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open2 acts very strange when STDOUT is redirected. #12748

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p5pRT opened this issue Jan 30, 2013 · 3 comments
Open

open2 acts very strange when STDOUT is redirected. #12748

p5pRT opened this issue Jan 30, 2013 · 3 comments

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@p5pRT
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p5pRT commented Jan 30, 2013

Migrated from rt.perl.org#116589 (status was 'open')

Searchable as RT116589$
@p5pRT
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p5pRT commented Jan 30, 2013

From xungeng@gmail.com

Created by xungeng@gmail.com

see this example​:
xungeng@​debian​:~/work/test/test$ cat foo
#!/usr/bin/perl
use FileHandle;
use IPC​::Open2;
use IO​::Handle;

#*STDOUT = IO​::Handle->new;

for (1..3) {
  print STDERR "Loop​: $_\n";
  $pid = open2( \*Reader, \*Writer, "cat" );
  Writer->autoflush(); # default here, actually
  print Writer "stuff\n";
  $got = <Reader>;
  print STDERR $got;
}
xungeng@​debian​:~/work/test/test$ cat bar
#!/usr/bin/perl
use FileHandle;
use IPC​::Open2;
use IO​::Handle;

*STDOUT = IO​::Handle->new;

for (1..3) {
  print STDERR "Loop​: $_\n";
  $pid = open2( \*Reader, \*Writer, "cat" );
  Writer->autoflush(); # default here, actually
  print Writer "stuff\n";
  $got = <Reader>;
  print STDERR $got;
}
xungeng@​debian​:~/work/test/test$ ./foo > /dev/null
Loop​: 1
stuff
Loop​: 2
stuff
Loop​: 3
stuff
xungeng@​debian​:~/work/test/test$ ./bar > /dev/null
Loop​: 1
Loop​: 2
Loop​: 3

the difference between foo and bar is only that STDOUT redirected or not.

when STDOUT is redirected, open2 make the child process write to the old
STDOUT,
not the child_out handler (\*Reader in this example).

Perl Info 

Flags:
    category=library
    severity=high
    module=IPC::Open2

Site configuration information for perl 5.14.2:

Configured by Debian Project at Mon Dec 10 13:17:22 UTC 2012.

Summary of my perl5 (revision 5 version 14 subversion 2) configuration:

  Platform:
    osname=linux, osvers=3.2.0-4-686-pae,
archname=i486-linux-gnu-thread-multi-64int
    uname='linux callisto 3.2.0-4-686-pae #1 smp debian 3.2.32-1 i686
gnulinux '
    config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN
-D_FORTIFY_SOURCE=2 -g -O2 -fstack-protector --param=ssp-buffer-size=4
-Wformat -Werror=format-security -Dldflags= -Wl,-z,relro
-Dlddlflags=-shared -Wl,-z,relro -Dcccdlflags=-fPIC
-Darchname=i486-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.14
-Darchlib=/usr/lib/perl/5.14 -Dvendorprefix=/usr
-Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/perl5
-Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.14.2
-Dsitearch=/usr/local/lib/perl/5.14.2 -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 -DDEBUGGING=-g -Doptimize=-O2 -Duseshrplib
-Dlibperl=libperl.so.5.14.2 -des'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=define, usemultiplicity=define
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
    use64bitint=define, use64bitall=undef, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN
-fstack-protector -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 -fstack-protector
-fno-strict-aliasing -pipe -I/usr/local/include'
    ccversion='', gccversion='4.7.2', gccosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=12345678
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
    ivtype='long long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t',
lseeksize=8
    alignbytes=4, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -fstack-protector -L/usr/local/lib'
    libpth=/usr/local/lib /lib/i386-linux-gnu /lib/../lib
/usr/lib/i386-linux-gnu /usr/lib/../lib /lib /usr/lib
    libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
    perllibs=-ldl -lm -lpthread -lc -lcrypt
    libc=, so=so, useshrplib=true, libperl=libperl.so.5.14.2
    gnulibc_version='2.13'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
    cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib
-fstack-protector'

Locally applied patches:



@INC for perl 5.14.2:
    /etc/perl
    /usr/local/lib/perl/5.14.2
    /usr/local/share/perl/5.14.2
    /usr/lib/perl5
    /usr/share/perl5
    /usr/lib/perl/5.14
    /usr/share/perl/5.14
    /usr/local/lib/site_perl
    .


Environment for perl 5.14.2:
    HOME=/home/xungeng
    LANG=en_US.UTF-8
    LANGUAGE=en_US:en
    LD_LIBRARY_PATH=/usr/local/lib/
    LOGDIR (unset)

PATH=/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games:/home/xungeng/bin:/usr/local/go/bin
    PERL_BADLANG (unset)
    SHELL=/bin/bash

@p5pRT
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p5pRT commented Jan 30, 2013

From @iabyn

On Tue, Jan 29, 2013 at 10​:38​:14PM -0800, Xungeng Lee wrote​:

see this example​:
xungeng@​debian​:~/work/test/test$ cat foo
#!/usr/bin/perl
use FileHandle;
use IPC​::Open2;
use IO​::Handle;

#*STDOUT = IO​::Handle->new;

for (1..3) {
print STDERR "Loop​: $_\n";
$pid = open2( \*Reader, \*Writer, "cat" );
Writer->autoflush(); # default here, actually
print Writer "stuff\n";
$got = <Reader>;
print STDERR $got;
}
xungeng@​debian​:~/work/test/test$ cat bar
#!/usr/bin/perl
use FileHandle;
use IPC​::Open2;
use IO​::Handle;

*STDOUT = IO​::Handle->new;

for (1..3) {
print STDERR "Loop​: $_\n";
$pid = open2( \*Reader, \*Writer, "cat" );
Writer->autoflush(); # default here, actually
print Writer "stuff\n";
$got = <Reader>;
print STDERR $got;
}
xungeng@​debian​:~/work/test/test$ ./foo > /dev/null
Loop​: 1
stuff
Loop​: 2
stuff
Loop​: 3
stuff
xungeng@​debian​:~/work/test/test$ ./bar > /dev/null
Loop​: 1
Loop​: 2
Loop​: 3

the difference between foo and bar is only that STDOUT redirected or not.

when STDOUT is redirected, open2 make the child process write to the old
STDOUT,
not the child_out handler (\*Reader in this example).

Well for stdout, IPC​::Open3 (which IPC​::Open2 calls out to) does
essentially​:

  pipe(\*Reader, \*FOO);
  fork();
  if (child) {
  close(Writer);
  open(STDOUT, ">&=" . fileno(FOO));
  exec($cmd);
  }

i.e. it dups the file handle of the pipe to whatever file handle STDOUT
maps to. If STDOUT doesn't point to file handle 1, then it won't work.

I don't understand Open3.pm enough to decide whether it's unreasonable for
it to handle a non-1 STDOUT (etc) or not.

--
Any [programming] language that doesn't occasionally surprise the
novice will pay for it by continually surprising the expert.
  -- Larry Wall

@p5pRT
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p5pRT commented Jan 30, 2013

The RT System itself - Status changed from 'new' to 'open'

@xenu xenu removed the Severity Low label Dec 29, 2021
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