External data representation

• boolean
• int (32 bit integer)
• hyper (64-bit integer)
• float
• double
• enumeration
• structure
• string
• fixed length arrays
• variable length arrays
• unformatted (raw) data

Different computers may have different internal representations of information. For example, a 32-bit Integer has 2 possible presentation forms:

• High to Low Byte Order (Motorola 68000)
• Direct Byte Order (Intel 80x86)

For some WinSock functions, their arguments (that is, function parameters) should be stored in reverse order.

• Server and client can exchange different types of data.
• If the server and the client are running on two corresponding machines using a different internal representation of the data, then they must coordinate exactly the representation of all the data transferred between them.
• Sun Microsystems has developed an external data representation (XDR) that defines the presentation for various data types (integer, enumeration ....)
• XDR has become the de facto standard for most client-server applications:

- The program converts messages from its internal representation to XDR for subsequent transmission. This is called coding. - The recipient converts the received message from XDR into its own representation. This is called decoding.

XDR indicates a view for most data types in C :

The encoded information contains only data; it does not contain information about the data type. For example, after encoding a 32-bit integer, the result will be a 32-bit integer in XDR. There will be no information that this is an integer. Clients and servers using XDR must agree on the data type of the messages they exchange.

• XDR defines a presentation for each data type.
• For example, a 32-bit integer has byte order from high to low.
• To assist programmers, XDR provides a library of standard programs for converting data representation.

• A sent message can consist of several data items.
• For example, the message contains information about the student. It consists of three points:

- name (character string) -ID (integer) - cumulative GPA (floating-point number)

• Before sending a message, the program (client or server) converts all the item information from the internal representation to XDR.
• Conversion steps
  1. Providing a buffer for storing all message information that should be sent.
  2. Call xdrmem_create () to initialize the XDR stream.

For example: xdrmem_create () returns a pointer to an empty stream.

• 1. Call a standard program in XDR to convert each item of information. She will append the encoded information to the end of the stream as follows:

  * # * put encoded information in the next available place in the buffer
        * # * update the internal pointer to the stream, placing it on a new available free space 

• 1. • For example, converting a 32-bit integer:

• 1. After encoding all items of the message, this message is sent.

• When a program receives a message in XDR, it converts each data item in the message from XDR to its internal representation.
• Steps:
  1. Call xdrmem_create () to initialize the XDR stream, specifying XDR_DECODE as the fourth argument.
  2. Putting the received message into the buffer.
  3. Calling a suitable standard conversion program to decode each data item of the received message.

For example, decoding a 32-bit integer:

• http://docs.freebsd.org/44doc/psd/25.xdrrfc/paper.pdf
• https://web.archive.org/web/20110303205629/http://www.comp.hkbu.edu.hk/~comp2650/lecture/notes/Chapter9.pdf