormat) determines
  if the specified data format is currently in the
  clipboard
• can be used without opening the clipboard
• an alternative to checking for a NULL result on
  the call to GetClipboardData()

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Example

• Clip1.cpp

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Multiple Clipboard Data Formats

• The clipboard can hold one block of data for each
  clipboard format
• The application that uses the clipboard can check
  each format stored to see if the data is as
  expected or can choose the format it can work
  with "best."

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Example

• Clip2.cpp

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Dynamic Link Libraries
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DLL

• Basic building block of Windows
• provide reusable code that can be shared by many
  applications
• provide reusable code that can be shared by many
  applications
• A file containing functions that can be called by
  other programs or DLLs
• Provides a way for a process to call a function
  that is not part of its   executable code

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Two methods of linking

• Implicit linking
• Also called
• Load-time linking
• Static loading
• Early binding
• Explicit linking
• Also called
• Run-time linking
• Dynamic loading
• Late binding

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Implicit linking

• DLL is automatically (implicitly) loaded (if not
  already  there) when the program that needs it is
  loaded
• The links to the DLL functions are established as
  soon as the app and the DLL  have been loaded
  into memory
• This requires that the executable module of the
  application be built by linking with the DLL's
  import library (.lib file)

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Explicit linking

• DLL can be loaded after execution of the app
  begins
• The application loads the DLL by calling
  LoadLibrary() when it is required
• When the application no longer needs the DLL it
  should call FreeLibrary()

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More about DLLs

• DLL functions are not directly executable, they
  must be called
• A DLL usually doesn't display a window or receive
  messages
• A DLL Can be accessed by any number of apps
• DLL Code is loaded only once any pgm using a ftn
  in an already-loaded

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Advantages of dynamic linking

• many processes can use a DLL simultaneously,
  sharing a single   copy in memory saves the
  space, swapping reduced
• Functions in a DLL can be changed without
  recompiling/relinking the applications  that use
  them
• Programs written in different programming
  languages can call the same DLL functions

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Potential disadvantages

• The app is not self-contained. The DLL module
  must be present
• Different applications may use the same name for
  their DLLs, or different versions of DLLs with
  the same name can exist

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DLL operation details

• DLLs don't have their own stack, instead use the
  stack of the calling program
• DLLs have their own local heap for static
  variables (may cause problem)
• If a DLL has to manage data for calling programs,
  it should allocate a separate memory block for
  each calling program's data
• However, global blocks allocated by a DLL
  "belong" to the calling program

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Build details

• The linker should know that DLL functions are
  available
• this is done by declaring DLL functions as
  "EXPORT
• EXPORT BOOL CALLBACK func()   
• EXPORT is a macro
• define EXPORT extern __declspec(dllexport)
• This will tell the linker that func() is
  exportable.

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Writing a DLL, DLLMain

• Most DLL's contain the function DllMain(), like
  WinMain()
• First function executed when DLL is loaded into
  memory by Windows
• Performs initialization stuff
• Runs only once (when DLL is loaded)
• int CALLBACK DllMain (HINSTANCE hInstance, DWORD
  fdwReason, PVOID  pvReserved)

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DLLMain( ) parameters

• hInstance DLL's instance handle
• fdwReason one of four values that indicate why
  Windows is calling DllMain().
• pReserved Reserved

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DllMain() details

• If initialization was a success, DllMain()
  return nonzero
• If initialization failed, Windows will not run
  the program that calls the DLL
• For most simple cases, all that DllMain() has to
  do is    return(1)

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Example

• dllcall.cpp
• revstr.cpp