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[perl #119935] "-0" inconsistent about signedness

From:
Zefram
Date:
September 21, 2013 21:11
Subject:
[perl #119935] "-0" inconsistent about signedness
Message ID:
rt-3.6.HEAD-1873-1379797884-243.119935-75-0@perl.org
# New Ticket Created by  Zefram 
# Please include the string:  [perl #119935]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org:443/rt3/Ticket/Display.html?id=119935 >



This is a bug report for perl from zefram@fysh.org,
generated with the help of perlbug 1.39 running under perl 5.18.1.


-----------------------------------------------------------------
[Please describe your issue here]

Since Perl 5.17.1, the string "-0" appears to be a negative zero if it is
examined directly as a number, but negating it produces an unsigned zero:

$ perl -lwe 'printf "%+.f%+.f\n", "-0", - - "-0"'
-0+0

This is unlike the pure floating-point value -0.0, which is consistently
signed:

$ perl -lwe 'printf "%+.f%+.f\n", -0.0, - - -0.0' 
-0-0

Also unlike the pure integer -0, which is consistently unsigned:

$ perl -lwe 'printf "%+.f%+.f\n", -0, - - -0'  
+0+0

"-0" is presumably meant to behave numerically like one or other of the
pure numbers.  Up to and including 5.17.0 it was consistently signed.

In fact, it's worse: whether a "-0" string appears signed or not
depends on which kind of numeric operation was performed on it first.
If it's passed to printf first then that string is a signed zero in all
subsequent operations.  If it's negated first then that string is an
unsigned zero in all subsequent operations:

$ perl -lwe '$a="-0"; sprintf "%+.f\n", $a; my $b = "-0"; -$b; printf "%+.f%+.f%+.f\n", $a,- $a, - -$a; printf "%+.f%+.f%+.f\n", $b,- $b, - -$b;'
Useless use of sprintf in void context at -e line 1.
Useless use of negation (-) in void context at -e line 1.
-0+0-0
+0+0+0

[Please do not change anything below this line]
-----------------------------------------------------------------
---
Flags:
    category=core
    severity=low
---
Site configuration information for perl 5.18.1:

Configured by zefram at Thu Aug 15 19:33:26 BST 2013.

Summary of my perl5 (revision 5 version 18 subversion 1) configuration:
   
  Platform:
    osname=linux, osvers=3.2.0-4-amd64, archname=x86_64-linux-thread-multi
    uname='linux barba.rous.org 3.2.0-4-amd64 #1 smp debian 3.2.46-1 x86_64 gnulinux '
    config_args='-des -Dprefix=/home/zefram/usr/perl/perl_install/perl-5.18.1-i64-f52 -Duselargefiles -Dusethreads -Uafs -Ud_csh -Uusesfio -Uusenm -Duseshrplib -Dusedevel -Uversiononly -Ui_db'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=define, usemultiplicity=define
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
    use64bitint=define, use64bitall=define, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='-O2',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
    ccversion='', gccversion='4.7.2', gccosandvers=''
    intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
    ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=8, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -fstack-protector -L/usr/local/lib'
    libpth=/usr/local/lib /lib/x86_64-linux-gnu /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib /usr/lib
    libs=-lnsl -ldb -ldl -lm -lcrypt -lutil -lpthread -lc
    perllibs=-lnsl -ldl -lm -lcrypt -lutil -lpthread -lc
    libc=, so=so, useshrplib=true, libperl=libperl.so
    gnulibc_version='2.15'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E -Wl,-rpath,/home/zefram/usr/perl/perl_install/perl-5.18.1-i64-f52/lib/5.18.1/x86_64-linux-thread-multi/CORE'
    cccdlflags='-fPIC', lddlflags='-shared -O2 -L/usr/local/lib -fstack-protector'

Locally applied patches:
    

---
@INC for perl 5.18.1:
    /home/zefram/usr/perl/perl_install/perl-5.18.1-i64-f52/lib/site_perl/5.18.1/x86_64-linux-thread-multi
    /home/zefram/usr/perl/perl_install/perl-5.18.1-i64-f52/lib/site_perl/5.18.1
    /home/zefram/usr/perl/perl_install/perl-5.18.1-i64-f52/lib/5.18.1/x86_64-linux-thread-multi
    /home/zefram/usr/perl/perl_install/perl-5.18.1-i64-f52/lib/5.18.1
    .

---
Environment for perl 5.18.1:
    HOME=/home/zefram
    LANG (unset)
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/zefram/usr/perl/perl_install/perl-5.18.1-i64-f52/bin:/home/zefram/usr/perl/util:/home/zefram/pub/x86_64-unknown-linux-gnu/bin:/home/zefram/pub/common/bin:/usr/bin:/bin:/usr/local/bin:/usr/games
    PERL_BADLANG (unset)
    SHELL=/usr/bin/zsh




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