PERLVMS(1)

NAME
perlvms - VMS-specific documentation for Perl

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 [.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@newman.upenn.edu.

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

Organization of Perl Images

  Core Images
During the installation process, three Perl images are produced. 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 PerlShr.Exe, which provides a core to which the Perl executable image and all Perl extensions are linked. You should place this image in Sys$Share, or define the logical name 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 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, 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.

  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 the perlxs manpage 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 "use" or "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 static configuration, the object code for the extension is linked directly into PerlShr.Exe, and is initialized whenever Perl is invoked. In the 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 "use"d or "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: Extshortname.xs (where Extshortname is the portion of the extension's name following the last "::"), containing the XS code, Extshortname.pm, the Perl library module for the extension, and Makefile.PL, a Perl script which uses the "MakeMaker" library modules supplied with Perl to generate a Descrip.MMS file for the extension.

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

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

  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 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 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
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 exceed 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 PGPLOT extension to Perl requires the PGPLOTSHR.EXE shareable image in order to properly link the Perl extension, then the line "PGPLOTSHR/Share" must be added to the linker options file PGPLOT.Opt produced during the build process for the Perl extension.

By default, the shareable image for an extension is placed [.lib.site_perl.autoArch.Extname] directory of the installed Perl directory tree (where Arch is VMS_VAX or VMS_AXP, and Extname is the name of the extension, with each "::" translated to "."). (See the MakeMaker documentation for more details on installation options for extensions.) However, it can be manually placed in any of several locations:
   - the [.Lib.Auto.Arch$PVersExtname] subdirectory
     of one of the directories in "@INC" (where PVers
     is the version of Perl you're using, as supplied in "$]",
     with '.' converted to '_'), or
   - one of the directories in "@INC", or
   - a directory which the extensions Perl library module
     passes to the DynaLoader when asking it to map
     the shareable image, or
   - Sys$Share or Sys$Library. If the shareable image isn't in any of these places, you'll need to define a logical name Extshortname, where Extshortname is the portion of the extension's name after the last "::", which translates to the full file specification of the shareable image.

File specifications

  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 (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 "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, etc. If you find instances of this in the Perl distribution itself, please let us know, so we can try to work around them.

  Wildcard expansion
File specifications containing wildcards are allowed both on the command line and within Perl globs (e.g. <C<*.c>>). 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.

In both cases, VMS wildcard expansion is performed. (csh-style wildcard expansion is available if you use "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 "[.t]*.*" will yield filespecs like "perl_root:[t]base.dir", while the expansion of "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.

  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 "system" operator (see below). In this case, Perl will wait for the subprocess to complete before continuing.

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

Command line

  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.

  Command line switches
The following command line switches behave differently under VMS than described in the perlrun manpage. 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.

-i  If the "-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.)

-S  If the "-S" switch is present 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 .pl, and a type of .com, in that order.

-u  The "-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.

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, 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
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 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

File tests
    The tests "-b", "-B", "-c", "-C", "-d", "-e", "-f", "-o", "-M", "-s", "-S", "-t", "-T", and "-z" work as advertised. The return values for "-r", "-w", and "-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, "-O", "-R", "-W", and "-X" are equivalent to "-o", "-r", "-w", and "-x". Similarly, several other tests, including "-A", "-g", "-k", "-l", "-p", and "-u", aren't particularly meaningful under VMS, and the values returned by these tests reflect whatever your CRTL "stat()" routine does to the equivalent bits in the st_mode field. Finally, "-d" returns true if passed a device specification without an explicit directory (e.g. "DUA1:"), as well as if passed a directory.

    Note: Some sites have reported problems when using the file-access tests ("-r", "-w", and "-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 "$!" indicating that the file does not exist. You can use "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 stat.h, in the mode value it returns, if you need an approximation of the file's protections.

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 "lib$spawn()", any valid DCL command string may be specified.

binmode FILEHANDLE
    The "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 "binmode" was called.

    Note that "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 "vmsfopen" function in the VMS::Stdio extension to gain finer control of I/O to files and devices with different record structures.

crypt PLAINTEXT, USER
    The "crypt" operator uses the "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 "crypt" may be compared against the encrypted password from the UAF returned by the "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 "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;
      }

dump
    Rather than causing Perl to abort and dump core, the "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 "dump", or, if no label was specified, back to the beginning of the program. All other state of the program (e.g. values of variables, open file handles) are not affected by calling "dump".

exec LIST
    The "exec" operator behaves in one of two different ways. If called after a call to "fork", it will invoke the CRTL "execv()" routine, passing its arguments to the subprocess created by "fork" for execution. In this case, it is subject to all limitations that affect "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 "exec" does not follow a call to "fork", it will cause Perl to exit, and to invoke the command given as an argument to "exec" via "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 .Exe and the process defaults to expand the filespec) and pass the rest of "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 "MCR" or a text file which should be passed to DCL as a command procedure.

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

fork
    The "fork" operator works in the same way as the CRTL "vfork()" routine, which is quite different under VMS than under Unix. Specifically, while "fork" returns 0 after it is called and the subprocess PID after "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 "exec".

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

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

gmtime
    The "gmtime" operator will function properly if you have a working CRTL "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 "undef" is returned.

kill
    In most cases, "kill" is implemented via the CRTL's "kill()" function, so it will behave according to that function's documentation. If you send a SIGKILL, however, the $DELPRC system service is called directly. This insures that the target process is actually deleted, if at all possible. (The CRTL's "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.

qx//
    See the entry on "backticks" above.

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

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 "st_dev" and "st_ino" fields of a "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.

system LIST
    The "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 "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. ":" or "]"), an attempt is made to expand it using a default type of .Exe and the process defaults, and if successful, the resulting file is invoked via "MCR". This allows you to invoke an image directly simply by passing the file specification to "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 "MCR" or a text file which should be passed to DCL as a command procedure.

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

    Perl waits for the subprocess to complete before continuing execution in the current process. As described in the perlfunc manpage, the return value of "system" is a fake "status" which follows POSIX semantics; see the description of "$?" in this document for more detail. The actual VMS exit status of the subprocess is available in "$^S" (as long as you haven't used another Perl function that resets "$?" and "$^S" in the meantime).

time
    The value returned by "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.

times
    The array returned by the "times" operator is divided up according to the same rules the CRTL "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 the times manpage keeps track of subprocesses separately. Note especially that the VAXCRTL (at least) keeps track only of subprocesses spawned using the fork manpage and the exec manpage; it will not accumulate the times of subprocesses spawned via pipes, the system manpage, or backticks.

unlink LIST
    "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 "unlink", no files with the names passed to "unlink" will exist. (Note: This can be changed at compile time; if you "use Config" and "$Config{'d_unlink_all_versions'}" is "define", then "unlink" will delete all versions of a file on the first call.)

    "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 "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 "VMS::Stdio::remove", if you've installed the VMS::Stdio extension distributed with Perl). If "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.

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).

waitpid PID,FLAGS
    If PID is a subprocess started by a piped "open()" (see the open manpage), "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, "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 "-w" switch, a warning will be issued.)

    The FLAGS argument is ignored in all cases.

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

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

    CRTL_ENV
        This string tells Perl to consult the CRTL's internal "environ" array of key-value pairs, using name as the key. In most cases, this contains only a few keys, but if Perl was invoked via the C "exec[lv]e()" function, as is the case for CGI processing by some HTTP servers, then the "environ" array may have been populated by the calling program.

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

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

    PERL_ENV_TABLES is translated once when Perl starts up; any changes you make while Perl is running do not affect the behavior of "%ENV". If PERL_ENV_TABLES is not defined, then Perl defaults to consulting first the logical name tables specified by LNM$FILE_DEV, and then the CRTL "environ" array.

    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.

    When an element of "%ENV" is read, the locations to which PERL_ENV_TABLES points are checked in order, and the value obtained from the first successful lookup is returned. If the name of the "%ENV" element contains a semi-colon, it and any characters after it are removed. These are ignored when the CRTL "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 "ONCE UPON A TIME THERE WAS", assuming, of course, that PERL_ENV_TABLES is set up so that the logical name "story" is found, rather than a CLI symbol or CRTL "environ" element with the same name.

    When an element of "%ENV" is set to a defined string, the corresponding definition is made in the location to which the first translation of 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 NUL (ASCII 00) character, since a logical name cannot translate to a zero-length string. (This restriction does not apply to CLI symbols or CRTL "environ" values; they are set to the empty string.) An element of the CRTL "environ" array can be set only if your copy of Perl knows about the CRTL's "setenv()" function. (This is present only in some versions of the DECCRTL; check "$Config{d_setenv}" to see whether your copy of Perl was built with a CRTL that has this function.)

    When an element of "%ENV" is set to "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 "environ" array is set to the empty string; this can only be done if your copy of Perl knows about the CRTL "setenv()" function.) Using "delete" to remove an element from "%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 "$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 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} = $temp;    # and define as logical name
        }
    (You can't just say "$ENV{$key} = $ENV{$key}", since the Perl optimizer is smart enough to elide the expression.)

    Don't try to clear "%ENV" by saying "%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 logicals were deleted.

    At present, the first time you iterate over %ENV using "keys", or "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 logicals representing process-permanent files, such as "SYS$INPUT" and "SYS$OUTPUT". The translations for these logicals 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 logicals, as the null byte acted as an end-of-string marker)

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

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

$?  The "status value" returned in "$?" 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 "$?" are always 0 under VMS, since the termination status of a process may or may not have been generated by an exception. The next 8 bits are derived from the severity portion of the subprocess' exit status: if the severity was success or informational, these bits are all 0; otherwise, they contain the severity value shifted left one bit. As a result, "$?" will always be zero if the subprocess' exit status indicated successful completion, and non-zero if a warning or error occurred. The actual VMS exit status may be found in "$^S" (q.v.).

$^S Under VMS, this is the 32-bit VMS status value returned by the last subprocess to complete. Unlike "$?", no manipulation is done to make this look like a POSIX wait(5) value, so it may be treated as a normal VMS status value.

$|  Setting "$|" for an I/O stream causes data to be flushed all the way to disk on each write (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.

Standard modules with VMS-specific differences

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

Revision date
This document was last updated on 26-Feb-2000, for Perl 5, patchlevel 6.

AUTHOR
Charles Bailey <bailey@cor.newman.upenn.edu> Dan Sugalski <dan@sidhe.org>