Provided by: perl-doc_5.26.2-7_all bug


       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 them.

       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

       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 ("FUNCTION IMPLEMENTATIONS").

       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

       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

           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 it.

           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

           $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 wallet.

           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 change:

           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

       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 characters.

       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 filenames.

       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

       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 first.

       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 undeleteable 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 port.

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

       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

       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 "sockaddr_in".

       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 portability.

   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, 70);

       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 ä 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

           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 operation.)

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

       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 notations.

       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:

       ·   Mailing list:

       ·   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 flavors:

           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 "opendir".

       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:

       ·   The djgpp environment for DOS, <> and perldos.

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

       ·   Build instructions for Win32 in perlwin32, or under the Cygnus environment in

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

       ·   The ActiveState Pages, <>

       ·   The Cygwin environment for Win32; README.cygwin (installed as perlcygwin),

       ·   The U/WIN environment for Win32, <>

       ·   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

           $ 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:

       ·   README.vms (installed as README_vms), perlvms

       ·   vmsperl list,

       ·   vmsperl on the web, <>

       ·   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 following:

           $ 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 characters.

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

       Also see:

       ·   README.vos (installed as perlvos)

       ·   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

       ·   Stratus Technologies on the web at <>

       ·   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
       email 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 systems.

       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 syntax.

       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

       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:

       ·   perlos390, perlos400, perlbs2000, perlebcdic.

       ·   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

       ·   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 limitations.

       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 slahes.

       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 body.)

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

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

       See also:

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

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

       ·   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 variations.

       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

               (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

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

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

               (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 at

       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.

       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 access.

               (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.

       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

               (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.

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

               (Symbian OS) Not supported.

       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

               (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

               (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

               (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

       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) Return values (especially for device and inode) may be bogus.

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

       open    (Win32, RISC OS) Open modes "|-" and "-|" are unsupported.

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

               (Win32, VMS, RISC OS) Not implemented.

       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

       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'

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

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

               (Win32) "dev" and "ino" are not meaningful.

               (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 reliable.

               (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")".

               (Win32) "stat" needs to open the file to determine the link count and update
               attributes that may have been changed through hard links.  Setting
               "${^WIN32_SLOPPY_STAT}" to a true value speeds up "stat" by not performing this

       symlink (Win32, RISC OS) Not implemented.

               (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).

               (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

               (VMS) 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

       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:

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

           ·   dist/I18N-Collate/t/I18N-Collate.t

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

       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
       Symbian (Series 60 v3, 3.2 and 5 - what else?)
       Stratus VOS / OpenVOS
           Perl now builds with FreeMiNT/Atari. It fails a few tests, that needs some

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


       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. König <>, 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 <>,
       André Pirard <>, Peter Prymmer <>, Hugo van der Sanden
       <>, Gurusamy Sarathy <>, Paul J. Schinder
       <>, Michael G Schwern <>, Dan Sugalski <>,
       Nathan Torkington <>, John Malmberg <>