Amiga® RKM Devices: 3 Clipboard Device

The clipboard device allows the exchange of data dynamically between one
application and another. It is responsible for caching data that has been
"cut" and providing data to "paste" in an application. A special
"post" mode allows an application to inform the clipboard device that
the application has data available. The clipboard device will request this
data only if the data is actually needed. The clipboard will cache the
data in RAM and will automatically spool the data to disk if necessary.

The clipboard device is implemented as an Exec-style device, and supports
random access reads and writes on data within the clipboard. All data in
the clipboard must be in IFF format.  A new library, IFFParse Library, has
been added to the Amiga libraries.  The routines in iffparse.library can
and should be used for reading and writing data to the clipboard.  This
chapter contains a brief discussion of IFF as it relates to the clipboard
(for more details see Appendix A).


                   Feature                 Description
               --------------             --------------
               CBD_CHANGEHOOK             Device Command

   Compatibility Warning:
   The new features for the 2.0 clipboard device are not backwards

 Clipboard Device Commands and Functions 
 Device Interface 
 Monitoring Clipboard Changes 
 Example Clipboard Programs 
 Support Functions Called from Example Programs 
 Include File for the Example Programs 
 Additional Information on the Clipboard Device 

3 Clipboard Device / Clipboard Device Commands and Functions

Command             Command Operation
-------             -----------------
CBD_CHANGEHOOK      Specify a hook to be called when the data on the
                    clipboard has changed (V36).

CBD_CURRENTREADID   Return the Clip ID of the current clip to read.  This
                    is used to determine if a clip posting is still the
                    latest cut.

CBD_CURRENTWRITEID  Return the Clip ID of the latest clip written.  This
                    is used to determine if the clip posting data is

CBD_POST            Post the availability of clip data.

CMD_READ            Read data from the clipboard for a paste.  Data can be
                    read from anywhere in the clipboard by specifying an
                    offset >0 in the I/O request.

CMD_UPDATE          Indicate that the data provided with a write command
                    is complete and available for subsequent read/pastes.

CMD_WRITE           Write data to the clipboard as a cut.

Exec Functions as Used in This Chapter
CloseDevice()       Relinquish use of the clipboard device.  All requests
                    must be complete before closing.

DoIO()              Initiate a command and wait for completion
                    (synchronous request).

GetMsg()            Get next message from a message port.

OpenDevice()        Obtain use of the clipboard device.

SendIO()            Initiate a command and return immediately
                    (asynchronous request).

Exec Support Functions as Used in This Chapter
CreateExtIO()       Create an I/O request structure of type IOClipReq.
                    This structure will be used to communicate commands to
                    the clipboard device.

CreatePort()        Create a signal message port for reply messages from
                    the clipboard device.  Exec will signal a task when a
                    message arrives at the port.

DeleteExtIO()       Delete an I/O request structure created by

DeletePort()        Delete the message port created by CreatePort().

3 Clipboard Device / Device Interface

The clipboard device operates like the other Amiga devices.  To use it,
you must first open the clipboard device, then send I/O requests to it,
and then close it when finished. See "Introduction to Amiga System Devices"
chapter for general information on device usage.

  struct IOClipReq
      struct  Message io_Message;
      struct  Device  *io_Device;     /* device node pointer  */
      struct  Unit    *io_Unit;       /* unit (driver private)*/
      UWORD   io_Command;             /* device command */
      UBYTE   io_Flags;               /* including QUICK and SATISFY */
      BYTE    io_Error;               /* error or warning num */
      ULONG   io_Actual;              /* number of bytes transferred */
      ULONG   io_Length;              /* number of bytes requested */
      STRPTR  io_Data;                /* either clip stream or post port */
      ULONG   io_Offset;              /* offset in clip stream */
      LONG    io_ClipID;              /* ordinal clip identifier */

See the include file devices/clipboard.h for the complete
structure definition.

The clipboard device I/O request, IOClipReq, looks like a standard
IORequest structure except for the addition of the io_ClipID field, which
is used by the device to identify clips. It must be set to zero by the
application for a post or an initial write or read, but preserved for
subsequent writes or reads, as the clipboard device uses this field
internally for bookkeeping purposes.

 Opening The Clipboard Device        Updating The Clipboard Device 
 Clipboard Data 		     Clipboard Messages 
 Multiple Clips 		     Reading From The Clipboard Device 
 Writing To The Clipboard Device     Closing The Clipboard Device 

3 / Device Interface / Opening The Clipboard Device

Three primary steps are required to open the clipboard device:

    *  Create a message port using CreatePort(). Reply messages from the
       device must be directed to a message port.

    *  Create an extended I/O request structure of type IOClipReq using

    *  Open the clipboard device. Call OpenDevice(), passing the IOClipReq.

    struct MsgPort  *ClipMP;          /* pointer to message port*/
    struct IOClipReq *ClipIO;         /* pointer to IORequest */

    if (ClipMP=CreatePort(0L,0L) )
        if (ClipIO=(struct IOClipReq *)
                    CreateExtIO(ClipMP,sizeof(struct IOClipReq)))
            if (OpenDevice("clipboard.device",0L,ClipIO,0))
                printf("clipboard.device did not open\n");
                 ... do device processing
            printf("Error: Could not create IORequest\n");
        printf("Error: Could not create message port\n");

3 / Device Interface / Clipboard Data

Data on the clipboard resides in one of three places. When an application
posts a cut, the data resides in the private memory space of that
application. When an application writes to the clipboard, either of its
own volition or in response to a message from the clipboard requesting
that it satisfy a post, the data is copied to the clipboard which is
either memory or a special disk file. When the clipboard is not open, the
data resides in the special disk file located in the directory specified
by the CLIPS: logical AmigaDOS assign.

Data on the clipboard is self-identifying. It must be a correct IFF
(Interchange File Format) file; the rest of this section refers to IFF
concepts. See the Appendix A in this manual for a complete description of
IFF. If the top-level chunk is of type CAT with an identifier of CLIP,
that indicates that the contained chunks are different representations of
the same data, in decreasing order of preference on the part of the
producer of the clip. Any other data is as defined elsewhere (probably a
single representation of the cut data produced by an application).

The IFFParse.Library also contains functions which simplify reading and
writing of IFF data to the clipboard device.  See the "IFFParse Library"
chapter of the Amiga ROM Kernel Reference Manual: Libraries for more

A clipboard tool, which is an application that allows a Workbench user to
view a clip, should understand the text (FTXT) and graphics (ILBM) form
types. Applications using the clipboard to export data should include at
least one of these types in a CAT CLIP so that their data can be
represented on the clipboard in some form for user feedback.

You should not, in any way, rely on the specifics of how files in CLIPS:
are handled or named.  The only proper way to read or write clipboard data
is via the clipboard device.

   Play Nice!
   Keep in mind that while your task is reading from or writing to a
   clipboard unit, other tasks cannot.  Therefore, it is important to be
   fast.  If possible, make a copy of the clipboard data in RAM instead
   of processing it while the read or write is in progress.

3 / Device Interface / Multiple Clips

The clipboard supports multiple clips, i.e., the clipboard device can
contain more than one distinct piece of data.  This is not to be confused
with the IFF CAT CLIP, which allows for different representation of the
same data.

The multiple clips are implemented as different units in the clipboard
device.  The unit is specified at OpenDevice() time.

    struct IOClipReq *ClipIO;
    LONG unit;

    OpenDevice("clipboard.device", unit, ClipIO, 0);

By default, applications should use clipboard unit 0. However, it is
recommended that each application provide a mechanism for selecting the
unit number which will be used when the clipboard is opened. This will
allow the user to create a convention for storing different types of data
in the clipboard. Applications should never write to clipboard unit 0
unless the user requests it (e.g., selecting COPY or CUT within an

Clipboard units 1-255 can be used by the more advanced user for:

   *  Sharing data between applications within an ARexx Script.

   *  Customizing applications to store different kinds of data in
      different clipboard units.

   *  Customizing an application to use multiple cut/copy/paste buffers.

   *  Specialized utilities which might display and/or automatically modify
      the contents of a clipboard unit.

All applications which provide CUT, COPY and PASTE capabilities, should,
at a minimum, provide support for clipboard unit 0.

3 / Device Interface / Writing To The Clipboard Device

You write to the clipboard device by passing an IOClipReq to the device
with CMD_WRITE set in io_Command, the number of bytes to be written set in
io_Length and the address of the write buffer set in io_Data.

    ClipIO->io_Data = (char *) data;
    ClipIO->io_Length = 4L;
    ClipIO->io_Command = CMD_WRITE;

An initial write should set io_Offset to zero.  Each time a write is done,
the device will increment io_Offset by the length of the write.

As previously stated, the data you write to the clipboard must be in IFF
format.  This requires a certain amount of preparation prior to actually
writing the data if it is not already in IFF format.  A brief explanation
of the IFF format will be helpful in this regard.

For our purposes, we will limit our discussion to a simple formatted text
(FTXT) IFF file.  An FTXT file looks like:


           Note: Uppercase characters above are literals.

Based on the above file format, a hex dump of an IFF FTXT file containing
 the string "Enterprise" would look like:

           0000   464F524D  FORM
           0004   00000016  (length of FTXT, CHRS, 0xA and data)
           0008   46545854  FTXT
           000C   43485253  CHRS
           0010   0000000A  (length of Enterprise)
           0014   456E7465  Ente
           0018   72707269  rpri
           001C   7365      se

A code fragment for doing this:

    LONG slen = strlen ("Enterprise");
    BOOL odd = (slen & 1);      /* pad byte flag */

    /* set length depending on whether string is odd or even length */
    LONG length = (odd) ? slen + 1 : slen;

    /* Reset the clip id */
    ClipIO->io_ClipID = 0;
    ClipIO->io_Offset = 0;

    error = writeLong ((LONG *) "FORM");/* "FORM" */

    /* add 12 bytes for FTXT,CHRS & length byte to string length */
    length += 12;
    error = writeLong (&length);
    error = writeLong ((LONG *) "FTXT");/* "FTXT" for example */
    error = writeLong ((LONG *) "CHRS");/* "CHRS" for example */
    error = writeLong (&slen);      /* #  (length of string) */

    ClipIO->io_Command = CMD_WRITE;
    ClipIO->io_Data = (char *) string;
    ClipIO->io_Length = slen;               /* length of string */
    error = (LONG) DoIO (clipIO);   /* text string */

    LONG writeLong (LONG * ldata)
        ClipIO->io_Command = CMD_WRITE;
        ClipIO->io_Data = (char *) ldata;
        ClipIO->io_Length = 4L;
        return ( (LONG) DoIO (clipIO) );

The fragment above does no error checking because it's a fragment.  You
should always error check.  See the example programs at the end of this
chapter for the proper method of error checking.

   Iffparse That Data!
   Keep in mind that the functions in the iffparse.library can be used
   to write data to the clipboard. See the "IFFParse Library" chapter
   of the Amiga ROM Kernel Reference Manual: Libraries for more

3 / Device Interface / Updating The Clipboard Device

When the final write is done, an update command must be sent to the device
to indicate that the writing is complete and the data is available. You
update the clipboard device by passing an IOClipReq to the device with
CMD_UPDATE set in io_Command.

    ClipIO->io_Command = CMD_UPDATE;

3 / Device Interface / Clipboard Messages

When an application performs a post, it must specify a message port for
the clipboard to send a message to if it needs the application to satisfy
the post with a write called the SatisfyMsg.

    struct SatisfyMsg
    struct  Message sm_Message; /* the length will be 6 */
    UWORD   sm_Unit;            /* 0 for the primary clip unit */
    LONG    sm_ClipID;          /* the clip identifier of the post */

This structure is defined in the include file devices/clipboard.h.

If the application wishes to determine if a post it has recently performed
is still the current clip, it should compare the io_ClipID found in the
post request upon return with that returned by the CBD_CURRENTREADID

If an application has a pending post and wishes to determine if it should
satisfy it (for example, before it exits), it should compare the io_ClipID
of the post I/O request with that of the CBD_CURRENTWRITEID command. If
the application receives a satisfy message from the clipboard device
(format described below), it must immediately perform the write with the
io_ClipID of the post. The satisfy message from the clipboard may be
removed from the application message port by the clipboard device at any
time (because it is re-used by the clipboard device). It is not dangerous
to spuriously satisfy a post, however, because it is identified by the

The cut data is provided to the clipboard device via either a write or a
post of the cut data. The write command accepts the data immediately and
copies it onto the clipboard. The post command allows an application to
inform the clipboard of a cut, but defers the write until the data is
actually required for a paste.

In the preceding discussion, references to the read and write commands of
the clipboard device actually refer to a sequence of read or write
commands, where the clip data is acquired and provided in pieces instead
of all at once.

The clipboard has an end-of-clip concept that is analogous to end-of-file
for both read and write. The read end-of-file must be triggered by the
user of the clipboard in order for the clipboard to move on to service
another application's requests, and consists of reading data past the end
of file. The write end-of-file is indicated by use of the update command,
which indicates to the clipboard that the previous write commands are

3 / Device Interface / Reading From The Clipboard Device

You read from the clipboard device by passing an IOClipReq to the device
with CMD_READ set in io_Command, the number of bytes to be read set in
io_Length and the address of the read buffer set in io_Data.

    ClipIO->io_Command = CMD_READ;
    ClipIO->io_Data = (char *) read_data;
    ClipIO->io_Length = 20L;

io_Offset must be set to zero for the first read of a paste sequence.  An
io_Actual that is less than the io_Length indicates that all the data has
been read.  After all the data has been read, a subsequent read must be
performed (one whose io_Actual returns zero) to indicate to the clipboard
device that all the data has been read. This allows random access of the
clip while reading. Providing only valid reads are performed, your program
can seek/read anywhere within the clip by setting the io_Offset field of
the I/O request appropriately.

   Tell The Clipboard You Are Finished Reading.
   Your application must perform an extra read (one whose io_Actual
   returns zero) to indicate to the clipboard device that all data has
   been read, if io_Actual is not already zero.

The data you read from the clipboard will be in IFF format.  Conversion
from IFF may be necessary depending on your application.

   Iffparse That Data!
   Keep in mind that the functions in the iffparse.library can be used
   to read data from the clipboard. See the "IFFParse Library"
   chapter of the Amiga ROM Kernel Reference Manual: Libraries for more

3 / Device Interface / Closing The Clipboard Device

Each OpenDevice() must eventually be matched by a call to


When the last task closes a clipboard unit with CloseDevice(), the
contents of the unit may be copied to a disk file in CLIPS: so that the
clipboard device can be expunged.

3 Clipboard Device / Monitoring Clipboard Changes

Some applications require notification of changes to data on the
clipboard. Typically, these applications will need to do some processing
when this occurs. You can set up such an environment through the
CBD_CHANGEHOOK command. CBD_CHANGEHOOK allows you to specify a hook to be
called when the data on the clipboard changes.

For example, a show clipboard utility would need to know when the data on
the clipboard is changed so that it can display the new data.  The hook it
would specify would read the new clipboard data and display it for the

You specify a hook for the clipboard device by initializing a Hook
structure and then passing an IOClipReq to the device with CBD_CHANGEHOOK
set in io_Command, 1 set in io_Length, and the address of the Hook
structure set in io_Data.

    ULONG HookEntry ();            /* Declare the hook assembly function */
    struct IOClipReq *ClipIO;      /* Declare the IOClipReq */
    struct Hook *ClipHook;         /* Declare the Hook */

    /* Prepare the hook */
    ClipHook->h_Entry = HookEntry; /* C intrfce in asmbly rout. HookEntry*/
    ClipHook->h_SubEntry = HookFunc;/* Call function when Hook activated */
    ClipHook->h_Data = FindTask(NULL);/* Set pointer to current task */

    ClipIO->io_Data = (char *) ClipHook;/* Point to hook struct */
    ClipIO->io_Length = 1;              /* Add hook to clipboard */
    ClipIO->io_Command = CBD_CHANGEHOOK;

The above code fragment assumes that an assembly language routine
HookEntry() has been coded:

    ; entry interface for C code
            move.l  a1,-(sp)            ; push message packet pointer
            move.l  a2,-(sp)            ; push object pointer
            move.l  a0,-(sp)            ; push hook pointer
            move.l  h_SubEntry(a0),a0   ; fetch C entry point ...
            jsr     (a0)                ; ... and call it
            lea     12(sp),sp           ; fix stack

It also assumes that the function HookFunc() has been coded.  One of the
example programs at the end of this chapter has hook processing in it.
See the include file utility/hooks.h and The Amiga ROM Kernel Reference
Manual: Libraries for further information on hooks.

You remove a hook by passing an IOClipReq to the device with the address
of the Hook structure set in io_Data, 0 set in io_Length and
CBD_CHANGEHOOK set in io_Command.

    ClipIO->io_Data = (char *) ClipHook;  /* point to hook struct */
    ClipIO->io_Length = 0;                /* Remove hook from clipboard */
    ClipIO->io_Command = CBD_CHANGEHOOK;
    (DoIO (clipIO))

You must remove the hook or it will continue indefinitely.


3 / Monitoring Clipboard Changes / Caveats For CBD_CHANGEHOOK

   *  CBD_CHANGEHOOK should only be used by a special application, such as
      a clipboard viewing program.  Most applications can check the
      contents of the clipboard when, and if, the user requests a paste.

   *  Do not add system overhead by blindly reading and parsing the
      clipboard everytime a user copies data to it.  If all applications
      did this, the system could become intolerably slow whenever an
      application wrote to the clipboard.  Only read and parse when it is

3 Clipboard Device / Additional Information on the Clipboard Device

Additional programming information on the clipboard device can be found in
the include files for the clipboard device, iffparse library and utility
library, and the Autodocs for all three. They are contained in the
Amiga ROM Kernel Reference Manual: Includes and Autodocs.

                     Clipboard Device Information
                   INCLUDES       devices/clipboard.h

                   AUTODOCS       clipboard.doc

Converted on 22 Apr 2000 with RexxDoesAmigaGuide2HTML 2.1 by Michael Ranner.