Lee Rhodes writes:
> Thinking about detection of endianness at *run-time* got me
> thinking...would this work, at least on machines with 32 bit IEEE 754 float,
> 32 bit unsigned long and 8 bit char?
>
> #include <iostream>
>
> union endian {
> float f;
> unsigned long i;
> unsigned char c[4];
> };
>
> inline int testEndian();
>
> int testEndian() {
> register endian e;
> register int out = 0;
> e.i = 1U << 24;
> if (e.c[3] == 1) out |= 1; //Little Endian Integers
> e.i = 3U << 30;
> if (e.f < 0) out |= 2; //Little Endian Float
> return out;
> }
>
> using namespace std;
> int main() {
> int t = testEndian();
> if ((t & 1) == 1) cout << "Little Endian Integer" << endl;
> else cout << "Big Endian Integer" << endl;
> if ((t & 2) == 2) cout << "Little Endian Float" << endl;
> else cout << "Big Endian Float" << endl;
> }
>
> This may not require a memory access if the compiler actually uses registers
> and if the machine instruction set utilized small immediate values within
> the instruction itself. I don't have a big-endian machine, but if one of
> you has one could you try it?
It would work on gcc, because gcc allows members of a union to be
written as one type and read as a different incompatible type. Other
C compilers allow this too. However, it's is not guaranteed by the
language standards, so it doesn't much help you with truly portable
code.
Andrew.
