1. A message processing device for sending and receiving a message composed of one or more elements between different kinds of devices, comprising: parse tree holding means having a function of holding a parse tree generated from syntax descriptions defining structure of a message sent or received; parse tree scanning means connected to said parse tree holding means and having a function of, as message data to be processed is applied, scanning said parse tree to encode and decode a value and outputting encoded and decoded values; preamble processing means connected to said parse tree scanning means and having a function of processing a preamble field of a message for use in indicating whether an element exists or not in message data sent or received; and index processing means connected to said parse tree scanning means and having a function of processing an index field of a message for use in indicating what number of element is selected in message data whose one of a plurality of elements is selected to be sent or received.
1. A message processing device for sending and receiving a message composed of one or more elements between different kinds of devices, comprising: parse tree holding means having a function of holding a parse tree generated from syntax descriptions defining structure of a message sent or received; parse tree scanning means connected to said parse tree holding means and having a function of, as message data to be processed is applied, scanning said parse tree to encode and decode a value and outputting encoded and decoded values; preamble processing means connected to said parse tree scanning means and having a function of processing a preamble field of a message for use in indicating whether an element exists or not in message data sent or received; and index processing means connected to said parse tree scanning means and having a function of processing an index field of a message for use in indicating what number of element is selected in message data whose one of a plurality of elements is selected to be sent or received. 8. The message processing device as set forth in claim 1, wherein said preamble processing means handles a preamble to instruct said parse tree scanning means to scan only a node of the parse tree corresponding to an element existing in message data sent or received, thereby eliminating the need of scanning of a node whose scanning is not required, the parse tree is stored in a continuous region on a storage region of said parse tree holding means, and in the decoding of a message, when message data is applied, first a preamble field is read and stored in said preamble processing means, a variable indicative of a bit position in the preamble field is initialized to 0, an address of a node being currently scanned is stored in a variable indicative of an address of a node of the parse tree, examination is made whether there exists an element yet to be decoded in the message data, when no element yet to be decoded exists, a decoding result of each element is output, when an element yet to be decoded exists, examination is made whether the element in question is an omissible element and when the element in question is not an omissible element, an address of the subsequent node is stored in the variable Indicative of an address of a node of the parse tree, the node of the parse tree having the address in question is scanned and the element in question is decoded to return to the examination whether an element yet to be decoded exists, when the element in question is an omissible element, if the contents of the current bit position in the preamble field indicate that "omissible element is omitted", the address of the subsequent node is stored in the variable indicative of an address of a node of the parse tree and 1 is added to a variable indicative of the current bit position to return to the examination whether an element yet to be decoded exists, and if the contents of the current bit position in the preamble field indicate that "omissible element is not omitted", the address of the subsequent node is stored in the variable indicative of an address of a node of the parse tree, the node of the parse tree having the address in question is scanned, the element in question is decoded and 1 is added to the variable indicative of the current bit position to return to the examination whether an element yet to be decoded exists.