Procedure Vput(var msg: message_packet_type; sender: boolean;
               force: boolean; var own: boolean; var stat: VI_status_type);
   EXTERNAL;
{*USER*
 
Look through our existing cache of field values to see if the
field value contained in MSG already exists.  If it does, compare
the values, then
update cache with the new value, and finally return the appropriate
VI_CHANGED or VI_NO_CHANGE in STAT to indicate if the values differed or not.
 
If the field value is not in cache, but FORCE is TRUE, add it,
and return VI_CHANGED.   If the field value is not in cache, and FORCE
is FALSE, then ignore value and return VI_NOT_FOUND.  The FORCE boolean
thus controls whether a new entry is made or not in cache if the
an existing entry is not found in cache.
 
If SENDER is true, the caller plans to send out the message.  Thus
if the caller is the owner of the field, we shall update the FIELD_STATUS
value, and check if it changed from the previous value.  If the caller
is not the owner, then we ignore the FIELD_STATUS value and leave the
one in cache as is.  If SENDER is false,
then the caller is receiving this message, and so if we are the owner, we
ignore the FIELD_STATUS, keeping the one we already have.  If we are not
the owner, we update the FIELD_STATUS value as it must be from the owner,
and check if it changed from the previous value.
 
If the field is "owned" by this program, return OWN as TRUE, else
return it as FALSE.  Naturally, it must be in cache to be "owned".
 
The FIELD_NAME and FIELD_VALUE fields of MSG must be correct before
making this call.  The FIELD_STATUS field need only be correct before
making this call if UPSTAT is true.  All other fields of MSG need not
be current for this call.
 
The message (MSG) variable will normally remain unchanged upon
return from VSEND.  The only change that can happen, is that the
message length (in MSG.FIELD_VALUE) will be adjusted to reflect the
length of the field value already in cache.  If needed the value
will be padded with blanks.  This is done, since the first VSEND,
which resulted in the field value being placed in cache, also determines
the "official" field length that will be use in subsequent sends.
 
 
*ERROR CODES*
 
VI_CHANGED - Field value was placed in cache, and either the
            supplied value was different that the value
            already in cache, or this is the first time the
            value has been placed in cache and the FORCE
            boolean was TRUE.
 
VI_NO_CHANGE - Field value was placed in cache, and the
               supplied value was the same as the value
               already in cache.
 
VI_NOT_FOUND - Field value was not found in cache, and since
               the FORCE boolean is FALSE, no entry was made
               for the field in cache.
 
VI_NO_MEMORY - Field value was not already in cache, and
               the FORCE boolean was TRUE, however, we were
               unable to place the supplied field value in
               cache as there was no more memory available
               in cache.
 
}