[ID 20010309.005] read, fork and exit mismatch file positions

To: perlbug@perl.com
Subject: read, fork and exit mismatch file positions
Reply-To: salvador.fandino@corp.vizzavi.net

This is a bug report for perl from salvador.fandino@corp.vizzavi.net,
generated with the help of perlbug 1.28 running under perl v5.6.0.


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

Hello,

I think I have found a bug. When using the read, fork, exit commands,
file positions get wrong. The script below will show:


---cut-here---
#!/usr/bin/perl

use strict;
use warnings;

use Getopt::Std;

our ($opt_s,$opt_w);
getopts("sw");

select STDERR; $|=1;
select STDOUT; $|=1;

open F, "<".$ARGV[0];

my $s;
my $t;

while ($s=read(F,$_,1024)) {
  $t+=$s;
  my $tell=tell F;
  seek(F,0,1) if $opt_s; # work around to solve the problem
  print "read: $s, total: $t, position: $tell\n";
  my $f=fork;
  if (defined($f) and $f == 0) {
    sleep 10 if $opt_w; # this delay hides the problem.
    exit(0);
  }
}

1 while ( wait != -1);

---cut-here---

if I run this test:

  [salva@sws0045eu:~]$ testfork.pl /bin/ls 
  read: 1024, total: 1024, position: 1024
  read: 1024, total: 2048, position: -5120
  read: 1024, total: 3072, position: -4096
  read: 1024, total: 4096, position: -3072
  read: 1024, total: 5120, position: -2048
  read: 1024, total: 6144, position: -1024
  read: 1024, total: 7168, position: 0
  read: 1024, total: 8192, position: 1024
  read: 1024, total: 9216, position: 2048
  read: 1024, total: 10240, position: 3072
  read: 1024, total: 11264, position: 4096
  read: 1024, total: 12288, position: 5120
  read: 1024, total: 13312, position: 6144
  read: 1024, total: 14336, position: 7168
  read: 1024, total: 15360, position: 8192
  read: 1024, total: 16384, position: 9216
  read: 1024, total: 17408, position: 10240
  read: 1024, total: 18432, position: 11264
  read: 1024, total: 19456, position: 12288
  read: 1024, total: 20480, position: 13312
  read: 1024, total: 21504, position: 14336
  read: 1024, total: 22528, position: 15360
  read: 1024, total: 23552, position: 16384
  read: 1024, total: 24576, position: 17408
  read: 32, total: 24608, position: 17440

As you can see the file position reported by the tell command gets
wrong after the first fork.

If I do a seek to the current position with "seek(F,0,SEEK_CUR)" the
problem disappears:

  [salva@sws0045eu:~]$ testfork.pl -s /bin/ls 
  read: 1024, total: 1024, position: 1024
  read: 1024, total: 2048, position: 2048
  read: 1024, total: 3072, position: 3072
  read: 1024, total: 4096, position: 4096
  read: 1024, total: 5120, position: 5120
  read: 1024, total: 6144, position: 6144
  read: 1024, total: 7168, position: 7168
  read: 1024, total: 8192, position: 8192
  read: 1024, total: 9216, position: 9216
  read: 1024, total: 10240, position: 10240
  read: 1024, total: 11264, position: 11264
  read: 1024, total: 12288, position: 12288
  read: 1024, total: 13312, position: 13312
  read: 1024, total: 14336, position: 14336
  read: 1024, total: 15360, position: 15360
  read: 1024, total: 16384, position: 16384
  read: 1024, total: 17408, position: 17408
  read: 32, total: 17440, position: 17440

Delaying the 'exit' call, everything works ok too, so the problems
seems to be between 'exit' doing something to open files and 'read'
letting the file state in an inconsistent way.

I have tried this script with the current perl 5.7.0 development version
obtaining the same result.


Bye,

- Salva


[Please do not change anything below this line]
-----------------------------------------------------------------
---
Flags:
    category=core
    severity=medium
---
Site configuration information for perl v5.6.0:

Configured by root at Mon Jul 24 21:57:26 BST 2000.

Summary of my perl5 (revision 5.0 version 6 subversion 0) configuration:
  Platform:
    osname=solaris, osvers=2.6, archname=sun4-solaris
    uname='sunos sfile01eu 5.6 generic_105181-21 sun4u sparc
sunw,ultra-4 '
    config_args='-Dprefix=/apps/perl/std -Dcc=gcc
-Dperladmin=sampo@iki.fi -des'
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=undef use5005threads=undef useithreads=undef
usemultiplicity=undef
    useperlio=undef d_sfio=undef uselargefiles=define 
    use64bitint=undef use64bitall=undef uselongdouble=undef
usesocks=undef
  Compiler:
    cc='gcc', optimize='-O', gccversion=2.95.2 19991024 (release)
    cppflags='-fno-strict-aliasing -I/usr/local/include'
    ccflags ='-fno-strict-aliasing -I/usr/local/include
-D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    stdchar='unsigned char', d_stdstdio=define, usevfork=false
    intsize=4, longsize=4, ptrsize=4, doublesize=8
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t',
lseeksize=8
    alignbytes=8, usemymalloc=y, prototype=define
  Linker and Libraries:
    ld='gcc', ldflags =' -L/usr/local/lib '
    libpth=/usr/local/lib /lib /usr/lib /usr/ccs/lib
    libs=-lsocket -lnsl -ldl -lm -lc -lcrypt -lsec
    libc=/lib/libc.so, so=so, useshrplib=false, libperl=libperl.a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags=' '
    cccdlflags='-fPIC', lddlflags='-G -L/usr/local/lib'

Locally applied patches:
    

---
@INC for perl v5.6.0:
    /home/salva
    /home/salva/apache
    /home/salva/lib/5.6.0/sun4-solaris
    /home/salva/lib/5.6.0/sun4-solaris
    /home/salva/lib/5.6.0
    /home/salva/lib/site_perl/5.6.0/sun4-solaris
    /home/salva/lib/site_perl/5.6.0/sun4-solaris
    /home/salva/lib/site_perl/5.6.0
    /home/salva/lib/site_perl/5.6.0/sun4-solaris
    /home/salva/lib/site_perl/5.6.0
    /home/salva/lib/site_perl
    /home/salva/apache/lib/5.6.0/sun4-solaris
    /home/salva/apache/lib/5.6.0/sun4-solaris
    /home/salva/apache/lib/5.6.0
    /home/salva/apache/lib/site_perl/5.6.0/sun4-solaris
    /home/salva/apache/lib/site_perl/5.6.0/sun4-solaris
    /home/salva/apache/lib/site_perl/5.6.0
    /home/salva/apache/lib/site_perl/5.6.0/sun4-solaris
    /home/salva/apache/lib/site_perl/5.6.0
    /home/salva/apache/lib/site_perl
    /apps/perl/std/lib/5.6.0/sun4-solaris
    /apps/perl/std/lib/5.6.0
    /apps/perl/std/lib/site_perl/5.6.0/sun4-solaris
    /apps/perl/std/lib/site_perl/5.6.0
    /apps/perl/std/lib/site_perl
    .

---
Environment for perl v5.6.0:
    HOME=/home/salva
    LANG=en_UK
    LANGUAGE (unset)
    LC_COLLATE=en_UK
    LC_CTYPE=en_UK
    LC_MESSAGES=C
    LC_MONETARY=en_UK
    LC_NUMERIC=en_UK
    LC_TIME=en_UK
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PERL5LIB=/home/salva:/home/salva/apache:\
    /home/salva/lib/5.6.0/sun4-solaris:/home/salva/lib/5.6.0:\
    /home/salva/lib/site_perl/5.6.0/sun4-solaris:\
    /home/salva/lib/site_perl/5.6.0:/home/salva/lib/site_perl:\
   
/home/salva/apache/lib/5.6.0/sun4-solaris:/home/salva/apache/lib/5.6.0:\
    /home/salva/apache/lib/site_perl/5.6.0/sun4-solaris:\
   
/home/salva/apache/lib/site_perl/5.6.0:/home/salva/apache/lib/site_perl
    PERL_BADLANG (unset)
    SHELL=/opt/VIZZutil/bin/bash


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• [ID 20010309.005] read, fork and exit mismatch file positions by salvador.fandino