PerlDoc perlport

NAME
perlport - Writing portable Perl

DESCRIPTION
Perl runs on numerous operating systems. While most of them share much
in common, they also have their own unique features.

This document is meant to help you to find out what constitutes portable
Perl code. That way once you make a decision to write portably, you know
where the lines are drawn, and you can stay within them.

There is a tradeoff between taking full advantage of one particular type
of computer and taking advantage of a full range of them. Naturally, as
you broaden your range and become more diverse, the common factors drop,
and you are left with an increasingly smaller area of common ground in
which you can operate to accomplish a particular task. Thus, when you
begin attacking a problem, it is important to consider under which part
of the tradeoff curve you want to operate. Specifically, you must decide
whether it is important that the task that you are coding have the full
generality of being portable, or whether to just get the job done right
now. This is the hardest choice to be made. The rest is easy, because
Perl provides many choices, whichever way you want to approach your
problem.

Looking at it another way, writing portable code is usually about
willfully limiting your available choices. Naturally, it takes
discipline and sacrifice to do that. The product of portability and
convenience may be a constant. You have been warned.

Be aware of two important points:

Not all Perl programs have to be portable
There is no reason you should not use Perl as a language to glue
Unix tools together, or to prototype a Macintosh application, or to
manage the Windows registry. If it makes no sense to aim for
portability for one reason or another in a given program, then don't
bother.

Nearly all of Perl already *is* portable
Don't be fooled into thinking that it is hard to create portable
Perl code. It isn't. Perl tries its level-best to bridge the gaps
between what's available on different platforms, and all the means
available to use those features. Thus almost all Perl code runs on
any machine without modification. But there are some significant
issues in writing portable code, and this document is entirely about
those issues.

Here's the general rule: When you approach a task commonly done using a
whole range of platforms, think about writing portable code. That way,
you don't sacrifice much by way of the implementation choices you can
avail yourself of, and at the same time you can give your users lots of
platform choices. On the other hand, when you have to take advantage of
some unique feature of a particular platform, as is often the case with
systems programming (whether for Unix, Windows, VMS, etc.), consider
writing platform-specific 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
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 "\012", one type of DOSish I/O uses "\015\012", and
Mac OS uses "\015".

Perl uses "\n" to represent the "logical" newline, where what is logical
may depend on the platform in use. In MacPerl, "\n" always means "\015".
In DOSish perls, "\n" usually means "\012", but when accessing a file in
"text" mode, perl uses the ":crlf" layer that translates it to (or from)
"\015\012", depending on whether you're reading or writing. Unix does
the same thing on ttys in canonical mode. "\015\012" is commonly
referred to as CRLF.

To trim trailing newlines from text lines use chomp(). With default
settings that function looks for a trailing "\n" character and thus
trims in a portable way.

When dealing with binary files (or text files in binary mode) be sure to
explicitly set $/ to the appropriate value for your file format before
using chomp().

Because of the "text" mode translation, DOSish perls have limitations in
using "seek" and "tell" on a file accessed in "text" mode. Stick to
"seek"-ing to locations you got from "tell" (and no others), and you are
usually free to use "seek" and "tell" even in "text" mode. Using "seek"
or "tell" or other file operations may be non-portable. If you use
"binmode" on a file, however, you can usually "seek" and "tell" with
arbitrary values 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 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/\015?\012/\n/g;
return $data;

Some of this may be confusing. Here's a handy reference to the ASCII CR
and LF characters. You can print it out and stick it in your wallet.

LF eq \012 eq \x0A eq \cJ eq chr(10) eq ASCII 10
CR eq \015 eq \x0D eq \cM eq chr(13) eq ASCII 13

| Unix | DOS | Mac |
---------------------------
\n | LF | LF | CR |
\r | CR | CR | LF |
\n * | LF | CRLF | CR |
\r * | CR | CR | LF |
---------------------------
* text-mode STDIO

The Unix column assumes that you are not accessing a serial line (like a
tty) in canonical mode. If you are, then CR on input becomes "\n", and
"\n" on output becomes CRLF.

These are just the most common definitions of "\n" and "\r" in Perl.
There may well be others. For example, on an EBCDIC implementation such
as z/OS (OS/390) or OS/400 (using the ILE, the PASE is ASCII-based) the
above material is similar to "Unix" but the code numbers change:

LF eq \025 eq \x15 eq \cU eq chr(21) eq CP-1047 21
LF eq \045 eq \x25 eq chr(37) eq CP-0037 37
CR eq \015 eq \x0D eq \cM eq chr(13) eq CP-1047 13
CR eq \015 eq \x0D eq \cM eq chr(13) eq CP-0037 13

| z/OS | OS/400 |
----------------------
\n | LF | LF |
\r | CR | CR |
\n * | LF | LF |
\r * | CR | CR |
----------------------
* text-mode STDIO

Numbers endianness and Width
Different CPUs store integers and floating point numbers in different
orders (called *endianness*) and widths (32-bit and 64-bit being the
most common today). This affects your programs when they attempt to
transfer numbers in binary format from one CPU architecture to another,
usually either "live" via network connection, or by storing the numbers
to secondary storage such as a disk file or tape.

Conflicting storage orders make utter mess out of the numbers. If a
little-endian host (Intel, VAX) stores 0x12345678 (305419896 in
decimal), a big-endian host (Motorola, Sparc, PA) reads it as 0x78563412
(2018915346 in decimal). Alpha and MIPS can be either: Digital/Compaq
used/uses them in little-endian mode; SGI/Cray uses them in big-endian
mode. To avoid this problem in network (socket) connections use the
"pack" and "unpack" formats "n" and "N", the "network" orders. These are
guaranteed to be portable.

As of perl 5.9.2, you can also use the ">" and "<" modifiers to force
big- or little-endian byte-order. This is useful if you want to store
signed integers or 64-bit integers, for example.

You can explore the endianness of your platform by unpacking a data
structure packed in native format such as:

print unpack("h*", pack("s2", 1, 2)), "\n";
# '10002000' on e.g. Intel x86 or Alpha 21064 in little-endian mode
# '00100020' on e.g. Motorola 68040

If you need to distinguish between endian architectures you could use
either of the variables set like so:

$is_big_endian = unpack("h*", pack("s", 1)) =~ /01/;
$is_little_endian = unpack("h*", pack("s", 1)) =~ /^1/;

Differing widths can cause truncation even between platforms of equal
endianness. The platform of shorter width loses the upper parts of the
number. There is no good solution for this problem except to avoid
transferring or storing raw binary numbers.

One can circumnavigate both these problems in two ways. Either transfer
and store numbers always in text format, instead of raw binary, or else
consider using modules like Data::Dumper (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
Most platforms these days structure files in a hierarchical fashion. So,
it is reasonably safe to assume that all platforms support the notion of
a "path" to uniquely identify a file on the system. How that path is
really written, though, differs considerably.

Although similar, file path specifications differ between Unix, Windows,
Mac OS, OS/2, VMS, VOS, RISC OS, and probably others. Unix, for example,
is one of the few OSes that has the elegant idea of a single root
directory.

DOS, OS/2, VMS, VOS, and Windows can work similarly to Unix with "/" as
path separator, or in their own idiosyncratic ways (such as having
several root directories and various "unrooted" device files such NIL:
and LPT:).

Mac OS 9 and earlier used ":" as a path separator instead of "/".

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

The filesystem may support neither access timestamp nor change timestamp
(meaning that about the only portable timestamp is the modification
timestamp), or one second granularity of any timestamps (e.g. the FAT
filesystem limits the time granularity to two 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
my $file = catfile(curdir(), 'temp', 'file.txt');
# on Unix and Win32, './temp/file.txt'
# on Mac OS Classic, ':temp:file.txt'
# on VMS, '[.temp]file.txt'

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

In general, production code should not have file paths hardcoded. Making
them user-supplied or read from a configuration file is better, keeping
in mind that file path syntax varies on different machines.

This is especially noticeable in scripts like Makefiles and test suites,
which often assume "/" as a path separator for subdirectories.

Also of use is File::Basename from the standard distribution, which
splits a pathname into pieces (base filename, full path to directory,
and file suffix).

Even when on a single platform (if you can call Unix a single platform),
remember not to count on the existence or the contents of particular
system-specific files or directories, like /etc/passwd,
/etc/sendmail.conf, /etc/resolv.conf, or even /tmp/. For example,
/etc/passwd may exist but not contain the encrypted passwords, because
the system is using some form of enhanced security. Or it may not
contain all the accounts, because the system is using NIS. If code does
need to rely on such a file, include a description of the file and its
format in the code's documentation, then make it easy for the user to
override the default location of the file.

Don't assume a text file will end with a newline. They should, but
people forget.

Do not have two files or directories of the same name with different
case, like 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 ODS-2) cannot have more than one "." in their
filenames.

Don't assume ">" won't be the first character of a filename. Always use
"<" explicitly to open a file for reading, or even better, use the
three-arg version of open, unless you want the user to be able to
specify a pipe open.

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

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

Don't use ":" as a part of a filename since many systems use that for
their own semantics (Mac OS Classic for separating pathname components,
many networking schemes and utilities for separating the nodename and
the pathname, and so on). For the same reasons, avoid "@", ";" and "|".

Don't assume that in pathnames you can collapse two leading slashes "//"
into one: some networking and clustering filesystems have special
semantics for that. Let the operating system to sort it out.

The *portable filename characters* as defined by ANSI C are

a b c d e f g h i j k l m n o p q r t u v w x y z
A B C D E F G H I J K L M N O P Q R T U V W X Y Z
0 1 2 3 4 5 6 7 8 9
. _ -

and the "-" shouldn't be the first character. If you want to be
hypercorrect, stay case-insensitive and within the 8.3 naming convention
(all the files and directories have to be unique within one directory if
their names are lowercased and truncated to eight characters before the
".", if any, and to three characters after the ".", if any). (And do not
use "."s in directory names.)

System Interaction
Not all platforms provide a command line. These are usually platforms
that rely primarily on a Graphical User Interface (GUI) for user
interaction. A program requiring a command line interface might not work
everywhere. This is probably for the user of the program to deal with,
so don't stay up late worrying about it.

Some platforms can't delete or rename files held open by the system,
this limitation may also apply to changing filesystem metainformation
like file permissions or owners. Remember to "close" files when you are
done with them. Don't "unlink" or "rename" an open file. Don't "tie" or
"open" a file already tied or opened; "untie" or "close" it first.

Don't open the same file more than once at a time for writing, as some
operating systems put mandatory locks on such files.

Don't assume that write/modify permission on a directory gives the right
to add or delete files/directories in that directory. That is filesystem
specific: in some filesystems you need write/modify permission also (or
even just) in the file/directory itself. In some filesystems (AFS, DFS)
the permission to add/delete directory entries is a completely separate
permission.

Don't assume that a single "unlink" completely gets rid of the file:
some filesystems (most notably the ones in VMS) have versioned
filesystems, and unlink() removes only the most recent one (it doesn't
remove all the versions because by default the native tools on those
platforms remove just the most recent version, too). The portable idiom
to remove all the versions of a file is

1 while unlink "file";

This will terminate if the file is undeleteable for some reason
(protected, not there, and so on).

Don't count on a specific environment variable existing in %ENV. Don't
count on %ENV entries being case-sensitive, or even case-preserving.
Don't try to clear %ENV by saying "%ENV = ();", or, if you really have
to, make it conditional on "$^O ne 'VMS'" since in VMS the %ENV table is
much more than a per-process key-value string table.

On VMS, some entries in the %ENV hash are dynamically created when their
key is used on a read if they did not previously exist. The values for
$ENV{HOME}, $ENV{TERM}, $ENV{HOME}, and $ENV{USER}, are known to be
dynamically generated. The specific names that are dynamically generated
may vary with the version of the C library on VMS, and more may exist
than is documented.

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

Don't count on signals or %SIG for anything.

Don't count on filename globbing. Use "opendir", "readdir", and
"closedir" instead.

Don't count on per-program environment variables, or per-program current
directories.

Don't count on specific values of $!, neither numeric nor especially the
strings values. Users may switch their locales causing error messages to
be translated into their languages. If you can trust a POSIXish
environment, you can portably use the symbols defined by the Errno
module, like ENOENT. And don't trust on the values of $! at all except
immediately after a failed system call.

Command names versus file pathnames
Don't assume that the name used to invoke a command or program with
"system" or "exec" can also be used to test for the existence of the
file that holds the executable code for that command or program. First,
many systems have "internal" commands that are built-in to the shell or
OS and while these commands can be invoked, there is no corresponding
file. Second, some operating systems (e.g., Cygwin, DJGPP, OS/2, and
VOS) have required suffixes for executable files; these suffixes are
generally permitted on the command name but are not required. Thus, a
command like "perl" might exist in a file named "perl", "perl.exe", or
"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;
my $thisperl = $^X;
if ($^O ne 'VMS')
{$thisperl .= $Config{_exe} unless $thisperl =~ m/$Config{_exe}$/i;}

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

use Config;
my $thisperl = $Config{perlpath};
if ($^O ne 'VMS')
{$thisperl .= $Config{_exe} unless $thisperl =~ m/$Config{_exe}$/i;}

Networking
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 that for
things such as DHCP and NAT, the hostname you get back might not be very
useful.

All the above "don't":s may look daunting, and they are, but the key is
to degrade gracefully if one cannot reach the particular network service
one wants. Croaking or hanging do not look very professional.

Interprocess Communication (IPC)
In general, don't directly access the system in code meant to be
portable. That means, no "system", "exec", "fork", "pipe", ``, "qx//",
"open" with a "|", nor any of the other things that makes being a perl
hacker worth being.

Commands that launch external processes are generally supported on most
platforms (though many of them do not support any type of forking). The
problem with using them arises from what you invoke them on. External
tools are often named differently on different platforms, may not be
available in the same location, might accept different arguments, can
behave differently, and often present their results in a
platform-dependent way. Thus, you should seldom depend on them to
produce consistent results. (Then again, if you're calling *netstat -a*,
you probably don't expect it to run on both Unix and CP/M.)

One especially common bit of Perl code is opening a pipe to sendmail:

open(MAIL, '|/usr/lib/sendmail -t')
or die "cannot fork sendmail: $!";

This is fine for systems programming when sendmail is known to be
available. But it is not fine for many non-Unix systems, and even some
Unix systems that may not have sendmail installed. If a portable
solution is needed, see the various distributions on CPAN that deal with
it. Mail::Mailer and Mail::Send in the MailTools distribution are
commonly used, and provide several mailing methods, including mail,
sendmail, and direct SMTP (via Net::SMTP) if a mail transfer agent is
not available. Mail::Sendmail is a standalone module that provides
simple, platform-independent mailing.

The Unix System V IPC ("msg*(), sem*(), shm*()") is not available even
on all Unix 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)
XS code can usually be made to work with any platform, but dependent
libraries, header files, etc., might not be readily available or
portable, or the XS code itself might be platform-specific, just as Perl
code might be. If the libraries and headers are portable, then it is
normally reasonable to make sure the XS code is portable, too.

A different type of portability issue arises when writing XS code:
availability of a C compiler on the end-user's system. C brings with it
its own portability issues, and writing XS code will expose you to some
of those. Writing purely in Perl is an easier way to achieve
portability.

Standard Modules
In general, the standard modules work across platforms. Notable
exceptions are the CPAN module (which currently makes connections to
external programs that may not be available), platform-specific modules
(like ExtUtils::MM_VMS), and DBM modules.

There is no one DBM module available on all platforms. SDBM_File and the
others are generally available on all Unix and DOSish ports, but not in
MacPerl, where only NBDM_File and DB_File are available.

The good news is that at least some DBM module should be available, and
AnyDBM_File will use whichever module it can find. Of course, then the
code needs to be fairly strict, dropping to the greatest common factor
(e.g., not exceeding 1K for each record), so that it will work with any
DBM module. See AnyDBM_File for more details.

Time and Date
The system's notion of time of day and calendar date is controlled in
widely different ways. Don't assume the timezone is stored in $ENV{TZ},
and even if it is, don't assume that you can control the timezone
through that variable. Don't assume anything about the three-letter
timezone abbreviations (for example that MST would be the Mountain
Standard Time, it's been known to stand for Moscow Standard Time). If
you need to use timezones, express them in some unambiguous format like
the exact number of minutes offset from UTC, or the POSIX timezone
format.

Don't assume that the epoch starts at 00:00:00, January 1, 1970, because
that is OS- and implementation-specific. It is better to store a date in
an unambiguous representation. The ISO 8601 standard defines YYYY-MM-DD
as the date format, or YYYY-MM-DDTHH:MM:SS (that's a literal "T"
separating the date from the time). Please do use the ISO 8601 instead
of making us guess what date 02/03/04 might be. ISO 8601 even sorts
nicely as-is. A text representation (like "1987-12-18") can be easily
converted into an OS-specific value using a module like Date::Parse. An
array of values, such as those returned by "localtime", can be converted
to an OS-specific representation using Time::Local.

When calculating specific times, such as for tests in time or date
modules, it may be appropriate to calculate an offset for the epoch.

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

The value for $offset in Unix will be 0, but in Mac OS Classic will be
some large number. $offset can then be added to a Unix time value to get
what should be the proper value on any system.

Character sets and character encoding
Assume very little about character sets.

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

Internationalisation
If you may assume POSIX (a rather large assumption), you may read more
about the POSIX locale system from perllocale. The locale system at
least attempts to make things a little bit more portable, or at least
more convenient and native-friendly for non-English users. The system
affects character sets and encoding, and date and time
formatting--amongst other things.

If you really want to be international, you should consider Unicode. See
perluniintro and perlunicode for more information.

If you want to use non-ASCII bytes (outside the bytes 0x00..0x7f) in the
"source code" of your code, to be portable you have to be explicit about
what bytes they are. Someone might for example be using your code under
a UTF-8 locale, in which case random native bytes might be illegal
("Malformed UTF-8 ...") This means that for example embedding ISO 8859-1
bytes beyond 0x7f into your strings might cause trouble later. If the
bytes are native 8-bit bytes, you can use the "bytes" pragma. If the
bytes are in a string (regular expression being a curious string), you
can often also use the "\xHH" notation instead of embedding the bytes
as-is. (If you want to write your code in UTF-8, you can use the
"utf8".) The "bytes" and "utf8" pragmata are available since Perl 5.6.0.

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:

my @lines = <$very_large_file>; # bad

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

The last two constructs may appear unintuitive to most people. The first
repeatedly grows a string, whereas the second allocates a large chunk of
memory in one go. On some systems, the second is more efficient that the
first.

Security
Most multi-user platforms provide basic levels of security, usually
implemented at the filesystem level. Some, however, unfortunately do
not. Thus the notion of user id, or "home" directory, or even the state
of being logged-in, may be unrecognizable on many platforms. If you
write programs that are security-conscious, it is usually best to know
what type of system you will be running under so that you can write code
explicitly for that platform (or class of platforms).

Don't assume the Unix filesystem access semantics: the operating system
or the filesystem may be using some ACL systems, which are richer
languages than the usual rwx. Even if the rwx exist, their semantics
might be different.

(From security viewpoint testing for permissions before attempting to do
something is silly anyway: if one tries this, there is potential for
race conditions. Someone or something might change the permissions
between the permissions check and the actual operation. Just try the
operation.)

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

Don't assume set-uid and set-gid semantics. (And even if you do, think
twice: set-uid and set-gid are a known can of security worms.)

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

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

Also see:

* Mailing list: cpan-testers-discuss@perl.org

* Testing results: <http://www.cpantesters.org/>

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

uname $^O $Config{'archname'}
--------------------------------------------
AIX aix aix
BSD/OS bsdos i386-bsdos
Darwin darwin darwin
dgux dgux AViiON-dgux
DYNIX/ptx dynixptx i386-dynixptx
FreeBSD freebsd freebsd-i386
Haiku haiku BePC-haiku
Linux linux arm-linux
Linux linux i386-linux
Linux linux i586-linux
Linux linux ppc-linux
HP-UX hpux PA-RISC1.1
IRIX irix irix
Mac OS X darwin darwin
NeXT 3 next next-fat
NeXT 4 next OPENSTEP-Mach
openbsd openbsd i386-openbsd
OSF1 dec_osf alpha-dec_osf
reliantunix-n svr4 RM400-svr4
SCO_SV sco_sv i386-sco_sv
SINIX-N svr4 RM400-svr4
sn4609 unicos CRAY_C90-unicos
sn6521 unicosmk t3e-unicosmk
sn9617 unicos CRAY_J90-unicos
SunOS solaris sun4-solaris
SunOS solaris i86pc-solaris
SunOS4 sunos sun4-sunos

Because the value of $Config{archname} may depend on the hardware
architecture, it can vary more than the value of $^O.

DOS and Derivatives
Perl has long been ported to Intel-style microcomputers running under
systems like PC-DOS, MS-DOS, OS/2, and most Windows platforms you can
bring yourself to mention (except for Windows CE, if you count that).
Users familiar with *COMMAND.COM* or *CMD.EXE* style shells should be
aware that each of these file specifications may have subtle
differences:

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

System calls accept either "/" or "\" as the path separator. However,
many command-line utilities of DOS vintage treat "/" as the option
prefix, so may get confused by filenames containing "/". Aside from
calling any external programs, "/" will work just fine, and probably
better, as it is more consistent with popular usage, and avoids the
problem of remembering what to backwhack and what not to.

The DOS FAT filesystem can accommodate only "8.3" style filenames. Under
the "case-insensitive, but case-preserving" HPFS (OS/2) and NTFS (NT)
filesystems you may have to be careful about case returned with
functions like "readdir" or used with functions like "open" or
"opendir".

DOS also treats several filenames as special, such as AUX, PRN, NUL,
CON, COM1, LPT1, LPT2, etc. Unfortunately, sometimes these filenames
won't even work if you include an explicit directory prefix. It is best
to avoid such filenames, if you want your code to be portable to DOS and
its derivatives. It's hard to know what these all are, 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 ("\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 Vista MSWin32 MSWin32-x86 2 6 00
Windows 7 MSWin32 MSWin32-x86 2 6 01
Windows 7 MSWin32 MSWin32-x64 2 6 01
Windows 2008 MSWin32 MSWin32-x86 2 6 01
Windows 2008 MSWin32 MSWin32-x64 2 6 01
Windows CE MSWin32 ? 3
Cygwin cygwin cygwin

The various MSWin32 Perl's can distinguish the OS they are running on
via the value of the fifth element of the list returned from
Win32::GetOSVersion(). For example:

if ($^O eq 'MSWin32') {
my @os_version_info = Win32::GetOSVersion();
print +('3.1','95','NT')[$os_version_info[4]],"\n";
}

There are also Win32::IsWinNT() and Win32::IsWin95(), try "perldoc
Win32", and as of libwin32 0.19 (not part of the core Perl distribution)
Win32::GetOSName(). The very portable POSIX::uname() will work too:

c:\> perl -MPOSIX -we "print join '|', uname"
Windows NT|moonru|5.0|Build 2195 (Service Pack 2)|x86

Also see:

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

* The EMX environment for DOS, OS/2, etc. emx@iaehv.nl,
<ftp://hobbes.nmsu.edu/pub/os2/dev/emx/> Also perlos2.

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

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

* The ActiveState Pages, <http://www.activestate.com/>

* The Cygwin environment for Win32; README.cygwin (installed as
perlcygwin), <http://www.cygwin.com/>

* The U/WIN environment for Win32,
<http://www.research.att.com/sw/tools/uwin/>

* Build instructions for OS/2, perlos2

VMS
Perl on VMS is discussed in perlvms in the perl distribution.

The official name of VMS as of this writing is OpenVMS.

Perl on VMS can accept either VMS- or Unix-style file specifications as
in either of the 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.\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__"
#!/usr/bin/perl

print "Hello from Perl!\n";

__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>;".

The VMS operating system has two filesystems, known as ODS-2 and ODS-5.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

RMS had an eight level limit on directory depths from any rooted logical
(allowing 16 levels overall) prior to VMS 7.2, and even with versions of
VMS on VAX up through 7.3. Hence "PERL_ROOT:[LIB.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/".

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

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

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

What "\n" represents depends on the type of file opened. It usually
represents "\012" but it could also be "\015", "\012", "\015\012",
"\000", "\040", or nothing depending on the file organization and record
format. The VMS::Stdio module provides access to the special fopen()
requirements of files with unusual attributes on VMS.

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

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

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

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

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

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

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

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

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

Also see:

* README.vms (installed as README_vms), perlvms

* vmsperl list, vmsperl-subscribe@perl.org

* vmsperl on the web, <http://www.sidhe.org/vmsperl/index.html>

VOS
Perl on VOS (also known as OpenVOS) is discussed in README.vos in the
perl distribution (installed as perlvos). Perl on VOS can accept either
VOS- or Unix-style file specifications as in either of the following:

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

or even a mixture of both as in:

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

Even though VOS allows the slash character to appear in object names,
because the VOS port of Perl interprets it as a pathname delimiting
character, VOS files, directories, or links whose names contain a slash
character cannot be processed. Such files must be renamed before they
can be processed by Perl.

Older releases of VOS (prior to OpenVOS Release 17.0) limit file names
to 32 or fewer characters, prohibit file names from starting with a "-"
character, and prohibit file names from containing any character
matching "tr/ !#%&'()*;<=>?//".

Newer releases of VOS (OpenVOS Release 17.0 or later) support a feature
known as extended names. On these releases, file names can contain up to
255 characters, are prohibited from starting with a "-" character, and
the set of prohibited characters is reduced to any character matching
"tr/#%*<>?//". There are restrictions involving spaces and apostrophes:
these characters must not begin or end a name, nor can they immediately
precede or follow a period. Additionally, a space must not immediately
precede another space or hyphen. Specifically, the following character
combinations are prohibited: space-space, space-hyphen, period-space,
space-period, period-apostrophe, apostrophe-period, leading or trailing
space, and leading or trailing apostrophe. Although an extended file
name is limited to 255 characters, a path name is still limited to 256
characters.

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";
die;
}

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 newsgroup, or use the contact
information located in the distribution files on the Stratus
Anonymous FTP site.

* VOS Perl on the web at
<http://ftp.stratus.com/pub/vos/posix/posix.html>

EBCDIC Platforms
Recent versions of Perl have been ported to platforms such as OS/400 on
AS/400 minicomputers as well as OS/390, VM/ESA, and BS2000 for S/390
Mainframes. Such computers use EBCDIC character sets internally (usually
Character Code Set ID 0037 for OS/400 and either 1047 or POSIX-BC for
S/390 systems). On the mainframe perl currently works under the "Unix
system services for OS/390" (formerly known as OpenEdition), VM/ESA
OpenEdition, or the BS200 POSIX-BC system (BS2000 is supported in perl
5.6 and greater). See perlos390 for details. Note that for OS/400 there
is also a port of Perl 5.8.1/5.9.0 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:

BEGIN
CALL PGM(PERL5/PERL) PARM('/QOpenSys/hello.pl')
ENDPGM

This will invoke the perl script hello.pl in the root of the QOpenSys
file system. On the AS/400 calls to "system" or backticks must use CL
syntax.

On these platforms, bear in mind that the EBCDIC character set may have
an effect on what happens with some perl functions (such as "chr",
"pack", "print", "printf", "ord", "sort", "sprintf", "unpack"), as well
as bit-fiddling with ASCII constants using operators like "^", "&" and
"|", not to mention dealing with socket interfaces to ASCII computers
(see "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 "\005") { print "EBCDIC may be spoken here!\n"; }

if (ord('A') == 193) { print "EBCDIC may be spoken here!\n"; }

if (chr(169) eq 'z') { print "EBCDIC may be spoken here!\n"; }

One thing you may not want to rely on is the EBCDIC encoding of
punctuation characters since these may differ from code page to code
page (and once your module or script is rumoured to work with EBCDIC,
folks will want it to work with all EBCDIC character sets).

Also see:

* perlos390, README.os390, perlbs2000, README.vmesa, perlebcdic.

* The perl-mvs@perl.org 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 majordomo@perl.org.

* AS/400 Perl information at <http://as400.rochester.ibm.com/> as well
as on CPAN in the ports/ directory.

Acorn RISC OS
Because Acorns use ASCII with newlines ("\n") in text files as "\012"
like Unix, and because Unix filename emulation is turned on by default,
most simple scripts will probably work "out of the box". The native
filesystem is modular, and individual filesystems are free to be
case-sensitive or insensitive, and are usually case-preserving. Some
native filesystems have name length limits, which file and directory
names are silently truncated to fit. Scripts should be aware that the
standard filesystem currently has a name length limit of 10 characters,
with up to 77 items in a directory, but other filesystems may not impose
such limitations.

Native filenames are of the form

Filesystem#Special_Field::DiskName.$.Directory.Directory.File

where

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
"System$Path" until a name is made that points to an object on disk.
Writing to a new file "System:Modules" would be allowed only if
"System$Path" contains a single item list. The filesystem will also
expand system variables in filenames if enclosed in angle brackets, so
"<System$Dir>.Modules" would look for the file
"$ENV{'System$Dir'} . 'Modules'". The obvious implication of this is
that fully qualified filenames can start with "<>" and should be
protected when "open" is used for input.

Because "." was in use as a directory separator and filenames could not
be assumed to be unique after 10 characters, Acorn implemented the C
compiler to strip the trailing ".c" ".h" ".s" and ".o" suffix from
filenames specified in source code and store the respective files in
subdirectories named after the suffix. Hence files are translated:

foo.h h.foo
C:foo.h C:h.foo (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 foo.bar.h.baz, and that "readdir" and "glob"
cannot and do not attempt to emulate the reverse mapping. Other "."'s in
filenames are translated to "/".

As implied above, the environment accessed through %ENV is global, and
the convention is that program specific environment variables are of the
form "Program$Name". Each filesystem maintains a current directory, and
the current filesystem's current directory is the global current
directory. Consequently, sociable programs don't change the current
directory but rely on full pathnames, and programs (and Makefiles)
cannot assume that they can spawn a child process which can change the
current directory without affecting its parent (and everyone else for
that matter).

Because native operating system filehandles are global and are currently
allocated down from 255, with 0 being a reserved value, the Unix
emulation library emulates Unix filehandles. Consequently, you can't
rely on passing "STDIN", "STDOUT", or "STDERR" to your children.

The desire of users to express filenames of the form "<Foo$Dir>.Bar" on
the command line unquoted causes problems, too: `` command output
capture has to perform a guessing game. It assumes that a string
"<[^<>]+\$[^<>]>" is a reference to an environment variable, whereas
anything else involving "<" or ">" is redirection, and generally manages
to be 99% right. Of course, the problem remains that scripts cannot rely
on any Unix tools being available, or that any tools found have
Unix-like command line arguments.

Extensions and XS are, in theory, buildable by anyone using free tools.
In practice, many don't, as users of the Acorn platform are used to
binary distributions. MakeMaker does run, but no available make
currently copes with MakeMaker's makefiles; even if and when this should
be fixed, the lack of a Unix-like shell will cause problems with
makefile rules, especially lines of the form "cd sdbm && make all", and
anything using quoting.

"RISC OS" is the proper name for the operating system, but the value in
$^O is "riscos" (because we don't like shouting).

Other perls
Perl has been ported to many platforms that do not fit into any of the
categories listed above. Some, such as AmigaOS, 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).

* Be OS, README.beos

* HP 300 MPE/iX, README.mpeix and Mark Bixby's web page
<http://www.bixby.org/mark/porting.html>

* A free perl5-based PERL.NLM for Novell Netware is available in
precompiled binary and source code form from
<http://www.novell.com/> as well as from CPAN.

* Plan 9, README.plan9

FUNCTION IMPLEMENTATIONS
Listed below are functions that are either completely unimplemented or
else have been implemented differently on various platforms. Following
each description will be, in parentheses, a list of platforms that the
description applies to.

The list may well be incomplete, or even wrong in some places. When in
doubt, consult the platform-specific README files in the Perl source
distribution, and any other documentation resources accompanying a given
port.

Be aware, moreover, that even among Unix-ish systems there are
variations.

For many functions, you can also query %Config, exported by default from
the Config module. For example, to check whether the platform has the
"lstat" call, check $Config{d_lstat}. See Config for a full description
of available variables.

Alphabetical Listing of Perl Functions
-X "-w" only inspects the read-only file attribute
(FILE_ATTRIBUTE_READONLY), which determines whether the
directory can be deleted, not whether it can be written to.
Directories always have read and write access unless denied by
discretionary access control lists (DACLs). (Win32)

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

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

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

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

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

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

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

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

alarm Emulated using timers that must be explicitly polled whenever
Perl wants to dispatch "safe signals" and therefore cannot
interrupt blocking system calls. (Win32)

atan2 Due to issues with various CPUs, math libraries, compilers, and
standards, results for "atan2()" may vary depending on any
combination of the above. Perl attempts to conform to the Open
Group/IEEE standards for the results returned from "atan2()",
but cannot force the issue if the system Perl is run on does not
allow it. (Tru64, HP-UX 10.20)

The current version of the standards for "atan2()" is available
at
<http://www.opengroup.org/onlinepubs/009695399/functions/atan2.h
tml>.

binmode Meaningless. (RISC OS)

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

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

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

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

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

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

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

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

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

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

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

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

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

dump Not useful. (RISC OS)

Not supported. (Cygwin, Win32)

Invokes VMS debugger. (VMS)

exec Implemented via Spawn. (VM/ESA)

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

Not supported. (Symbian OS)

exit Emulates Unix exit() (which considers "exit 1" to indicate an
error) by mapping the 1 to SS$_ABORT (44). This behavior may be
overridden with the pragma "use vmsish 'exit'". As with the
CRTL's exit() function, "exit 0" is also mapped to an exit
status of SS$_NORMAL (1); this mapping cannot be overridden. Any
other argument to exit() is used directly as Perl's exit status.
On VMS, unless the future POSIX_EXIT mode is enabled, the exit
code should always be a valid VMS exit code and not a generic
number. When the POSIX_EXIT mode is enabled, a generic number
will be encoded in a method compatible with the C library
_POSIX_EXIT macro so that it can be decoded by other programs,
particularly ones written in C, like the GNV package. (VMS)

"exit()" resets file pointers, which is a problem when called
from a child process (created by "fork()") in "BEGIN". A
workaround is to use "POSIX::_exit". (Solaris)

exit unless $Config{archname} =~ /\bsolaris\b/;
require POSIX and POSIX::_exit(0);

fcntl Not implemented. (Win32)

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

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

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

Emulated using multiple interpreters. See perlfork. (Win32)

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

getlogin
Not implemented. (RISC OS)

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

getppid Not implemented. (Win32, RISC OS)

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

getpwnam
Not implemented. (Win32)

Not useful. (RISC OS)

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

getnetbyname
Not implemented. (Win32, Plan 9)

getpwuid
Not implemented. (Win32)

Not useful. (RISC OS)

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

getnetbyaddr
Not implemented. (Win32, Plan 9)

getprotobynumber
getservbyport
getpwent
Not implemented. (Win32, VM/ESA)

getgrent
Not implemented. (Win32, VMS, VM/ESA)

gethostbyname
"gethostbyname('localhost')" does not work everywhere: you may
have to use "gethostbyname('127.0.0.1')". (Irix 5)

gethostent
Not implemented. (Win32)

getnetent
Not implemented. (Win32, Plan 9)

getprotoent
Not implemented. (Win32, Plan 9)

getservent
Not implemented. (Win32, Plan 9)

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

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

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

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

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

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

endhostent
Not implemented. (Win32)

endnetent
Not implemented. (Win32, Plan 9)

endprotoent
Not implemented. (Win32, Plan 9)

endservent
Not implemented. (Plan 9, Win32)

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

glob This operator is implemented via the File::Glob extension on
most platforms. See File::Glob for portability information.

gmtime In theory, gmtime() is reliable from -2**63 to 2**63-1. However,
because work arounds in the implementation use floating point
numbers, it will become inaccurate as the time gets larger. This
is a bug and will be fixed in the future.

On VOS, time values are 32-bit quantities.

ioctl FILEHANDLE,FUNCTION,SCALAR
Not implemented. (VMS)

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

Available only for socket handles. (RISC OS)

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

"kill()" doesn't have the semantics of "raise()", i.e. it
doesn't send a signal to the identified process like it does on
Unix platforms. Instead "kill($sig, $pid)" terminates the
process identified by $pid, and makes it exit immediately with
exit status $sig. As in Unix, if $sig is 0 and the specified
process exists, it returns true without actually terminating it.
(Win32)

"kill(-9, $pid)" will terminate the process specified by $pid
and recursively all child processes owned by it. This is
different from the Unix semantics, where the signal will be
delivered to all processes in the same process group as the
process specified by $pid. (Win32)

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

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

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

Hard links are implemented on Win32 under NTFS only. They are
natively supported on Windows 2000 and later. On Windows NT they
are implemented using the Windows POSIX subsystem support and
the Perl process will need Administrator or Backup Operator
privileges to create hard links.

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

localtime
localtime() has the same range as "gmtime", but because time
zone rules change its accuracy for historical and future times
may degrade but usually by no more than an hour.

lstat Not implemented. (RISC OS)

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

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

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

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

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

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

rewinddir
Will not cause readdir() to re-read the directory stream. The
entries already read before the rewinddir() call will just be
returned again from a cache buffer. (Win32)

select Only implemented on sockets. (Win32, VMS)

Only reliable on sockets. (RISC OS)

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

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

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

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

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

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

setsockopt
Not implemented. (Plan 9)

shmctl
shmget
shmread
shmwrite
Not implemented. (Win32, VMS, RISC OS, VOS)

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

socketpair
Not implemented. (RISC OS, VM/ESA)

Available on OpenVOS Release 17.0 or later. (VOS)

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

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

ctime not supported on UFS (Mac OS X).

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

device and inode are not meaningful. (Win32)

device and inode are not necessarily reliable. (VMS)

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

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

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

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

symlink Not implemented. (Win32, RISC OS)

Implemented on 64 bit VMS 8.3. VMS requires the symbolic link to
be in Unix syntax if it is intended to resolve to a valid path.

syscall Not implemented. (Win32, VMS, RISC OS, VOS, 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 As an optimization, may not call the command shell specified in
$ENV{PERL5SHELL}. "system(1, @args)" spawns an external process
and immediately returns its process designator, without waiting
for it to terminate. Return value may be used subsequently in
"wait" or "waitpid". Failure to spawn() a subprocess is
indicated by setting $? to "255 << 8". $? is set in a way
compatible with Unix (i.e. the exitstatus of the subprocess is
obtained by "$? >> 8", as described in the documentation).
(Win32)

There is no shell to process metacharacters, and the native
standard is to pass a command line terminated by "\n" "\r" or
"\0" to the spawned program. Redirection such as "> foo" is
performed (if at all) by the run time library of the spawned
program. "system" *list* will call the Unix emulation library's
"exec" emulation, which attempts to provide emulation of the
stdin, stdout, stderr in force in the parent, providing the
child program uses a compatible version of the emulation
library. *scalar* will call the native command line direct and
no such emulation of a child Unix program will exists. Mileage
will vary. (RISC OS)

Does not automatically flush output handles on some platforms.
(SunOS, Solaris, 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'"). If the native condition code is one that
has a POSIX value encoded, the POSIX value will be decoded to
extract the expected exit value. For more details see "$?" in
perlvms. (VMS)

times "cumulative" times will be bogus. On anything other than Windows
NT or Windows 2000, "system" time will be bogus, and "user" time
is actually the time returned by the clock() function in the C
runtime library. (Win32)

Not useful. (RISC OS)

truncate
Not implemented. (Older versions of VMS)

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

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

umask Returns undef where unavailable, 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, 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)

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

Not useful. (RISC OS)

Supported Platforms
The following platforms are known to build Perl 5.12 (as of April 2010,
its release date) from the standard source code distribution available
at <http://www.cpan.org/src>

Linux (x86, ARM, IA64)
HP-UX
AIX
Win32

Windows 2000
Windows XP
Windows Server 2003
Windows Vista
Windows Server 2008
Windows 7

Cygwin
Solaris (x86, SPARC)
OpenVMS

Alpha (7.2 and later)
I64 (8.2 and later)

Symbian
NetBSD
FreeBSD
Debian GNU/kFreeBSD
Haiku
Irix (6.5. What else?)
OpenBSD
Dragonfly BSD
QNX Neutrino RTOS (6.5.0)
MirOS BSD
Caveats:

time_t issues that may or may not be fixed

Symbian (Series 60 v3, 3.2 and 5 - what else?)
Stratus VOS / OpenVOS
AIX

EOL Platforms (Perl 5.14)
The following platforms were supported by a previous version of Perl but
have been officially removed from Perl's source code as of 5.12:

Atari MiNT
Apollo Domain/OS
Apple Mac OS 8/9
Tenon Machten

The following platforms were supported up to 5.10. They may still have
worked in 5.12, but supporting code has been removed for 5.14:

Windows 95
Windows 98
Windows ME
Windows NT4

Supported Platforms (Perl 5.8)
As of July 2002 (the Perl release 5.8.0), the following platforms were
able to build Perl from the standard source code distribution available
at <http://www.cpan.org/src/>

AIX
BeOS
BSD/OS (BSDi)
Cygwin
DG/UX
DOS DJGPP 1)
DYNIX/ptx
EPOC R5
FreeBSD
HI-UXMPP (Hitachi) (5.8.0 worked but we didn't know it)
HP-UX
IRIX
Linux
Mac OS Classic
Mac OS X (Darwin)
MPE/iX
NetBSD
NetWare
NonStop-UX
ReliantUNIX (formerly SINIX)
OpenBSD
OpenVMS (formerly VMS)
Open UNIX (Unixware) (since Perl 5.8.1/5.9.0)
OS/2
OS/400 (using the PASE) (since Perl 5.8.1/5.9.0)
PowerUX
POSIX-BC (formerly BS2000)
QNX
Solaris
SunOS 4
SUPER-UX (NEC)
Tru64 UNIX (formerly DEC OSF/1, Digital UNIX)
UNICOS
UNICOS/mk
UTS
VOS
Win95/98/ME/2K/XP 2)
WinCE
z/OS (formerly OS/390)
VM/ESA

1) in DOS mode either the DOS or OS/2 ports can be used
2) compilers: Borland, MinGW (GCC), VC6

The following platforms worked with the previous releases (5.6 and 5.7),
but we did not manage either to fix or to test these in time for the
5.8.0 release. There is a very good chance that many of these will work
fine with the 5.8.0.

BSD/OS
DomainOS
Hurd
LynxOS
MachTen
PowerMAX
SCO SV
SVR4
Unixware
Windows 3.1

Known to be broken for 5.8.0 (but 5.6.1 and 5.7.2 can be used):

AmigaOS

The following platforms have been known to build Perl from source in the
past (5.005_03 and earlier), but we haven't been able to verify their
status for the current release, either because the hardware/software
platforms are rare or because we don't have an active champion on these
platforms--or both. They used to work, though, so go ahead and try
compiling them, and let perlbug@perl.org of any trouble.

3b1
A/UX
ConvexOS
CX/UX
DC/OSx
DDE SMES
DOS EMX
Dynix
EP/IX
ESIX
FPS
GENIX
Greenhills
ISC
MachTen 68k
MPC
NEWS-OS
NextSTEP
OpenSTEP
Opus
Plan 9
RISC/os
SCO ODT/OSR
Stellar
SVR2
TI1500
TitanOS
Ultrix
Unisys Dynix

The following platforms have their own source code distributions and
binaries available via <http://www.cpan.org/ports/>

Perl release

OS/400 (ILE) 5.005_02
Tandem Guardian 5.004

The following platforms have only binaries available via
<http://www.cpan.org/ports/index.html> :

Perl release

Acorn RISCOS 5.005_02
AOS 5.002
LynxOS 5.004_02

Although we do suggest that you always build your own Perl from the
source code, both for maximal configurability and for security, in case
you are in a hurry you can check <http://www.cpan.org/ports/index.html>
for binary distributions.

SEE ALSO
perlaix, perlamiga, perlbeos, perlbs2000, perlce, perlcygwin, perldgux,
perldos, perlepoc, perlebcdic, perlfreebsd, perlhurd, perlhpux,
perlirix, perlmacos, perlmacosx, perlmpeix, perlnetware, perlos2,
perlos390, perlos400, perlplan9, perlqnx, perlsolaris, perltru64,
perlunicode, perlvmesa, perlvms, perlvos, perlwin32, and Win32.

AUTHORS / CONTRIBUTORS
Abigail <abigail@foad.org>, Charles Bailey <bailey@newman.upenn.edu>,
Graham Barr <gbarr@pobox.com>, Tom Christiansen <tchrist@perl.com>,
Nicholas Clark <nick@ccl4.org>, Thomas Dorner <Thomas.Dorner@start.de>,
Andy Dougherty <doughera@lafayette.edu>, Dominic Dunlop
<domo@computer.org>, Neale Ferguson <neale@vma.tabnsw.com.au>, David J.
Fiander <davidf@mks.com>, Paul Green <Paul.Green@stratus.com>, M.J.T.
Guy <mjtg@cam.ac.uk>, Jarkko Hietaniemi <jhi@iki.fi>, Luther Huffman
<lutherh@stratcom.com>, Nick Ing-Simmons <nick@ing-simmons.net>, Andreas
J. K�nig <a.koenig@mind.de>, Markus Laker <mlaker@contax.co.uk>, Andrew
M. Langmead <aml@world.std.com>, Larry Moore <ljmoore@freespace.net>,
Paul Moore <Paul.Moore@uk.origin-it.com>, Chris Nandor
<pudge@pobox.com>, Matthias Neeracher <neeracher@mac.com>, Philip Newton
<pne@cpan.org>, Gary Ng <71564.1743@CompuServe.COM>, Tom Phoenix
<rootbeer@teleport.com>, Andr� Pirard <A.Pirard@ulg.ac.be>, Peter
Prymmer <pvhp@forte.com>, Hugo van der Sanden <hv@crypt0.demon.co.uk>,
Gurusamy Sarathy <gsar@activestate.com>, Paul J. Schinder
<schinder@pobox.com>, Michael G Schwern <schwern@pobox.com>, Dan
Sugalski <dan@sidhe.org>, Nathan Torkington <gnat@frii.com>, John
Malmberg <wb8tyw@qsl.net>

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