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

       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 par-
       ticular 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, Mac OS, VMS, etc.), consider writing platform-specific code.

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

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

       This information should not be considered complete; it includes possibly transient information about idiosyn-
       crasies 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, STDIO 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.

       A common cause of unportable programs is the misuse of chop() to trim newlines:

           # XXX UNPORTABLE!
           while(<FILE>) {
               @array = split(/:/);

       You can get away with this on Unix and Mac OS (they have a single character end-of-line), but the same program
       will break under DOSish perls because you're only chop()ing half the end-of-line.  Instead, chomp() should be
       used to trim newlines.  The Dunce::Files module can help audit your code for misuses of chop().

       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 safety.

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

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

       However, using "\015\012" (or "\cM\cJ", or "\x0D\x0A") can be tedious and unsightly, as well as confusing to
       those maintaining the code.  As such, the Socket module supplies the Right Thing for those who want 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 trans-
       late newlines before returning the data, if they've not yet been translated to the local newline representa-
       tion.  A single line of code will often suffice:

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

       Some of this may be confusing.  Here's a handy reference to the ASCII CR and LF characters.  You can print it
       out and stick it in your 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 num-
       bers 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 5.8.5, 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 trans-
       ferring or storing raw binary numbers.

       One can circumnavigate both these problems in two ways.  Either transfer and store numbers always in text for-
       mat, instead of raw binary, or else consider using modules like Data::Dumper (included in the standard distri-
       bution as of Perl 5.005) and Storable (included as of perl 5.8).  Keeping all data as text significantly sim-
       plifies 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 direc-

       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

       Mac OS uses ":" as a path separator instead of "/".

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

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

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

       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 some-
       times 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 pro-

           use File::Spec::Functions;
           chdir(updir());        # go up one directory
           $file = catfile(curdir(), 'temp', 'file.txt');
           # on Unix and Win32, './temp/file.txt'
           # on Mac OS, ':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 sepa-
       rator for subdirectories.

       Also of use is File::Basename from the standard distribution, which splits a pathname into pieces (base file-
       name, 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

       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 charac-
       ters (except for ".") in the names, and keep them to the 8.3 convention, for maximum portability, onerous a
       burden though this may appear.

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

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

       Many systems (DOS, VMS) 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(FILE, '<', $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 path-
       name, 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 cluster-
       ing 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 ver-
       sions 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-sensi-
       tive, 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

       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 envi-
       ronment, 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 suf-
       fixes 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 possibil-
       ities, say:

         use Config;
         $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;
         $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 things like 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

       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 platforms.

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

       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 cur-
       rently makes connections to external programs that may not be available), platform-specific modules (like ExtU-
       tils::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

       Don't assume that the epoch starts at 00:00:00, January 1, 1970, because that is OS- and implementation-spe-
       cific.  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 to 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;
           $offset = Time::Local::timegm(0, 0, 0, 1, 0, 70);

       The value for $offset in Unix will be 0, but in Mac OS 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.

       On Windows (at least), you shouldn't pass a negative value to "gmtime" or "localtime".

       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 "[:print:]".

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

       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

       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 they are
       in some particular legacy encoding (ether single-byte or something more complicated), you can use the "encod-
       ing" pragma.  (If you want to write your code in UTF-8, you can use either the "utf8" pragma, or the "encoding"
       pragma.)  The "bytes" and "utf8" pragmata are available since Perl 5.6.0, and the "encoding" pragma since Perl

       System Resources

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

           # NOTE: this is no longer "bad" in perl5.005
           for (0..10000000) {}                       # bad
           for (my $x = 0; $x <= 10000000; ++$x) {}   # good

           @lines = <VERY_LARGE_FILE>;                # bad

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


       Most multi-user platforms provide basic levels of security, usually implemented at the filesystem level.  Some,
       however, do not-- unfortunately.  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 dif-

       (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 "PLATFORMS".

       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

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 plat-
       form), 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:

       As of version 5.002, 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 $Con-
       fig{osname}.  Of course, to get more detailed information about the system, looking into %Config is certainly

       %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


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

           uname         $^O        $Config{'archname'}
           AIX           aix        aix
           BSD/OS        bsdos      i386-bsdos
           Darwin        darwin     darwin
           dgux          dgux       AViiON-dgux
           DYNIX/ptx     dynixptx   i386-dynixptx
           FreeBSD       freebsd    freebsd-i386
           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
           MachTen PPC   machten    powerpc-machten
           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:

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

       System calls accept either "/" or "\" as the path separator.  However, many command-line utilities of DOS vin-
       tage 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-pre-
       serving" 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.  Unfortu-
       nately, 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, unfortunately.

       Users of these operating systems may also wish to make use of scripts such as pl2bat.bat or pl2cmd to put wrap-
       pers 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 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 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 perlcygwin.

       ?   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

       Mac OS

       Any module requiring XS compilation is right out for most people, because MacPerl is built using non-free (and
       non-cheap!) compilers.  Some XS modules that can work with MacPerl are built and distributed in binary form on

       Directories are specified as:

           volume:folder:file              for absolute pathnames
           volume:folder:                  for absolute pathnames
           :folder:file                    for relative pathnames
           :folder:                        for relative pathnames
           :file                           for relative pathnames
           file                            for relative pathnames

       Files are stored in the directory in alphabetical order.  Filenames are limited to 31 characters, and may
       include any character except for null and ":", which is reserved as the path separator.

       Instead of "flock", see "FSpSetFLock" and "FSpRstFLock" in the Mac::Files module, or "chmod(0444, ...)" and
       "chmod(0666, ...)".

       In the MacPerl application, you can't run a program from the command line; programs that expect @ARGV to be
       populated can be edited with something like the following, which brings up a dialog box asking for the command
       line arguments.

           if (!@ARGV) {
               @ARGV = split /\s+/, MacPerl::Ask('Arguments?');

       A MacPerl script saved as a "droplet" will populate @ARGV with the full pathnames of the files dropped onto the

       Mac users can run programs under a type of command line interface under MPW (Macintosh Programmer's Workshop, a
       free development environment from Apple).  MacPerl was first introduced as an MPW tool, and MPW can be used
       like a shell:

           perl myscript.plx some arguments

       ToolServer is another app from Apple that provides access to MPW tools from MPW and the MacPerl app, which
       allows MacPerl programs to use "system", backticks, and piped "open".

       "Mac OS" is the proper name for the operating system, but the value in $^O is "MacOS".  To determine architec-
       ture, version, or whether the application or MPW tool version is running, check:

           $is_app    = $MacPerl::Version =~ /App/;
           $is_tool   = $MacPerl::Version =~ /MPW/;
           ($version) = $MacPerl::Version =~ /^(\S+)/;
           $is_ppc    = $MacPerl::Architecture eq 'MacPPC';
           $is_68k    = $MacPerl::Architecture eq 'Mac68K';

       Mac OS X, based on NeXT's OpenStep OS, runs MacPerl natively, under the "Classic" environment.  There is no
       "Carbon" version of MacPerl to run under the primary Mac OS X environment.  Mac OS X and its Open Source ver-
       sion, Darwin, both run Unix perl natively.

       Also see:

       ?   MacPerl Development, .

       ?   The MacPerl Pages, .

       ?   The MacPerl mailing lists, .

       ?   MPW,


       Perl on VMS is discussed in perlvms in the perl distribution.  Perl on VMS can accept either VMS- or Unix-style
       file specifications as in either of the following:

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

       but not a mixture of both as in:

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

       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>;".

       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 charac-
       ters are "/[A-Z0-9$_-]/".

       VMS's RMS filesystem is case-insensitive and does not preserve case.  "readdir" returns lowercased filenames,
       but specifying a file for opening remains case-insensitive.  Files without extensions have a trailing period on
       them, so doing a "readdir" with a file named A.;5 will return a. (though that file could be opened with
       "open(FH, 'A')").

       RMS had an eight level limit on directory depths from any rooted logical (allowing 16 levels overall) prior to
       VMS 7.2.  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 "/PERL_ROOT/lib/2/3/4/5/6/7/8/".

       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.

       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

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

       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";

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

       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,

           (Put the words "subscribe vmsperl" in message body.)

       ?   vmsperl on the web,


       Perl on VOS 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:

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

       or even a mixture of both as in:

           C<< $ 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.  Note that VOS limits
       file names to 32 or fewer characters.

       Perl on VOS can be built using two different compilers and two different versions of the POSIX runtime.  The
       recommended method for building full Perl is with the GNU C compiler and the generally-available version of VOS
       POSIX support.  See README.vos (installed as perlvos) for restrictions that apply when Perl is built using the
       VOS Standard C compiler or the alpha version of VOS POSIX support.

       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";

           if (grep(/860/, @INC)) {
               print "This box is a Stratus XA/R!\n";

           } elsif (grep(/7100/, @INC)) {
               print "This box is a Stratus HP 7100 or 8xxx!\n";

           } elsif (grep(/8000/, @INC)) {
               print "This box is a Stratus HP 8xxx!\n";

           } else {
               print "This box is a Stratus 68K!\n";

       Also see:

       ?   README.vos (installed as perlvos)

       ?   The VOS mailing list.

           There is no specific mailing list for Perl on VOS.  You can post comments to the comp.sys.stratus news-
           group, or subscribe to the general Stratus mailing list.  Send a letter with "subscribe Info-Stratus" in
           the message body to

       ?   VOS Perl 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 Charac-
       ter Code Set ID 0037 for OS/400 and either 1047 or POSIX-BC for S/390 systems).  On the mainframe perl cur-
       rently 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 5.8.1/5.9.0 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 inter-
       faces to ASCII computers (see "Newlines").

       Fortunately, most web servers for the mainframe will correctly translate the "\n" in the following statement to
       its ASCII equivalent ("\r" is the same under both Unix and OS/390 & VM/ESA):

           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
           VM/ESA        vmesa      vmesa

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

           if ("\t" eq "\05")   { 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, README.vmesa, 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 emu-
       lation 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-pre-
       serving.  Some native filesystems have name length limits, which file and directory names are silently trun-
       cated 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 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 "Sys-
       tem$Path" until a name is made that points to an object on disk.  Writing to a new file "System:Modules" would
       be allowed only if "System$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{'Sys-
       tem$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 char-
       acters, 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 pass-
       ing "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 prob-
       lems, 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

       Other perls

       Perl has been ported to many platforms that do not fit into any of the categories listed above.  Some, such as
       AmigaOS, Atari MiNT, BeOS, HP MPE/iX, 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 include:

           OS            $^O        $Config{'archname'}
           Amiga DOS     amigaos    m68k-amigos
           BeOS          beos
           MPE/iX        mpeix      PA-RISC1.1

       See also:

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

       ?   Atari, and Guido Flohr's web page

       ?   Be OS, README.beos

       ?   HP 300 MPE/iX, README.mpeix and Mark Bixby's web page

       ?   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      "-r", "-w", and "-x" have a limited meaning only; directories and applications are executable, and
               there are no uid/gid considerations.  "-o" is not supported.  (Mac OS)

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

               "-s" returns the size of the data fork, not the total size of data fork plus resource fork.  (Mac OS).

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

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

               "-b", "-c", "-k", "-g", "-p", "-u", "-A" are not implemented.  (Mac OS)

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

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

               "-T" and "-B" are implemented, but might misclassify Mac text files with foreign characters; this is
               the case will all platforms, but may affect Mac OS often.  (Mac OS)

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

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

       atan2 Y,X
               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 stan-
               dards 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 <

       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 stan-
               dards 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.  (Mac OS, 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 limited meaning.  Disabling/enabling write permission is mapped to locking/unlocking the file.
               (Mac OS)

               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.

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

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

       chroot  Not implemented. (Mac OS, Win32, VMS, Plan 9, RISC OS, VOS, VM/ESA)

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

               Not implemented. (VOS)

               Not implemented. (VMS, Plan 9, VOS)

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

       dump    Not useful. (Mac OS, RISC OS)

               Not implemented. (Win32)

               Invokes VMS debugger. (VMS)

       exec    Not implemented. (Mac OS)

               Implemented via Spawn. (VM/ESA)

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

       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 overrid-
               den.  Any other argument to exit() is used directly as Perl's exit status. (VMS)

       fcntl   Not implemented. (Win32, VMS)

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

               Available only on Windows NT (not on Windows 95). (Win32)

       fork    Not implemented. (Mac OS, AmigaOS, RISC OS, VOS, VM/ESA, VMS)

               Emulated using multiple interpreters.  See perlfork.  (Win32)

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

               Not implemented. (Mac OS, RISC OS)

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

       getppid Not implemented. (Mac OS, Win32, RISC OS)

               Not implemented. (Mac OS, Win32, VMS, RISC OS, VOS, VM/ESA)

               Not implemented. (Mac OS, Win32)

               Not useful. (RISC OS)

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

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

               Not implemented. (Mac OS, Win32)

               Not useful. (RISC OS)

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

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

               Not implemented. (Mac OS)

               Not implemented. (Mac OS)

               Not implemented. (Mac OS, Win32, VM/ESA)

               Not implemented. (Mac OS, Win32, VMS, VM/ESA)

               "gethostbyname('localhost')" does not work everywhere: you may have to use "gethostby-
               name('')". (Mac OS, Irix 5)

               Not implemented. (Mac OS, Win32)

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

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

               Not implemented. (Win32, Plan 9)

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

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

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

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

               Not implemented. (Mac OS, MPE/iX, VM/ESA, Win32)

               Not implemented. (Mac OS, MPE/iX, RISC OS, VM/ESA, VMS, Win32)

               Not implemented. (Mac OS, Win32)

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

               Not implemented. (Mac OS, 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 porta-
               bility information.

       gmtime  Same portability caveats as localtime.

               Not implemented. (VMS)

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

               Available only for socket handles. (RISC OS)

       kill    "kill(0, LIST)" is implemented for the sake of taint checking; use with other signals is unimplemented.
               (Mac OS)

               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 pro-
               cess 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)

       link    Not implemented. (Mac OS, MPE/iX, VMS, RISC OS)

               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 (Windows NT and Windows 2000) under NTFS only.

               Because Perl currently relies on the native standard C localtime() function, it is only safe to use
               times between 0 and (2**31)-1.  Times outside this range may result in unexpected behavior depending on
               your operating system's implementation of localtime().

       lstat   Not implemented. (VMS, RISC OS)

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

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

       open    The "|" variants are supported only if ToolServer is installed.  (Mac OS)

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

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

       pipe    Very limited functionality. (MiNT)

               Not implemented. (Win32, VMS, RISC OS)

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

       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. (Mac OS, Win32, VMS, RISC OS, VOS)

               Not implemented. (Mac OS, MPE/iX, VMS, Win32, RISC OS)

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

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

               Not implemented. (Mac OS, MPE/iX, Win32, RISC OS)

               Not implemented. (Plan 9)

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

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

               Not implemented. (Win32, VMS, RISC OS, VOS, VM/ESA)

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

               mtime and atime are the same thing, and ctime is creation time instead of inode change time. (Mac OS).

               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)

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

       syscall Not implemented. (Mac OS, Win32, VMS, RISC OS, VOS, VM/ESA)

       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, VM/ESA)

       system  In general, do not assume the UNIX/POSIX semantics that you can shift $? right by eight to get the exit
               value, or that "$? & 127" would give you the number of the signal that terminated the program, or that
               "$? & 128" would test true if the program was terminated by a coredump.  Instead, use the POSIX W*()
               interfaces: for example, use WIFEXITED($?) and WEXITVALUE($?) to test for a normal exit and the exit
               value, WIFSIGNALED($?) and WTERMSIG($?) for a signal exit and the signal.  Core dumping is not a
               portable concept, so there's no portable way to test for that.

               Only implemented if ToolServer is installed. (Mac OS)

               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 subpro-
               cess 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).  (Win32)

               There is no shell to process metacharacters, and the native standard is to pass a command line termi-
               nated 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)

               Far from being POSIX compliant.  Because there may be no underlying /bin/sh tries to work around the
               problem by forking and execing the first token in its argument string.  Handles basic redirection ("<"
               or ">") on its own behalf. (MiNT)

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

               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'").  For more details see "$?" in perlvms. (VMS)

       times   Only the first entry returned is nonzero. (Mac OS)

               "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 zero-length 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, as of version 5.005.

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

       utime   Only the modification time is updated. (BeOS, Mac OS, 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 Not implemented. (Mac OS, VOS)

               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
       As of September 2003 (the Perl release 5.8.1), the following platforms are 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)
               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 5.8.1 release.  There is a very good chance that many of these will work fine
       with the 5.8.1.

               SCO SV
               Windows 3.1

       Known to be broken for 5.8.0 and 5.8.1 (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 plat-
       forms 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 configurabil-
       ity and for security, in case you are in a hurry you can check for binary

       perlaix, perlamiga, perlapollo, perlbeos, perlbs2000, perlce, perlcygwin, perldgux, perldos, perlepoc, per-
       lebcdic, perlfreebsd, perlhurd, perlhpux, perlirix, perlmachten, perlmacos, perlmacosx, perlmint, perlmpeix,
       perlnetware, perlos2, perlos390, perlos400, perlplan9, perlqnx, perlsolaris, perltru64, perlunicode, perlvmesa,
       perlvms, perlvos, perlwin32, and Win32.

       Abigail <>, Charles Bailey <>, Graham Barr <>, Tom Chris-
       tiansen <>, 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
       <71564.1743ATCompuServe.COM>, Tom Phoenix <>, Andre Pirard <>, Peter
       Prymmer <>, Hugo van der Sanden <>, Gurusamy Sarathy <>,
       Paul J. Schinder <>, Michael G Schwern <>, Dan Sugalski <>,
       Nathan Torkington <>.

perl v5.8.8                       2006-01-07                       PERLPORT(1)