my $result = unpack "l", pack "l", 1729080737 + 72 * 14 * 425567;

In this case the arithmetic gives an unsigned 32-bit integer, and all is well...

...though I note it works because pack('l', ...) silently accepts a value outside -0x8000_0000..+0x7FFF_FFFF and packs away the LS 4 bytes of it's machine representation. I haven't found where that is documented... In this case that can be avoided by using pack('L', ...).

I wondered about more general/extreme cases, for example:

my $result = 1729080737 + 72 * 14 * 425567  + 0xFFFF_FFFF ;

print $result, "\n" ;
print $result & 0xFFFF_FFFF, "\n" ;
print unpack('l', pack('l', $result)), "\n" ;

[download]
which works fine on 64 bit integer systems, but on a 32 bit one the result is:
6453019568
4294967295
-1
because Perl happily does the arithmetic in ~54 bits of "integer in floating point form" (assuming IEEE-754 double), but when an integer in integer form is required, gives 0xFFFF_FFFF for anything > 0xFFFF_FFFF (and -0x8000_0000 for anything < -0x8000_0000) even if the operation would happily mask down to 32 bits :-( I don't know of a simple way to persuade Perl to give the LS 32 bits of an integer which may or may not be held as a float.

use integer wrapped around the arithmetic gives the expected result on both 32 and 64 bit systems.

Avoiding pack (in order to avoid giving an out of range argument) is a bit messy, but I believe this works for both 64 and 32 bit systems:

  { my $q   = $result & 0xFFFF_FFFF ;
    $result = $q <= 0x7FFF_FFFF ? $q : -1 - ($q ^ 0xFFFF_FFFF) ;
  } ;

[download]

In reply to Re^2: Integer overflow by gone2015
in thread Integer overflow by terranullis

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.