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PERLPORT(1)

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NAME    [Toc]    [Back]

       perlport - Writing portable Perl

DESCRIPTION    [Toc]    [Back]

       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, 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-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">").

ISSUES    [Toc]    [Back]

       Newlines

       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 " 12", one type of
       DOSish I/O uses " 15 12", and Mac OS uses " 15".

       Perl uses "0 to represent the "logical" newline, where
       what is logical may depend on the platform in use.  In
       MacPerl, "0 always means " 15".  In DOSish perls, "0
       usually means " 12", but when accessing a file in "text"
       mode, STDIO translates it to (or from) " 15 12", depending
 on whether you're reading or writing.  Unix does the
       same thing on ttys in canonical mode.  " 15 12" is commonly
 referred to as CRLF.

       A common cause of unportable programs is the misuse of
       chop() to trim newlines:
           # XXX UNPORTABLE!
           while(<FILE>) {
               chop;
               @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 "0
       eq " 12" everywhere.  When using protocols such as common
       Internet protocols, " 12" and " 15" are called for  "
       specifically, and the values of the logical "0 and "
       (carriage return) are not reliable.
                                          0;      # WRONG
           print SOCKET "Hi there, client!
           print SOCKET "Hi there, client! 15 12";  # RIGHT

       However, using " 15 12" (or "        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 "0, but robust socket code
       will recognize as either " 12" or " 15 12" 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  12

           while (<SOCKET>) {
               s/$CR?$LF/0;    #  not  sure  if socket uses LF or
CRLF, OK
           #   s/ 15? 12/0; # same thing
           }

       This example is preferred over the previous one--even for
       Unix platforms--because now any " 15"'s ("        stripped
out (and there was much rejoicing).

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

           $data =~ s/ 15? 12/0g;
           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   12  eq  A  eq             CR  eq   15  eq  D
eq
                    | Unix | DOS  | Mac  |
               ---------------------------
                 || LCR || LCR || CLF ||

               **|| LCR ||CRCR || CLF ||

               ---------------------------
               * 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 "0, and "0 on output becomes
       CRLF.

       T"ein Perl.usThere maytwellmbe others.ioForoexample, 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   25  eq  5  eq             LF  eq   45  eq   5
eq           chr(37)  eq  CP-0037 37
           CR   eq   15  eq  D  eq             CR  eq   15  eq  D
eq
                    | z/OS | OS/400 |
               ----------------------
                 || LCR || LCR   ||

               **|| LCR || LCR   ||

               ----------------------
               * text-mode STDIO

       Numbers endianness and Width    [Toc]    [Back]

       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.

       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)), "0;
           # '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 (included in the standard distribution as of
       Perl 5.005) 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    [Toc]    [Back]

       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 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
       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
           $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 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 test.pl and Test.pl, 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 appear.

       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) 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 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    [Toc]    [Back]

       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.

       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 perprogram
 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    [Toc]    [Back]

       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 "perl.pm", 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;
         $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;}

       Networking    [Toc]    [Back]

       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 '127.0.0.1'.  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)    [Toc]    [Back]

       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 nonUnix
 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, platformindependent
 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 implement
 it with platform-specific code, but expose a common
       interface).

       External Subroutines (XS)    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       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 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    [Toc]    [Back]

       Assume very little about character sets.

       Assume nothing about numerical values ("ord", "chr") of
       characters.  Do not use explicit code point ranges (like
       -); 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 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'.

       Internationalisation    [Toc]    [Back]

       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 "" 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 "encoding"
       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 5.8.0.

       System Resources    [Toc]    [Back]

       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 first.

       Security    [Toc]    [Back]

       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 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.)

       Style    [Toc]    [Back]

       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 value.

CPAN Testers    [Toc]    [Back]

       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.

       Mailing list: cpan-testers@perl.org
       Testing results: http://testers.cpan.org/

PLATFORMS    [Toc]    [Back]

       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 $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.

       Unix    [Toc]    [Back]

       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
           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:     r
                            l
           $filespec0 = "c:/eoo/bar/file.txt";
           $filespec1 = "c:\.oo\bar\file.txt";
           $filespec2 = 'c:ot
           $filespec3 = 'c:\foo\bar\file.txt';
                            t
       System calls accept either "/" or "
       tor.  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,
       unfortunately.

       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 ("0) is translated as " 15 12" by STDIO when
       reading from and writing to files (see "Newlines").  "binmode(FILEHANDLE)"
 will keep "0 translated as " 12" 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
xx
            Windows XP    MSWin32    MSWin32-x86       2      ?
            Windows CE    MSWin32    ?                 3
            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]],"0;
           }

       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:

       o   The djgpp environment for DOS, http://www.delo-
           rie.com/djgpp/ and perldos.

       o   The EMX environment for DOS, OS/2, etc. emx@iaehv.nl,
           http://www.leo.org/pub/comp/os/os2/leo/gnu/emx+gcc/index.html
           or ftp://hobbes.nmsu.edu/pub/os2/dev/emx/  Also perlos2.


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

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

       o   The ActiveState Pages, http://www.activestate.com/

       o   The Cygwin environment for Win32; README.cygwin
           (installed as perlcygwin), http://www.cygwin.com/
       o   The U/WIN environment for Win32,
           http://www.research.att.com/sw/tools/uwin/

       o   Build instructions for OS/2, perlos2

       Mac OS    [Toc]    [Back]

       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  CPAN.

       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 /, MacPerl::Ask('Arguments?');
           }

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

       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 architecture,
       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 =~ /^()/;
           $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 version,
       Darwin, both run Unix perl natively.

       Also see:

       o   MacPerl Development, http://dev.macperl.org/ .

       o   The MacPerl Pages, http://www.macperl.com/ .

       o   The MacPerl mailing lists, http://lists.perl.org/ .

       VMS    [Toc]    [Back]

       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/login.com

       but not a mixture of both as in:

           $  perl  -ne  "print  if /perl_setup/i" sys$login:/login.com
           Can't open  sys$login:/login.com:  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.0""
           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__"
           #!/usr/bin/perl

           print "Hello from Perl!0;

           __END__
           $ 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 characters 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.2.3.4.5.6.7.8]" is a valid
       directory specification but
       "PERL_ROOT:[LIB.2.3.4.5.6.7.8.9]" 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 "0 represents depends on the type of file opened.
       It usually represents " 12" but it could also be " 15",
       " 12", " 15 12", " 00", " 40", or nothing depending
       on the file organiztion 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 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!0;

           } elsif (grep(/VMS_VAX/, @INC)) {
               print "I'm on VAX!0;

           } else {
               print "I'm not so sure about where $^O is...0;
           }

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

       Also see:

       o   README.vms (installed as README_vms), perlvms

       o   vmsperl list, majordomo@perl.org

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

       o   vmsperl on the web,
           http://www.sidhe.org/vmsperl/index.html

       VOS    [Toc]    [Back]

       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!0;
           } else {
               print "I'm not on a Stratus box!0;
               die;
           }

           if (grep(/860/, @INC)) {
               print "This box is a Stratus XA/R!0;

           } elsif (grep(/7100/, @INC)) {
               print "This box is a Stratus HP 7100 or 8xxx!0;

           } elsif (grep(/8000/, @INC)) {
               print "This box is a Stratus HP 8xxx!0;

           } else {
               print "This box is a Stratus 68K!0;
           }

       Also see:

       o   README.vos (installed as perlvos)

       o   The VOS mailing list.

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

       o   VOS Perl on the web at http://ftp.stra-
           tus.com/pub/vos/posix/posix.html

       EBCDIC Platforms    [Toc]    [Back]

       Recent versions of Perl have been ported to platforms such
       as OS/400

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