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       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 have 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", and Mac OS uses "\015".

       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".  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

       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 in

       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>) {
               # ...

       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 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.

       The v-strings are portable only up to v2147483647 (0x7FFFFFFF), that's how far EBCDIC, or
       more precisely UTF-EBCDIC will go.

   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.

       If all this is intimidating, have no (well, maybe only a little) fear.  There are modules
       that can help.  The File::Spec modules provide methods to do the Right Thing on whatever
       platform happens to be running the program.

           use File::Spec::Functions;
           chdir(updir());        # go up one directory
           my $file = catfile(curdir(), 'temp', 'file.txt');
           # on Unix and Win32, './temp/file.txt'
           # on Mac OS Classic, ':temp:file.txt'
           # on VMS, '[.temp]file.txt'

       File::Spec is available in the standard distribution as of version 5.004_05.
       File::Spec::Functions is only in File::Spec 0.7 and later, and some versions of perl come
       with version 0.6.  If File::Spec is not updated to 0.7 or later, you must use the object-
       oriented interface from File::Spec (or upgrade File::Spec).

       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 "<" explicitly to
       open a file for reading, or even better, use the three-arg version of open, unless you
       want the user to be able to specify a pipe open.

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

       If filenames might use strange characters, it is safest to open it with "sysopen" instead
       of "open".  "open" is magic and can translate characters like ">", "<", and "|", which may
       be the wrong thing to do.  (Sometimes, though, it's the right thing.)  Three-arg open can
       also help protect against this translation in cases where it is undesirable.

       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 to 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 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 T U V W X Y Z
        0 1 2 3 4 5 6 7 8 9
        . _ -

       and the "-" 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{HOME},
       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 is documented.

       On VMS by default, changes to the %ENV hash are persistent after the process exits.  This
       can cause unintended issues.

       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 strings 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 "_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 $thisperl =~ m/$Config{_exe}$/i;}

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

        use Config;
        my $thisperl = $Config{perlpath};
        if ($^O ne 'VMS')
           {$thisperl .= $Config{_exe} unless $thisperl =~ m/$Config{_exe}$/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't":s 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", "``", "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(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 extension, 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 expose 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 NBDM_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 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.

           require Time::Local;
           my $offset = Time::Local::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); use for example symbolic character classes like

       Do not assume that the alphabetic characters are encoded contiguously (in the numeric
       sense).  There may be gaps.

       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".

       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.

       If you want to use non-ASCII bytes (outside the bytes 0x00..0x7f) in the "source code" of
       your code, to be portable you have to be explicit about what bytes they are.  Someone
       might for example be using your code under a UTF-8 locale, in which case random native
       bytes might be illegal ("Malformed UTF-8 ...")  This means that for example embedding ISO
       8859-1 bytes beyond 0x7f into your strings might cause trouble later.  If the bytes are
       native 8-bit bytes, you can use the "bytes" pragma.  If the bytes are in a string (regular
       expression being a curious string), you can often also use the "\xHH" notation instead of
       embedding the bytes as-is.  If you want to write your code in UTF-8, you can use the

   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 that 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 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 the $< and $> (or
       the $( 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

       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
           dgux          dgux       AViiON-dgux
           DYNIX/ptx     dynixptx   i386-dynixptx
           FreeBSD       freebsd    freebsd-i386
           Haiku         haiku      BePC-haiku
           Linux         linux      arm-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
       or pl2cmd to put wrappers around your scripts.

       Newline ("\n") is translated as "\015\012" by STDIO when reading from and writing to files
       (see "Newlines").  "binmode(FILEHANDLE)" will keep "\n" translated as "\012" for that
       filehandle.  Since it is a no-op on other systems, "binmode" should be used for cross-
       platform 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

       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() and Win32::IsWin95(), try "perldoc Win32", and as of
       libwin32 0.19 (not part of the core Perl distribution) Win32::GetOSName().  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

       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.

       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

       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, known as ODS-2 and ODS-5.

       For ODS-2, filenames are in the format "name.extension;version".  The maximum length for
       filenames is 39 characters, and the maximum length for extensions is also 39 characters.
       Version is a number from 1 to 32767.  Valid characters are "/[A-Z0-9$_-]/".

       The ODS-2 filesystem is case-insensitive and does not preserve case.  Perl simulates this
       by converting all filenames to lowercase internally.

       For ODS-5, filenames may have almost any character in them and can include Unicode
       characters.  Characters that could be misinterpreted by the DCL shell or file parsing
       utilities need to be prefixed with the "^" character, or replaced with hexadecimal
       characters prefixed with the "^" character.  Such prefixing is only needed with the
       pathnames are in VMS format in applications.  Programs that can accept the Unix format of
       pathnames do not need the escape characters.  The maximum length for filenames is 255
       characters.  The ODS-5 file system can handle both a case preserved and a case sensitive

       ODS-5 is only available on the OpenVMS for 64 bit platforms.

       Support for the extended file specifications is being done as optional settings to
       preserve backward compatibility with Perl scripts that assume the previous VMS

       In general routines on VMS that get a Unix format file specification should return it in a
       Unix format, and when they get a VMS format specification they should return a VMS format
       unless they are documented to do a conversion.

       For routines that generate return a file specification, VMS allows setting if the C
       library which Perl is built on if it will be returned in VMS format or in Unix format.

       With the ODS-2 file system, there is not much difference in syntax of filenames without
       paths for VMS or Unix.  With the extended character set available with ODS-5 there can be
       a significant difference.

       Because of this, existing Perl scripts written for VMS were sometimes treating VMS and
       Unix filenames interchangeably.  Without the extended character set enabled, this behavior
       will mostly be maintained for backwards compatibility.

       When extended characters are enabled with ODS-5, the handling of Unix formatted file
       specifications is to that of a Unix system.

       VMS file specifications without extensions have a trailing dot.  An equivalent Unix file
       specification should not show the trailing dot.

       The result of all of this, is that for VMS, for portable scripts, you can not depend on
       Perl to present the filenames in lowercase, to be case sensitive, and that the filenames
       could be returned in either Unix or VMS format.

       And if a routine returns a file specification, unless it is intended to convert it, it
       should return it in the same format as it found it.

       "readdir" by default has traditionally returned lowercased filenames.  When the ODS-5
       support is enabled, it will return the exact case of the filename on the disk.

       Files without extensions have a trailing period on them, so doing a "readdir" in the
       default mode with a file named A.;5 will return a. when VMS is (though that file could be
       opened with "open(FH, 'A')").

       With support for extended file specifications and if "opendir" was given a Unix format
       directory, a file named A.;5 will return a and optionally in the exact case on the disk.
       When "opendir" is given a VMS format directory, then "readdir" should return a., and again
       with the optionally the exact case.

       RMS had an eight level limit on directory depths from any rooted logical (allowing 16
       levels overall) prior to VMS 7.2, and even with versions of VMS on VAX up through 7.3.
       Hence "PERL_ROOT:[LIB.]" is a valid directory specification but
       "PERL_ROOT:[LIB.]" is not.  Makefile.PL authors might have to take this
       into account, but at least they can refer to the former as

       Pumpkings and module integrators can easily see whether files with too many directory
       levels have snuck into the core by running the following in the top-level source

        $ perl -ne "$_=~s/\s+.*//; print if scalar(split /\//) > 8;" < MANIFEST

       The VMS::Filespec module, which gets installed as part of the build process on VMS, is a
       pure Perl module that can easily be installed on non-VMS platforms and can be helpful for
       conversions to and from RMS native formats.  It is also now the only way that you should
       check to see if VMS is in a case sensitive mode.

       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.

       TCP/IP stacks are optional on VMS, so socket routines might not be implemented.  UDP
       sockets may not be supported.

       The TCP/IP library support for all current versions of VMS is dynamically loaded if
       present, so even if the routines are configured, they may return a status indicating that
       they are not implemented.

       The value of $^O on OpenVMS is "VMS".  To determine the architecture that you are running
       on without resorting to loading all of %Config you can examine the content of the @INC
       array like so:

           if (grep(/VMS_AXP/, @INC)) {
               print "I'm on Alpha!\n";

           } elsif (grep(/VMS_VAX/, @INC)) {
               print "I'm on VAX!\n";

           } elsif (grep(/VMS_IA64/, @INC)) {
               print "I'm on IA64!\n";

           } else {
               print "I'm not so sure about where $^O is...\n";

       In general, the significant differences should only be if Perl is running on VMS_VAX or
       one of the 64 bit OpenVMS platforms.

       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, <>

       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 any character matching "tr/ !#%&'()*;<=>?//".

       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 any
       character matching "tr/#%*<>?//".  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
       without resorting to loading all of %Config you can examine the content of the @INC array
       like so:

           if ($^O =~ /vos/) {
               print "I'm on a Stratus box!\n";
           } else {
               print "I'm not on a Stratus box!\n";

       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
       Recent versions of Perl have been ported to platforms such as OS/400 on AS/400
       minicomputers as well as OS/390, 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).  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 OS/390):

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

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

           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, README.os390, 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|/.|./|;"

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

       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 should be protected when "open" is used for input.

       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.  Following 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      "-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).  (Win32)

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

               "-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.
               (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, VMS,
               RISC OS)

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

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

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

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

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

       atan2   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.  (Tru64, HP-UX 10.20)

               The current version of the standards for "atan2()" is available at

       binmode Meaningless.  (RISC OS)

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

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

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

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

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

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

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

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

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

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

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

               Not implemented. (VMS, Plan 9, VOS)

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

       dump    Not useful. (RISC OS)

               Not supported. (Cygwin, Win32)

               Invokes VMS debugger. (VMS)

       exec    Does not automatically flush output handles on some platforms.  (SunOS, Solaris,

               Not supported. (Symbian OS)

       exit    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.  (VMS)

               "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 and POSIX::_exit(0);

       fcntl   Not implemented. (Win32)

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

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

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

               Emulated using multiple interpreters.  See perlfork.  (Win32)

               Does not automatically flush output handles on some platforms.  (SunOS, Solaris,

               Not implemented. (RISC OS)

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

       getppid Not implemented. (Win32, RISC OS)

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

               Not implemented. (Win32)

               Not useful. (RISC OS)

               Not implemented. (Win32, VMS, RISC OS)

               Not implemented. (Win32, Plan 9)

               Not implemented. (Win32)

               Not useful. (RISC OS)

               Not implemented. (Win32, VMS, RISC OS)

               Not implemented. (Win32, Plan 9)

               Not implemented. (Win32)

               Not implemented. (Win32, VMS)

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

               Not implemented. (Win32)

               Not implemented. (Win32, Plan 9)

               Not implemented. (Win32, Plan 9)

               Not implemented. (Win32, Plan 9)

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

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

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

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

               Not implemented. (Win32)

               Not implemented. (RISC OS, VMS, Win32)

               Not implemented. (Win32)

               Not implemented. (Win32, Plan 9)

               Not implemented. (Win32, Plan 9)

               Not implemented. (Plan 9, Win32)

       getsockopt SOCKET,LEVEL,OPTNAME
               Not implemented. (Plan 9)

       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

               On VOS, time values are 32-bit quantities.

               Not implemented. (VMS)

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

               Available only for socket handles. (RISC OS)

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

               "kill()" doesn't have the semantics of "raise()", i.e. it 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. (Win32)

               Is not supported for process identification number of 0 or negative numbers. (VMS)

       link    Not implemented. (RISC OS, VOS)

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

               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.

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

               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   Not implemented. (RISC OS)

               Return values (especially for device and inode) may be bogus. (Win32)

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

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

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

               Not implemented. (Win32, VMS, RISC OS)

       rename  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  Only implemented on sockets. (Win32, VMS)

               Only reliable on sockets. (RISC OS)

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

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

               Not implemented. (VMS, Win32, RISC OS)

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

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

               Not implemented. (Win32, RISC OS)

               Not implemented. (Plan 9)

               Not implemented. (Win32, VMS, RISC OS)

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

               A relatively recent addition to socket functions, may not be implemented even in
               Unix platforms.

               Not implemented. (RISC OS)

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

       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'

               ctime not supported on UFS (Mac OS X).

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

               device and inode are not meaningful.  (Win32)

               device and inode are not necessarily reliable.  (VMS)

               mtime, atime and ctime all return the last modification time.  Device and inode
               are not necessarily reliable.  (RISC OS)

               dev, rdev, blksize, and blocks are not available.  inode is not meaningful and
               will differ between stat calls on the same file.  (os2)

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

               On 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
               operation. (Win32)

       symlink Not implemented. (Win32, RISC OS)

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

       sysopen 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.  (Mac OS, OS/390)

       system  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".  $? is set in a way compatible with Unix (i.e. the
               exitstatus of the subprocess is obtained by "$? >> 8", as described in the
               documentation).  (Win32)

               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, providing the child program uses a compatible version of the emulation
               library.  scalar will call the native command line direct and no such emulation of
               a child Unix program will exists.  Mileage will vary.  (RISC OS)

               Does not automatically flush output handles on some platforms.  (SunOS, Solaris,

               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. (VMS)

       times   "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. (Win32)

               Not useful. (RISC OS)

               Not implemented. (Older versions of VMS)

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

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

       umask   Returns undef where unavailable.

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

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

               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. (Win32)

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

               Not useful. (RISC OS)

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
       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

EOL Platforms (Perl 5.14)

       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

       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

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):


       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, perldgux, 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. Koenig <>, Markus Laker
       <>, Andrew M. Langmead <>, 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 <>