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                    ³          DB/C Newsletter          ³
                    ³           October 1993            ³
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                              Editor's Notes  

     Subject, Wills & Company is investigating the possibility of moving the
DB/C BBS to the Internet.  If we do this, we will still offer a dial-up line
in our Oak Brook offices, but the interface will not be the existing BBS.  If
there is interest, we might also offer access into the Internet (there would
be a fee for this service).  Another aspect of moving to the Internet is that
E-mail could originate anywhere that can send mail to the Internet and SWC
could send mail back to your electronic address (which might be on Compuserve,
America Online, etc.).  What do you think?  Please send your thoughts to the
Sysop on the DB/C BBS.

     The topic for this month's newsletter is Windows Dynamic Data Exchange.
DDE is the protocol by which many client/server software products communicate.
It is currently available on Windows, Windows NT and OS/2.  The only version
of DB/C that currently supports DDE is the Windows 3.1 version, but support
for other platforms will be added in the future.  I hope that this discussion
helps you understand DDE a little better.
                                                                    DNW


                            Dynamic Data Exchange     

     Microsoft Windows provides a program-to-program communications facility
known as Dynamic Data Exchange (DDE).  DDE provides a way for Windows programs
to exchange data and execute procedures in other programs.  These exchanges are
implemented with a client/server arrangement where the DDE server provides data
and other services based upon requests from the DDE client program.

I. A DDE server provides the following services to a DDE client:
  1. send data as a response to a query (DATA)
  2. notify a client that a change in data has occurred (ADVISE)
  3. send data to a client containing changed values (ADVISE)
  4. allow a client to change the value of items
  5. execute a series of instructions given by a client

II. A DDE client can issue the following instructions to a DDE server:
  1. request the value of a particular data item (REQUEST)
  2. request to be notified whenever a data item's value changes (WARMLINK)
  3. request to be sent the new value of a data item whenever it changes
     (HOTLINK)
  4. instruct a server to discontinue notification messages for a data item
     (UNLINK)
  5. request that a server execute a series of commands (EXECUTE)
  6. instruct the server to change the value of a data item (DATA)

     Connections between a DDE server and a DDE client are based upon an
application name and a topic name.  The application name is usually related to
the name of the server program.  For instance, the application name for the
program Microsoft Excel is 'Excel'.  The topic name for a DDE conversation
refers to the category of information that will be exchanged.  In the case of
Microsoft Excel, the topic could be the name of the worksheet or data file.

     The DB/C code to establish a DDE connection is a follows:

For a DDE server:
dde  device
	prep	dde,"DDESERVER!appname!topicname"

For a DDE client:
dde  device
	prep	dde,"DDECLIENT!appname!topicname"

where 'appname' is the application name and 'topicname' is the topic name.

     When a connection has been established between the client and server based
on an application name and topic name, the client can obtain information about
data items or cause the server to execute a series of commands.  The client
refers to specific data item by an item name.  For our Excel example, one use
for the item name would be to refer to a specific cell in the worksheet.  The
item name 'r5c6' refers to value in the worksheet cell in row 5, column 6.

     The following code establishes a DDE connection to an Excel worksheet
named 'Sheet1' and queries the value of the cell 'B9' (row 9, column 2).

dde       device
data      char 20
msgq      queue size=120

msgdata   list
header    char 8
function  char 4
appname   char 32
topicname char 32
item      char 22
data      char 20
          listend

     prep dde, "DDECLIENT!Excel!Sheet1"
     link dde, msgq
     change dde, "REQUEST.r9c2"
     wait msgq
     get msgq; msgdata
     close dde

     A DDE client can also request the DDE server to change the value of a data
item.  The DDE client does this by sending a DATA message for a specific item.
In other documentation, this operation is referred to as a POKE operation.
This sample code changes the value in row 9, column 2 in 'Sheet1' to "123":

dde       device

     prep dde, "DDECLIENT!Excel!Sheet1"
     change dde, "DATA.r9c2"; "123"

     As mentioned earlier, a DDE client can also request to be notified when a
data value changes.  The DDE client accomplishes this by establishing a warm or
hot link.  When a warm link is active, the server to notifies the client
whenever data has changed.  A hot link is the same as a warm link except that
the notification message includes the new data value.

     DB/C stores the notification messages resulting from warm links and hot
links in a DB/C queue variable.  The following code sets up a hot link with
cell 'A1' (row 1, column 1) of the Excel worksheet named 'Sheet1'.  Whenever
the value of this cell changes, the new value is printed.  The code uses the
data variables declared in the examples above.

     prep dde, "DDECLIENT!Excel!Sheet1"
     link dde, msgq
     change dde, "HOTLINK.r1c1"
     loop
          wait	dde
          get dde; msgdata
          print "DDE Message ", data
     repeat

     A DDE client can request a DDE server to execute a series of commands.
For example, a DDE client connected to Excel might want to cause Excel to open
an existing worksheet.  The following example causes Excel to open a worksheet
named 'datafile.xls' by using the special topic name 'System'.

     prep dde, "DDECLIENT!Excel!System"
     change dde, "EXECUTE"; "[open(#"datafile.xls#")]"

     DB/C programs can also be DDE servers.  A DDE server program may receive a
variety of requests from one or more DDE clients.  A typical DDE server must
keep track of warm and hot links.  Additionally, a server must be able to
respond to requests for item values, to changes to item values, and to execute
commands given by the client.  The only type of messages that can be returned
to a client program from a DDE server are DATA and ADVISE messages.  A DATA
message return the value of an item.  An ADVISE message that notify the client
that an item value has changed.

     Refer back to the sample code that defines the 'msgdata' list variable.
The variable named 'header' will always contain the string "DDEMSG  ".  The
variable named 'function' will contain a value appropriate to the message being
sent.  The variable named 'item' contain the item name, if applicable.

     For a server, the values of the variable 'function' may be:
"REQU"  client requests the value 'item'
"DATA"  client requests server to change value of 'item' to the value in 'data'
"HOTL"  client requests establishment of a hot link with 'item'
"WARM"  client requests establishment of a warm link with 'item'
"NOLI"  client requests discontinuance of a warm or hot link with 'item'
"EXEC"  client requests server to execute the commands in 'data'

     For a client, the values of the variable 'function' may be:
"DATA"  server is returning requested information about 'item' in 'data'
"ADVI"  notification that 'item' value has changed, 'data' is value if hot link

     There are two limitations in the current implementation of DDE in DB/C:
 1. DB/C does not support timeout values for DDE operations.
 2. DB/C attempts to automatically acknowledge most DDE messages received by
    clients and servers.  Therefore, a DDE server or client has no opportunity
    to notify or be notified if a referenced item does not exist.


                              DB/C Class Schedule   

     The next DB/C classes scheduled for January.  The classes are held in the
Oak Brook, Illinois office of Subject, Wills & Company.  For more information,
contact Judi Tamkevic at (708) 572-0240.