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[perl #57950] int(some_expr) sometimes returns different results than some_expr, when result is an integer

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From:
perlbug-followup
Date:
August 15, 2008 17:27
Subject:
[perl #57950] int(some_expr) sometimes returns different results than some_expr, when result is an integer
Message ID:
rt-3.6.HEAD-29762-1218826194-1389.57950-75-0@perl.org
# New Ticket Created by  elakin@infohell.net 
# Please include the string:  [perl #57950]
# in the subject line of all future correspondence about this issue. 
# <URL: http://rt.perl.org/rt3/Ticket/Display.html?id=57950 >



This is a bug report for perl from elakin@infohell.net,
generated with the help of perlbug 1.35 running under perl v5.8.5.


-----------------------------------------------------------------
[Please enter your report here]

Running the following returns the expected result:

perl -e 'print 32.400451 * 1000000, "\n"'     
32400451

but if int() is wrapped around the entire math expression, it returns a 
different result:

perl -e 'print int(32.400451 * 1000000), "\n"' 
32400450

looks like some sort of floating-point precision loss, but I'd expect the
results to be the same. In the actual program this was seen in, 32.400451 is
actually a variable of unknown precision, and only 6 places are desired.

I've also tried the following, with the same results:
This is perl, v5.8.6 built for sun4-solaris-thread-multi
This is perl, v5.8.8 built for amd64-freebsd


[Please do not change anything below this line]
-----------------------------------------------------------------
---
Flags:
    category=library
    severity=low
---
This perlbug was built using Perl v5.8.5 in the Red Hat build system.
It is being executed now by Perl v5.8.5 - Fri Dec 16 14:48:17 EST 2005.

Site configuration information for perl v5.8.5:

Configured by Red Hat, Inc. at Fri Dec 16 14:48:17 EST 2005.

Summary of my perl5 (revision 5 version 8 subversion 5) configuration:
  Platform:
    osname=linux, osvers=2.6.9-1.906_elsmp, archname=i386-linux-thread-multi
    uname='linux tweety.build.redhat.com 2.6.9-1.906_elsmp #1 smp sun dec 12 22:58:08 est 2004 i686 i686 i386 gnulinux '
    config_args='-des -Doptimize=-O2 -g -pipe -m32 -march=i386 -mtune=pentium4 -Dversion=5.8.5 -Dmyhostname=localhost -Dperladmin=root@localhost -Dcc=gcc -Dcf_by=Red Hat, Inc. -Dinstallprefix=/usr -Dprefix=/usr -Darchname=i386-linux -Dvendorprefix=/usr -Dsiteprefix=/usr -Duseshrplib -Dusethreads -Duseithreads -Duselargefiles -Dd_dosuid -Dd_semctl_semun -Di_db -Ui_ndbm -Di_gdbm -Di_shadow -Di_syslog -Dman3ext=3pm -Duseperlio -Dinstallusrbinperl -Ubincompat5005 -Uversiononly -Dpager=/usr/bin/less -isr -Dinc_version_list=5.8.4 5.8.3 5.8.2 5.8.1 5.8.0'
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=define use5005threads=undef useithreads=define usemultiplicity=define
    useperlio=define d_sfio=undef uselargefiles=define usesocks=undef
    use64bitint=undef use64bitall=undef uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='gcc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS -DDEBUGGING -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -I/usr/include/gdbm',
    optimize='-O2 -g -pipe -m32 -march=i386 -mtune=pentium4',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS -DDEBUGGING -fno-strict-aliasing -pipe -I/usr/local/include -I/usr/include/gdbm'
    ccversion='', gccversion='3.4.4 20050721 (Red Hat 3.4.4-2)', gccosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=4, prototype=define
  Linker and Libraries:
    ld='gcc', ldflags =' -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib
    libs=-lresolv -lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lpthread -lc
    perllibs=-lresolv -lnsl -ldl -lm -lcrypt -lutil -lpthread -lc
    libc=/lib/libc-2.3.6.so, so=so, useshrplib=true, libperl=libperl.so
    gnulibc_version='2.3.6'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E -Wl,-rpath,/usr/lib/perl5/5.8.5/i386-linux-thread-multi/CORE'
    cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib'

Locally applied patches:
    

---
@INC for perl v5.8.5:
    /usr/lib/perl5/5.8.5/i386-linux-thread-multi
    /usr/lib/perl5/5.8.5
    /usr/lib/perl5/site_perl/5.8.5/i386-linux-thread-multi
    /usr/lib/perl5/site_perl/5.8.4/i386-linux-thread-multi
    /usr/lib/perl5/site_perl/5.8.3/i386-linux-thread-multi
    /usr/lib/perl5/site_perl/5.8.2/i386-linux-thread-multi
    /usr/lib/perl5/site_perl/5.8.1/i386-linux-thread-multi
    /usr/lib/perl5/site_perl/5.8.0/i386-linux-thread-multi
    /usr/lib/perl5/site_perl/5.8.5
    /usr/lib/perl5/site_perl/5.8.4
    /usr/lib/perl5/site_perl/5.8.3
    /usr/lib/perl5/site_perl/5.8.2
    /usr/lib/perl5/site_perl/5.8.1
    /usr/lib/perl5/site_perl/5.8.0
    /usr/lib/perl5/site_perl
    /usr/lib/perl5/vendor_perl/5.8.5/i386-linux-thread-multi
    /usr/lib/perl5/vendor_perl/5.8.4/i386-linux-thread-multi
    /usr/lib/perl5/vendor_perl/5.8.3/i386-linux-thread-multi
    /usr/lib/perl5/vendor_perl/5.8.2/i386-linux-thread-multi
    /usr/lib/perl5/vendor_perl/5.8.1/i386-linux-thread-multi
    /usr/lib/perl5/vendor_perl/5.8.0/i386-linux-thread-multi
    /usr/lib/perl5/vendor_perl/5.8.5
    /usr/lib/perl5/vendor_perl/5.8.4
    /usr/lib/perl5/vendor_perl/5.8.3
    /usr/lib/perl5/vendor_perl/5.8.2
    /usr/lib/perl5/vendor_perl/5.8.1
    /usr/lib/perl5/vendor_perl/5.8.0
    /usr/lib/perl5/vendor_perl
    .

---
Environment for perl v5.8.5:
    HOME=/home/elakin
    LANG (unset)
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/local/sbin:/usr/X11R6/bin:/usr/bin/X11:/usr/games:/usr/local/games:/etc:/usr/etc:/usr/local/pgsql/bin
    PERL_BADLANG (unset)
    SHELL=/bin/ksh


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