PerlIO::Via: check arg is non-NULL before using it.

[perl5.git] / pod / perlvms.pod

=head1 NAME

perlvms - VMS-specific documentation for Perl

=head1 DESCRIPTION

Gathered below are notes describing details of Perl 5's 
behavior on VMS.  They are a supplement to the regular Perl 5 
documentation, so we have focussed on the ways in which Perl 
5 functions differently under VMS than it does under Unix, 
and on the interactions between Perl and the rest of the 
operating system.  We haven't tried to duplicate complete 
descriptions of Perl features from the main Perl 
documentation, which can be found in the F<[.pod]> 
subdirectory of the Perl distribution.

We hope these notes will save you from confusion and lost 
sleep when writing Perl scripts on VMS.  If you find we've 
missed something you think should appear here, please don't 
hesitate to drop a line to vmsperl@perl.org.

=head1 Installation

Directions for building and installing Perl 5 can be found in 
the file F<README.vms> in the main source directory of the 
Perl distribution.

=head1 Organization of Perl Images

=head2 Core Images

During the build process, three Perl images are produced.
F<Miniperl.Exe> is an executable image which contains all of
the basic functionality of Perl, but cannot take advantage of
Perl XS extensions and has a hard-wired list of library locations
for loading pure-Perl modules.  It is used extensively to build and
test Perl and various extensions, but is not installed.

Most of the complete Perl resides in the shareable image F<PerlShr.Exe>,
which provides a core to which the Perl executable image and all Perl
extensions are linked. It is generally located via the logical name
F<PERLSHR>.  While it's possible to put the image in F<SYS$SHARE> to
make it loadable, that's not recommended. And while you may wish to
INSTALL the image for performance reasons, you should not install it
with privileges; if you do, the result will not be what you expect as
image privileges are disabled during Perl start-up.

Finally, F<Perl.Exe> is an executable image containing the main
entry point for Perl, as well as some initialization code.  It
should be placed in a public directory, and made world executable.
In order to run Perl with command line arguments, you should
define a foreign command to invoke this image.

=head2 Perl Extensions

Perl extensions are packages which provide both XS and Perl code
to add new functionality to perl.  (XS is a meta-language which
simplifies writing C code which interacts with Perl, see
L<perlxs> for more details.)  The Perl code for an
extension is treated like any other library module - it's
made available in your script through the appropriate
C<use> or C<require> statement, and usually defines a Perl
package containing the extension.

The portion of the extension provided by the XS code may be
connected to the rest of Perl in either of two ways.  In the
B<static> configuration, the object code for the extension is
linked directly into F<PerlShr.Exe>, and is initialized whenever
Perl is invoked.  In the B<dynamic> configuration, the extension's
machine code is placed into a separate shareable image, which is
mapped by Perl's DynaLoader when the extension is C<use>d or
C<require>d in your script.  This allows you to maintain the
extension as a separate entity, at the cost of keeping track of the
additional shareable image.  Most extensions can be set up as either
static or dynamic.

The source code for an extension usually resides in its own
directory.  At least three files are generally provided:
I<Extshortname>F<.xs> (where I<Extshortname> is the portion of
the extension's name following the last C<::>), containing
the XS code, I<Extshortname>F<.pm>, the Perl library module
for the extension, and F<Makefile.PL>, a Perl script which uses
the C<MakeMaker> library modules supplied with Perl to generate
a F<Descrip.MMS> file for the extension.

=head2 Installing static extensions

Since static extensions are incorporated directly into
F<PerlShr.Exe>, you'll have to rebuild Perl to incorporate a
new extension.  You should edit the main F<Descrip.MMS> or F<Makefile>
you use to build Perl, adding the extension's name to the C<ext>
macro, and the extension's object file to the C<extobj> macro.
You'll also need to build the extension's object file, either
by adding dependencies to the main F<Descrip.MMS>, or using a
separate F<Descrip.MMS> for the extension.  Then, rebuild
F<PerlShr.Exe> to incorporate the new code.

Finally, you'll need to copy the extension's Perl library
module to the F<[.>I<Extname>F<]> subdirectory under one
of the directories in C<@INC>, where I<Extname> is the name
of the extension, with all C<::> replaced by C<.> (e.g.
the library module for extension Foo::Bar would be copied
to a F<[.Foo.Bar]> subdirectory).

=head2 Installing dynamic extensions

In general, the distributed kit for a Perl extension includes
a file named Makefile.PL, which is a Perl program which is used
to create a F<Descrip.MMS> file which can be used to build and
install the files required by the extension.  The kit should be
unpacked into a directory tree B<not> under the main Perl source
directory, and the procedure for building the extension is simply

    $ perl Makefile.PL  ! Create Descrip.MMS
    $ mmk               ! Build necessary files
    $ mmk test          ! Run test code, if supplied
    $ mmk install       ! Install into public Perl tree

VMS support for this process in the current release of Perl
is sufficient to handle most extensions.  (See the MakeMaker
documentation for more details on installation options for
extensions.)

=over 4

=item *

the F<[.Lib.Auto.>I<Arch>I<$PVers>I<Extname>F<]> subdirectory
of one of the directories in C<@INC> (where I<PVers>
is the version of Perl you're using, as supplied in C<$]>,
with '.' converted to '_'), or

=item *

one of the directories in C<@INC>, or

=item *

a directory which the extensions Perl library module
passes to the DynaLoader when asking it to map
the shareable image, or

=item *

F<Sys$Share> or F<Sys$Library>.

=back

If the shareable image isn't in any of these places, you'll need
to define a logical name I<Extshortname>, where I<Extshortname>
is the portion of the extension's name after the last C<::>, which
translates to the full file specification of the shareable image.

=head1 File specifications

=head2 Syntax

We have tried to make Perl aware of both VMS-style and Unix-style file
specifications wherever possible.  You may use either style, or both,
on the command line and in scripts, but you may not combine the two
styles within a single file specification.  VMS Perl interprets Unix
pathnames in much the same way as the CRTL (I<e.g.> the first component
of an absolute path is read as the device name for the VMS file
specification).  There are a set of functions provided in the
C<VMS::Filespec> package for explicit interconversion between VMS and
Unix syntax; its documentation provides more details.

We've tried to minimize the dependence of Perl library
modules on Unix syntax, but you may find that some of these,
as well as some scripts written for Unix systems, will
require that you use Unix syntax, since they will assume that
'/' is the directory separator, I<etc.>  If you find instances
of this in the Perl distribution itself, please let us know,
so we can try to work around them.

Also when working on Perl programs on VMS, if you need a syntax
in a specific operating system format, then you need either to
check the appropriate DECC$ feature logical, or call a conversion
routine to force it to that format.

The feature logical name DECC$FILENAME_UNIX_REPORT modifies traditional
Perl behavior in the conversion of file specifications from Unix to VMS
format in order to follow the extended character handling rules now
expected by the CRTL.  Specifically, when this feature is in effect, the
C<./.../> in a Unix path is now translated to C<[.^.^.^.]> instead of
the traditional VMS C<[...]>.  To be compatible with what MakeMaker
expects, if a VMS path cannot be translated to a Unix path, it is
passed through unchanged, so C<unixify("[...]")> will return C<[...]>.

There are several ambiguous cases where a conversion routine cannot
determine whether an input filename is in Unix format or in VMS format,
since now both VMS and Unix file specifications may have characters in
them that could be mistaken for syntax delimiters of the other type. So
some pathnames simply cannot be used in a mode that allows either type
of pathname to be present.  Perl will tend to assume that an ambiguous
filename is in Unix format.

Allowing "." as a version delimiter is simply incompatible with
determining whether a pathname is in VMS format or in Unix format with
extended file syntax.  There is no way to know whether "perl-5.8.6" is a
Unix "perl-5.8.6" or a VMS "perl-5.8;6" when passing it to unixify() or
vmsify().

The DECC$FILENAME_UNIX_REPORT logical name controls how Perl interprets
filenames to the extent that Perl uses the CRTL internally for many
purposes, and attempts to follow CRTL conventions for reporting
filenames.  The DECC$FILENAME_UNIX_ONLY feature differs in that it
expects all filenames passed to the C run-time to be already in Unix
format.  This feature is not yet supported in Perl since Perl uses
traditional OpenVMS file specifications internally and in the test
harness, and it is not yet clear whether this mode will be useful or
useable.  The feature logical name DECC$POSIX_COMPLIANT_PATHNAMES is new
with the RMS Symbolic Link SDK and included with OpenVMS v8.3, but is
not yet supported in Perl.

=head2 Filename Case

Perl enables DECC$EFS_CASE_PRESERVE and DECC$ARGV_PARSE_STYLE by
default.  Note that the latter only takes effect when extended parse
is set in the process in which Perl is running.  When these features
are explicitly disabled in the environment or the CRTL does not support
them, Perl follows the traditional CRTL behavior of downcasing command-line
arguments and returning file specifications in lower case only.

I<N. B.>  It is very easy to get tripped up using a mixture of other
programs, external utilities, and Perl scripts that are in varying
states of being able to handle case preservation.  For example, a file
created by an older version of an archive utility or a build utility
such as MMK or MMS may generate a filename in all upper case even on an
ODS-5 volume.  If this filename is later retrieved by a Perl script or
module in a case preserving environment, that upper case name may not
match the mixed-case or lower-case expectations of the Perl code.  Your
best bet is to follow an all-or-nothing approach to case preservation:
either don't use it at all, or make sure your entire toolchain and
application environment support and use it.

OpenVMS Alpha v7.3-1 and later and all version of OpenVMS I64 support
case sensitivity as a process setting (see C<SET PROCESS
/CASE_LOOKUP=SENSITIVE>). Perl does not currently support case
sensitivity on VMS, but it may in the future, so Perl programs should
use the C<< File::Spec->case_tolerant >> method to determine the state, and
not the C<$^O> variable.

=head2 Symbolic Links

When built on an ODS-5 volume with symbolic links enabled, Perl by
default supports symbolic links when the requisite support is available
in the filesystem and CRTL (generally 64-bit OpenVMS v8.3 and later). 
There are a number of limitations and caveats to be aware of when
working with symbolic links on VMS.  Most notably, the target of a valid
symbolic link must be expressed as a Unix-style path and it must exist
on a volume visible from your POSIX root (see the C<SHOW ROOT> command
in DCL help).  For further details on symbolic link capabilities and
requirements, see chapter 12 of the CRTL manual that ships with OpenVMS
v8.3 or later.

=head2 Wildcard expansion

File specifications containing wildcards are allowed both on 
the command line and within Perl globs (e.g. C<E<lt>*.cE<gt>>).  If
the wildcard filespec uses VMS syntax, the resultant 
filespecs will follow VMS syntax; if a Unix-style filespec is 
passed in, Unix-style filespecs will be returned.
Similar to the behavior of wildcard globbing for a Unix shell,
one can escape command line wildcards with double quotation
marks C<"> around a perl program command line argument.  However,
owing to the stripping of C<"> characters carried out by the C
handling of argv you will need to escape a construct such as
this one (in a directory containing the files F<PERL.C>, F<PERL.EXE>,
F<PERL.H>, and F<PERL.OBJ>):

    $ perl -e "print join(' ',@ARGV)" perl.*
    perl.c perl.exe perl.h perl.obj

in the following triple quoted manner:

    $ perl -e "print join(' ',@ARGV)" """perl.*"""
    perl.*

In both the case of unquoted command line arguments or in calls
to C<glob()> VMS wildcard expansion is performed. (csh-style
wildcard expansion is available if you use C<File::Glob::glob>.)
If the wildcard filespec contains a device or directory 
specification, then the resultant filespecs will also contain 
a device and directory; otherwise, device and directory 
information are removed.  VMS-style resultant filespecs will 
contain a full device and directory, while Unix-style 
resultant filespecs will contain only as much of a directory 
path as was present in the input filespec.  For example, if 
your default directory is Perl_Root:[000000], the expansion 
of C<[.t]*.*> will yield filespecs  like 
"perl_root:[t]base.dir", while the expansion of C<t/*/*> will 
yield filespecs like "t/base.dir".  (This is done to match 
the behavior of glob expansion performed by Unix shells.) 

Similarly, the resultant filespec will contain the file version
only if one was present in the input filespec.


=head2 Pipes

Input and output pipes to Perl filehandles are supported; the 
"file name" is passed to lib$spawn() for asynchronous 
execution.  You should be careful to close any pipes you have 
opened in a Perl script, lest you leave any "orphaned" 
subprocesses around when Perl exits. 

You may also use backticks to invoke a DCL subprocess, whose 
output is used as the return value of the expression.  The 
string between the backticks is handled as if it were the
argument to the C<system> operator (see below).  In this case,
Perl will wait for the subprocess to complete before continuing. 

The mailbox (MBX) that perl can create to communicate with a pipe
defaults to a buffer size of 8192 on 64-bit systems, 512 on VAX.  The
default buffer size is adjustable via the logical name PERL_MBX_SIZE
provided that the value falls between 128 and the SYSGEN parameter
MAXBUF inclusive.  For example, to set the mailbox size to 32767 use
C<$ENV{'PERL_MBX_SIZE'} = 32767;> and then open and use pipe constructs. 
An alternative would be to issue the command:

    $ Define PERL_MBX_SIZE 32767

before running your wide record pipe program.  A larger value may
improve performance at the expense of the BYTLM UAF quota.

=head1 PERL5LIB and PERLLIB

The PERL5LIB and PERLLIB environment elements work as documented in L<perl>,
except that the element separator is, by default, '|' instead of ':'.
However, when running under a Unix shell as determined by the logical
name C<GNV$UNIX_SHELL>, the separator will be ':' as on Unix systems. The
directory specifications may use either VMS or Unix syntax.

=head1 The Perl Forked Debugger

The Perl forked debugger places the debugger commands and output in a
separate X-11 terminal window so that commands and output from multiple
processes are not mixed together.

Perl on VMS supports an emulation of the forked debugger when Perl is
run on a VMS system that has X11 support installed.

To use the forked debugger, you need to have the default display set to an
X-11 Server and some environment variables set that Unix expects.

The forked debugger requires the environment variable C<TERM> to be C<xterm>,
and the environment variable C<DISPLAY> to exist.  C<xterm> must be in
lower case.

  $define TERM "xterm"

  $define DISPLAY "hostname:0.0"

Currently the value of C<DISPLAY> is ignored.  It is recommended that it be set
to be the hostname of the display, the server and screen in Unix notation.  In
the future the value of DISPLAY may be honored by Perl instead of using the
default display.

It may be helpful to always use the forked debugger so that script I/O is
separated from debugger I/O.  You can force the debugger to be forked by
assigning a value to the logical name <PERLDB_PIDS> that is not a process
identification number.

  $define PERLDB_PIDS XXXX


=head1 PERL_VMS_EXCEPTION_DEBUG

The PERL_VMS_EXCEPTION_DEBUG being defined as "ENABLE" will cause the VMS
debugger to be invoked if a fatal exception that is not otherwise
handled is raised.  The purpose of this is to allow debugging of
internal Perl problems that would cause such a condition.

This allows the programmer to look at the execution stack and variables to
find out the cause of the exception.  As the debugger is being invoked as
the Perl interpreter is about to do a fatal exit, continuing the execution
in debug mode is usually not practical.

Starting Perl in the VMS debugger may change the program execution
profile in a way that such problems are not reproduced.

The C<kill> function can be used to test this functionality from within
a program.

In typical VMS style, only the first letter of the value of this logical
name is actually checked in a case insensitive mode, and it is considered
enabled if it is the value "T","1" or "E".

This logical name must be defined before Perl is started.

=head1 Command line

=head2 I/O redirection and backgrounding

Perl for VMS supports redirection of input and output on the 
command line, using a subset of Bourne shell syntax:

=over 4

=item *

C<E<lt>file> reads stdin from C<file>,

=item *

C<E<gt>file> writes stdout to C<file>,

=item *

C<E<gt>E<gt>file> appends stdout to C<file>,

=item *

C<2E<gt>file> writes stderr to C<file>,

=item *

C<2E<gt>E<gt>file> appends stderr to C<file>, and

=item *

C<< 2>&1 >> redirects stderr to stdout.

=back

In addition, output may be piped to a subprocess, using the  
character '|'.  Anything after this character on the command 
line is passed to a subprocess for execution; the subprocess 
takes the output of Perl as its input.

Finally, if the command line ends with '&', the entire 
command is run in the background as an asynchronous 
subprocess.

=head2 Command line switches

The following command line switches behave differently under
VMS than described in L<perlrun>.  Note also that in order
to pass uppercase switches to Perl, you need to enclose
them in double-quotes on the command line, since the CRTL
downcases all unquoted strings.

On newer 64 bit versions of OpenVMS, a process setting now
controls if the quoting is needed to preserve the case of
command line arguments.

=over 4

=item -i

If the C<-i> switch is present but no extension for a backup
copy is given, then inplace editing creates a new version of
a file; the existing copy is not deleted.  (Note that if
an extension is given, an existing file is renamed to the backup
file, as is the case under other operating systems, so it does
not remain as a previous version under the original filename.)

=item -S

If the C<"-S"> or C<-"S"> switch is present I<and> the script
name does not contain a directory, then Perl translates the
logical name DCL$PATH as a searchlist, using each translation
as a directory in which to look for the script.  In addition,
if no file type is specified, Perl looks in each directory
for a file matching the name specified, with a blank type,
a type of F<.pl>, and a type of F<.com>, in that order.

=item -u

The C<-u> switch causes the VMS debugger to be invoked
after the Perl program is compiled, but before it has
run.  It does not create a core dump file.

=back

=head1 Perl functions

As of the time this document was last revised, the following 
Perl functions were implemented in the VMS port of Perl 
(functions marked with * are discussed in more detail below):

    file tests*, abs, alarm, atan, backticks*, binmode*, bless,
    caller, chdir, chmod, chown, chomp, chop, chr,
    close, closedir, cos, crypt*, defined, delete, die, do, dump*, 
    each, endgrent, endpwent, eof, eval, exec*, exists, exit, exp, 
    fileno, flock  getc, getgrent*, getgrgid*, getgrnam, getlogin,
    getppid, getpwent*, getpwnam*, getpwuid*, glob, gmtime*, goto,
    grep, hex, ioctl, import, index, int, join, keys, kill*,
    last, lc, lcfirst, lchown*, length, link*, local, localtime, log,
    lstat, m//, map, mkdir, my, next, no, oct, open, opendir, ord,
    pack, pipe, pop, pos, print, printf, push, q//, qq//, qw//,
    qx//*, quotemeta, rand, read, readdir, readlink*, redo, ref,
    rename, require, reset, return, reverse, rewinddir, rindex,
    rmdir, s///, scalar, seek, seekdir, select(internal),
    select (system call)*, setgrent, setpwent, shift, sin, sleep,
    socketpair, sort, splice, split, sprintf, sqrt, srand, stat,
    study, substr, symlink*, sysread, system*, syswrite, tell,
    telldir, tie, time, times*, tr///, uc, ucfirst, umask,
    undef, unlink*, unpack, untie, unshift, use, utime*,
    values, vec, wait, waitpid*, wantarray, warn, write, y///

The following functions were not implemented in the VMS port, 
and calling them produces a fatal error (usually) or 
undefined behavior (rarely, we hope):

    chroot, dbmclose, dbmopen, fork*, getpgrp, getpriority,  
    msgctl, msgget, msgsend, msgrcv, semctl,
    semget, semop, setpgrp, setpriority, shmctl, shmget,
    shmread, shmwrite, syscall

The following functions are available on Perls compiled with Dec C
5.2 or greater and running VMS 7.0 or greater:

    truncate

The following functions are available on Perls built on VMS 7.2 or
greater:

    fcntl (without locking)

The following functions may or may not be implemented, 
depending on what type of socket support you've built into 
your copy of Perl:

    accept, bind, connect, getpeername,
    gethostbyname, getnetbyname, getprotobyname,
    getservbyname, gethostbyaddr, getnetbyaddr,
    getprotobynumber, getservbyport, gethostent,
    getnetent, getprotoent, getservent, sethostent,
    setnetent, setprotoent, setservent, endhostent,
    endnetent, endprotoent, endservent, getsockname,
    getsockopt, listen, recv, select(system call)*,
    send, setsockopt, shutdown, socket

The following function is available on Perls built on 64 bit OpenVMS v8.2
with hard links enabled on an ODS-5 formatted build disk.  CRTL support
is in principle available as of OpenVMS v7.3-1, and better configuration
support could detect this.

    link

The following functions are available on Perls built on 64 bit OpenVMS
v8.2 and later.  CRTL support is in principle available as of OpenVMS
v7.3-2, and better configuration support could detect this.

   getgrgid, getgrnam, getpwnam, getpwuid,
   setgrent, ttyname

The following functions are available on Perls built on 64 bit OpenVMS v8.2
and later.  

   statvfs, socketpair

=over 4

=item File tests

The tests C<-b>, C<-B>, C<-c>, C<-C>, C<-d>, C<-e>, C<-f>,
C<-o>, C<-M>, C<-s>, C<-S>, C<-t>, C<-T>, and C<-z> work as
advertised.  The return values for C<-r>, C<-w>, and C<-x>
tell you whether you can actually access the file; this may
not reflect the UIC-based file protections.  Since real and
effective UIC don't differ under VMS, C<-O>, C<-R>, C<-W>,
and C<-X> are equivalent to C<-o>, C<-r>, C<-w>, and C<-x>.
Similarly, several other tests, including C<-A>, C<-g>, C<-k>,
C<-l>, C<-p>, and C<-u>, aren't particularly meaningful under
VMS, and the values returned by these tests reflect whatever
your CRTL C<stat()> routine does to the equivalent bits in the
st_mode field.  Finally, C<-d> returns true if passed a device
specification without an explicit directory (e.g. C<DUA1:>), as
well as if passed a directory.

There are DECC feature logical names AND ODS-5 volume attributes that
also control what values are returned for the date fields.

Note: Some sites have reported problems when using the file-access
tests (C<-r>, C<-w>, and C<-x>) on files accessed via DEC's DFS.
Specifically, since DFS does not currently provide access to the
extended file header of files on remote volumes, attempts to
examine the ACL fail, and the file tests will return false,
with C<$!> indicating that the file does not exist.  You can
use C<stat> on these files, since that checks UIC-based protection
only, and then manually check the appropriate bits, as defined by
your C compiler's F<stat.h>, in the mode value it returns, if you
need an approximation of the file's protections.

=item backticks

Backticks create a subprocess, and pass the enclosed string
to it for execution as a DCL command.  Since the subprocess is
created directly via C<lib$spawn()>, any valid DCL command string
may be specified.

=item binmode FILEHANDLE

The C<binmode> operator will attempt to insure that no translation
of carriage control occurs on input from or output to this filehandle.
Since this involves reopening the file and then restoring its
file position indicator, if this function returns FALSE, the
underlying filehandle may no longer point to an open file, or may
point to a different position in the file than before C<binmode>
was called.

Note that C<binmode> is generally not necessary when using normal
filehandles; it is provided so that you can control I/O to existing
record-structured files when necessary.  You can also use the
C<vmsfopen> function in the VMS::Stdio extension to gain finer
control of I/O to files and devices with different record structures.

=item crypt PLAINTEXT, USER

The C<crypt> operator uses the C<sys$hash_password> system
service to generate the hashed representation of PLAINTEXT.
If USER is a valid username, the algorithm and salt values
are taken from that user's UAF record.  If it is not, then
the preferred algorithm and a salt of 0 are used.  The
quadword encrypted value is returned as an 8-character string.

The value returned by C<crypt> may be compared against
the encrypted password from the UAF returned by the C<getpw*>
functions, in order to authenticate users.  If you're
going to do this, remember that the encrypted password in
the UAF was generated using uppercase username and
password strings; you'll have to upcase the arguments to
C<crypt> to insure that you'll get the proper value:

    sub validate_passwd {
        my($user,$passwd) = @_;
        my($pwdhash);
        if ( !($pwdhash = (getpwnam($user))[1]) ||
               $pwdhash ne crypt("\U$passwd","\U$name") ) {
            intruder_alert($name);
        }
        return 1;
    }


=item die

C<die> will force the native VMS exit status to be an SS$_ABORT code
if neither of the $! or $? status values are ones that would cause
the native status to be interpreted as being what VMS classifies as
SEVERE_ERROR severity for DCL error handling.

When C<PERL_VMS_POSIX_EXIT> is active (see L</"$?"> below), the native VMS exit
status value will have either one of the C<$!> or C<$?> or C<$^E> or
the Unix value 255 encoded into it in a way that the effective original
value can be decoded by other programs written in C, including Perl
and the GNV package.  As per the normal non-VMS behavior of C<die> if
either C<$!> or C<$?> are non-zero, one of those values will be
encoded into a native VMS status value.  If both of the Unix status
values are 0, and the C<$^E> value is set one of ERROR or SEVERE_ERROR
severity, then the C<$^E> value will be used as the exit code as is.
If none of the above apply, the Unix value of 255 will be encoded into
a native VMS exit status value.

Please note a significant difference in the behavior of C<die> in
the C<PERL_VMS_POSIX_EXIT> mode is that it does not force a VMS
SEVERE_ERROR status on exit.  The Unix exit values of 2 through
255 will be encoded in VMS status values with severity levels of
SUCCESS.  The Unix exit value of 1 will be encoded in a VMS status
value with a severity level of ERROR.  This is to be compatible with
how the VMS C library encodes these values.

The minimum severity level set by C<die> in C<PERL_VMS_POSIX_EXIT> mode
may be changed to be ERROR or higher in the future depending on the 
results of testing and further review.

See L</"$?"> for a description of the encoding of the Unix value to
produce a native VMS status containing it.

=item dump

Rather than causing Perl to abort and dump core, the C<dump>
operator invokes the VMS debugger.  If you continue to
execute the Perl program under the debugger, control will
be transferred to the label specified as the argument to
C<dump>, or, if no label was specified, back to the
beginning of the program.  All other state of the program
(I<e.g.> values of variables, open file handles) are not
affected by calling C<dump>.

=item exec LIST

A call to C<exec> will cause Perl to exit, and to invoke the command
given as an argument to C<exec> via C<lib$do_command>.  If the
argument begins with '@' or '$' (other than as part of a filespec),
then it is executed as a DCL command.  Otherwise, the first token on
the command line is treated as the filespec of an image to run, and
an attempt is made to invoke it (using F<.Exe> and the process
defaults to expand the filespec) and pass the rest of C<exec>'s
argument to it as parameters.  If the token has no file type, and
matches a file with null type, then an attempt is made to determine
whether the file is an executable image which should be invoked
using C<MCR> or a text file which should be passed to DCL as a
command procedure.

=item fork

While in principle the C<fork> operator could be implemented via
(and with the same rather severe limitations as) the CRTL C<vfork()>
routine, and while some internal support to do just that is in
place, the implementation has never been completed, making C<fork>
currently unavailable.  A true kernel C<fork()> is expected in a
future version of VMS, and the pseudo-fork based on interpreter
threads may be available in a future version of Perl on VMS (see
L<perlfork>).  In the meantime, use C<system>, backticks, or piped
filehandles to create subprocesses.

=item getpwent

=item getpwnam

=item getpwuid

These operators obtain the information described in L<perlfunc>,
if you have the privileges necessary to retrieve the named user's
UAF information via C<sys$getuai>.  If not, then only the C<$name>,
C<$uid>, and C<$gid> items are returned.  The C<$dir> item contains
the login directory in VMS syntax, while the C<$comment> item
contains the login directory in Unix syntax. The C<$gcos> item
contains the owner field from the UAF record.  The C<$quota>
item is not used.

=item gmtime

The C<gmtime> operator will function properly if you have a
working CRTL C<gmtime()> routine, or if the logical name
SYS$TIMEZONE_DIFFERENTIAL is defined as the number of seconds
which must be added to UTC to yield local time.  (This logical
name is defined automatically if you are running a version of
VMS with built-in UTC support.)  If neither of these cases is
true, a warning message is printed, and C<undef> is returned.

=item kill

In most cases, C<kill> is implemented via the undocumented system
service C<$SIGPRC>, which has the same calling sequence as C<$FORCEX>, but
throws an exception in the target process rather than forcing it to call
C<$EXIT>.  Generally speaking, C<kill> follows the behavior of the
CRTL's C<kill()> function, but unlike that function can be called from
within a signal handler.  Also, unlike the C<kill> in some versions of
the CRTL, Perl's C<kill> checks the validity of the signal passed in and
returns an error rather than attempting to send an unrecognized signal.

Also, negative signal values don't do anything special under
VMS; they're just converted to the corresponding positive value.

=item qx//

See the entry on C<backticks> above.

=item select (system call)

If Perl was not built with socket support, the system call
version of C<select> is not available at all.  If socket
support is present, then the system call version of
C<select> functions only for file descriptors attached
to sockets.  It will not provide information about regular
files or pipes, since the CRTL C<select()> routine does not
provide this functionality.

=item stat EXPR

Since VMS keeps track of files according to a different scheme
than Unix, it's not really possible to represent the file's ID
in the C<st_dev> and C<st_ino> fields of a C<struct stat>.  Perl
tries its best, though, and the values it uses are pretty unlikely
to be the same for two different files.  We can't guarantee this,
though, so caveat scriptor.

=item system LIST

The C<system> operator creates a subprocess, and passes its 
arguments to the subprocess for execution as a DCL command.  
Since the subprocess is created directly via C<lib$spawn()>, any 
valid DCL command string may be specified.  If the string begins with
'@', it is treated as a DCL command unconditionally.  Otherwise, if
the first token contains a character used as a delimiter in file
specification (e.g. C<:> or C<]>), an attempt is made to expand it
using  a default type of F<.Exe> and the process defaults, and if
successful, the resulting file is invoked via C<MCR>. This allows you
to invoke an image directly simply by passing the file specification
to C<system>, a common Unixish idiom.  If the token has no file type,
and matches a file with null type, then an attempt is made to
determine whether the file is an executable image which should be
invoked using C<MCR> or a text file which should be passed to DCL
as a command procedure.

If LIST consists of the empty string, C<system> spawns an
interactive DCL subprocess, in the same fashion as typing
B<SPAWN> at the DCL prompt.

Perl waits for the subprocess to complete before continuing
execution in the current process.  As described in L<perlfunc>,
the return value of C<system> is a fake "status" which follows
POSIX semantics unless the pragma C<use vmsish 'status'> is in
effect; see the description of C<$?> in this document for more 
detail.  

=item time

The value returned by C<time> is the offset in seconds from
01-JAN-1970 00:00:00 (just like the CRTL's times() routine), in order
to make life easier for code coming in from the POSIX/Unix world.

=item times

The array returned by the C<times> operator is divided up 
according to the same rules the CRTL C<times()> routine.  
Therefore, the "system time" elements will always be 0, since 
there is no difference between "user time" and "system" time 
under VMS, and the time accumulated by a subprocess may or may 
not appear separately in the "child time" field, depending on 
whether C<times()> keeps track of subprocesses separately.  Note
especially that the VAXCRTL (at least) keeps track only of
subprocesses spawned using C<fork()> and C<exec()>; it will not
accumulate the times of subprocesses spawned via pipes, C<system()>,
or backticks.

=item unlink LIST

C<unlink> will delete the highest version of a file only; in
order to delete all versions, you need to say

    1 while unlink LIST;

You may need to make this change to scripts written for a
Unix system which expect that after a call to C<unlink>,
no files with the names passed to C<unlink> will exist.
(Note: This can be changed at compile time; if you
C<use Config> and C<$Config{'d_unlink_all_versions'}> is
C<define>, then C<unlink> will delete all versions of a
file on the first call.)

C<unlink> will delete a file if at all possible, even if it
requires changing file protection (though it won't try to
change the protection of the parent directory).  You can tell
whether you've got explicit delete access to a file by using the
C<VMS::Filespec::candelete> operator.  For instance, in order
to delete only files to which you have delete access, you could
say something like

    sub safe_unlink {
        my($file,$num);
        foreach $file (@_) {
            next unless VMS::Filespec::candelete($file);
            $num += unlink $file;
        }
        $num;
    }

(or you could just use C<VMS::Stdio::remove>, if you've installed
the VMS::Stdio extension distributed with Perl). If C<unlink> has to
change the file protection to delete the file, and you interrupt it
in midstream, the file may be left intact, but with a changed ACL
allowing you delete access.

This behavior of C<unlink> is to be compatible with POSIX behavior
and not traditional VMS behavior.

=item utime LIST

This operator changes only the modification time of the file (VMS 
revision date) on ODS-2 volumes and ODS-5 volumes without access 
dates enabled. On ODS-5 volumes with access dates enabled, the 
true access time is modified.

=item waitpid PID,FLAGS

If PID is a subprocess started by a piped C<open()> (see L<open>), 
C<waitpid> will wait for that subprocess, and return its final status
value in C<$?>.  If PID is a subprocess created in some other way (e.g.
SPAWNed before Perl was invoked), C<waitpid> will simply check once per
second whether the process has completed, and return when it has.  (If
PID specifies a process that isn't a subprocess of the current process,
and you invoked Perl with the C<-w> switch, a warning will be issued.)

Returns PID on success, -1 on error.  The FLAGS argument is ignored
in all cases.

=back

=head1 Perl variables

The following VMS-specific information applies to the indicated
"special" Perl variables, in addition to the general information
in L<perlvar>.  Where there is a conflict, this information
takes precedence.

=over 4

=item %ENV 

The operation of the C<%ENV> array depends on the translation
of the logical name F<PERL_ENV_TABLES>.  If defined, it should
be a search list, each element of which specifies a location
for C<%ENV> elements.  If you tell Perl to read or set the
element C<$ENV{>I<name>C<}>, then Perl uses the translations of
F<PERL_ENV_TABLES> as follows:

=over 4

=item CRTL_ENV

This string tells Perl to consult the CRTL's internal C<environ> array
of key-value pairs, using I<name> as the key.  In most cases, this
contains only a few keys, but if Perl was invoked via the C
C<exec[lv]e()> function, as is the case for some embedded Perl
applications or when running under a shell such as GNV bash, the
C<environ> array may have been populated by the calling program.

=item CLISYM_[LOCAL]

A string beginning with C<CLISYM_>tells Perl to consult the CLI's
symbol tables, using I<name> as the name of the symbol.  When reading
an element of C<%ENV>, the local symbol table is scanned first, followed
by the global symbol table..  The characters following C<CLISYM_> are
significant when an element of C<%ENV> is set or deleted: if the
complete string is C<CLISYM_LOCAL>, the change is made in the local
symbol table; otherwise the global symbol table is changed.

=item Any other string

If an element of F<PERL_ENV_TABLES> translates to any other string,
that string is used as the name of a logical name table, which is
consulted using I<name> as the logical name.  The normal search
order of access modes is used.

=back

F<PERL_ENV_TABLES> is translated once when Perl starts up; any changes
you make while Perl is running do not affect the behavior of C<%ENV>.
If F<PERL_ENV_TABLES> is not defined, then Perl defaults to consulting
first the logical name tables specified by F<LNM$FILE_DEV>, and then
the CRTL C<environ> array.  This default order is reversed when the
logical name F<GNV$UNIX_SHELL> is defined, such as when running under
GNV bash.

For operations on %ENV entries based on logical names or DCL symbols, the
key string is treated as if it were entirely uppercase, regardless of the
case actually specified in the Perl expression. Entries in %ENV based on the
CRTL's environ array preserve the case of the key string when stored, and
lookups are case sensitive.

When an element of C<%ENV> is read, the locations to which
F<PERL_ENV_TABLES> points are checked in order, and the value
obtained from the first successful lookup is returned.  If the
name of the C<%ENV> element contains a semi-colon, it and
any characters after it are removed.  These are ignored when
the CRTL C<environ> array or a CLI symbol table is consulted.
However, the name is looked up in a logical name table, the
suffix after the semi-colon is treated as the translation index
to be used for the lookup.   This lets you look up successive values
for search list logical names.  For instance, if you say

   $  Define STORY  once,upon,a,time,there,was
   $  perl -e "for ($i = 0; $i <= 6; $i++) " -
   _$ -e "{ print $ENV{'story;'.$i},' '}"

Perl will print C<ONCE UPON A TIME THERE WAS>, assuming, of course,
that F<PERL_ENV_TABLES> is set up so that the logical name C<story>
is found, rather than a CLI symbol or CRTL C<environ> element with
the same name.

When an element of C<%ENV> is set to a defined string, the
corresponding definition is made in the location to which the
first translation of F<PERL_ENV_TABLES> points.  If this causes a
logical name to be created, it is defined in supervisor mode.
(The same is done if an existing logical name was defined in
executive or kernel mode; an existing user or supervisor mode
logical name is reset to the new value.)  If the value is an empty
string, the logical name's translation is defined as a single C<NUL>
(ASCII C<\0>) character, since a logical name cannot translate to a
zero-length string.  (This restriction does not apply to CLI symbols
or CRTL C<environ> values; they are set to the empty string.)

When an element of C<%ENV> is set to C<undef>, the element is looked
up as if it were being read, and if it is found, it is deleted.  (An
item "deleted" from the CRTL C<environ> array is set to the empty
string.)  Using C<delete> to remove an element from C<%ENV> has a
similar effect, but after the element is deleted, another attempt is
made to look up the element, so an inner-mode logical name or a name
in another location will replace the logical name just deleted. In
either case, only the first value found searching PERL_ENV_TABLES is
altered.  It is not possible at present to define a search list
logical name via %ENV.

The element C<$ENV{DEFAULT}> is special: when read, it returns
Perl's current default device and directory, and when set, it
resets them, regardless of the definition of F<PERL_ENV_TABLES>.
It cannot be cleared or deleted; attempts to do so are silently
ignored.

Note that if you want to pass on any elements of the
C-local environ array to a subprocess which isn't
started by fork/exec, or isn't running a C program, you
can "promote" them to logical names in the current
process, which will then be inherited by all subprocesses,
by saying

    foreach my $key (qw[C-local keys you want promoted]) {
        my $temp = $ENV{$key}; # read from C-local array
        $ENV{$key} = $temp;    # and define as logical name
    }

(You can't just say C<$ENV{$key} = $ENV{$key}>, since the
Perl optimizer is smart enough to elide the expression.)

Don't try to clear C<%ENV> by saying C<%ENV = ();>, it will throw
a fatal error.  This is equivalent to doing the following from DCL:

    DELETE/LOGICAL *

You can imagine how bad things would be if, for example, the SYS$MANAGER
or SYS$SYSTEM logical names were deleted.

At present, the first time you iterate over %ENV using
C<keys>, or C<values>,  you will incur a time penalty as all
logical names are read, in order to fully populate %ENV.
Subsequent iterations will not reread logical names, so they
won't be as slow, but they also won't reflect any changes
to logical name tables caused by other programs.

You do need to be careful with the logical names representing
process-permanent files, such as C<SYS$INPUT> and C<SYS$OUTPUT>.
The translations for these logical names are prepended with a
two-byte binary value (0x1B 0x00) that needs to be stripped off
if you want to use it. (In previous versions of Perl it wasn't
possible to get the values of these logical names, as the null
byte acted as an end-of-string marker)

=item $!

The string value of C<$!> is that returned by the CRTL's
strerror() function, so it will include the VMS message for
VMS-specific errors.  The numeric value of C<$!> is the
value of C<errno>, except if errno is EVMSERR, in which
case C<$!> contains the value of vaxc$errno.  Setting C<$!>
always sets errno to the value specified.  If this value is
EVMSERR, it also sets vaxc$errno to 4 (NONAME-F-NOMSG), so
that the string value of C<$!> won't reflect the VMS error
message from before C<$!> was set.

=item $^E

This variable provides direct access to VMS status values
in vaxc$errno, which are often more specific than the
generic Unix-style error messages in C<$!>.  Its numeric value
is the value of vaxc$errno, and its string value is the
corresponding VMS message string, as retrieved by sys$getmsg().
Setting C<$^E> sets vaxc$errno to the value specified.

While Perl attempts to keep the vaxc$errno value to be current, if
errno is not EVMSERR, it may not be from the current operation.

=item $?

The "status value" returned in C<$?> is synthesized from the
actual exit status of the subprocess in a way that approximates
POSIX wait(5) semantics, in order to allow Perl programs to
portably test for successful completion of subprocesses.  The
low order 8 bits of C<$?> are always 0 under VMS, since the
termination status of a process may or may not have been
generated by an exception.

The next 8 bits contain the termination status of the program.

If the child process follows the convention of C programs
compiled with the _POSIX_EXIT macro set, the status value will
contain the actual value of 0 to 255 returned by that program
on a normal exit.

With the _POSIX_EXIT macro set, the Unix exit value of zero is
represented as a VMS native status of 1, and the Unix values
from 2 to 255 are encoded by the equation:

   VMS_status = 0x35a000 + (unix_value * 8) + 1.

And in the special case of Unix value 1 the encoding is:

   VMS_status = 0x35a000 + 8 + 2 + 0x10000000.

For other termination statuses, the severity portion of the
subprocess's exit status is used: if the severity was success or
informational, these bits are all 0; if the severity was
warning, they contain a value of 1; if the severity was
error or fatal error, they contain the actual severity bits,
which turns out to be a value of 2 for error and 4 for severe_error.
Fatal is another term for the severe_error status.

As a result, C<$?> will always be zero if the subprocess's exit
status indicated successful completion, and non-zero if a
warning or error occurred or a program compliant with encoding
_POSIX_EXIT values was run and set a status.

How can you tell the difference between a non-zero status that is
the result of a VMS native error status or an encoded Unix status?
You can not unless you look at the ${^CHILD_ERROR_NATIVE} value.
The ${^CHILD_ERROR_NATIVE} value returns the actual VMS status value
and check the severity bits. If the severity bits are equal to 1,
then if the numeric value for C<$?> is between 2 and 255 or 0, then
C<$?> accurately reflects a value passed back from a Unix application.
If C<$?> is 1, and the severity bits indicate a VMS error (2), then
C<$?> is from a Unix application exit value.

In practice, Perl scripts that call programs that return _POSIX_EXIT
type status values will be expecting those values, and programs that
call traditional VMS programs will either be expecting the previous
behavior or just checking for a non-zero status.

And success is always the value 0 in all behaviors.

When the actual VMS termination status of the child is an error,
internally the C<$!> value will be set to the closest Unix errno
value to that error so that Perl scripts that test for error
messages will see the expected Unix style error message instead
of a VMS message.

Conversely, when setting C<$?> in an END block, an attempt is made
to convert the POSIX value into a native status intelligible to
the operating system upon exiting Perl.  What this boils down to
is that setting C<$?> to zero results in the generic success value
SS$_NORMAL, and setting C<$?> to a non-zero value results in the
generic failure status SS$_ABORT.  See also L<perlport/exit>.

With the C<PERL_VMS_POSIX_EXIT> logical name defined as "ENABLE",
setting C<$?> will cause the new value to be encoded into C<$^E>
so that either the original parent or child exit status values 
 0 to 255 can be automatically recovered by C programs expecting
_POSIX_EXIT behavior.  If both a parent and a child exit value are
non-zero, then it will be assumed that this is actually a VMS native
status value to be passed through.  The special value of 0xFFFF is
almost a NOOP as it will cause the current native VMS status in the
C library to become the current native Perl VMS status, and is handled
this way as it is known to not be a valid native VMS status value.
It is recommend that only values in the range of normal Unix parent or
child status numbers, 0 to 255 are used.

The pragma C<use vmsish 'status'> makes C<$?> reflect the actual 
VMS exit status instead of the default emulation of POSIX status 
described above.  This pragma also disables the conversion of
non-zero values to SS$_ABORT when setting C<$?> in an END
block (but zero will still be converted to SS$_NORMAL).

Do not use the pragma C<use vmsish 'status'> with C<PERL_VMS_POSIX_EXIT>
enabled, as they are at times requesting conflicting actions and the
consequence of ignoring this advice will be undefined to allow future
improvements in the POSIX exit handling.

In general, with C<PERL_VMS_POSIX_EXIT> enabled, more detailed information
will be available in the exit status for DCL scripts or other native VMS tools,
and will give the expected information for Posix programs.  It has not been
made the default in order to preserve backward compatibility.

N.B. Setting C<DECC$FILENAME_UNIX_REPORT> implicitly enables 
C<PERL_VMS_POSIX_EXIT>.

=item $|

Setting C<$|> for an I/O stream causes data to be flushed
all the way to disk on each write (I<i.e.> not just to
the underlying RMS buffers for a file).  In other words,
it's equivalent to calling fflush() and fsync() from C.

=back

=head1 Standard modules with VMS-specific differences

=head2 SDBM_File

SDBM_File works properly on VMS. It has, however, one minor
difference. The database directory file created has a F<.sdbm_dir>
extension rather than a F<.dir> extension. F<.dir> files are VMS filesystem
directory files, and using them for other purposes could cause unacceptable
problems.

=head1 Revision date

Please see the git repository for revision history.

=head1 AUTHOR

Charles Bailey  bailey@cor.newman.upenn.edu
Craig Berry  craigberry@mac.com
Dan Sugalski  dan@sidhe.org
John Malmberg wb8tyw@qsl.net