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133 Common Data Representation
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GIOP defines a Common Data Representation that determines the binary layout of IDL types for transmission CDR has the following main characteristics CDR supports both big-endian and little-endian representation CDR-encoded data is tagged to indicate the byte ordering of the data This means that both big-endian and little-endian machines can send data in their native format If the sender and receiver use different byte ordering, the receiver is responsible for byteswapping This model, called receiver makes it right, has the advantage that if both sender and receiver have the same endianness, they can communicate using the native data representation of their respective machines This is preferable to encodings such as XDR, which require big-endian encoding on the wire and therefore penalize communication if both sender and receiver use little-endian machines CDR aligns primitive types on natural boundaries CDR aligns primitive data types on byte boundaries that are natural for most machine architectures For example, short values are aligned on a 2-byte boundary, long values are aligned on a 4-byte boundary, and double values are aligned on an 8-byte boundary Encoding data according to these alignments wastes some bandwidth because part of a CDR-encoded byte stream consists of padding bytes However, despite the padding, CDR is more efficient than a more compact encoding because, in many cases, data can be marshaled and unmarshaled simply by pointing at a value that is stored in memory in its natural binary representation This approach avoids expensive data copying during marshaling CDR-encoded data is not self-identifying CDR is a binary encoding that is not self-identifying For example, if an operation requires two in parameters, a long followed by a double, the marshaled data consists of 16 bytes The first 4 bytes contain the long value, the next 4 bytes are padding with undefined contents to maintain alignment, and the final 8 bytes contain the double value The receiver simply sees 16 bytes of data and must know in advance that these 16 bytes contain a long followed by a double in order to correctly unmarshal the parameters This means that CDR encoding requires an agreement between sender and receiver about the types of data that are to be exchanged This agreement is established by the IDL definitions that are used to define the interface between sender and receiver The receiver has no way to prevent misinterpretation of data if the agreement is violated For example, if the sender sends two double values instead of a long followed by a double, the receiver still gets 16 bytes of data but will silently misinterpret the first 4 bytes of the first double value as a long value CDR encoding is a compromise that favors efficiency Because CDR supports both littleendian and big-endian representations and aligns data on natural boundaries, marshaling
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IT-SC book: Advanced CORBA Programming with C++
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is both simple and efficient The downside of CDR is that certain type mismatches cannot be detected at run time In practice, this is rarely a problem because the stubs and skeletons generated by the C++ mapping make it impossible to send data of the wrong type However, if you use the DII or DSI, you must take care not to send data of the wrong type as operation parameters because, at least in some cases, the type mismatch will go undetected at run time Other encodings do not suffer from this problem For example, the Basic Encoding Rules (BER) used by ASN1 use a Tag-Length-Value (TLV) encoding, which tags each primitive data item with both its type and its length Such encodings provide better type safety at run time but are less efficient in both marshaling overhead and bandwidth For this reason, most modern RPC mechanisms use encodings similar to CDR, in which data is not tagged with its type during transmission 1331 CDR Data Alignment This section presents an overview of the CDR encoding rules Again, we do not cover all of CDR here Instead, we show the encoding of a few IDL types to illustrate the basic ideas
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