Re: 64-bit arithmetic in scripts?

From: Dan Nelson (dnelson_at_allantgroup.com)
Date: 09/30/04

  • Next message: Aaron Siegel: "Re: FreeBSD hardware specifications" 
    Date: Thu, 30 Sep 2004 15:48:09 -0500
To: Andrew <infofarmer@mail.ru>

    In the last episode (Oct 01), Andrew said:
    > Thanks! I haven't thought about using expr.
    >
    > How come that my expr(1) manpage has nothing to say about -e option?
    > In fact my expr(1) does not accept it. I have FreeBSD 4.10. I've just
    > looked into a current manpage from www.freebsd.org, and it says
    > something about 4.x compatibility.
    >
    > What is the best way to go if I need to write scripts now, but I'm
    > planning to switch to 5.x later? Can I upgrade expr(1) now? If not,
    > what should I do?

    In 4.x, expr does 64-bit math by default. Apparently POSIX requires
    that expr use whatever the systems' "signed long" size is, so the
    default was changed for 5.x, and -e was added to get the old behaviour.

    If you want your script to work on both, you'll have to do a feature
    test. I started out just testing expr and expr -e, but it sort of
    grew... The following script will check the shell's math, two ways of
    calling expr, and finally fall back on calling bc. As long as you just
    use the basic math operators, quote your "*"'s, and put spaces between
    everything, all the methods should be compatible, and your script will
    work on any bourne-compatible shell :)

    #! /bin/sh

    if [ x$(( 65536 * 65536 )) = "x4294967296" ] ; then
      shellarith() { echo $(( $@ )) ; }
      MATH="shellarith"
    else
      if [ x`expr 65536 "*" 65536` = "x4294967296" ] ; then
        MATH="expr"
      else
        if [ x`expr -e 65536 "*" 65536 2>/dev/null` = x"4294967296" ] ; then
          MATH="expr -e"
        else
          if [ x`echo 65536 "*" 65536 | bc` = "x4294967296" ] ; then
            bcfunc() { echo "$@" | bc ; }
            MATH="bcfunc"
          else
            echo "Can't do 64-bit math noway nohow"
          fi
        fi
      fi
    fi

    echo "Using $MATH"
    bigval=`$MATH 65536 "*" 65536`
    echo $bigval 

    -- 
	Dan Nelson
	dnelson@allantgroup.com
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