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Endianness.h File Reference

Provides definitions of the ENDIANNESS of the target machine. The symbols ENDIANNESS_IS_LITTLE and ENDIANNESS_IS_BIG should be used in preprocessing directives to determine the endianness of the machine, while the symbol ENDIANNESS can be more easily used in 'C' code. More...

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Defines

#define ENDIANNESS_K_UNKNOWN   0
 Value of ENDIANNESS the endianness of the target is undefined.

#define ENDIANNESS_K_LITTLE   1
 Value of ENDIANNESS if the target is a little endian machine.

#define ENDIANNESS_K_BIG   2
 Value of ENDIANNESS if the target is a big endian machine machine.

#define ENDIANNESS   ENDIANNESS_K_UNKNOWN
 ENDINANESS assumes one of the values,
  • ENDIANNESS_K_LITTLE
  • ENDIANNESS_K_BIG
  • ENDIANNESS_K_UNKNOWN.


#define ENDIANNESS_IS_LITTLE   ENDIANNESS_K_UNKNOWN
 Defined and set to a non-zero value, ENDIANNESS_K_LITTLE, if the target is a little endian machine.

#define ENDIANNESS_IS_BIG   ENDIANNESS_K_UNKNOWN
 Defined and set to a non-zero value, ENDIANNESS_BIG, if the target is a big endian machine.

Detailed Description

Provides definitions of the ENDIANNESS of the target machine. The symbols ENDIANNESS_IS_LITTLE and ENDIANNESS_IS_BIG should be used in preprocessing directives to determine the endianness of the machine, while the symbol ENDIANNESS can be more easily used in 'C' code.

Author:
JJRussell - russell@slac.stanford.edu

   CVS $Id: Endianness.h,v 1.3 2004/02/17 01:30:08 russell Exp $

SYNOPSIS
One of two symbols are defined
As a convenience to the programmer, an overall symbol, ENDIANNESS is also defined. It will take on one of the three values.

USAGE
One should use this as follows 
    #if   ENDIANNESS_IS_LITTLE

      / * Do little endian kind of things * /

    #elif ENDIANNESS_IS_BIG

      / * Do big endian kind of things * /

    #else

    #error _FILE_ Endianness not determined in this file

    #endif
It is recommended that both BIG and LITTLE conditions are tested for. This can save one from embarrassing typographical errors. For example, consider the consequences of the following 
    #if ENDIANESS_IS_LITTLE

       / * Do little endian kind of things * /

    #else

       / * Do big endian kind of things * /

    #endif
It takes a keen eye to note the misspelling of ENDIANNESS_IS_LITTLE as ENDIANESS_IS_LITTLE. Since there is no such symbol as ENDIANESS_IS_LITTLE, the 'big' endian kind of things will always be done.
It is also recommended that one use #if preprocessor directive over the #ifdef form. Either works with the current implementation, but future implementations reserves the right to support only the # if form.
TARGET PLATFORMS
All FSW supported platforms.

Define Documentation

#define ENDIANNESS   ENDIANNESS_K_UNKNOWN
 

ENDINANESS assumes one of the values,

  • ENDIANNESS_K_LITTLE
  • ENDIANNESS_K_BIG
  • ENDIANNESS_K_UNKNOWN.

Note:
Since DOXYGEN does not have access to the full set of compile time preprocessor symbols, ENDIANNESS will appear to be defined as ENDIANNESS_K_UNKNOWN.
Warning:
It is not recommended that this symbol be used in preprocessor directives to determine the endianness of the machine. Use ENDIANNESS_IS_LITTLE or ENDIANNESS_IS_BIG. Why? To do this correctly one must use the following long-winded style: 
   if   defined (ENDIANNESS) && (ENDIANNESS == ENDIANNESS_K_LITTLE)
   {
      .
      .
   }
   elif defined (ENDIANNESS) && (ENDIANNESS == ENDIANNESS_K_BIG)
   {
      .
      .
   }
   else
   {
      .
      / * Endianness unknown * /
   }
The check for ENDIANNESS being defined is mandatory for robustness. If the user forgets to include "PBI/Endiannness.h", all the ENDIANNESS symbols are, in fact, undefined. Without the check that ENDIANNESS is defined, the simple check of (ENDIANNESS == ENDIANNESS_K_LITTLE) will always succeed, causing the big endian if clause to always be taken. If the code is developed on a big endian machine, it is only when the code is ported to a little endian machine will the problem show up as malfunctioning code which the poor developer will eventually trace to the ill-constructed endianness determination.

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