It would be helpful to show some code

Yes, it might have been clearer if I'd phrased the question in terms of pack/unpack.
On my 64-bit little-endian machine (Ubuntu-18.04), running a perl whose NV is the extended precision (80-bit) long double, I get: 
$ perl -le 'print unpack "h*", pack "D", sqrt 2;'
4846ed9f333f405bfff3000000000000

[download]
If I ran the identical one-liner on a 64-bit *big-endian* machine,running a perl whose NV is the extended precision (80-bit) long double, where would that substring of 12 consecutive zeroes appear ? Would it still appear at the end of the string (as was the case on my little-endian machine), or would it appear at the beginning ?

I have long-standing C code in Math-MPFR (MPFR.xs) that presumes that the substring in question will appear at the end of the string, irrespective of endianess: 
....
  n = bits == 64 ? 10 : 16;
#ifdef MPFR_HAVE_BENDIAN /* Big Endian architecture */
  for (i = 0; i < n; i++) {
#else
  for (i = n - 1; i >= 0; i--) {
#endif
  // code that does something with byte i

[download]
but I don't recall having ever established the correctness of that code for the big-endian case.

If it were incorrect, then the test suite should expose the error - but then I don't know if Math::MPFR has ever been built on a big-endian machine, using a perl whose NV is the 80-bit long double.

Cheers,
Rob

In reply to Re^2: [OT] Endianness and extended precision (80-bit) long doubles by syphilis
in thread [OT] Endianness and extended precision (80-bit) long doubles by syphilis

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