perlport - Online in the Cloud

PROGRAM:

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 has 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", Mac OS uses "\015", and z/OS uses "\025".

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". On EBCDIC platforms, "\n" could
be "\025" or "\045". 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
safely.

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>) { # NOT ADVISABLE!
# ...
}

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

As of Perl 5.10.0, you can also use the ">" and "<" modifiers to force big- or little-
endian byte-order. This is useful if you want to store signed integers or 64-bit
integers, for example.

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

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

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

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

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

One can circumnavigate both these problems in two ways. Either transfer and store numbers
always in text format, instead of raw binary, or else consider using modules like
"Data::Dumper" and "Storable" (included as of Perl 5.8). Keeping all data as text
significantly simplifies matters.

The v-strings are portable only up to v2147483647 (0x7FFF_FFFF), 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 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{PATH},
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 are documented.

On VMS by default, changes to the %ENV hash persist after perl exits. Subsequent
invocations of perl in the same process can inadvertently inherit environment settings
that were meant to be temporary.

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 string 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 exposes 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
"NDBM_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)". However, starting in Perl v5.22, regular expression
pattern bracketed character class ranges specified like "qr/[\N{U+HH}-\N{U+HH}]/" are
portable. You can portably use symbolic character classes like "[:print:]".

Do not assume that the alphabetic characters are encoded contiguously (in the numeric
sense). There may be gaps. Special coding in Perl, however, guarantees that all subsets
of "qr/[A-Z]/", "qr/[a-z]/", and "qr/[0-9]/" behave as expected. "tr///" behaves the same
for these ranges. In patterns, any ranges specified with end points using the "\N{...}"
notations ensures character set portability, but it is a bug in Perl v5.22, that this
isn't true of "tr///".

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". Unicode::Collate can be used to sort this
all out.

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
expressions being curious strings), you can often also use the "\xHH" or more portably,
the "\N{U+HH}" notations instead of embedding the bytes as-is. If you want to write your
code in UTF-8, you can use utf8.

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 than 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 the 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 $< and $> (or $(
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

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
DYNIX/ptx dynixptx i386-dynixptx
FreeBSD freebsd freebsd-i386
Haiku haiku BePC-haiku
Linux linux arm-linux
Linux linux armv5tel-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.

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, designated by their on-disk structure (ODS)
level: ODS-2 and its successor ODS-5. The initial port of Perl to VMS pre-dates ODS-5,
but all current testing and development assumes ODS-5 and its capabilities, including case
preservation, extended characters in filespecs, and names up to 8192 bytes long.

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

In general, the easiest path to portability is always to specify filenames in Unix format
unless they will need to be processed by native commands or utilities. Because of this
latter consideration, the File::Spec module by default returns native format
specifications regardless of input format. This default may be reversed so that filenames
are always reported in Unix format by specifying the "DECC$FILENAME_UNIX_REPORT" feature
logical in the environment.

The file type, or extension, is always present in a VMS-format file specification even if
it's zero-length. This means that, by default, "readdir" will return a trailing dot on a
file with no extension, so where you would see "a" on Unix you'll see "a." on VMS.
However, the trailing dot may be suppressed by enabling the "DECC$READDIR_DROPDOTNOTYPE"
feature in the environment (see the CRTL documentation on feature logical names).

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.

The value of $^O on OpenVMS is "VMS". To determine the architecture that you are running
on refer to $Config{'archname'}.

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>

· VMS Software Inc. web site, <http://www.vmssoftware.com>

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 contact the Stratus
Technologies Customer Assistance Center (CAC) for your region, or you can use the
contact information located in the distribution files on the Stratus Anonymous FTP
site.

· Stratus Technologies on the web at <http://www.stratus.com>

· VOS Open-Source Software on the web at <http://ftp.stratus.com/pub/vos/vos.html>

EBCDIC Platforms
v5.22 core Perl runs on z/OS (formerly OS/390). Theoretically it could run on the
successors of OS/400 on AS/400 minicomputers as well as 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).

The rest of this section may need updating, but we don't know what it should say. Please
email comments to perlbug@perl.org <mailto:perlbug@perl.org>.

On the mainframe Perl currently works under the "Unix system services for OS/390"
(formerly known as OpenEdition), VM/ESA OpenEdition, or the BS200 POSIX-BC system (BS2000
is supported in Perl 5.6 and greater). See perlos390 for details. Note that for OS/400
there is also a port of Perl 5.8.1/5.10.0 or later to the PASE which is ASCII-based (as
opposed to ILE which is EBCDIC-based), see perlos400.

As of R2.5 of USS for OS/390 and Version 2.3 of VM/ESA these Unix sub-systems do not
support the "#!" shebang trick for script invocation. Hence, on OS/390 and VM/ESA Perl
scripts can be executed with a header similar to the following simple script:

: # use perl
eval 'exec /usr/local/bin/perl -S $0 ${1+"$@"}'
if 0;
#!/usr/local/bin/perl # just a comment really

print "Hello from perl!\n";

OS/390 will support the "#!" shebang trick in release 2.8 and beyond. Calls to "system"
and backticks can use POSIX shell syntax on all S/390 systems.

On the AS/400, if PERL5 is in your library list, you may need to wrap your Perl scripts in
a CL procedure to invoke them like so:

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 z/OS):

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

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

uname $^O $Config{'archname'}
--------------------------------------------
OS/390 os390 os390
OS400 os400 os400
POSIX-BC posix-bc BS2000-posix-bc

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

if ("\t" eq "\005") { print "EBCDIC may be spoken here!\n"; }

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

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

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

Also see:

· perlos390, perlos400, perlbs2000, 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, 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

See also:

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

· A free perl5-based PERL.NLM for Novell Netware is available in precompiled binary and
source code form from <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.html>.

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)

Setting the exec bit on some locations (generally /sdcard) will return true but
not actually set the bit. (Android)

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

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

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

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

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

Not implemented. (Android)

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 "exec LIST" without the use of indirect object syntax ("exec PROGRAM LIST") may
fall back to trying the shell if the first "spawn()" fails. (Win32)

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, 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)

getpwnam
Not implemented. (Win32)

Not useful. (RISC OS)

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

getnetbyname
Not implemented. (Android, Win32, Plan 9)

getpwuid
Not implemented. (Win32)

Not useful. (RISC OS)

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

getnetbyaddr
Not implemented. (Android, Win32, Plan 9)

getprotobynumber
Not implemented. (Android)

getservbyport
getpwent
Not implemented. (Android, Win32)

getgrent
Not implemented. (Android, Win32, VMS)

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. (Android, Win32, Plan 9)

getprotoent
Not implemented. (Android, Win32, Plan 9)

getservent
Not implemented. (Win32, Plan 9)

seekdir Not implemented. (Android)

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

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

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

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

endpwent
Not implemented. (Win32)

Either not implemented or a no-op. (Android)

endgrent
Not implemented. (Android, RISC OS, VMS, Win32)

endhostent
Not implemented. (Android, Win32)

endnetent
Not implemented. (Android, Win32, Plan 9)

endprotoent
Not implemented. (Android, 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. (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. (Android, 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. (Android, Win32, VMS, RISC OS)

setgrent
Not implemented. (Android, VMS, Win32, RISC OS)

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

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

setpwent
Not implemented. (Android, Win32, RISC OS)

setsockopt
Not implemented. (Plan 9)

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

sleep Emulated using synchronization functions such that it can be interrupted by
"alarm()", and limited to a maximum of 4294967 seconds, approximately 49 days.
(Win32)

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

socketpair
Not implemented. (RISC OS)

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

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

sysopen The traditional "0", "1", and "2" MODEs are implemented with different numeric
values on some systems. The flags exported by "Fcntl" (O_RDONLY, O_WRONLY,
O_RDWR) should work everywhere though. (Mac OS, OS/390)

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

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

"system LIST" without the use of indirect object syntax ("system PROGRAM LIST")
may fall back to trying the shell if the first "spawn()" fails. (Win32)

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)

telldir Not implemented. (Android)

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.

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

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

May not behave as expected. Behavior depends on the C runtime library's
implementation of "utime()", and the filesystem being used. The FAT filesystem
typically does not support an "access time" field, and it may limit timestamps to
a granularity of two seconds. (Win32)

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
Some tests are known to fail:

· ext/XS-APItes/t/call_checker.t - see
<https://rt.perl.org/Ticket/Display.html?id=78502>

· dist/I18N-Collate/t/I18N-Collate.t

· ext/Win32CORE/t/win32core.t - may fail on recent cygwin installs.

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
Midnight BSD
QNX Neutrino RTOS (6.5.0)
MirOS BSD
Stratus OpenVOS (17.0 or later)
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
Android
FreeMINT
Perl now builds with FreeMiNT/Atari. It fails a few tests, that needs some
investigation.

The FreeMiNT port uses GNU dld for loadable module capabilities. So ensure you have
that library installed when building perl.

EOL Platforms

(Perl 5.20)
The following platforms were supported by a previous version of Perl but have been
officially removed from Perl's source code as of 5.20:

AT&T 3b1

(Perl 5.14)
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

(Perl 5.12)
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

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 / OpenVOS
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.

Use perlport online using onworks.net services