Fcntl and fcntl() woes

[p5pRT]

Migrated from rt.perl.org#3502 (status was 'resolved')

Searchable as RT3502$

[p5pRT]

From [email protected]

Perhaps I'm misunderstanding the use of fcntl(), but I can't get it to do
what I'd like it to. I've read the fcntl(2) and fcntl(5) man pages, but I
don't know if I'm supposed to unpack() the flags somehow.

[jefpin@​towers ~]$ perl
use Fcntl;
fcntl(STDIN, F_GETFL, $mode = '');
print $mode;
Ý@​

[jefpin@​towers ~]$ perl
use Fcntl;
fcntl(STDIN, F_GETFL, $mode = '');
print $mode & O_RDONLY;
print $mode & O_WRONLY;
print $mode & O_RDWR;
000

[jefpin@​towers ~]$

Perl Info

Site configuration information for perl 5.00503:

Configured by root at Wed Feb  2 15:34:50 EST 2000.

Summary of my perl5 (5.0 patchlevel 5 subversion 3) configuration:
  Platform:
    osname=linux, osvers=2.2.5-22smp, archname=i386-linux
    uname='linux porky.devel.redhat.com 2.2.5-22smp #1 smp wed jun 2 09:11:51 edt 1999 i686 unknown '
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=undef useperlio=undef d_sfio=undef
  Compiler:
    cc='cc', optimize='-O2 -m486 -fno-strength-reduce', gccversion=egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)
    cppflags='-Dbool=char -DHAS_BOOL -I/usr/local/include'
    ccflags ='-Dbool=char -DHAS_BOOL -I/usr/local/include'
    stdchar='char', d_stdstdio=undef, usevfork=false
    intsize=4, longsize=4, ptrsize=4, doublesize=8
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
    alignbytes=4, usemymalloc=n, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib
    libs=-lnsl -ldl -lm -lc -lposix -lcrypt
    libc=, so=so, useshrplib=false, libperl=libperl.a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynamic'
    cccdlflags='-fpic', lddlflags='-shared -L/usr/local/lib'

Locally applied patches:
    


@INC for perl 5.00503:
    /usr/lib/perl5/5.00503/i386-linux
    /usr/lib/perl5/5.00503
    /usr/lib/perl5/site_perl/5.005/i386-linux
    /usr/lib/perl5/site_perl/5.005
    .


Environment for perl 5.00503:
    HOME=/home/jefpin
    LANG=en_US
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin
    PERL_BADLANG (unset)
    SHELL=/bin/tcsh

[p5pRT]

From [Unknown Contact. See original ticket]

On Thu, Jul 13, 2000 at 07​:32​:27PM -0400, Jeff Pinyan wrote​:

Perhaps I'm misunderstanding the use of fcntl(), but I can't get it to do
what I'd like it to. I've read the fcntl(2) and fcntl(5) man pages, but I
don't know if I'm supposed to unpack() the flags somehow.

[jefpin@​towers ~]$ perl
use Fcntl;
fcntl(STDIN, F_GETFL, $mode = '');
print $mode;
Ý@​

Unfortunately you are misunderstanding the use of fcntl. This isn't
surprising, considering the requirement for a third argument by perl, and
the differences from ioctl. This could very well be considered a Perl bug,
because fcntl(2) reveals a third argument should be omitted in this case,
and strace reveals perl is indeed omitting said third argument.

  use Fcntl;
  $mode = fcntl(STDIN, F_GETFL, 0) || die("fcntl failed​: $!");
 
  print $mode;

Michael
--
Administrator www.shoebox.net
Programmer, System Administrator www.gallanttech.com

[p5pRT]

From [Unknown Contact. See original ticket]

On Jul 13, Michael Fowler said​:

On Thu, Jul 13, 2000 at 07​:32​:27PM -0400, Jeff Pinyan wrote​:

use Fcntl;
fcntl(STDIN, F_GETFL, $mode = '');

Unfortunately you are misunderstanding the use of fcntl. This isn't
surprising, considering the requirement for a third argument by perl, and
the differences from ioctl. This could very well be considered a Perl bug,
because fcntl(2) reveals a third argument should be omitted in this case,
and strace reveals perl is indeed omitting said third argument.

use Fcntl;
$mode = fcntl(STDIN, F_GETFL, 0) || die("fcntl failed​: $!");
print $mode;

Then the Perl documentation is wrong​:

  use Fcntl;
  fcntl($filehandle, F_GETFL, $packed_return_buffer)
  or die "can't fcntl F_GETFL​: $!";

And as for the return value... what flags can I test against it?

jeffp@​hut [12​:05am] ~ #102> perl
use Fcntl;
print fcntl($_, F_GETFL, 0) . "\n" for *STDIN, *STDOUT, *STDERR;
__END__
2
2
2

I'd provide a documentation patch if I had any idea what was truth.

[p5pRT]

From [Unknown Contact. See original ticket]

On Fri, Jul 14, 2000 at 12​:07​:17AM -0400, Jeff Pinyan wrote​:

And as for the return value... what flags can I test against it?

jeffp@​hut [12​:05am] ~ #102> perl
use Fcntl;
print fcntl($_, F_GETFL, 0) . "\n" for *STDIN, *STDOUT, *STDERR;
__END__
2
2
2

I'd provide a documentation patch if I had any idea what was truth.

It appears testing against STDIN, STDOUT, and STDERR is giving you results
you don't expect (I don't expect them to be open O_RDWR either..).

  use Fcntl;

  sysopen(IN, "foo.txt", O_RDONLY) || die( "open in​: $!");
  sysopen(OUT, "foo.txt", O_WRONLY) || die( "open out​: $!");
  sysopen(BOTH, "foo.txt", O_RDWR) || die("open both​: $!");

  foreach my $fh (*STDIN, *STDOUT, *STDERR, *IN, *OUT, *BOTH) {
  printf(
  "handle​: %-6s mode​: %d\n",
  *$fh{NAME}, fcntl($fh, F_GETFL, 0) || die("fcntl​: $!")
  );
  }

  __END__
  handle​: STDIN mode​: 2 # O_RDWR
  handle​: STDOUT mode​: 2 # O_RDWR
  handle​: STDERR mode​: 2 # O_RDWR
  handle​: IN mode​: 0 # O_RDONLY
  handle​: OUT mode​: 1 # O_WRONLY
  handle​: BOTH mode​: 2 # O_RDWR

So it produces what's expected, it was only the test case chosen causing
confusion.

Michael
--
Administrator www.shoebox.net
Programmer, System Administrator www.gallanttech.com

[p5pRT]

From [Unknown Contact. See original ticket]

On Jul 13, Michael Fowler said​:

use Fcntl;
sysopen(IN, "foo.txt", O_RDONLY) || die( "open in​: $!");
sysopen(OUT, "foo.txt", O_WRONLY) || die( "open out​: $!");
sysopen(BOTH, "foo.txt", O_RDWR) || die("open both​: $!");
foreach my $fh (*STDIN, *STDOUT, *STDERR, *IN, *OUT, *BOTH) {
printf(
"handle​: %-6s mode​: %d\n",
*$fh{NAME}, fcntl($fh, F_GETFL, 0) || die("fcntl​: $!")
);
}

Thank you for the clarification. :)

[p5pRT]

@smpeters - Status changed from 'open' to 'resolved'