"Kevin Kenny" <kenn...@acm.org> wrote in message

> Ihug wrote:
>> I am trying to write IEEE 754 64 bit numbers.
>> The following wiki page has a procedure "float2IEEE" that writes a 32 bit
>> number.

>> http://wiki.tcl.tk/756

>> How would I modify this to output 64 bit?
>> Is this possible/easy?

> Do you need hexadecimal?  Binary?  Simply to ensure that the
> number that you read matches the number you wrote?

> In particular, have you a need to support a machine whose native
> format is *not* IEEE-754?  There are few such machines left, and
> if you *do* have such a need, I need to know, since right now the
> plan is to desupport them in 8.5.

> From easiest to most difficult:

> (1) If all you need is assurance that a number will be bit-for-bit
>     the same on input that it was on output, format it to precisely
>     seventeen decimal digits, no more, no less.  Either
>         set ::tcl_precision 17; puts $theFloat
>     or
>         puts [format %.17g $theFloat]
>     will do the trick.  (Caveat: This presumes that your sscanf
>     and sprintf are standards-compliant.  Not all are.)  In 8.5,
>     simply [puts $theFloat] will do just fine without messing
>     with ::tcl_precision; the default value of tcl_precision
>     becomes "as many digits as are needed to guarantee correct
>     reconstruction of the number on input."

> (2) If you need the number in binary because you're putting it
>     to external media, then
>         [binary format d $theFloat]
>     will give the floating point number as a string of eight
>     bytes in the native byte order.

> (3) If you need a specific endianity,
>     things get a trifle more complicated in 8.4.  You have to
>     determine the machine's endianity:
>         binary scan [binary format d 1.0] w test
>         switch -exact [format %16lx $test] {
>             3ff0000000000000 {
>                 set big false
>             }
>             0000000000000f3f {
>                 set big true
>             }
>             default {
>                 error "machine does not have IEEE-754"
>             }
>         }
>      Now, to write a float out big-endian:
>          if {$big} {
>             binary scan [binary format d $theFloat] W bits
>          } else {
>             binary scan [binary format d $theFloat] w bits
>          }
>          set result [binary format W $bits]
>      Change the 'W' in the last line to a 'w' to write
>      little-endian instead, or to write hexadecimal, change
>      it to:
>          puts [format %16x $bits]

>      8.5 makes this process somewhat easier, because the [binary]
>      command adds the 'q' and 'Q' format groups to format 'double's
>      in a specific endianity.

> I hope this has covered everything you need; feel free to
> ask again if it hasn't.

> --
> 73 de ke9tv/2, Kevin