From @jimav

Created by @jimav

use Fcntl;
printf "O_RDWR is %d\n", &O_RDWR; # no warning or error
die "Error, O_RDWR not defined" unless defined (&O_RDWR); #
***DIES HERE***

This is inconsistent, and is a real user problem if a symbol may or
may
not actually be exported by the package used. If the symbol is not
actually exported, the program must not use it; but the program
can't
test for this condition! For example​:

  use strict;
  use Fcntl;
  if (defined &O_FOOBAR) {
  printf "O_FOOBAR = %s\n", &O_FOOBAR; # case 1
  } else {
  die "O_FOOBAR not defined, so I won't use it\n"; # case 2
  }

if O_FOOBAR is undefined, it hits case 2 (correct).
if O_FOOBAR is a locally-defined sub, it hits case 1 (correct).
if O_FOOBAR is replaced by O_RDWR, it hits case 2 (WRONG).

I hit this problem trying to use O_BINARY on Doshish platforms.
sysopen *must* be passed O_BINARY to avoid CR-LF translations.
But O_BINARY presumably might not be defined on Unix platforms.
So I tried this​:
  use Fcntl;
  if (defined &O_BINARY) {
  sysopen(FILE,$path, O_RDONLY|O_BINARY) ...
  } else {
  sysopen(FILE,$path, O_RDONLY) ...
  }
This FAILED because the second branch was always taken, even on
Windows
where O_BINARY is in fact defined.

I'm not certain whether or not O_BINARY is defined on all
platforms,
but that's the whole point... programs should be able to test
whether
or not a constant is in fact exported from a package which it is
using.

Cheers.

Flags:
    category=core
    severity=high

Site configuration information for perl v5.6.1:

Configured by root at Mon Mar  4 11:19:37 PST 2002.

Summary of my perl5 (revision 5.0 version 6 subversion 1)
configuration:
  Platform:
    osname=linux, osvers=2.4.7-10smp, archname=i686-linux
    uname='linux tribble.simplex.com 2.4.7-10smp #1 smp thu sep 6
17:09:31 edt 2
001 i686 unknown '
    config_args=''
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=undef use5005threads=undef useithreads=undef
usemultiplicity=unde
f
    useperlio=undef d_sfio=undef uselargefiles=define
usesocks=undef
    use64bitint=undef use64bitall=undef uselongdouble=undef
  Compiler:
    cc='gcc', ccflags ='-fno-strict-aliasing -D_LARGEFILE_SOURCE
-D_FILE_OFFSET_
BITS=64',
    optimize='-O2',
    cppflags='-fno-strict-aliasing'
    ccversion='', gccversion='2.96 20000731 (Red Hat Linux 7.1
2.96-98)', gccosa
ndvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
    d_longlong=define, longlongsize=8, d_longdbl=define,
longdblsize=12
    ivtype='long', ivsize=4, nvtype='double', nvsize=8,
Off_t='off_t', lseeksize
=8
    alignbytes=4, usemymalloc=n, prototype=define
  Linker and Libraries:
    ld='gcc', ldflags =''
    libpth=/lib /usr/lib
    libs=-lnsl -lgdbm -ldb -ldl -lm -lc -lcrypt -lutil
    perllibs=-lnsl -ldl -lm -lc -lcrypt -lutil
    libc=/lib/libc-2.2.4.so, so=so, useshrplib=false,
libperl=libperl.a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef,
ccdlflags='-rdynamic'
    cccdlflags='-fpic', lddlflags='-shared'

Locally applied patches:
    


@INC for perl v5.6.1:
    /usr/local/lib/perl5/5.6.1/i686-linux
    /usr/local/lib/perl5/5.6.1
    /usr/local/lib/perl5/site_perl/5.6.1/i686-linux
    /usr/local/lib/perl5/site_perl/5.6.1
    /usr/local/lib/perl5/site_perl
    .


Environment for perl v5.6.1:
    HOME=/home/jima
    LANG=en_US
    LANGUAGE (unset)
    LC_COLLATE=POSIX
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
   
PATH=output/linux_x86/bin:libdft/src:../../output/linux_x86/bin:../../libdft
/src:/home/jima/work-branch/output/linux_x86/bin:/home/jima/work-branch/libdft/s
rc:/home/jima/work/output/linux_x86/bin:/home/jima/work/libdft/src:/home/jima/wo
rk.mess/output/linux_x86/bin:/home/jima/work.mess/libdft/src:/home/jima/bin/linu
x_x86:/home/jima/bin:/home/jima/jima_tools/linux_x86/bin:/usr/local/bin:/usr/loc
al/gcc/3.1/bin:/usr/local/gcc/gdb-5.2/bin:/bin:/usr/bin/X11:/usr/sbin:/opt/vnc/b
in:/opt/mozilla:/opt/jima/bin:/usr/bin:/usr/X11R6/bin:/usr/games:/opt/kde3/bin:/
usr/lib/java/bin:/opt/gnome/bin:.
    PERL_BADLANG (unset)
    SHELL=/bin/bash



=====
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From @rgs

Jim Avera (via RT) <perlbug@​perl.org> wrote​:

use Fcntl;
printf "O_RDWR is %d\n", &O_RDWR; # no warning or error
die "Error, O_RDWR not defined" unless defined (&O_RDWR); #
***DIES HERE***

In fact, the Fcntl constants are AUTOLOAD'ed.
To check whether they exist, you should use exists() rather than
defined().
HTH.

From @eserte

Jim Avera (via RT) <perlbug@​perl.org> writes​:

# New Ticket Created by Jim Avera
# Please include the string​: [perl #18046]
# in the subject line of all future correspondence about this issue.
# <URL​: http​://rt.perl.org/rt2/Ticket/Display.html?id=18046 >

This is a bug report for perl from james_avera@​yahoo.com,
generated with the help of perlbug 1.33 running under perl v5.6.1.

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

use Fcntl;
printf "O_RDWR is %d\n", &O_RDWR; # no warning or error
die "Error, O_RDWR not defined" unless defined (&O_RDWR); #
***DIES HERE***

This is inconsistent, and is a real user problem if a symbol may or
may
not actually be exported by the package used. If the symbol is not
actually exported, the program must not use it; but the program
can't
test for this condition! For example​:

use strict;
use Fcntl;
if (defined &O_FOOBAR) {
printf "O_FOOBAR = %s\n", &O_FOOBAR; # case 1
} else {
die "O_FOOBAR not defined, so I won't use it\n"; # case 2
}

if O_FOOBAR is undefined, it hits case 2 (correct).
if O_FOOBAR is a locally-defined sub, it hits case 1 (correct).
if O_FOOBAR is replaced by O_RDWR, it hits case 2 (WRONG).

I hit this problem trying to use O_BINARY on Doshish platforms.
sysopen *must* be passed O_BINARY to avoid CR-LF translations.
But O_BINARY presumably might not be defined on Unix platforms.
So I tried this​:
use Fcntl;
if (defined &O_BINARY) {
sysopen(FILE,$path, O_RDONLY|O_BINARY) ...
} else {
sysopen(FILE,$path, O_RDONLY) ...
}
This FAILED because the second branch was always taken, even on
Windows
where O_BINARY is in fact defined.

I'm not certain whether or not O_BINARY is defined on all
platforms,
but that's the whole point... programs should be able to test
whether
or not a constant is in fact exported from a package which it is
using.

This is documented in the perlsub documentation for "defined"​:

| Note that a subroutine which is not defined may still be callable​: its
| package may have an AUTOLOAD method that makes it spring into
| existence the first time that it is called -- see the perlsub manpage.

You can use "exists" instead, because "exists" checks whether a
subroutine is declared, which seems to be the case for exported
subroutines and constants. As a last resort, you can always test for
constants and subroutines in a eval { } block.

Rehards,
  Slaven

--
Slaven Rezic - slaven.rezic@​berlin.de

  tkruler - Perl/Tk program for measuring screen distances
  http​://ptktools.sourceforge.net/#tkruler

From @jimav

Thanks. But...

perlfunc (under 'exists') says​:
  "Note that a subroutine which does not exist may still be
  callable​: its package may have an "AUTOLOAD" method that makes
  it spring into existence the first time that it is called..."

This implies that exists(&subname) returns FALSE for an
AUTOLOADED sub which has not yet been loaded.

This contradicts reality.
Perhaps perlfunc could be clarified at this point.

-Jim

--- Rafael Garcia-Suarez <perlbug@​perl.org> wrote​:

Jim Avera (via RT) <perlbug@​perl.org> wrote​:

use Fcntl;
printf "O_RDWR is %d\n", &O_RDWR; # no warning or error
die "Error, O_RDWR not defined" unless defined (&O_RDWR); #
***DIES HERE***

In fact, the Fcntl constants are AUTOLOAD'ed.
To check whether they exist, you should use exists() rather than
defined().
HTH.

=====
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Please do not put me on a mailing list or give my
address(es) to anyone else.

__________________________________________________
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From @cpansprout

On Wed Oct 23 22​:44​:42 2002, jimav wrote​:

Thanks. But...

perlfunc (under 'exists') says​:
"Note that a subroutine which does not exist may still be
callable​: its package may have an "AUTOLOAD" method that makes
it spring into existence the first time that it is called..."

This implies that exists(&subname) returns FALSE for an
AUTOLOADED sub which has not yet been loaded.

This contradicts reality.

It’s true that non-existent functions can be AUTOLOADed. But you can’t
export a non-existent function. It has to exist for that, but it does
not have to be defined.

So, if Fcntl does not export FROMP, then &FROMP will call
main​::AUTOLOAD, if present.
If it does export FROMP then &FROMP will call Fcntl​::AUTOLOAD.

Perhaps perlfunc could be clarified at this point.

Any suggestions?