Subroutine to determine big/little endian

[RRick]

This was a question that showed up in a job interview once. (And to answer your next question: No, I didn't :))

Write a subroutine that returns a bool on whether a system supports big endian numbers (true) or little endian (false). I tried something like the following, but never did find out if it was correct.

There are two assumptions here: First, big endian direction from small to large is ordered like a string. Second, endian is byte based. On a PC, I get a true value. Does anyone know what goes on with the PC architecture?

bool isBigEndian( void)
{
    union Endian
    {
        int iValue;
        char bValue;
    };
 
    Endian endian;
    endian.iValue = 1;
    return endian.bValue;
}
 

[Banfa]

On a PC, I get a true value. Does anyone know what goes on with the PC architecture?

Depends on the processor but Intel is little endian.

Your code returns the direct inverse of what it says it returns. In big endian the most significant byte comes first in memory (in the lowest location) and your most significant byte will be 0 for a value of 1 so bValue will be 1 on a little endian system.

[RRick]

Does big/little endian always occur at the byte level? I thought I heard of endian also occuring on the 16-bit work level.

[Banfa]

The thing to remember is that there are more than 2 ways of byte ordering. It is just that most significant to least significant and least significant to most significant are the most common 2 ways.

If you think about it if I have a 4 byte value (DWORD) calling the least significant byte byte 1 and and most significant byte 4 then in the memory map of a little endian processor they will appear

1 2 3 4

on a big endian prcessor it will be

4 3 2 1

not only are the bytes swapped but the words are swapped as well.