[p5sagit/p5-mst-13.2.git] / vms / 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@genetics.upenn.edu.

=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 installation 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 extensions.  It is used to generate several files needed
to build the complete Perl and various extensions.  Once you've
finished installing Perl, you can delete this image.

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.  You should place this
image in F<Sys$Share>, or define the logical name F<PerlShr> to
translate to the full file specification of this image.  It should
be world readable.  (Remember that if a user has execute only access
to F<PerlShr>, VMS will treat it as if it were a privileged shareable
image, and will therefore require all downstream shareable images to be
INSTALLed, etc.)


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<perlapi> 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

I<N.B.> The procedure by which extensions are built and
tested creates several levels (at least 4) under the
directory in which the extension's source files live.
For this reason, you shouldn't nest the source directory
too deeply in your directory structure, lest you eccedd RMS'
maximum of 8 levels of subdirectory in a filespec.  (You
can use rooted logical names to get another 8 levels of
nesting, if you can't place the files near the top of
the physical directory structure.)

VMS support for this process in the current release of Perl
is sufficient to handle most extensions.  However, it does
not yet recognize extra libraries required to build shareable
images which are part of an extension, so these must be added
to the linker options file for the extension by hand.  For
instance, if the F<PGPLOT> extension to Perl requires the
F<PGPLOTSHR.EXE> shareable image in order to properly link
the Perl extension, then the line C<PGPLOTSHR/Share> must
be added to the linker options file F<PGPLOT.Opt> produced
during the build process for the Perl extension.

By default, the shareable image for an extension is placed
in the F<[.Lib.Auto.>I<Arch>.I<Extname>F<]> directory of the
installed Perl directory tree (where I<Arch> is F<VMS_VAX> or
F<VMS_AXP>, followed by the Perl version number, and I<Extname>
is the name of the extension, with each C<::> translated to C<.>).
However, it can be manually placed in any of several locations:
   - the F<[.Lib.Auto.>I<Extname>F<]> subdirectory of one of
     the directories in C<@INC>, or
   - one of the directories in C<@INC>, or
   - a directory which the extensions Perl library module
     passes to the DynaLoader when asking it to map
     the shareable image, or
   - F<Sys$Share> or F<Sys$Library>.
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 fle 
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.

Filenames are, of course, still case-insensitive.  For
consistency, most Perl routines return  filespecs using
lower case letters only, regardless of the case used in
the arguments passed to them.  (This is true  only when
running under VMS; Perl respects the case-sensitivity
of OSs like Unix.)

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. 

=head2 Wildcard expansion

File specifications containing wildcards are allowed both on 
the command line and within Perl globs (e.g. <CE<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.

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 passed directly to lib$spawn 
as the command to execute.  In this case, Perl will wait for 
the subprocess to complete before continuing. 

=head1 PERL5LIB and PERLLIB

The PERL5LIB and PERLLIB logical names work as documented L<perl>,
except that the element separator is '|' instead of ':'.  The
directory specifications may use either VMS or Unix syntax.

=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:
    <F<file> reads stdin from F<file>,
    >F<file> writes stdout to F<file>,
    >>F<file> appends stdout to F<file>,
    2>F<file> writes stderr to F<file>, and
    2>>F<file> appends stderr to F<file>. 

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.

=item -S

If the 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.

=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, binmode*, bless,
    caller, chdir, chmod, chown, chomp, chop, chr,
    close, closedir, cos, crypt*, defined, delete,
    die, do, dump*, each, endpwent, eof, eval, exec*,
    exists, exit, exp, fileno, fork*, getc, getlogin,
    getpwent*, getpwnam*, getpwuid*, glob, gmtime*, goto,
    grep, hex, import, index, int, join, keys, kill*,
    last, lc, lcfirst, length, local, localtime, log, 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, redo, ref, rename,
    require, reset, return, reverse, rewinddir, rindex,
    rmdir, s///, scalar, seek, seekdir, select(internal),
    select (system call)*, setpwent, shift, sin, sleep,
    sort, splice, split, sprintf, sqrt, srand, stat,
    study, substr, 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, fcntl, flock,
    getpgrp, getppid, getpriority, getgrent, getgrgid,
    getgrnam, setgrent, endgrent, ioctl, link, lstat,
    msgctl, msgget, msgsend, msgrcv, readlink, semctl,
    semget, semop, setpgrp, setpriority, shmctl, shmget,
    shmread, shmwrite, socketpair, symlink, syscall, truncate

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


=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.

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 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 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

The C<exec> operator behaves in one of two different ways.  
If called after a call to C<fork>, it will invoke the CRTL 
C<execv()> routine, passing its arguments to the subprocess 
created by C<fork> for execution.  In this case, it is 
subject to all limitations that affect C<execv()>.  (In 
particular, this usually means that the command executed in 
the subprocess must be an image compiled from C source code, 
and that your options for passing file descriptors and signal 
handlers to the subprocess are limited.)

If the call to C<exec> does not follow a call to C<fork>, it 
will cause Perl to exit, and to invoke the command given as 
an argument to C<exec> via C<lib$do_command>.  If the argument 
begins with a '$' (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.

You can use C<exec> in both ways within the same script, as 
long as you call C<fork> and C<exec> in pairs.  Perl
keeps track of how many times C<fork> and C<exec> have been
called, and will call the CRTL C<execv()> routine if there have
previously been more calls to C<fork> than to C<exec>.

=item fork

The C<fork> operator works in the same way as the CRTL 
C<vfork()> routine, which is quite different under VMS than 
under Unix.  Specifically, while C<fork> returns 0 after it 
is called and the subprocess PID after C<exec> is called, in 
both cases the thread of execution is within the parent 
process, so there is no opportunity to perform operations in 
the subprocess before calling C<exec>.

In general, the use of C<fork> and C<exec> to create 
subprocess is not recommended under VMS; wherever possible, 
use the C<system> operator or piped filehandles instead.

=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> kill is implemented via the CRTL's C<kill()>
function, so it will behave according to that function's
documentation.  If you send a SIGKILL, however, the $DELPRC system
service is is called directly.  This insures that the target
process is actually deleted, if at all possible.  (The CRTL's C<kill()>
function is presently implemented via $FORCEX, which is ignored by
supervisor-mode images like DCL.)

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

=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 LIST consists
of the empty string, C<system> spawns an interactive DCL subprocess,
in the same fashion as typiing B<SPAWN> at the DCL prompt.
Perl waits for the subprocess to complete before continuing
execution in the current process.

=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 subprocess may or may 
not appear separately in the "child time" field, depending on 
whether L<times> keeps track of subprocesses separately.  Note
especially that the VAXCRTL (at least) keeps track only of
subprocesses spawned using L<fork> and L<exec>; it will not
accumulate the times of suprocesses spawned via pipes, L<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.

=item utime LIST

Since ODS-2, the VMS file structure for disk files, does not keep
track of access times, this operator changes only the modification
time of the file (VMS revision date).

=item waitpid PID,FLAGS

If PID is a subprocess started by a piped L<open>, C<waitpid>
will wait for that subprocess, and return its final
status value.  If PID is a subprocess created in some other way
(e.g. SPAWNed before Perl was invoked), or is not a subprocess of
the current process, C<waitpid> will check once per second whether
the process has completed, and when it has, will return 0.  (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.)

The FLAGS argument is ignored in all cases.

=head1 Perl variables

=item %ENV 

Reading the elements of the %ENV array returns the 
translation of the logical name specified by the key, 
according to the normal search order of access modes and 
logical name tables.  If you append a semicolon to the
logical name, followed by an integer, that integer is
used as the translation index for the logical name,
so that you can 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>.

The %ENV keys C<home>, C<path>,C<term>, and C<user>
return the CRTL "environment variables" of the same
names, if these logical names are not defined.  The
key C<default> returns the current default device
and directory specification, regardless of whether
there is a logical name DEFAULT defined..

Setting an element of %ENV defines a supervisor-mode logical 
name in the process logical name table.  C<Undef>ing or 
C<delete>ing an element of %ENV deletes the equivalent user-
mode or supervisor-mode logical name from the process logical 
name table.  If you use C<undef>, the %ENV element remains 
empty.  If you use C<delete>, another attempt is made at 
logical name translation after the deletion, so an inner-mode 
logical name or a name in another logical name table will 
replace the logical name just deleted.  It is not possible
at present to define a search list logical name via %ENV.

At present, the first time you iterate over %ENV using
C<keys>, C<values>, or C<each>, 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.

In all operations on %ENV, the key string is treated as if it 
were entirely uppercase, regardless of the case actually 
specified in the Perl expression.

=item $?

Since VMS status values are 32 bits wide, the value of C<$?>
is simply the final status value of the last subprocess to
complete.  This differs from the behavior of C<$?> under Unix,
and under VMS' POSIX environment, in that the low-order 8 bits
of C<$?> do not specify whether the process terminated normally
or due to a signal, and you do not need to shift C<$?> 8 bits
to the right in order to find the process' exit status.

=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.

=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.

=head1 Revision date

This document was last updated on 28-Feb-1996, for Perl 5, 
patchlevel 2.

=head1 AUTHOR

Charles Bailey  bailey@genetics.upenn.edu