The type of discriminant is either "int", "unsigned int", or an enumerated type, such as "bool". The component types are called "arms" of the union, and are preceded by the value of the discriminant which implies their encoding. The default arm is optional. If it is not specified, then a valid encoding of the union cannot take on unspecified discriminant values.
The size of the implied arm is always a multiple of four bytes. The discriminated union is encoded as its discriminant followed by the encoding of the implied arm.
Voids are useful for describing operations that take no data as input or no data as output. They are also useful in unions, where some arms may contain data and others do not. The symbolic constant may be used anywhere a regular constant may be used. The syntax is: typedef declaration; The new type name is actually the variable name in the declaration part of the typedef.
For example, the following defines a new type called "eggbox" using an existing type called "egg": typedef egg eggbox[DOZEN]; Variables declared using the new type name have the same type as the new type name would have in the typedef, if it was considered a variable. For example, the following two declarations are equivalent in declaring the variable "fresheggs": eggbox fresheggs; egg fresheggs[DOZEN]; When a typedef involves a struct, enum, or union definition, there is another preferred syntax that may be used to define the same type.
The optional-data type also has a close correlation to how recursive data structures are represented in high-level languages such as Pascal or C by use of pointers. In fact, the syntax is the same as that of the C language for pointers. Also missing are packed or binary-coded decimals. The intent of the XDR standard was not to describe every kind of data that people have ever sent or will ever want to send from machine to machine.
Rather, it only describes the most commonly used data-types of high-level languages such as Pascal or C so that applications written in these languages will be able to communicate easily over some medium. The minimum necessary for this are support for different block sizes and byte-orders. What's wrong with diagrams? There are many advantages in using a data-description language such as XDR versus using diagrams. Languages are more formal than diagrams and lead to less ambiguous descriptions of data.
Languages are also easier to understand and allow one to think of other issues instead of the low-level details of bit-encoding. Also, there is a close analogy between the types of XDR and a high-level language such as C or Pascal. This makes the implementation of XDR encoding and decoding modules an easier task. Finally, the language specification itself is an ASCII string that can be passed from machine to machine to perform on-the-fly data interpretation.
Supporting two byte-orderings requires a higher level protocol for determining in which byte-order the data is encoded. Since XDR is not a protocol, this can't be done. The advantage of this, though, is that data in XDR format can be written to a magnetic tape, for example, and any machine will be able to interpret it, since no higher level protocol is necessary for determining the byte-order.
Isn't this unfair to little-endian machines such as the VAX r , which has to convert from one form to the other? Yes, it is unfair, but having only one byte-order means you have to be unfair to somebody. There is a tradeoff in choosing the XDR unit size.
Choosing a small size such as two makes the encoded data small, but causes alignment problems for machines that aren't aligned on these boundaries. A large size such as eight means the data will be aligned on virtually every machine, but causes the encoded data to grow too big.
If you are aware of any additional file formats that use the XDR extension, please let us know. The best way to open an XDR file is to simply double-click it and let the default assoisated application open the file. If you are unable to open the file this way, it may be because you do not have the correct application associated with the extension to view or edit the XDR file.
This file format was added to our database by a visitor to this site, but no additional information was provided. We have yet to investigate this file type further, or there was not enough information available at the time to report accurately on the format. Please check back soon for more information as we are constantly updating our file descriptions based on search frequency. XDR extension. The XDR file extension indicates to your device which app can open the file.
However, different apps may use the same file extension for different types of data. In this case, look through both formats to make sure you find the best suited XDR viewer. These apps are known to open certain types of XDR files. Remember, different programs may use XDR files for different purposes, so you may need to try out a few of them to be able to open your specific file. Not sure exactly what type of file you are trying to open? Try our new File Analyzer.
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