Byte Order

[Muhammad Farooq-i-Azam <farooq_a_azam () yahoo com>]

Well, this question relates to the byte order. I have
searched the archives of this list but still feel like
in need of some enlightenment.

Please let me explain what I understand about the byte
order and then my question at the end.

1- In the case of little endian format, least
significant byte is stored at the least significant
position (or lower address position) and reverse is
true for the big endian format. Same is true for bit
positions within a byte but we are not concerned about
it as our compiler takes care of this.

2- Conversion from network(big endian) to host byte
order is required only when a data item is stored in
more than one byte. For items in which each byte is
treated individually, we need not bother about the
byte order. For example, in the case of character
strings, where each character occupies a single byte,
we do not need the conversion.

The Question Part:
I have this RIPv1 header before me as below:

0                   1                   2             
        3      
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 ~ 9 0     
         1      
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-~-+-~-+-+-+   
  |  command (1)| version (1)   | must be zero (2)  | 
   
+-------------+---------------+---------+---~-----+
|                                                 |
~                  RIP Entry (20)                 ~   
                                                      
       |                                              
  |
+-------------+---------------+---------+---~-----+

I think I can define a structure as below for the
fixed part of the header:

typedef struct header_ripv1 {
   u_int8_t   command;
   u_int8_t   version;
   u_int16_t  zeros;
} header_ripv1_t;

My question is can I use the above structure for both
little endian and big endian systems with the only
conversion required below:

header_ripv1_t ripv1;

ripv1.zeros = ntohs(ripv1.zeros);

Or do I have to do something like this (which I
*doubt* has a double error, and will result in
problems on a big endian system):

typedef struct header_ripv1{
#if defined (WORDS_BIGENDIAN)
  u_int16_t   zeros;
  u_int8_t    version;
  u_int8_t    command;
#else
  u_int8_t    command;
  u_int8_t    version;
  u_int16_t   zeros;
#endif
} header_ripv1_t;

Not a libpcap specific question, but I hope that I
will not be disappointed.

Much thanks,

Muhammad Farooq-i-Azam

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