Apart from the usual corncern about endianness, the main problem with packing/unpacking floating point numbers is that there is no universal representation of how they are stored. However, the majority of modern platforms use the IEEE specification.*

Therefore, if you know in advance that you are writing/reading a IEEE floating point number in a particular endianness you could use a scheme like the following to guarantee some level of portability: 
#!/usr/bin/perl -w

use strict;


# Check if "pack" gives the required IEEE 64bit float
#
my $little_endian;

# Some test data
my $teststr = pack "d", 1.2345;
my @hexdata =(0x8D, 0x97, 0x6E, 0x12, 0x83, 0xC0, 0xF3, 0x3F);
my $number  = pack "C8", @hexdata;


if ($number eq $teststr) {
    # Little Endian
    $little_endian = 1;
}
elsif ($number eq reverse($teststr)) {
    # Big Endian
    $little_endian = 0;
}
else {
    # Give up, gracefully or otherwise.
    die "Required floating point format not supported on this platform
+.";
}


# Now use this information to pack a double
#
my $double = 3.1415;
my $packed_double;

if ($little_endian) {
    $packed_double = pack "d", $double;
}
else {
    $packed_double = reverse pack "d", $double;
}

[download]
If you are sure of the IEEE issue you can can probably just determine the endianness using the Config module and pack or reverse pack as necessary: 
#!/usr/bin/perl -w

use strict;
use Config;

my $double = 3.1415;
my $packed_double;

if ($Config{byteorder} == 1234) {
    $packed_double = pack "d", $double;
}
else {
    $packed_double = reverse pack "d", $double;
}

[download]


John.
--

* Does anyone have an example of modern hardware that doesn't support this format?

In reply to Re: newbie: unpacking real numbers by jmcnamara
in thread newbie: unpacking real numbers by perchance

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.