perlport.pod

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    require Time::Local;
    $offset = Time::Local::timegm(0, 0, 0, 1, 0, 70);

The value for C<$offset> in Unix will be C<0>, but in Mac OS will be
some large number.  C<$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 C<gmtime> or
C<localtime>.

=head2 Character sets and character encoding

Assume very little about character sets.

Assume nothing about numerical values (C<ord>, C<chr>) of characters.
Do not use explicit code point ranges (like \xHH-\xHH); use for
example symbolic character classes like C<[: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 E<auml> comes before "b".

=head2 Internationalisation

If you may assume POSIX (a rather large assumption), you may read
more about the POSIX locale system from L<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 L<perluniintro> and L<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 C<bytes>
pragma.  If the bytes are in a string (regular expression being a
curious string), you can often also use the C<\xHH> notation instead
of embedding the bytes as-is.  (If you want to write your code in UTF-8,
you can use the C<utf8>.) The C<bytes> and C<utf8> pragmata are
available since Perl 5.6.0.

=head2 System Resources

If your code is destined for systems with severely constrained (or
missing!) virtual memory systems then you want to be I<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.

=head2 Security

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 C<< $< >> and C<< $> >> (or the C<$(> and C<$)>) 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.)

=head2 Style

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 C<$^O> to differentiate platforms, as described in
L<"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
C<$!> after a failed system call.  Using C<$!> 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.

=head1 CPAN Testers

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

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

Also see: 

=over 4

=item *

Mailing list: [e-mail]

=item *

Testing results: http://testers.cpan.org/

=back

=head1 PLATFORMS

As of version 5.002, Perl is built with a C<$^O> variable that
indicates the operating system it was built on.  This was implemented
to help speed up code that would otherwise have to C<use Config>
and use the value of C<$Config{osname}>.  Of course, to get more
detailed information about the system, looking into C<%Config> is
certainly recommended.

C<%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.

=head2 Unix

Perl works on a bewildering variety of Unix and Unix-like platforms (see
e.g. most of the files in the F<hints/> directory in the source code kit).
On most of these systems, the value of C<$^O> (hence C<$Config{'osname'}>,
too) is determined either by lowercasing and stripping punctuation from the
first field of the string returned by typing C<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 C<$Config{archname}> may depend on the
hardware architecture, it can vary more than the value of C<$^O>.

=head2 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 I<COMMAND.COM> or I<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 C</> or C<\> as the path separator.
However, many command-line utilities of DOS vintage treat C</> as
the option prefix, so may get confused by filenames containing C</>.
Aside from calling any external programs, C</> 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 C<readdir> or used with functions like C<open> or C<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 I<pl2bat.bat> or I<pl2cmd> to
put wrappers around your scripts.

Newline (C<\n>) is translated as C<\015\012> by STDIO when reading from
and writing to files (see L<"Newlines">).  C<binmode(FILEHANDLE)>
will keep C<\n> translated as C<\012> for that filehandle.  Since it is a
no-op on other systems, C<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 C<$^O> variable and the C<$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 C<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:

=over 4

=item *

The djgpp environment for DOS, http://www.delorie.com/djgpp/
and L<perldos>.

=item *

The EMX environment for DOS, OS/2, etc. [e-mail],
http://www.leo.org/pub/comp/os/os2/leo/gnu/emx+gcc/index.html or
ftp://hobbes.nmsu.edu/pub/os2/dev/emx/  Also L<perlos2>.

=item *

Build instructions for Win32 in L<perlwin32>, or under the Cygnus environment
in L<perlcygwin>.  

=item *

The C<Win32::*> modules in L<Win32>.

=item *

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

=item *