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PERLPORT(1pm)          Perl Programmers Reference Guide          PERLPORT(1pm)


       perlport - Writing portable Perl


       Perl runs on numerous operating systems.  While most of them share much
       in common, they also have their own unique features.

       This document is meant to help you to find out what constitutes
       portable Perl code.  That way once you make a decision to write
       portably, you know where the lines are drawn, and you can stay within

       There is a tradeoff between taking full advantage of one particular
       type of computer and taking advantage of a full range of them.
       Naturally, as you broaden your range and become more diverse, the
       common factors drop, and you are left with an increasingly smaller area
       of common ground in which you can operate to accomplish a particular
       task.  Thus, when you begin attacking a problem, it is important to
       consider under which part of the tradeoff curve you want to operate.
       Specifically, you must decide whether it is important that the task
       that you are coding has the full generality of being portable, or
       whether to just get the job done right now.  This is the hardest choice
       to be made.  The rest is easy, because Perl provides many choices,
       whichever way you want to approach your problem.

       Looking at it another way, writing portable code is usually about
       willfully limiting your available choices.  Naturally, it takes
       discipline and sacrifice to do that.  The product of portability and
       convenience may be a constant.  You have been warned.

       Be aware of two important points:

       Not all Perl programs have to be portable
           There is no reason you should not use Perl as a language to glue
           Unix tools together, or to prototype a Macintosh application, or to
           manage the Windows registry.  If it makes no sense to aim for
           portability for one reason or another in a given program, then
           don't bother.

       Nearly all of Perl already is portable
           Don't be fooled into thinking that it is hard to create portable
           Perl code.  It isn't.  Perl tries its level-best to bridge the gaps
           between what's available on different platforms, and all the means
           available to use those features.  Thus almost all Perl code runs on
           any machine without modification.  But there are some significant
           issues in writing portable code, and this document is entirely
           about those issues.

       Here's the general rule: When you approach a task commonly done using a
       whole range of platforms, think about writing portable code.  That way,
       you don't sacrifice much by way of the implementation choices you can
       avail yourself of, and at the same time you can give your users lots of
       platform choices.  On the other hand, when you have to take advantage
       of some unique feature of a particular platform, as is often the case
       with systems programming (whether for Unix, Windows, VMS, etc.),
       consider writing platform-specific code.

       When the code will run on only two or three operating systems, you may
       need to consider only the differences of those particular systems.  The
       important thing is to decide where the code will run and to be
       deliberate in your decision.

       The material below is separated into three main sections: main issues
       of portability ("ISSUES"), platform-specific issues ("PLATFORMS"), and
       built-in Perl functions that behave differently on various ports

       This information should not be considered complete; it includes
       possibly transient information about idiosyncrasies of some of the
       ports, almost all of which are in a state of constant evolution.  Thus,
       this material should be considered a perpetual work in progress ("<IMG
       SRC="yellow_sign.gif" ALT="Under Construction">").


       In most operating systems, lines in files are terminated by newlines.
       Just what is used as a newline may vary from OS to OS.  Unix
       traditionally uses "\012", one type of DOSish I/O uses "\015\012",
       Mac OS uses "\015", and z/OS uses "\025".

       Perl uses "\n" to represent the "logical" newline, where what is
       logical may depend on the platform in use.  In MacPerl, "\n" always
       means "\015".  On EBCDIC platforms, "\n" could be "\025" or "\045".  In
       DOSish perls, "\n" usually means "\012", but when accessing a file in
       "text" mode, perl uses the ":crlf" layer that translates it to (or
       from) "\015\012", depending on whether you're reading or writing. Unix
       does the same thing on ttys in canonical mode.  "\015\012" is commonly
       referred to as CRLF.

       To trim trailing newlines from text lines use "chomp".  With default
       settings that function looks for a trailing "\n" character and thus
       trims in a portable way.

       When dealing with binary files (or text files in binary mode) be sure
       to explicitly set $/ to the appropriate value for your file format
       before using "chomp".

       Because of the "text" mode translation, DOSish perls have limitations
       in using "seek" and "tell" on a file accessed in "text" mode.  Stick to
       "seek"-ing to locations you got from "tell" (and no others), and you
       are usually free to use "seek" and "tell" even in "text" mode.  Using
       "seek" or "tell" or other file operations may be non-portable.  If you
       use "binmode" on a file, however, you can usually "seek" and "tell"
       with arbitrary values safely.

       A common misconception in socket programming is that "\n eq \012"
       everywhere.  When using protocols such as common Internet protocols,
       "\012" and "\015" are called for specifically, and the values of the
       logical "\n" and "\r" (carriage return) are not reliable.

           print $socket "Hi there, client!\r\n";      # WRONG
           print $socket "Hi there, client!\015\012";  # RIGHT

       However, using "\015\012" (or "\cM\cJ", or "\x0D\x0A") can be tedious
       and unsightly, as well as confusing to those maintaining the code.  As
       such, the "Socket" module supplies the Right Thing for those who want

           use Socket qw(:DEFAULT :crlf);
           print $socket "Hi there, client!$CRLF"      # RIGHT

       When reading from a socket, remember that the default input record
       separator $/ is "\n", but robust socket code will recognize as either
       "\012" or "\015\012" as end of line:

           while (<$socket>) {  # NOT ADVISABLE!
               # ...

       Because both CRLF and LF end in LF, the input record separator can be
       set to LF and any CR stripped later.  Better to write:

           use Socket qw(:DEFAULT :crlf);
           local($/) = LF;      # not needed if $/ is already \012

           while (<$socket>) {
               s/$CR?$LF/\n/;   # not sure if socket uses LF or CRLF, OK
           #   s/\015?\012/\n/; # same thing

       This example is preferred over the previous one--even for Unix
       platforms--because now any "\015"'s ("\cM"'s) are stripped out (and
       there was much rejoicing).

       Similarly, functions that return text data--such as a function that
       fetches a web page--should sometimes translate newlines before
       returning the data, if they've not yet been translated to the local
       newline representation.  A single line of code will often suffice:

           $data =~ s/\015?\012/\n/g;
           return $data;

       Some of this may be confusing.  Here's a handy reference to the ASCII
       CR and LF characters.  You can print it out and stick it in your

           LF  eq  \012  eq  \x0A  eq  \cJ  eq  chr(10)  eq  ASCII 10
           CR  eq  \015  eq  \x0D  eq  \cM  eq  chr(13)  eq  ASCII 13

                    | Unix | DOS  | Mac  |
               \n   |  LF  |  LF  |  CR  |
               \r   |  CR  |  CR  |  LF  |
               \n * |  LF  | CRLF |  CR  |
               \r * |  CR  |  CR  |  LF  |
               * text-mode STDIO

       The Unix column assumes that you are not accessing a serial line (like
       a tty) in canonical mode.  If you are, then CR on input becomes "\n",
       and "\n" on output becomes CRLF.

       These are just the most common definitions of "\n" and "\r" in Perl.
       There may well be others.  For example, on an EBCDIC implementation
       such as z/OS (OS/390) or OS/400 (using the ILE, the PASE is ASCII-
       based) the above material is similar to "Unix" but the code numbers

           LF  eq  \025  eq  \x15  eq  \cU  eq  chr(21)  eq  CP-1047 21
           LF  eq  \045  eq  \x25  eq           chr(37)  eq  CP-0037 37
           CR  eq  \015  eq  \x0D  eq  \cM  eq  chr(13)  eq  CP-1047 13
           CR  eq  \015  eq  \x0D  eq  \cM  eq  chr(13)  eq  CP-0037 13

                    | z/OS | OS/400 |
               \n   |  LF  |  LF    |
               \r   |  CR  |  CR    |
               \n * |  LF  |  LF    |
               \r * |  CR  |  CR    |
               * text-mode STDIO

   Numbers endianness and Width
       Different CPUs store integers and floating point numbers in different
       orders (called endianness) and widths (32-bit and 64-bit being the most
       common today).  This affects your programs when they attempt to
       transfer numbers in binary format from one CPU architecture to another,
       usually either "live" via network connection, or by storing the numbers
       to secondary storage such as a disk file or tape.

       Conflicting storage orders make an utter mess out of the numbers.  If a
       little-endian host (Intel, VAX) stores 0x12345678 (305419896 in
       decimal), a big-endian host (Motorola, Sparc, PA) reads it as
       0x78563412 (2018915346 in decimal).  Alpha and MIPS can be either:
       Digital/Compaq used/uses them in little-endian mode; SGI/Cray uses them
       in big-endian mode.  To avoid this problem in network (socket)
       connections use the "pack" and "unpack" formats "n" and "N", the
       "network" orders.  These are guaranteed to be portable.

       As of Perl 5.10.0, you can also use the ">" and "<" modifiers to force
       big- or little-endian byte-order.  This is useful if you want to store
       signed integers or 64-bit integers, for example.

       You can explore the endianness of your platform by unpacking a data
       structure packed in native format such as:

           print unpack("h*", pack("s2", 1, 2)), "\n";
           # '10002000' on e.g. Intel x86 or Alpha 21064 in little-endian mode
           # '00100020' on e.g. Motorola 68040

       If you need to distinguish between endian architectures you could use
       either of the variables set like so:

           $is_big_endian   = unpack("h*", pack("s", 1)) =~ /01/;
           $is_little_endian = unpack("h*", pack("s", 1)) =~ /^1/;

       Differing widths can cause truncation even between platforms of equal
       endianness.  The platform of shorter width loses the upper parts of the
       number.  There is no good solution for this problem except to avoid
       transferring or storing raw binary numbers.

       One can circumnavigate both these problems in two ways.  Either
       transfer and store numbers always in text format, instead of raw
       binary, or else consider using modules like "Data::Dumper" and
       "Storable" (included as of Perl 5.8).  Keeping all data as text
       significantly simplifies matters.

   Files and Filesystems
       Most platforms these days structure files in a hierarchical fashion.
       So, it is reasonably safe to assume that all platforms support the
       notion of a "path" to uniquely identify a file on the system.  How that
       path is really written, though, differs considerably.

       Although similar, file path specifications differ between Unix,
       Windows, Mac OS, OS/2, VMS, VOS, RISC OS, and probably others.  Unix,
       for example, is one of the few OSes that has the elegant idea of a
       single root directory.

       DOS, OS/2, VMS, VOS, and Windows can work similarly to Unix with "/" as
       path separator, or in their own idiosyncratic ways (such as having
       several root directories and various "unrooted" device files such NIL:
       and LPT:).

       Mac OS 9 and earlier used ":" as a path separator instead of "/".

       The filesystem may support neither hard links ("link") nor symbolic
       links ("symlink", "readlink", "lstat").

       The filesystem may support neither access timestamp nor change
       timestamp (meaning that about the only portable timestamp is the
       modification timestamp), or one second granularity of any timestamps
       (e.g. the FAT filesystem limits the time granularity to two seconds).

       The "inode change timestamp" (the "-C" filetest) may really be the
       "creation timestamp" (which it is not in Unix).

       VOS perl can emulate Unix filenames with "/" as path separator.  The
       native pathname characters greater-than, less-than, number-sign, and
       percent-sign are always accepted.

       RISC OS perl can emulate Unix filenames with "/" as path separator, or
       go native and use "." for path separator and ":" to signal filesystems
       and disk names.

       Don't assume Unix filesystem access semantics: that read, write, and
       execute are all the permissions there are, and even if they exist, that
       their semantics (for example what do "r", "w", and "x" mean on a
       directory) are the Unix ones.  The various Unix/POSIX compatibility
       layers usually try to make interfaces like "chmod" work, but sometimes
       there simply is no good mapping.

       The "File::Spec" modules provide methods to manipulate path
       specifications and return the results in native format for each
       platform.  This is often unnecessary as Unix-style paths are understood
       by Perl on every supported platform, but if you need to produce native
       paths for a native utility that does not understand Unix syntax, or if
       you are operating on paths or path components in unknown (and thus
       possibly native) syntax, "File::Spec" is your friend.  Here are two
       brief examples:

           use File::Spec::Functions;
           chdir(updir());        # go up one directory

           # Concatenate a path from its components
           my $file = catfile(updir(), 'temp', 'file.txt');
           # on Unix:    '../temp/file.txt'
           # on Win32:   '..\temp\file.txt'
           # on VMS:     '[-.temp]file.txt'

       In general, production code should not have file paths hardcoded.
       Making them user-supplied or read from a configuration file is better,
       keeping in mind that file path syntax varies on different machines.

       This is especially noticeable in scripts like Makefiles and test
       suites, which often assume "/" as a path separator for subdirectories.

       Also of use is "File::Basename" from the standard distribution, which
       splits a pathname into pieces (base filename, full path to directory,
       and file suffix).

       Even when on a single platform (if you can call Unix a single
       platform), remember not to count on the existence or the contents of
       particular system-specific files or directories, like /etc/passwd,
       /etc/sendmail.conf, /etc/resolv.conf, or even /tmp/.  For example,
       /etc/passwd may exist but not contain the encrypted passwords, because
       the system is using some form of enhanced security.  Or it may not
       contain all the accounts, because the system is using NIS.  If code
       does need to rely on such a file, include a description of the file and
       its format in the code's documentation, then make it easy for the user
       to override the default location of the file.

       Don't assume a text file will end with a newline.  They should, but
       people forget.

       Do not have two files or directories of the same name with different
       case, like and, as many platforms have case-insensitive
       (or at least case-forgiving) filenames.  Also, try not to have non-word
       characters (except for ".") in the names, and keep them to the 8.3
       convention, for maximum portability, onerous a burden though this may

       Likewise, when using the "AutoSplit" module, try to keep your functions
       to 8.3 naming and case-insensitive conventions; or, at the least, make
       it so the resulting files have a unique (case-insensitively) first 8

       Whitespace in filenames is tolerated on most systems, but not all, and
       even on systems where it might be tolerated, some utilities might
       become confused by such whitespace.

       Many systems (DOS, VMS ODS-2) cannot have more than one "." in their

       Don't assume ">" won't be the first character of a filename.  Always
       use the three-arg version of "open":

           open my $fh, '<', $existing_file) or die $!;

       Two-arg "open" is magic and can translate characters like ">", "<", and
       "|" in filenames, which is usually the wrong thing to do.  "sysopen"
       and three-arg "open" don't have this problem.

       Don't use ":" as a part of a filename since many systems use that for
       their own semantics (Mac OS Classic for separating pathname components,
       many networking schemes and utilities for separating the nodename and
       the pathname, and so on).  For the same reasons, avoid "@", ";" and

       Don't assume that in pathnames you can collapse two leading slashes
       "//" into one: some networking and clustering filesystems have special
       semantics for that.  Let the operating system sort it out.

       The portable filename characters as defined by ANSI C are

        a b c d e f g h i j k l m n o p q r s t u v w x y z
        A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
        0 1 2 3 4 5 6 7 8 9
        . _ -

       and "-" shouldn't be the first character.  If you want to be
       hypercorrect, stay case-insensitive and within the 8.3 naming
       convention (all the files and directories have to be unique within one
       directory if their names are lowercased and truncated to eight
       characters before the ".", if any, and to three characters after the
       ".", if any).  (And do not use "."s in directory names.)

   System Interaction
       Not all platforms provide a command line.  These are usually platforms
       that rely primarily on a Graphical User Interface (GUI) for user
       interaction.  A program requiring a command line interface might not
       work everywhere.  This is probably for the user of the program to deal
       with, so don't stay up late worrying about it.

       Some platforms can't delete or rename files held open by the system,
       this limitation may also apply to changing filesystem metainformation
       like file permissions or owners.  Remember to "close" files when you
       are done with them.  Don't "unlink" or "rename" an open file.  Don't
       "tie" or "open" a file already tied or opened; "untie" or "close" it

       Don't open the same file more than once at a time for writing, as some
       operating systems put mandatory locks on such files.

       Don't assume that write/modify permission on a directory gives the
       right to add or delete files/directories in that directory.  That is
       filesystem specific: in some filesystems you need write/modify
       permission also (or even just) in the file/directory itself.  In some
       filesystems (AFS, DFS) the permission to add/delete directory entries
       is a completely separate permission.

       Don't assume that a single "unlink" completely gets rid of the file:
       some filesystems (most notably the ones in VMS) have versioned
       filesystems, and "unlink" removes only the most recent one (it doesn't
       remove all the versions because by default the native tools on those
       platforms remove just the most recent version, too).  The portable
       idiom to remove all the versions of a file is

           1 while unlink "file";

       This will terminate if the file is undeletable for some reason
       (protected, not there, and so on).

       Don't count on a specific environment variable existing in %ENV.  Don't
       count on %ENV entries being case-sensitive, or even case-preserving.
       Don't try to clear %ENV by saying "%ENV = ();", or, if you really have
       to, make it conditional on "$^O ne 'VMS'" since in VMS the %ENV table
       is much more than a per-process key-value string table.

       On VMS, some entries in the %ENV hash are dynamically created when
       their key is used on a read if they did not previously exist.  The
       values for $ENV{HOME}, $ENV{TERM}, $ENV{PATH}, and $ENV{USER}, are
       known to be dynamically generated.  The specific names that are
       dynamically generated may vary with the version of the C library on
       VMS, and more may exist than are documented.

       On VMS by default, changes to the %ENV hash persist after perl exits.
       Subsequent invocations of perl in the same process can inadvertently
       inherit environment settings that were meant to be temporary.

       Don't count on signals or %SIG for anything.

       Don't count on filename globbing.  Use "opendir", "readdir", and
       "closedir" instead.

       Don't count on per-program environment variables, or per-program
       current directories.

       Don't count on specific values of $!, neither numeric nor especially
       the string values. Users may switch their locales causing error
       messages to be translated into their languages.  If you can trust a
       POSIXish environment, you can portably use the symbols defined by the
       "Errno" module, like "ENOENT".  And don't trust on the values of $! at
       all except immediately after a failed system call.

   Command names versus file pathnames
       Don't assume that the name used to invoke a command or program with
       "system" or "exec" can also be used to test for the existence of the
       file that holds the executable code for that command or program.
       First, many systems have "internal" commands that are built-in to the
       shell or OS and while these commands can be invoked, there is no
       corresponding file.  Second, some operating systems (e.g., Cygwin,
       DJGPP, OS/2, and VOS) have required suffixes for executable files;
       these suffixes are generally permitted on the command name but are not
       required.  Thus, a command like "perl" might exist in a file named
       perl, perl.exe, or, depending on the operating system.  The
       variable $Config{_exe} in the "Config" module holds the executable
       suffix, if any.  Third, the VMS port carefully sets up $^X and
       $Config{perlpath} so that no further processing is required.  This is
       just as well, because the matching regular expression used below would
       then have to deal with a possible trailing version number in the VMS
       file name.

       To convert $^X to a file pathname, taking account of the requirements
       of the various operating system possibilities, say:

        use Config;
        my $thisperl = $^X;
        if ($^O ne 'VMS') {
            $thisperl .= $Config{_exe}
                unless $thisperl =~ m/\Q$Config{_exe}\E$/i;

       To convert $Config{perlpath} to a file pathname, say:

        use Config;
        my $thisperl = $Config{perlpath};
        if ($^O ne 'VMS') {
            $thisperl .= $Config{_exe}
                unless $thisperl =~ m/\Q$Config{_exe}\E$/i;

       Don't assume that you can reach the public Internet.

       Don't assume that there is only one way to get through firewalls to the
       public Internet.

       Don't assume that you can reach outside world through any other port
       than 80, or some web proxy.  ftp is blocked by many firewalls.

       Don't assume that you can send email by connecting to the local SMTP

       Don't assume that you can reach yourself or any node by the name
       'localhost'.  The same goes for ''.  You will have to try

       Don't assume that the host has only one network card, or that it can't
       bind to many virtual IP addresses.

       Don't assume a particular network device name.

       Don't assume a particular set of "ioctl"s will work.

       Don't assume that you can ping hosts and get replies.

       Don't assume that any particular port (service) will respond.

       Don't assume that "Sys::Hostname" (or any other API or command) returns
       either a fully qualified hostname or a non-qualified hostname: it all
       depends on how the system had been configured.  Also remember that for
       things such as DHCP and NAT, the hostname you get back might not be
       very useful.

       All the above don'ts may look daunting, and they are, but the key is to
       degrade gracefully if one cannot reach the particular network service
       one wants.  Croaking or hanging do not look very professional.

   Interprocess Communication (IPC)
       In general, don't directly access the system in code meant to be
       portable.  That means, no "system", "exec", "fork", "pipe", "``" or
       "qx//", "open" with a "|", nor any of the other things that makes being
       a Perl hacker worth being.

       Commands that launch external processes are generally supported on most
       platforms (though many of them do not support any type of forking).
       The problem with using them arises from what you invoke them on.
       External tools are often named differently on different platforms, may
       not be available in the same location, might accept different
       arguments, can behave differently, and often present their results in a
       platform-dependent way.  Thus, you should seldom depend on them to
       produce consistent results.  (Then again, if you're calling "netstat
       -a", you probably don't expect it to run on both Unix and CP/M.)

       One especially common bit of Perl code is opening a pipe to sendmail:

           open(my $mail, '|-', '/usr/lib/sendmail -t')
               or die "cannot fork sendmail: $!";

       This is fine for systems programming when sendmail is known to be
       available.  But it is not fine for many non-Unix systems, and even some
       Unix systems that may not have sendmail installed.  If a portable
       solution is needed, see the various distributions on CPAN that deal
       with it.  "Mail::Mailer" and "Mail::Send" in the "MailTools"
       distribution are commonly used, and provide several mailing methods,
       including "mail", "sendmail", and direct SMTP (via "Net::SMTP") if a
       mail transfer agent is not available.  "Mail::Sendmail" is a standalone
       module that provides simple, platform-independent mailing.

       The Unix System V IPC ("msg*(), sem*(), shm*()") is not available even
       on all Unix platforms.

       Do not use either the bare result of "pack("N", 10, 20, 30, 40)" or
       bare v-strings (such as "v10.20.30.40") to represent IPv4 addresses:
       both forms just pack the four bytes into network order.  That this
       would be equal to the C language "in_addr" struct (which is what the
       socket code internally uses) is not guaranteed.  To be portable use the
       routines of the "Socket" module, such as "inet_aton", "inet_ntoa", and

       The rule of thumb for portable code is: Do it all in portable Perl, or
       use a module (that may internally implement it with platform-specific
       code, but exposes a common interface).

   External Subroutines (XS)
       XS code can usually be made to work with any platform, but dependent
       libraries, header files, etc., might not be readily available or
       portable, or the XS code itself might be platform-specific, just as
       Perl code might be.  If the libraries and headers are portable, then it
       is normally reasonable to make sure the XS code is portable, too.

       A different type of portability issue arises when writing XS code:
       availability of a C compiler on the end-user's system.  C brings with
       it its own portability issues, and writing XS code will expose you to
       some of those.  Writing purely in Perl is an easier way to achieve

   Standard Modules
       In general, the standard modules work across platforms.  Notable
       exceptions are the "CPAN" module (which currently makes connections to
       external programs that may not be available), platform-specific modules
       (like "ExtUtils::MM_VMS"), and DBM modules.

       There is no one DBM module available on all platforms.  "SDBM_File" and
       the others are generally available on all Unix and DOSish ports, but
       not in MacPerl, where only "NDBM_File" and "DB_File" are available.

       The good news is that at least some DBM module should be available, and
       "AnyDBM_File" will use whichever module it can find.  Of course, then
       the code needs to be fairly strict, dropping to the greatest common
       factor (e.g., not exceeding 1K for each record), so that it will work
       with any DBM module.  See AnyDBM_File for more details.

   Time and Date
       The system's notion of time of day and calendar date is controlled in
       widely different ways.  Don't assume the timezone is stored in
       $ENV{TZ}, and even if it is, don't assume that you can control the
       timezone through that variable.  Don't assume anything about the three-
       letter timezone abbreviations (for example that MST would be the
       Mountain Standard Time, it's been known to stand for Moscow Standard
       Time).  If you need to use timezones, express them in some unambiguous
       format like the exact number of minutes offset from UTC, or the POSIX
       timezone format.

       Don't assume that the epoch starts at 00:00:00, January 1, 1970,
       because that is OS- and implementation-specific.  It is better to store
       a date in an unambiguous representation.  The ISO 8601 standard defines
       YYYY-MM-DD as the date format, or YYYY-MM-DDTHH:MM:SS (that's a literal
       "T" separating the date from the time).  Please do use the ISO 8601
       instead of making us guess what date 02/03/04 might be.  ISO 8601 even
       sorts nicely as-is.  A text representation (like "1987-12-18") can be
       easily converted into an OS-specific value using a module like
       "Time::Piece" (see "Date Parsing" in Time::Piece) or "Date::Parse".  An
       array of values, such as those returned by "localtime", can be
       converted to an OS-specific representation using "Time::Local".

       When calculating specific times, such as for tests in time or date
       modules, it may be appropriate to calculate an offset for the epoch.

           use Time::Local qw(timegm);
           my $offset = timegm(0, 0, 0, 1, 0, 1970);

       The value for $offset in Unix will be 0, but in Mac OS Classic will be
       some large number.  $offset can then be added to a Unix time value to
       get what should be the proper value on any system.

   Character sets and character encoding
       Assume very little about character sets.

       Assume nothing about numerical values ("ord", "chr") of characters.  Do
       not use explicit code point ranges (like "\xHH-\xHH)".  However,
       starting in Perl v5.22, regular expression pattern bracketed character
       class ranges specified like "qr/[\N{U+HH}-\N{U+HH}]/" are portable, and
       starting in Perl v5.24, the same ranges are portable in "tr///".  You
       can portably use symbolic character classes like "[:print:]".

       Do not assume that the alphabetic characters are encoded contiguously
       (in the numeric sense).  There may be gaps.  Special coding in Perl,
       however, guarantees that all subsets of "qr/[A-Z]/", "qr/[a-z]/", and
       "qr/[0-9]/" behave as expected.  "tr///" behaves the same for these
       ranges.  In patterns, any ranges specified with end points using the
       "\N{...}" notations ensures character set portability, but it is a bug
       in Perl v5.22 that this isn't true of "tr///", fixed in v5.24.

       Do not assume anything about the ordering of the characters.  The
       lowercase letters may come before or after the uppercase letters; the
       lowercase and uppercase may be interlaced so that both "a" and "A" come
       before "b"; the accented and other international characters may be
       interlaced so that ae comes before "b".  Unicode::Collate can be used
       to sort this all out.

       If you may assume POSIX (a rather large assumption), you may read more
       about the POSIX locale system from perllocale.  The locale system at
       least attempts to make things a little bit more portable, or at least
       more convenient and native-friendly for non-English users.  The system
       affects character sets and encoding, and date and time
       formatting--amongst other things.

       If you really want to be international, you should consider Unicode.
       See perluniintro and perlunicode for more information.

       By default Perl assumes your source code is written in an 8-bit ASCII
       superset. To embed Unicode characters in your strings and regexes, you
       can use the "\x{HH}" or (more portably) "\N{U+HH}" notations. You can
       also use the "utf8" pragma and write your code in UTF-8, which lets you
       use Unicode characters directly (not just in quoted constructs but also
       in identifiers).

   System Resources
       If your code is destined for systems with severely constrained (or
       missing!) virtual memory systems then you want to be especially mindful
       of avoiding wasteful constructs such as:

           my @lines = <$very_large_file>;            # bad

           while (<$fh>) {$file .= $_}                # sometimes bad
           my $file = join('', <$fh>);                # better

       The last two constructs may appear unintuitive to most people.  The
       first repeatedly grows a string, whereas the second allocates a large
       chunk of memory in one go.  On some systems, the second is more
       efficient than the first.

       Most multi-user platforms provide basic levels of security, usually
       implemented at the filesystem level.  Some, however, unfortunately do
       not.  Thus the notion of user id, or "home" directory, or even the
       state of being logged-in, may be unrecognizable on many platforms.  If
       you write programs that are security-conscious, it is usually best to
       know what type of system you will be running under so that you can
       write code explicitly for that platform (or class of platforms).

       Don't assume the Unix filesystem access semantics: the operating system
       or the filesystem may be using some ACL systems, which are richer
       languages than the usual "rwx".  Even if the "rwx" exist, their
       semantics might be different.

       (From the security viewpoint, testing for permissions before attempting
       to do something is silly anyway: if one tries this, there is potential
       for race conditions. Someone or something might change the permissions
       between the permissions check and the actual operation.  Just try the

       Don't assume the Unix user and group semantics: especially, don't
       expect $< and $> (or $( and $)) to work for switching identities (or

       Don't assume set-uid and set-gid semantics.  (And even if you do, think
       twice: set-uid and set-gid are a known can of security worms.)

       For those times when it is necessary to have platform-specific code,
       consider keeping the platform-specific code in one place, making
       porting to other platforms easier.  Use the "Config" module and the
       special variable $^O to differentiate platforms, as described in

       Beware of the "else syndrome":

         if ($^O eq 'MSWin32') {
           # code that assumes Windows
         } else {
           # code that assumes Linux

       The "else" branch should be used for the really ultimate fallback, not
       for code specific to some platform.

       Be careful in the tests you supply with your module or programs.
       Module code may be fully portable, but its tests might not be.  This
       often happens when tests spawn off other processes or call external
       programs to aid in the testing, or when (as noted above) the tests
       assume certain things about the filesystem and paths.  Be careful not
       to depend on a specific output style for errors, such as when checking
       $! after a failed system call.  Using $! for anything else than
       displaying it as output is doubtful (though see the "Errno" module for
       testing reasonably portably for error value). Some platforms expect a
       certain output format, and Perl on those platforms may have been
       adjusted accordingly.  Most specifically, don't anchor a regex when
       testing an error value.

CPAN Testers

       Modules uploaded to CPAN are tested by a variety of volunteers on
       different platforms.  These CPAN testers are notified by mail of each
       new upload, and reply to the list with PASS, FAIL, NA (not applicable
       to this platform), or UNKNOWN (unknown), along with any relevant

       The purpose of the testing is twofold: one, to help developers fix any
       problems in their code that crop up because of lack of testing on other
       platforms; two, to provide users with information about whether a given
       module works on a given platform.

       Also see:

       o   Mailing list:

       o   Testing results: <>


       Perl is built with a $^O variable that indicates the operating system
       it was built on.  This was implemented to help speed up code that would
       otherwise have to "use Config" and use the value of $Config{osname}.
       Of course, to get more detailed information about the system, looking
       into %Config is certainly recommended.

       %Config cannot always be trusted, however, because it was built at
       compile time.  If perl was built in one place, then transferred
       elsewhere, some values may be wrong.  The values may even have been
       edited after the fact.

       Perl works on a bewildering variety of Unix and Unix-like platforms
       (see e.g. most of the files in the hints/ directory in the source code
       kit).  On most of these systems, the value of $^O (hence
       $Config{osname}, too) is determined either by lowercasing and stripping
       punctuation from the first field of the string returned by typing
       "uname -a" (or a similar command) at the shell prompt or by testing the
       file system for the presence of uniquely named files such as a kernel
       or header file.  Here, for example, are a few of the more popular Unix

           uname         $^O        $Config{archname}
           AIX           aix        aix
           BSD/OS        bsdos      i386-bsdos
           Darwin        darwin     darwin
           DYNIX/ptx     dynixptx   i386-dynixptx
           FreeBSD       freebsd    freebsd-i386
           Haiku         haiku      BePC-haiku
           Linux         linux      arm-linux
           Linux         linux      armv5tel-linux
           Linux         linux      i386-linux
           Linux         linux      i586-linux
           Linux         linux      ppc-linux
           HP-UX         hpux       PA-RISC1.1
           IRIX          irix       irix
           Mac OS X      darwin     darwin
           NeXT 3        next       next-fat
           NeXT 4        next       OPENSTEP-Mach
           openbsd       openbsd    i386-openbsd
           OSF1          dec_osf    alpha-dec_osf
           reliantunix-n svr4       RM400-svr4
           SCO_SV        sco_sv     i386-sco_sv
           SINIX-N       svr4       RM400-svr4
           sn4609        unicos     CRAY_C90-unicos
           sn6521        unicosmk   t3e-unicosmk
           sn9617        unicos     CRAY_J90-unicos
           SunOS         solaris    sun4-solaris
           SunOS         solaris    i86pc-solaris
           SunOS4        sunos      sun4-sunos

       Because the value of $Config{archname} may depend on the hardware
       architecture, it can vary more than the value of $^O.

   DOS and Derivatives
       Perl has long been ported to Intel-style microcomputers running under
       systems like PC-DOS, MS-DOS, OS/2, and most Windows platforms you can
       bring yourself to mention (except for Windows CE, if you count that).
       Users familiar with COMMAND.COM or CMD.EXE style shells should be aware
       that each of these file specifications may have subtle differences:

           my $filespec0 = "c:/foo/bar/file.txt";
           my $filespec1 = "c:\\foo\\bar\\file.txt";
           my $filespec2 = 'c:\foo\bar\file.txt';
           my $filespec3 = 'c:\\foo\\bar\\file.txt';

       System calls accept either "/" or "\" as the path separator.  However,
       many command-line utilities of DOS vintage treat "/" as the option
       prefix, so may get confused by filenames containing "/".  Aside from
       calling any external programs, "/" will work just fine, and probably
       better, as it is more consistent with popular usage, and avoids the
       problem of remembering what to backwhack and what not to.

       The DOS FAT filesystem can accommodate only "8.3" style filenames.
       Under the "case-insensitive, but case-preserving" HPFS (OS/2) and NTFS
       (NT) filesystems you may have to be careful about case returned with
       functions like "readdir" or used with functions like "open" or

       DOS also treats several filenames as special, such as AUX, PRN, NUL,
       CON, COM1, LPT1, LPT2, etc.  Unfortunately, sometimes these filenames
       won't even work if you include an explicit directory prefix.  It is
       best to avoid such filenames, if you want your code to be portable to
       DOS and its derivatives.  It's hard to know what these all are,

       Users of these operating systems may also wish to make use of scripts
       such as pl2bat.bat to put wrappers around your scripts.

       Newline ("\n") is translated as "\015\012" by the I/O system when
       reading from and writing to files (see "Newlines").
       "binmode($filehandle)" will keep "\n" translated as "\012" for that
       filehandle.  "binmode" should always be used for code that deals with
       binary data.  That's assuming you realize in advance that your data is
       in binary.  General-purpose programs should often assume nothing about
       their data.

       The $^O variable and the $Config{archname} values for various DOSish
       perls are as follows:

           OS             $^O       $Config{archname}  ID    Version
           MS-DOS         dos       ?
           PC-DOS         dos       ?
           OS/2           os2       ?
           Windows 3.1    ?         ?                  0     3 01
           Windows 95     MSWin32   MSWin32-x86        1     4 00
           Windows 98     MSWin32   MSWin32-x86        1     4 10
           Windows ME     MSWin32   MSWin32-x86        1     ?
           Windows NT     MSWin32   MSWin32-x86        2     4 xx
           Windows NT     MSWin32   MSWin32-ALPHA      2     4 xx
           Windows NT     MSWin32   MSWin32-ppc        2     4 xx
           Windows 2000   MSWin32   MSWin32-x86        2     5 00
           Windows XP     MSWin32   MSWin32-x86        2     5 01
           Windows 2003   MSWin32   MSWin32-x86        2     5 02
           Windows Vista  MSWin32   MSWin32-x86        2     6 00
           Windows 7      MSWin32   MSWin32-x86        2     6 01
           Windows 7      MSWin32   MSWin32-x64        2     6 01
           Windows 2008   MSWin32   MSWin32-x86        2     6 01
           Windows 2008   MSWin32   MSWin32-x64        2     6 01
           Windows CE     MSWin32   ?                  3
           Cygwin         cygwin    cygwin

       The various MSWin32 Perl's can distinguish the OS they are running on
       via the value of the fifth element of the list returned from
       "Win32::GetOSVersion()".  For example:

           if ($^O eq 'MSWin32') {
               my @os_version_info = Win32::GetOSVersion();
               print +('3.1','95','NT')[$os_version_info[4]],"\n";

       There are also "Win32::IsWinNT()|Win32/Win32::IsWinNT()",
       "Win32::IsWin95()|Win32/Win32::IsWin95()", and "Win32::GetOSName()";
       try "perldoc Win32".  The very portable "POSIX::uname()" will work too:

           c:\> perl -MPOSIX -we "print join '|', uname"
           Windows NT|moonru|5.0|Build 2195 (Service Pack 2)|x86

       Errors set by Winsock functions are now put directly into $^E, and the
       relevant "WSAE*" error codes are now exported from the Errno and POSIX
       modules for testing this against.

       The previous behavior of putting the errors (converted to POSIX-style
       "E*" error codes since Perl 5.20.0) into $! was buggy due to the non-
       equivalence of like-named Winsock and POSIX error constants, a
       relationship between which has unfortunately been established in one
       way or another since Perl 5.8.0.

       The new behavior provides a much more robust solution for checking
       Winsock errors in portable software without accidentally matching POSIX
       tests that were intended for other OSes and may have different meanings
       for Winsock.

       The old behavior is currently retained, warts and all, for backwards
       compatibility, but users are encouraged to change any code that tests
       $! against "E*" constants for Winsock errors to instead test $^E
       against "WSAE*" constants.  After a suitable deprecation period, which
       started with Perl 5.24, the old behavior may be removed, leaving $!
       unchanged after Winsock function calls, to avoid any possible confusion
       over which error variable to check.

       Also see:

       o   The djgpp environment for DOS, <> and

       o   The EMX environment for DOS, OS/2, etc.,
           <>  Also perlos2.

       o   Build instructions for Win32 in perlwin32, or under the Cygnus
           environment in perlcygwin.

       o   The "Win32::*" modules in Win32.

       o   The ActiveState Pages, <>

       o   The Cygwin environment for Win32; README.cygwin (installed as
           perlcygwin), <>

       o   The U/WIN environment for Win32,

       o   Build instructions for OS/2, perlos2

       Perl on VMS is discussed in perlvms in the Perl distribution.

       The official name of VMS as of this writing is OpenVMS.

       Interacting with Perl from the Digital Command Language (DCL) shell
       often requires a different set of quotation marks than Unix shells do.
       For example:

           $ perl -e "print ""Hello, world.\n"""
           Hello, world.

       There are several ways to wrap your Perl scripts in DCL .COM files, if
       you are so inclined.  For example:

           $ write sys$output "Hello from DCL!"
           $ if p1 .eqs. ""
           $ then perl -x 'f$environment("PROCEDURE")
           $ else perl -x - 'p1 'p2 'p3 'p4 'p5 'p6 'p7 'p8
           $ deck/dollars="__END__"

           print "Hello from Perl!\n";

           $ endif

       Do take care with "$ ASSIGN/nolog/user SYS$COMMAND: SYS$INPUT" if your
       Perl-in-DCL script expects to do things like "$read = <STDIN>;".

       The VMS operating system has two filesystems, designated by their on-
       disk structure (ODS) level: ODS-2 and its successor ODS-5.  The initial
       port of Perl to VMS pre-dates ODS-5, but all current testing and
       development assumes ODS-5 and its capabilities, including case
       preservation, extended characters in filespecs, and names up to 8192
       bytes long.

       Perl on VMS can accept either VMS- or Unix-style file specifications as
       in either of the following:

           $ perl -ne "print if /perl_setup/i" SYS$LOGIN:LOGIN.COM
           $ perl -ne "print if /perl_setup/i" /sys$login/

       but not a mixture of both as in:

           $ perl -ne "print if /perl_setup/i" sys$login:/
           Can't open sys$login:/ file specification syntax error

       In general, the easiest path to portability is always to specify
       filenames in Unix format unless they will need to be processed by
       native commands or utilities.  Because of this latter consideration,
       the File::Spec module by default returns native format specifications
       regardless of input format.  This default may be reversed so that
       filenames are always reported in Unix format by specifying the
       "DECC$FILENAME_UNIX_REPORT" feature logical in the environment.

       The file type, or extension, is always present in a VMS-format file
       specification even if it's zero-length.  This means that, by default,
       "readdir" will return a trailing dot on a file with no extension, so
       where you would see "a" on Unix you'll see "a." on VMS.  However, the
       trailing dot may be suppressed by enabling the
       "DECC$READDIR_DROPDOTNOTYPE" feature in the environment (see the CRTL
       documentation on feature logical names).

       What "\n" represents depends on the type of file opened.  It usually
       represents "\012" but it could also be "\015", "\012", "\015\012",
       "\000", "\040", or nothing depending on the file organization and
       record format.  The "VMS::Stdio" module provides access to the special
       "fopen()" requirements of files with unusual attributes on VMS.

       The value of $^O on OpenVMS is "VMS".  To determine the architecture
       that you are running on refer to $Config{archname}.

       On VMS, perl determines the UTC offset from the
       "SYS$TIMEZONE_DIFFERENTIAL" logical name.  Although the VMS epoch began
       at 17-NOV-1858 00:00:00.00, calls to "localtime" are adjusted to count
       offsets from 01-JAN-1970 00:00:00.00, just like Unix.

       Also see:

       o   README.vms (installed as README_vms), perlvms

       o   vmsperl list,

       o   vmsperl on the web, <>

       o   VMS Software Inc. web site, <>

       Perl on VOS (also known as OpenVOS) is discussed in README.vos in the
       Perl distribution (installed as perlvos).  Perl on VOS can accept
       either VOS- or Unix-style file specifications as in either of the

           $ perl -ne "print if /perl_setup/i" >system>notices
           $ perl -ne "print if /perl_setup/i" /system/notices

       or even a mixture of both as in:

           $ perl -ne "print if /perl_setup/i" >system/notices

       Even though VOS allows the slash character to appear in object names,
       because the VOS port of Perl interprets it as a pathname delimiting
       character, VOS files, directories, or links whose names contain a slash
       character cannot be processed.  Such files must be renamed before they
       can be processed by Perl.

       Older releases of VOS (prior to OpenVOS Release 17.0) limit file names
       to 32 or fewer characters, prohibit file names from starting with a "-"
       character, and prohibit file names from containing " " (space) or any
       character from the set "!#%&'()*;<=>?".

       Newer releases of VOS (OpenVOS Release 17.0 or later) support a feature
       known as extended names.  On these releases, file names can contain up
       to 255 characters, are prohibited from starting with a "-" character,
       and the set of prohibited characters is reduced to "#%*<>?".  There are
       restrictions involving spaces and apostrophes:  these characters must
       not begin or end a name, nor can they immediately precede or follow a
       period.  Additionally, a space must not immediately precede another
       space or hyphen.  Specifically, the following character combinations
       are prohibited:  space-space, space-hyphen, period-space, space-period,
       period-apostrophe, apostrophe-period, leading or trailing space, and
       leading or trailing apostrophe.  Although an extended file name is
       limited to 255 characters, a path name is still limited to 256

       The value of $^O on VOS is "vos".  To determine the architecture that
       you are running on refer to $Config{archname}.

       Also see:

       o   README.vos (installed as perlvos)

       o   The VOS mailing list.

           There is no specific mailing list for Perl on VOS.  You can contact
           the Stratus Technologies Customer Assistance Center (CAC) for your
           region, or you can use the contact information located in the
           distribution files on the Stratus Anonymous FTP site.

       o   Stratus Technologies on the web at <>

       o   VOS Open-Source Software on the web at

   EBCDIC Platforms
       v5.22 core Perl runs on z/OS (formerly OS/390).  Theoretically it could
       run on the successors of OS/400 on AS/400 minicomputers as well as
       VM/ESA, and BS2000 for S/390 Mainframes.  Such computers use EBCDIC
       character sets internally (usually Character Code Set ID 0037 for
       OS/400 and either 1047 or POSIX-BC for S/390 systems).

       The rest of this section may need updating, but we don't know what it
       should say.  Please submit comments to

       On the mainframe Perl currently works under the "Unix system services
       for OS/390" (formerly known as OpenEdition), VM/ESA OpenEdition, or the
       BS200 POSIX-BC system (BS2000 is supported in Perl 5.6 and greater).
       See perlos390 for details.  Note that for OS/400 there is also a port
       of Perl 5.8.1/5.10.0 or later to the PASE which is ASCII-based (as
       opposed to ILE which is EBCDIC-based), see perlos400.

       As of R2.5 of USS for OS/390 and Version 2.3 of VM/ESA these Unix sub-
       systems do not support the "#!" shebang trick for script invocation.
       Hence, on OS/390 and VM/ESA Perl scripts can be executed with a header
       similar to the following simple script:

           : # use perl
               eval 'exec /usr/local/bin/perl -S $0 ${1+"$@"}'
                   if 0;
           #!/usr/local/bin/perl     # just a comment really

           print "Hello from perl!\n";

       OS/390 will support the "#!" shebang trick in release 2.8 and beyond.
       Calls to "system" and backticks can use POSIX shell syntax on all S/390

       On the AS/400, if PERL5 is in your library list, you may need to wrap
       your Perl scripts in a CL procedure to invoke them like so:

             CALL PGM(PERL5/PERL) PARM('/QOpenSys/')

       This will invoke the Perl script in the root of the QOpenSys
       file system.  On the AS/400 calls to "system" or backticks must use CL

       On these platforms, bear in mind that the EBCDIC character set may have
       an effect on what happens with some Perl functions (such as "chr",
       "pack", "print", "printf", "ord", "sort", "sprintf", "unpack"), as well
       as bit-fiddling with ASCII constants using operators like "^", "&" and
       "|", not to mention dealing with socket interfaces to ASCII computers
       (see "Newlines").

       Fortunately, most web servers for the mainframe will correctly
       translate the "\n" in the following statement to its ASCII equivalent
       ("\r" is the same under both Unix and z/OS):

           print "Content-type: text/html\r\n\r\n";

       The values of $^O on some of these platforms include:

           uname         $^O        $Config{archname}
           OS/390        os390      os390
           OS400         os400      os400
           POSIX-BC      posix-bc   BS2000-posix-bc

       Some simple tricks for determining if you are running on an EBCDIC
       platform could include any of the following (perhaps all):

           if ("\t" eq "\005")  { print "EBCDIC may be spoken here!\n"; }

           if (ord('A') == 193) { print "EBCDIC may be spoken here!\n"; }

           if (chr(169) eq 'z') { print "EBCDIC may be spoken here!\n"; }

       One thing you may not want to rely on is the EBCDIC encoding of
       punctuation characters since these may differ from code page to code
       page (and once your module or script is rumoured to work with EBCDIC,
       folks will want it to work with all EBCDIC character sets).

       Also see:

       o   perlos390, perlos400, perlbs2000, perlebcdic.

       o   The list is for discussion of porting issues as
           well as general usage issues for all EBCDIC Perls.  Send a message
           body of "subscribe perl-mvs" to

       o   AS/400 Perl information at <> as
           well as on CPAN in the ports/ directory.

   Acorn RISC OS
       Because Acorns use ASCII with newlines ("\n") in text files as "\012"
       like Unix, and because Unix filename emulation is turned on by default,
       most simple scripts will probably work "out of the box".  The native
       filesystem is modular, and individual filesystems are free to be case-
       sensitive or insensitive, and are usually case-preserving.  Some native
       filesystems have name length limits, which file and directory names are
       silently truncated to fit.  Scripts should be aware that the standard
       filesystem currently has a name length limit of 10 characters, with up
       to 77 items in a directory, but other filesystems may not impose such

       Native filenames are of the form



           Special_Field is not usually present, but may contain . and $ .
           Filesystem =~ m|[A-Za-z0-9_]|
           DsicName   =~ m|[A-Za-z0-9_/]|
           $ represents the root directory
           . is the path separator
           @ is the current directory (per filesystem but machine global)
           ^ is the parent directory
           Directory and File =~ m|[^\0- "\.\$\%\&:\@\\^\|\177]+|

       The default filename translation is roughly "tr|/.|./|", swapping dots
       and slashes.

       Note that ""ADFS::HardDisk.$.File" ne 'ADFS::HardDisk.$.File'" and that
       the second stage of "$" interpolation in regular expressions will fall
       foul of the $. variable if scripts are not careful.

       Logical paths specified by system variables containing comma-separated
       search lists are also allowed; hence "System:Modules" is a valid
       filename, and the filesystem will prefix "Modules" with each section of
       "System$Path" until a name is made that points to an object on disk.
       Writing to a new file "System:Modules" would be allowed only if
       "System$Path" contains a single item list.  The filesystem will also
       expand system variables in filenames if enclosed in angle brackets, so
       "<System$Dir>.Modules" would look for the file
       "$ENV{'System$Dir'} . 'Modules'".  The obvious implication of this is
       that fully qualified filenames can start with "<>" and the three-
       argument form of "open" should always be used.

       Because "." was in use as a directory separator and filenames could not
       be assumed to be unique after 10 characters, Acorn implemented the C
       compiler to strip the trailing ".c" ".h" ".s" and ".o" suffix from
       filenames specified in source code and store the respective files in
       subdirectories named after the suffix.  Hence files are translated:

           C:foo.h        (logical path variable)
           sys/os.h        sys.h.os       (C compiler groks Unix-speak)
           10charname.c    c.10charname
           10charname.o    o.10charname
           11charname_.c   c.11charname   (assuming filesystem truncates at 10)

       The Unix emulation library's translation of filenames to native assumes
       that this sort of translation is required, and it allows a user-defined
       list of known suffixes that it will transpose in this fashion.  This
       may seem transparent, but consider that with these rules foo/bar/baz.h
       and foo/bar/h/baz both map to, and that "readdir" and
       "glob" cannot and do not attempt to emulate the reverse mapping.  Other
       "."'s in filenames are translated to "/".

       As implied above, the environment accessed through %ENV is global, and
       the convention is that program specific environment variables are of
       the form "Program$Name".  Each filesystem maintains a current
       directory, and the current filesystem's current directory is the global
       current directory.  Consequently, sociable programs don't change the
       current directory but rely on full pathnames, and programs (and
       Makefiles) cannot assume that they can spawn a child process which can
       change the current directory without affecting its parent (and everyone
       else for that matter).

       Because native operating system filehandles are global and are
       currently allocated down from 255, with 0 being a reserved value, the
       Unix emulation library emulates Unix filehandles.  Consequently, you
       can't rely on passing "STDIN", "STDOUT", or "STDERR" to your children.

       The desire of users to express filenames of the form "<Foo$Dir>.Bar" on
       the command line unquoted causes problems, too: "``" command output
       capture has to perform a guessing game.  It assumes that a string
       "<[^<>]+\$[^<>]>" is a reference to an environment variable, whereas
       anything else involving "<" or ">" is redirection, and generally
       manages to be 99% right.  Of course, the problem remains that scripts
       cannot rely on any Unix tools being available, or that any tools found
       have Unix-like command line arguments.

       Extensions and XS are, in theory, buildable by anyone using free tools.
       In practice, many don't, as users of the Acorn platform are used to
       binary distributions.  MakeMaker does run, but no available make
       currently copes with MakeMaker's makefiles; even if and when this
       should be fixed, the lack of a Unix-like shell will cause problems with
       makefile rules, especially lines of the form "cd sdbm && make all", and
       anything using quoting.

       "RISC OS" is the proper name for the operating system, but the value in
       $^O is "riscos" (because we don't like shouting).

   Other perls
       Perl has been ported to many platforms that do not fit into any of the
       categories listed above.  Some, such as AmigaOS, QNX, Plan 9, and VOS,
       have been well-integrated into the standard Perl source code kit.  You
       may need to see the ports/ directory on CPAN for information, and
       possibly binaries, for the likes of: aos, Atari ST, lynxos, riscos,
       Novell Netware, Tandem Guardian, etc.  (Yes, we know that some of these
       OSes may fall under the Unix category, but we are not a standards

       Some approximate operating system names and their $^O values in the
       "OTHER" category include:

           OS            $^O        $Config{archname}
           Amiga DOS     amigaos    m68k-amigos

       See also:

       o   Amiga, README.amiga (installed as perlamiga).

       o   A free perl5-based PERL.NLM for Novell Netware is available in
           precompiled binary and source code form from
           <> as well as from CPAN.

       o   Plan 9, README.plan9


       Listed below are functions that are either completely unimplemented or
       else have been implemented differently on various platforms.  Preceding
       each description will be, in parentheses, a list of platforms that the
       description applies to.

       The list may well be incomplete, or even wrong in some places.  When in
       doubt, consult the platform-specific README files in the Perl source
       distribution, and any other documentation resources accompanying a
       given port.

       Be aware, moreover, that even among Unix-ish systems there are

       For many functions, you can also query %Config, exported by default
       from the "Config" module.  For example, to check whether the platform
       has the "lstat" call, check $Config{d_lstat}.  See Config for a full
       description of available variables.

   Alphabetical Listing of Perl Functions
       -X      (Win32) "-w" only inspects the read-only file attribute
               (FILE_ATTRIBUTE_READONLY), which determines whether the
               directory can be deleted, not whether it can be written to.
               Directories always have read and write access unless denied by
               discretionary access control lists (DACLs).

               (VMS) "-r", "-w", "-x", and "-o" tell whether the file is
               accessible, which may not reflect UIC-based file protections.

               (RISC OS) "-s" by name on an open file will return the space
               reserved on disk, rather than the current extent.  "-s" on an
               open filehandle returns the current size.

               (Win32, VMS, RISC OS) "-R", "-W", "-X", "-O" are
               indistinguishable from "-r", "-w", "-x", "-o".

               (Win32, VMS, RISC OS) "-g", "-k", "-l", "-u", "-A" are not
               particularly meaningful.

               (Win32) "-l" returns true for both symlinks and directory

               (VMS, RISC OS) "-p" is not particularly meaningful.

               (VMS) "-d" is true if passed a device spec without an explicit

               (Win32) "-x" (or "-X") determine if a file ends in one of the
               executable suffixes.  "-S" is meaningless.

               (RISC OS) "-x" (or "-X") determine if a file has an executable
               file type.

       alarm   (Win32) Emulated using timers that must be explicitly polled
               whenever Perl wants to dispatch "safe signals" and therefore
               cannot interrupt blocking system calls.

       atan2   (Tru64, HP-UX 10.20) Due to issues with various CPUs, math
               libraries, compilers, and standards, results for "atan2" may
               vary depending on any combination of the above.  Perl attempts
               to conform to the Open Group/IEEE standards for the results
               returned from "atan2", but cannot force the issue if the system
               Perl is run on does not allow it.

               The current version of the standards for "atan2" is available

       binmode (RISC OS) Meaningless.

               (VMS) Reopens file and restores pointer; if function fails,
               underlying filehandle may be closed, or pointer may be in a
               different position.

               (Win32) The value returned by "tell" may be affected after the
               call, and the filehandle may be flushed.

       chdir   (Win32) The current directory reported by the system may
               include any symbolic links specified to chdir().

       chmod   (Win32) Only good for changing "owner" read-write access;
               "group" and "other" bits are meaningless.

               (RISC OS) Only good for changing "owner" and "other" read-write

               (VOS) Access permissions are mapped onto VOS access-control
               list changes.

               (Cygwin) The actual permissions set depend on the value of the
               "CYGWIN" variable in the SYSTEM environment settings.

               (Android) Setting the exec bit on some locations (generally
               /sdcard) will return true but not actually set the bit.

               (VMS) A mode argument of zero sets permissions to the user's
               default permission mask rather than disabling all permissions.

       chown   (Plan 9, RISC OS) Not implemented.

               (Win32) Does nothing, but won't fail.

               (VOS) A little funky, because VOS's notion of ownership is a
               little funky.

       chroot  (Win32, VMS, Plan 9, RISC OS, VOS) Not implemented.

       crypt   (Win32) May not be available if library or source was not
               provided when building perl.

               (Android) Not implemented.

               (VMS, Plan 9, VOS) Not implemented.

       dbmopen (VMS, Plan 9, VOS) Not implemented.

       dump    (RISC OS) Not useful.

               (Cygwin, Win32) Not supported.

               (VMS) Invokes VMS debugger.

       exec    (Win32) "exec LIST" without the use of indirect object syntax
               ("exec PROGRAM LIST") may fall back to trying the shell if the
               first "spawn()" fails.

               Note that the list form of exec() is emulated since the Win32
               API CreateProcess() accepts a simple string rather than an
               array of command-line arguments.  This may have security
               implications for your code.

               (SunOS, Solaris, HP-UX) Does not automatically flush output
               handles on some platforms.

       exit    (VMS) Emulates Unix "exit" (which considers "exit 1" to
               indicate an error) by mapping the 1 to "SS$_ABORT" (44).  This
               behavior may be overridden with the pragma "use vmsish 'exit'".
               As with the CRTL's "exit()" function, "exit 0" is also mapped
               to an exit status of "SS$_NORMAL" (1); this mapping cannot be
               overridden.  Any other argument to "exit" is used directly as
               Perl's exit status.  On VMS, unless the future POSIX_EXIT mode
               is enabled, the exit code should always be a valid VMS exit
               code and not a generic number.  When the POSIX_EXIT mode is
               enabled, a generic number will be encoded in a method
               compatible with the C library _POSIX_EXIT macro so that it can
               be decoded by other programs, particularly ones written in C,
               like the GNV package.

               (Solaris) "exit" resets file pointers, which is a problem when
               called from a child process (created by "fork") in "BEGIN".  A
               workaround is to use "POSIX::_exit".

                   exit unless $Config{archname} =~ /\bsolaris\b/;
                   require POSIX;

       fcntl   (Win32) Not implemented.

               (VMS) Some functions available based on the version of VMS.

       flock   (VMS, RISC OS, VOS) Not implemented.

       fork    (AmigaOS, RISC OS, VMS) Not implemented.

               (Win32) Emulated using multiple interpreters.  See perlfork.

               (SunOS, Solaris, HP-UX) Does not automatically flush output
               handles on some platforms.

               (RISC OS) Not implemented.

       getpgrp (Win32, VMS, RISC OS) Not implemented.

       getppid (Win32, RISC OS) Not implemented.

               (Win32, VMS, RISC OS, VOS) Not implemented.

               (Win32) Not implemented.

               (RISC OS) Not useful.

               (Win32, VMS, RISC OS) Not implemented.

               (Android, Win32, Plan 9) Not implemented.

               (Win32) Not implemented.

               (RISC OS) Not useful.

               (Win32, VMS, RISC OS) Not implemented.

               (Android, Win32, Plan 9) Not implemented.

               (Android) Not implemented.

               (Android, Win32) Not implemented.

               (Android, Win32, VMS) Not implemented.

               (Irix 5) "gethostbyname('localhost')" does not work everywhere:
               you may have to use "gethostbyname('')".

               (Win32) Not implemented.

               (Android, Win32, Plan 9) Not implemented.

               (Android, Win32, Plan 9) Not implemented.

               (Win32, Plan 9) Not implemented.

       seekdir (Android) Not implemented.

               (Android, Win32, Plan 9, RISC OS) Not implemented.

               (Win32, Plan 9, RISC OS) Not implemented.

               (Android, Win32, Plan 9, RISC OS) Not implemented.

               (Plan 9, Win32, RISC OS) Not implemented.

               (Win32) Not implemented.

               (Android) Either not implemented or a no-op.

               (Android, RISC OS, VMS, Win32) Not implemented.

               (Android, Win32) Not implemented.

               (Android, Win32, Plan 9) Not implemented.

               (Android, Win32, Plan 9) Not implemented.

               (Plan 9, Win32) Not implemented.

               (Plan 9) Not implemented.

       glob    This operator is implemented via the "File::Glob" extension on
               most platforms.  See File::Glob for portability information.

       gmtime  In theory, "gmtime" is reliable from -2**63 to 2**63-1.
               However, because work-arounds in the implementation use
               floating point numbers, it will become inaccurate as the time
               gets larger.  This is a bug and will be fixed in the future.

               (VOS) Time values are 32-bit quantities.

       ioctl   (VMS) Not implemented.

               (Win32) Available only for socket handles, and it does what the
               "ioctlsocket()" call in the Winsock API does.

               (RISC OS) Available only for socket handles.

       kill    (RISC OS) Not implemented, hence not useful for taint checking.

               (Win32) "kill" doesn't send a signal to the identified process
               like it does on Unix platforms.  Instead "kill($sig, $pid)"
               terminates the process identified by $pid, and makes it exit
               immediately with exit status $sig.  As in Unix, if $sig is 0
               and the specified process exists, it returns true without
               actually terminating it.

               (Win32) "kill(-9, $pid)" will terminate the process specified
               by $pid and recursively all child processes owned by it.  This
               is different from the Unix semantics, where the signal will be
               delivered to all processes in the same process group as the
               process specified by $pid.

               (VMS) A pid of -1 indicating all processes on the system is not
               currently supported.

       link    (RISC OS, VOS) Not implemented.

               (AmigaOS) Link count not updated because hard links are not
               quite that hard (They are sort of half-way between hard and
               soft links).

               (Win32) Hard links are implemented on Win32 under NTFS only.
               They are natively supported on Windows 2000 and later.  On
               Windows NT they are implemented using the Windows POSIX
               subsystem support and the Perl process will need Administrator
               or Backup Operator privileges to create hard links.

               (VMS) Available on 64 bit OpenVMS 8.2 and later.

               "localtime" has the same range as "gmtime", but because time
               zone rules change, its accuracy for historical and future times
               may degrade but usually by no more than an hour.

       lstat   (RISC OS) Not implemented.

               (Win32) Treats directory junctions as symlinks.

       msgrcv  (Android, Win32, VMS, Plan 9, RISC OS, VOS) Not implemented.

       open    (RISC OS) Open modes "|-" and "-|" are unsupported.

               (SunOS, Solaris, HP-UX) Opening a process does not
               automatically flush output handles on some platforms.

               (Win32) Both of modes "|-" and "-|" are supported, but the list
               form is emulated since the Win32 API CreateProcess() accepts a
               simple string rather than an array of arguments.  This may have
               security implications for your code.

               (VMS, RISC OS) Not implemented.

               (Win32) readlink() on a directory junction returns the object
               name, not a simple path.

       rename  (Win32) Can't move directories between directories on different
               logical volumes.

               (Win32) Will not cause "readdir" to re-read the directory
               stream.  The entries already read before the "rewinddir" call
               will just be returned again from a cache buffer.

       select  (Win32, VMS) Only implemented on sockets.

               (RISC OS) Only reliable on sockets.

               Note that the "select FILEHANDLE" form is generally portable.

       semop   (Android, Win32, VMS, RISC OS) Not implemented.

               (Android, VMS, Win32, RISC OS) Not implemented.

       setpgrp (Win32, VMS, RISC OS, VOS) Not implemented.

               (Win32, VMS, RISC OS, VOS) Not implemented.

               (Android, Win32, RISC OS) Not implemented.

               (Plan 9) Not implemented.

               (Android, Win32, VMS, RISC OS) Not implemented.

       sleep   (Win32) Emulated using synchronization functions such that it
               can be interrupted by "alarm", and limited to a maximum of
               4294967 seconds, approximately 49 days.

               (RISC OS) Not implemented.

               (VMS) Available on 64 bit OpenVMS 8.2 and later.

       stat    Platforms that do not have "rdev", "blksize", or "blocks" will
               return these as '', so numeric comparison or manipulation of
               these fields may cause 'not numeric' warnings.

               (Mac OS X) "ctime" not supported on UFS.

               (Win32) "ctime" is creation time instead of inode change time.

               (VMS) "dev" and "ino" are not necessarily reliable.

               (RISC OS) "mtime", "atime" and "ctime" all return the last
               modification time.  "dev" and "ino" are not necessarily

               (OS/2) "dev", "rdev", "blksize", and "blocks" are not
               available.  "ino" is not meaningful and will differ between
               stat calls on the same file.

               (Cygwin) Some versions of cygwin when doing a "stat("foo")" and
               not finding it may then attempt to "stat("foo.exe")".

       symlink (RISC OS) Not implemented.

               (Win32) Requires either elevated permissions or developer mode
               and a sufficiently recent version of Windows 10. You can check
               whether the current process has the required privileges using
               the Win32::IsSymlinkCreationAllowed() function.

               Since Windows needs to know whether the target is a directory
               or not when creating the link the target Perl will only create
               the link as a directory link when the target exists and is a

               (VMS) Implemented on 64 bit VMS 8.3.  VMS requires the symbolic
               link to be in Unix syntax if it is intended to resolve to a
               valid path.

       syscall (Win32, VMS, RISC OS, VOS) Not implemented.

       sysopen (Mac OS, OS/390) The traditional 0, 1, and 2 MODEs are
               implemented with different numeric values on some systems.  The
               flags exported by "Fcntl" ("O_RDONLY", "O_WRONLY", "O_RDWR")
               should work everywhere though.

       system  (Win32) As an optimization, may not call the command shell
               specified in $ENV{PERL5SHELL}.  "system(1, @args)" spawns an
               external process and immediately returns its process
               designator, without waiting for it to terminate.  Return value
               may be used subsequently in "wait" or "waitpid".  Failure to
               "spawn()" a subprocess is indicated by setting $? to "255 <<
               8".  $? is set in a way compatible with Unix (i.e. the exit
               status of the subprocess is obtained by "$? >> 8", as described
               in the documentation).

               Note that the list form of system() is emulated since the Win32
               API CreateProcess() accepts a simple string rather than an
               array of command-line arguments.  This may have security
               implications for your code.

               (RISC OS) There is no shell to process metacharacters, and the
               native standard is to pass a command line terminated by "\n"
               "\r" or "\0" to the spawned program.  Redirection such as ">
               foo" is performed (if at all) by the run time library of the
               spawned program.  "system LIST" will call the Unix emulation
               library's "exec" emulation, which attempts to provide emulation
               of the stdin, stdout, stderr in force in the parent, provided
               the child program uses a compatible version of the emulation
               library.  "system SCALAR" will call the native command line
               directly and no such emulation of a child Unix program will
               occur.  Mileage will vary.

               (Win32) "system LIST" without the use of indirect object syntax
               ("system PROGRAM LIST") may fall back to trying the shell if
               the first "spawn()" fails.

               (SunOS, Solaris, HP-UX) Does not automatically flush output
               handles on some platforms.

               (VMS) As with Win32, "system(1, @args)" spawns an external
               process and immediately returns its process designator without
               waiting for the process to terminate.  In this case the return
               value may be used subsequently in "wait" or "waitpid".
               Otherwise the return value is POSIX-like (shifted up by 8
               bits), which only allows room for a made-up value derived from
               the severity bits of the native 32-bit condition code (unless
               overridden by "use vmsish 'status'").  If the native condition
               code is one that has a POSIX value encoded, the POSIX value
               will be decoded to extract the expected exit value.  For more
               details see "$?" in perlvms.

       telldir (Android) Not implemented.

       times   (Win32) "Cumulative" times will be bogus.  On anything other
               than Windows NT or Windows 2000, "system" time will be bogus,
               and "user" time is actually the time returned by the "clock()"
               function in the C runtime library.

               (RISC OS) Not useful.

               (Older versions of VMS) Not implemented.

               (VOS) Truncation to same-or-shorter lengths only.

               (Win32) If a FILEHANDLE is supplied, it must be writable and
               opened in append mode (i.e., use "open(my $fh, '>>',
               'filename')" or "sysopen(my $fh, ..., O_APPEND|O_RDWR)".  If a
               filename is supplied, it should not be held open elsewhere.

       umask   Returns "undef" where unavailable.

               (AmigaOS) "umask" works but the correct permissions are set
               only when the file is finally closed.

       utime   (VMS, RISC OS) Only the modification time is updated.

               (Win32) May not behave as expected.  Behavior depends on the C
               runtime library's implementation of "utime()", and the
               filesystem being used.  The FAT filesystem typically does not
               support an "access time" field, and it may limit timestamps to
               a granularity of two seconds.

       waitpid (Win32) Can only be applied to process handles returned for
               processes spawned using "system(1, ...)" or pseudo processes
               created with "fork".

               (RISC OS) Not useful.

Supported Platforms

       The following platforms are known to build Perl 5.12 (as of April 2010,
       its release date) from the standard source code distribution available
       at <>

       Linux (x86, ARM, IA64)
           Windows 2000
           Windows XP
           Windows Server 2003
           Windows Vista
           Windows Server 2008
           Windows 7
           Some tests are known to fail:

           o   ext/XS-APItest/t/call_checker.t - see

           o   dist/I18N-Collate/t/I18N-Collate.t

           o   ext/Win32CORE/t/win32core.t - may fail on recent cygwin

       Solaris (x86, SPARC)
           Alpha (7.2 and later)
           I64 (8.2 and later)
       Debian GNU/kFreeBSD
       Irix (6.5. What else?)
       Dragonfly BSD
       Midnight BSD
       QNX Neutrino RTOS (6.5.0)
       MirOS BSD
       Stratus OpenVOS (17.0 or later)

           time_t issues that may or may not be fixed
       Stratus VOS / OpenVOS
           Perl now builds with FreeMiNT/Atari. It fails a few tests, that
           needs some investigation.

           The FreeMiNT port uses GNU dld for loadable module capabilities. So
           ensure you have that library installed when building perl.

EOL Platforms

   (Perl 5.20)
       The following platforms were supported by a previous version of Perl
       but have been officially removed from Perl's source code as of 5.20:

       AT&T 3b1

   (Perl 5.14)
       The following platforms were supported up to 5.10.  They may still have
       worked in 5.12, but supporting code has been removed for 5.14:

       Windows 95
       Windows 98
       Windows ME
       Windows NT4

   (Perl 5.12)
       The following platforms were supported by a previous version of Perl
       but have been officially removed from Perl's source code as of 5.12:

       Atari MiNT
       Apollo Domain/OS
       Apple Mac OS 8/9
       Tenon Machten

Supported Platforms (Perl 5.8)

       As of July 2002 (the Perl release 5.8.0), the following platforms were
       able to build Perl from the standard source code distribution available
       at <>

               BSD/OS          (BSDi)
               DOS DJGPP       1)
               EPOC R5
               HI-UXMPP        (Hitachi) (5.8.0 worked but we didn't know it)
               Mac OS Classic
               Mac OS X        (Darwin)
               ReliantUNIX     (formerly SINIX)
               OpenVMS         (formerly VMS)
               Open UNIX       (Unixware) (since Perl 5.8.1/5.9.0)
               OS/400          (using the PASE) (since Perl 5.8.1/5.9.0)
               POSIX-BC        (formerly BS2000)
               SunOS 4
               SUPER-UX        (NEC)
               Tru64 UNIX      (formerly DEC OSF/1, Digital UNIX)
               VOS / OpenVOS
               Win95/98/ME/2K/XP 2)
               z/OS            (formerly OS/390)

               1) in DOS mode either the DOS or OS/2 ports can be used
               2) compilers: Borland, MinGW (GCC), VC6

       The following platforms worked with the previous releases (5.6 and
       5.7), but we did not manage either to fix or to test these in time for
       the 5.8.0 release.  There is a very good chance that many of these will
       work fine with the 5.8.0.

               SCO SV
               Windows 3.1

       Known to be broken for 5.8.0 (but 5.6.1 and 5.7.2 can be used):

               AmigaOS 3

       The following platforms have been known to build Perl from source in
       the past (5.005_03 and earlier), but we haven't been able to verify
       their status for the current release, either because the
       hardware/software platforms are rare or because we don't have an active
       champion on these platforms--or both.  They used to work, though, so go
       ahead and try compiling them, and let
       <> know of any trouble.

               DDE SMES
               DOS EMX
               MachTen 68k
               Plan 9
               SCO ODT/OSR
               Unisys Dynix

       The following platforms have their own source code distributions and
       binaries available via <>

                                       Perl release

               OS/400 (ILE)            5.005_02
               Tandem Guardian         5.004

       The following platforms have only binaries available via
       <> :

                                       Perl release

               Acorn RISCOS            5.005_02
               AOS                     5.002
               LynxOS                  5.004_02

       Although we do suggest that you always build your own Perl from the
       source code, both for maximal configurability and for security, in case
       you are in a hurry you can check <>
       for binary distributions.


       perlaix(1), perlamiga(1), perlbs2000(1), perlcygwin(1), perldos(1),
       perlebcdic(1), perlfreebsd(1), perlhurd(1), perlhpux(1), perlirix(1),
       perlmacos(1), perlmacosx(1), perlnetware(1), perlos2(1), perlos390(1),
       perlos400(1), perlplan9(1), perlqnx(1), perlsolaris(1), perltru64(1),
       perlunicode(1), perlvms(1), perlvos(1), perlwin32(1), and Win32(1).


       Abigail <>, Charles Bailey <>,
       Graham Barr <>, Tom Christiansen <>,
       Nicholas Clark <>, Thomas Dorner <>,
       Andy Dougherty <>, Dominic Dunlop
       <>, Neale Ferguson <>, David J.
       Fiander <>, Paul Green <>, M.J.T.
       Guy <>, Jarkko Hietaniemi <>, Luther Huffman
       <>, Nick Ing-Simmons <>,
       Andreas J. Koenig <>, Markus Laker
       <>, Andrew M. Langmead <>, Lukas
       Mai <>, Larry Moore <>, Paul Moore
       <>, Chris Nandor <>, Matthias
       Neeracher <>, Philip Newton <>, Gary Ng
       <71564.1743@CompuServe.COM>, Tom Phoenix <>, Andre
       Pirard <>, Peter Prymmer <>, Hugo van
       der Sanden <>, Gurusamy Sarathy
       <>, Paul J. Schinder <>, Michael
       G Schwern <>, Dan Sugalski <>, Nathan
       Torkington <>, John Malmberg <>

perl v5.34.0                      2021-05-04                     PERLPORT(1pm)

perl 5.34.0 - Generated Sun Feb 27 11:08:12 CST 2022
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