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[perl #133984] Devel::Peek docs may contain incorrect examples

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From:
Jim Avera
Date:
April 3, 2019 21:51
Subject:
[perl #133984] Devel::Peek docs may contain incorrect examples
Message ID:
rt-4.0.24-32330-1554328291-445.133984-75-0@perl.org
# New Ticket Created by  Jim Avera 
# Please include the string:  [perl #133984]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=133984 >



This is a bug report for perl from jim.avera@gmail.com,
generated with the help of perlbug 1.40 running under perl 5.26.2.


-----------------------------------------------------------------
The pod for Devel::Peek contains many illustrations of references
where the RV value is different than the "at ..." address of the
referenced thing.  For example:

 "A reference to a simple scalar

	    use Devel::Peek;
	    $a = 42;
	    $b = \$a;
	    Dump $b;

    The output:
	    SV = IV(0xf041c) at 0xbe9a0
	      REFCNT = 1
	      FLAGS = (ROK)
	      RV = 0xbab08	
	      SV = IV(0xbe860) at 0xbe9a8  ### Not the same as 0xbab08 !?!
		REFCNT = 2
		FLAGS = (IOK,pIOK)
		IV = 42

This confused me mightly because the examples consistently showed
different values for RV and the referenced SV address, which contradicted 
my notions of what bits are stored in the IV of a reference. 

I had to download the perl source code and root around in it for 
a while, eventually coming to Perl_do_sv_dump() which, I think,
proves that my original notions were correct.  It was a lot of work 
but very educational and ultimately worthwhile. 

Still, those examples in the Devel::Peek pod cast darkness rather than light
because the sample addresses are inconsistent (RV can never be different
than the referenced-SV address).  It would be great if the examples were 
updated so the next person traveling this path will not be confused as I was.

Of course, I may well be missing something fundamental and/or mis-reading
the code in Perl_do_sv_dump(); in that case maybe the pod could mention 
under what circumstances those values might be different.

Thanks,
-Jim

-----------------------------------------------------------------
---
Flags:
    category=library
    severity=low
    module=Devel::Peek
---
Site configuration information for perl 5.26.2:

Configured by Ubuntu at Mon Nov 19 15:05:42 UTC 2018.

Summary of my perl5 (revision 5 version 26 subversion 2) configuration:
   
  Platform:
    osname=linux
    osvers=4.9.0
    archname=x86_64-linux-gnu-thread-multi
    uname='linux localhost 4.9.0 #1 smp debian 4.9.0 x86_64 gnulinux '
    config_args='-Dusethreads -Duselargefiles -Dcc=x86_64-linux-gnu-gcc -Dcpp=x86_64-linux-gnu-cpp -Dld=x86_64-linux-gnu-gcc -Dccflags=-DDEBIAN -Wdate-time -D_FORTIFY_SOURCE=2 -g -O2 -fdebug-prefix-map=/build/perl-6mb423/perl-5.26.2=. -fstack-protector-strong -Wformat -Werror=format-security -Dldflags= -Wl,-Bsymbolic-functions -Wl,-z,relro -Dlddlflags=-shared -Wl,-Bsymbolic-functions -Wl,-z,relro -Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.26 -Darchlib=/usr/lib/x86_64-linux-gnu/perl/5.26 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/x86_64-linux-gnu/perl5/5.26 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.26.2 -Dsitearch=/usr/local/lib/x86_64-linux-gnu/perl/5.26.2 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Duse64bitint
-Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -Ui_libutil -Ui_xlocale -Uversiononly -DDEBUGGING=-g -Doptimize=-O2 -dEs -Duseshrplib -Dlibperl=libperl.so.5.26.2'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=define
    usemultiplicity=define
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
    bincompat5005=undef
  Compiler:
    cc='x86_64-linux-gnu-gcc'
    ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    optimize='-O2 -g'
    cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include'
    ccversion=''
    gccversion='8.2.0'
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=12345678
    doublekind=3
    d_longlong=define
    longlongsize=8
    d_longdbl=define
    longdblsize=16
    longdblkind=3
    ivtype='long'
    ivsize=8
    nvtype='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='x86_64-linux-gnu-gcc'
    ldflags =' -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib/gcc/x86_64-linux-gnu/8/include-fixed /usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib
    libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
    perllibs=-ldl -lm -lpthread -lc -lcrypt
    libc=libc-2.28.so
    so=so
    useshrplib=true
    libperl=libperl.so.5.26
    gnulibc_version='2.28'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='-Wl,-E'
    cccdlflags='-fPIC'
    lddlflags='-shared -L/usr/local/lib -fstack-protector-strong'

Locally applied patches:
    DEBPKG:debian/cpan_definstalldirs - Provide a sensible INSTALLDIRS default for modules installed from CPAN.
    DEBPKG:debian/db_file_ver - https://bugs.debian.org/340047 Remove overly restrictive DB_File version check.
    DEBPKG:debian/doc_info - Replace generic man(1) instructions with Debian-specific information.
    DEBPKG:debian/enc2xs_inc - https://bugs.debian.org/290336 Tweak enc2xs to follow symlinks and ignore missing @INC directories.
    DEBPKG:debian/errno_ver - https://bugs.debian.org/343351 Remove Errno version check due to upgrade problems with long-running processes.
    DEBPKG:debian/libperl_embed_doc - https://bugs.debian.org/186778 Note that libperl-dev package is required for embedded linking
    DEBPKG:fixes/respect_umask - Respect umask during installation
    DEBPKG:debian/writable_site_dirs - Set umask approproately for site install directories
    DEBPKG:debian/extutils_set_libperl_path - EU:MM: set location of libperl.a under /usr/lib
    DEBPKG:debian/no_packlist_perllocal - Don't install .packlist or perllocal.pod for perl or vendor
    DEBPKG:debian/fakeroot - Postpone LD_LIBRARY_PATH evaluation to the binary targets.
    DEBPKG:debian/instmodsh_doc - Debian policy doesn't install .packlist files for core or vendor.
    DEBPKG:debian/ld_run_path - Remove standard libs from LD_RUN_PATH as per Debian policy.
    DEBPKG:debian/libnet_config_path - Set location of libnet.cfg to /etc/perl/Net as /usr may not be writable.
    DEBPKG:debian/perlivp - https://bugs.debian.org/510895 Make perlivp skip include directories in /usr/local
    DEBPKG:debian/deprecate-with-apt - https://bugs.debian.org/747628 Point users to Debian packages of deprecated core modules
    DEBPKG:debian/squelch-locale-warnings - https://bugs.debian.org/508764 Squelch locale warnings in Debian package maintainer scripts
    DEBPKG:debian/patchlevel - https://bugs.debian.org/567489 List packaged patches for 5.26.2-7ubuntu0.1 in patchlevel.h
    DEBPKG:fixes/document_makemaker_ccflags - https://bugs.debian.org/628522 [rt.cpan.org #68613] Document that CCFLAGS should include $Config{ccflags}
    DEBPKG:debian/find_html2text - https://bugs.debian.org/640479 Configure CPAN::Distribution with correct name of html2text
    DEBPKG:debian/perl5db-x-terminal-emulator.patch - https://bugs.debian.org/668490 Invoke x-terminal-emulator rather than xterm in perl5db.pl
    DEBPKG:debian/cpan-missing-site-dirs - https://bugs.debian.org/688842 Fix CPAN::FirstTime defaults with nonexisting site dirs if a parent is writable
    DEBPKG:fixes/memoize_storable_nstore - [rt.cpan.org #77790] https://bugs.debian.org/587650 Memoize::Storable: respect 'nstore' option not respected
    DEBPKG:debian/makemaker-pasthru - https://bugs.debian.org/758471 Pass LD settings through to subdirectories
    DEBPKG:debian/makemaker-manext - https://bugs.debian.org/247370 Make EU::MakeMaker honour MANnEXT settings in generated manpage headers
    DEBPKG:debian/kfreebsd-softupdates - https://bugs.debian.org/796798 Work around Debian Bug#796798
    DEBPKG:fixes/autodie-scope - https://bugs.debian.org/798096 Fix a scoping issue with "no autodie" and the "system" sub
    DEBPKG:fixes/memoize-pod - [rt.cpan.org #89441] Fix POD errors in Memoize
    DEBPKG:debian/hurd-softupdates - https://bugs.debian.org/822735 Fix t/op/stat.t failures on hurd
    DEBPKG:fixes/math_complex_doc_great_circle - https://bugs.debian.org/697567 [rt.cpan.org #114104] Math::Trig: clarify definition of great_circle_midpoint
    DEBPKG:fixes/math_complex_doc_see_also - https://bugs.debian.org/697568 [rt.cpan.org #114105] Math::Trig: add missing SEE ALSO
    DEBPKG:fixes/math_complex_doc_angle_units - https://bugs.debian.org/731505 [rt.cpan.org #114106] Math::Trig: document angle units
    DEBPKG:fixes/cpan_web_link - https://bugs.debian.org/367291 CPAN: Add link to main CPAN web site
    DEBPKG:fixes/time_piece_doc - https://bugs.debian.org/817925 Time::Piece: Improve documentation for add_months and add_years
    DEBPKG:fixes/extutils_makemaker_reproducible - https://bugs.debian.org/835815 https://bugs.debian.org/834190 Make perllocal.pod files reproducible
    DEBPKG:fixes/file_path_hurd_errno - File-Path: Fix test failure in Hurd due to hard-coded ENOENT
    DEBPKG:debian/hppa_op_optimize_workaround - https://bugs.debian.org/838613 Temporarily lower the optimization of op.c on hppa due to gcc-6 problems
    DEBPKG:debian/installman-utf8 - https://bugs.debian.org/840211 Generate man pages with UTF-8 characters
    DEBPKG:fixes/file_path_chmod_race - https://bugs.debian.org/863870 [rt.cpan.org #121951] Prevent directory chmod race attack.
    DEBPKG:fixes/extutils_file_path_compat - Correct the order of tests of chmod(). (#294)
    DEBPKG:fixes/getopt-long-2 - [rt.cpan.org #120300] Withdraw part of commit 5d9947fb445327c7299d8beb009d609bc70066c0, which tries to implement more GNU getopt_long campatibility. GNU
    DEBPKG:fixes/getopt-long-3 - provide a default value for optional arguments
    DEBPKG:fixes/getopt-long-4 - https://bugs.debian.org/864544 [rt.cpan.org #122068] Fix issue #122068.
    DEBPKG:fixes/test-builder-reset - https://bugs.debian.org/865894 Reset inside subtest maintains parent
    DEBPKG:debian/hppa_opmini_optimize_workaround - https://bugs.debian.org/869122 Lower the optimization level of opmini.c on hppa
    DEBPKG:debian/sh4_op_optimize_workaround - https://bugs.debian.org/869373 Also lower the optimization level of op.c and opmini.c on sh4
    DEBPKG:fixes/json-pp-example - [rt.cpan.org #92793] https://bugs.debian.org/871837 fix RT-92793: bug in SYNOPSIS
    DEBPKG:debian/perldoc-pager - https://bugs.debian.org/870340 [rt.cpan.org #120229] Fix perldoc terminal escapes when sensible-pager is less
    DEBPKG:debian/prune_libs - https://bugs.debian.org/128355 Prune the list of libraries wanted to what we actually need.
    DEBPKG:fixes/rename-filexp.U-phase1 - regen-configure: rename filexp.U to filexp_path.U, phase 1
    DEBPKG:fixes/rename-filexp.U-phase2 - regen-configure: rename filexp.U to filexp_path.U, phase 2
    DEBPKG:fixes/packaging_test_skips - Skip various tests if PERL_BUILD_PACKAGING is set
    DEBPKG:debian/mod_paths - Tweak @INC ordering for Debian
    DEBPKG:fixes/encode-alias-regexp - https://bugs.debian.org/880085 fix https://github.com/dankogai/p5-encode/issues/127
    DEBPKG:fixes/CVE-2018-6797-testcase - [perl #132227] (perl #132227) restart a node if we change to uni rules within the node and encounter a sharp S
    DEBPKG:fixes/test-printf-null - https://bugs.debian.org/893601 Fix pointer to integer cast in null_printf check
    DEBPKG:debian/configure-regen - https://bugs.debian.org/762638 Regenerate Configure et al. after probe unit changes
    DEBPKG:fixes/CVE-2018-12015-Archive-Tar-directory-traversal - https://bugs.debian.org/900834 [rt.cpan.org #125523] Remove existing files before overwriting them
    DEBPKG:debian/gdbm-fatal - [perl #133295] https://bugs.debian.org/904005 Temporarily skip GDBM_File fatal.t for gdbm >= 1.15 compatibility
    DEBPKG:fixes/CVE-2018-18311.patch - [PATCH] Perl_my_setenv(); handle integer wrap
    DEBPKG:fixes/CVE-2018-18312.patch - [PATCH 242/242] PATCH: [perl #133423] for 5.26 maint
    DEBPKG:fixes/CVE-2018-18313.patch - [PATCH] regcomp.c: Convert some strchr to memchr
    DEBPKG:fixes/CVE-2018-18314.patch - [PATCH] fix #131649 - extended charclass can trigger assert

---
@INC for perl 5.26.2:
    /home/jima/lib/perl
    /home/jima/perl5/lib/perl5/x86_64-linux-gnu-thread-multi
    /home/jima/perl5/lib/perl5/5.26.2/x86_64-linux-gnu-thread-multi
    /home/jima/perl5/lib/perl5/5.26.2
    /home/jima/perl5/lib/perl5/x86_64-linux-gnu-thread-multi
    /home/jima/perl5/lib/perl5
    /etc/perl
    /usr/local/lib/x86_64-linux-gnu/perl/5.26.2
    /usr/local/share/perl/5.26.2
    /usr/lib/x86_64-linux-gnu/perl5/5.26
    /usr/share/perl5
    /usr/lib/x86_64-linux-gnu/perl/5.26
    /usr/share/perl/5.26
    /home/jima/perl5/lib/perl5/5.26.1
    /home/jima/perl5/lib/perl5/5.26.1/x86_64-linux-gnu-thread-multi
    /home/jima/perl5/lib/perl5/5.26.0
    /home/jima/perl5/lib/perl5/5.26.0/x86_64-linux-gnu-thread-multi
    /usr/local/lib/site_perl
    /usr/lib/x86_64-linux-gnu/perl-base

---
Environment for perl 5.26.2:
    HOME=/home/jima
    LANG=en_US.UTF-8
    LANGUAGE (unset)
    LC_COLLATE=C
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/jima/.local/bin:/home/jima/perl5/bin:/home/jima/bin:/home/jima/jima_tools/x86_64/bin:/home/jima/jima_tools/bin:/opt/Qt/5.12.1/gcc_64/bin:/opt/Qt/Tools/QtCreator/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/bin/X11:/usr/local/bin:/usr/local/sbin:/usr/games:/usr/local/games:/snap/bin:/usr/lib/jvm/java-8-oracle/bin:/usr/lib/jvm/java-8-oracle/jre/bin:.
    PERL5LIB=/home/jima/lib/perl:/home/jima/perl5/lib/perl5/x86_64-linux-gnu-thread-multi:/home/jima/perl5/lib/perl5
    PERL_BADLANG (unset)
    PERL_LOCAL_LIB_ROOT=/home/jima/perl5
    PERL_MB_OPT=--install_base /home/jima/perl5
    PERL_MM_OPT=INSTALL_BASE=/home/jima/perl5
    SHELL=/bin/bash


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