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perlport.pod
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=head1 NAME
perlport - Writing portable Perl
=head1 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:
=over 4
=item 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.
=item Nearly all of Perl already I<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.
=back
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 (L</"ISSUES">), platform-specific issues (L</"PLATFORMS">), and
built-in Perl functions that behave differently on various ports
(L</"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
(C<< <IMG SRC="yellow_sign.gif" ALT="Under Construction"> >>).
=head1 ISSUES
=head2 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 C<\012>, one type of DOSish I/O uses C<\015\012>,
S<Mac OS> uses C<\015>, and z/OS uses C<\025>.
Perl uses C<\n> to represent the "logical" newline, where what is
logical may depend on the platform in use. In MacPerl, C<\n> always
means C<\015>. On EBCDIC platforms, C<\n> could be C<\025> or C<\045>.
In DOSish perls, C<\n> usually means C<\012>, but when
accessing a file in "text" mode, perl uses the C<:crlf> layer that
translates it to (or from) C<\015\012>, depending on whether you're
reading or writing. Unix does the same thing on ttys in canonical
mode. C<\015\012> is commonly referred to as CRLF.
To trim trailing newlines from text lines use
L<C<chomp>|perlfunc/chomp VARIABLE>. With default settings that function
looks for a trailing C<\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 L<C<$E<sol>>|perlvar/$E<sol>> to the appropriate value for
your file format before using L<C<chomp>|perlfunc/chomp VARIABLE>.
Because of the "text" mode translation, DOSish perls have limitations in
using L<C<seek>|perlfunc/seek FILEHANDLE,POSITION,WHENCE> and
L<C<tell>|perlfunc/tell FILEHANDLE> on a file accessed in "text" mode.
Stick to L<C<seek>|perlfunc/seek FILEHANDLE,POSITION,WHENCE>-ing to
locations you got from L<C<tell>|perlfunc/tell FILEHANDLE> (and no
others), and you are usually free to use
L<C<seek>|perlfunc/seek FILEHANDLE,POSITION,WHENCE> and
L<C<tell>|perlfunc/tell FILEHANDLE> even in "text" mode. Using
L<C<seek>|perlfunc/seek FILEHANDLE,POSITION,WHENCE> or
L<C<tell>|perlfunc/tell FILEHANDLE> or other file operations may be
non-portable. If you use L<C<binmode>|perlfunc/binmode FILEHANDLE> on a
file, however, you can usually
L<C<seek>|perlfunc/seek FILEHANDLE,POSITION,WHENCE> and
L<C<tell>|perlfunc/tell FILEHANDLE> with arbitrary values safely.
A common misconception in socket programming is that S<C<\n eq \012>>
everywhere. When using protocols such as common Internet protocols,
C<\012> and C<\015> are called for specifically, and the values of
the logical C<\n> and C<\r> (carriage return) are not reliable.
print $socket "Hi there, client!\r\n"; # WRONG
print $socket "Hi there, client!\015\012"; # RIGHT
However, using C<\015\012> (or C<\cM\cJ>, or C<\x0D\x0A>) can be tedious
and unsightly, as well as confusing to those maintaining the code. As
such, the L<C<Socket>|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 L<C<$E<sol>>|perlvar/$E<sol>> is C<\n>, but robust socket code
will recognize as either C<\012> or C<\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 C<\015>'s (C<\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 C<\n> and C<\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
=head2 Numbers endianness and Width
Different CPUs store integers and floating point numbers in different
orders (called I<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 L<C<pack>|perlfunc/pack TEMPLATE,LIST> and
L<C<unpack>|perlfunc/unpack TEMPLATE,EXPR> formats C<n> and C<N>, the
"network" orders. These are guaranteed to be portable.
As of Perl 5.10.0, you can also use the C<E<gt>> and C<E<lt>> 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
L<C<Data::Dumper>|Data::Dumper> and L<C<Storable>|Storable> (included as
of Perl 5.8). Keeping all data as text significantly simplifies matters.
=head2 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, S<Mac OS>, OS/2, VMS, VOS, S<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 C</>
as path separator, or in their own idiosyncratic ways (such as having
several root directories and various "unrooted" device files such NIL:
and LPT:).
S<Mac OS> 9 and earlier used C<:> as a path separator instead of C</>.
The filesystem may support neither hard links
(L<C<link>|perlfunc/link OLDFILE,NEWFILE>) nor symbolic links
(L<C<symlink>|perlfunc/symlink OLDFILE,NEWFILE>,
L<C<readlink>|perlfunc/readlink EXPR>,
L<C<lstat>|perlfunc/lstat FILEHANDLE>).
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 L<C<-C>|perlfunc/-X FILEHANDLE>
filetest) may really be the "creation timestamp" (which it is not in
Unix).
VOS perl can emulate Unix filenames with C</> as path separator. The
native pathname characters greater-than, less-than, number-sign, and
percent-sign are always accepted.
S<RISC OS> perl can emulate Unix filenames with C</> as path
separator, or go native and use C<.> for path separator and C<:> 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 C<r>, C<w>, and C<x> mean on
a directory) are the Unix ones. The various Unix/POSIX compatibility
layers usually try to make interfaces like L<C<chmod>|perlfunc/chmod LIST>
work, but sometimes there simply is no good mapping.
The L<C<File::Spec>|File::Spec> modules provide methods to manipulate path
specifications and return the results in native format for each
platform. This is often unnecessary as Unix-style paths are
understood by Perl on every supported platform, but if you need to
produce native paths for a native utility that does not understand
Unix syntax, or if you are operating on paths or path components
in unknown (and thus possibly native) syntax, L<C<File::Spec>|File::Spec>
is your friend. Here are two brief examples:
use File::Spec::Functions;
chdir(updir()); # go up one directory
# Concatenate a path from its components
my $file = catfile(updir(), 'temp', 'file.txt');
# on Unix: '../temp/file.txt'
# on Win32: '..\temp\file.txt'
# on VMS: '[-.temp]file.txt'
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 C</> as a path separator for subdirectories.
Also of use is L<C<File::Basename>|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 F</etc/passwd>,
F</etc/sendmail.conf>, F</etc/resolv.conf>, or even F</tmp/>. For
example, F</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 F<test.pl> and F<Test.pl>, as many platforms have
case-insensitive (or at least case-forgiving) filenames. Also, try
not to have non-word characters (except for C<.>) 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 L<C<AutoSplit>|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 C<.> in their
filenames.
Don't assume C<< > >> won't be the first character of a filename.
Always use the three-arg version of
L<C<open>|perlfunc/open FILEHANDLE,MODE,EXPR>:
open my $fh, '<', $existing_file) or die $!;
Two-arg L<C<open>|perlfunc/open FILEHANDLE,MODE,EXPR> is magic and can
translate characters like C<< > >>, C<< < >>, and C<|> in filenames,
which is usually the wrong thing to do.
L<C<sysopen>|perlfunc/sysopen FILEHANDLE,FILENAME,MODE> and three-arg
L<C<open>|perlfunc/open FILEHANDLE,MODE,EXPR> don't have this problem.
Don't use C<:> 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 C<@>, C<;> and
C<|>.
Don't assume that in pathnames you can collapse two leading slashes
C<//> into one: some networking and clustering filesystems have special
semantics for that. Let the operating system sort it out.
The I<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 s 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 S T U V W X Y Z
0 1 2 3 4 5 6 7 8 9
. _ -
and C<-> 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 C<.>, if any, and to three characters after the
C<.>, if any). (And do not use C<.>s in directory names.)
On Windows extra C<.>s at the end of a file or directory name are
ignored in most circumstances, and a directory name containing only
three or more C<.>s are treated as the current directory by some APIs.
=head2 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
L<C<close>|perlfunc/close FILEHANDLE> files when you are done with them.
Don't L<C<unlink>|perlfunc/unlink LIST> or
L<C<rename>|perlfunc/rename OLDNAME,NEWNAME> an open file. Don't
L<C<tie>|perlfunc/tie VARIABLE,CLASSNAME,LIST> or
L<C<open>|perlfunc/open FILEHANDLE,MODE,EXPR> a file already tied or opened;
L<C<untie>|perlfunc/untie VARIABLE> or
L<C<close>|perlfunc/close FILEHANDLE> 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 L<C<unlink>|perlfunc/unlink LIST> completely
gets rid of the file: some filesystems (most notably the ones in VMS) have
versioned filesystems, and L<C<unlink>|perlfunc/unlink LIST> 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 undeletable for some reason
(protected, not there, and so on).
Don't count on a specific environment variable existing in
L<C<%ENV>|perlvar/%ENV>. Don't count on L<C<%ENV>|perlvar/%ENV> entries
being case-sensitive, or even case-preserving. Don't try to clear
L<C<%ENV>|perlvar/%ENV> by saying C<%ENV = ();>, or, if you really have
to, make it conditional on C<$^O ne 'VMS'> since in VMS the
L<C<%ENV>|perlvar/%ENV> table is much more than a per-process key-value
string table.
On VMS, some entries in the L<C<%ENV>|perlvar/%ENV> hash are dynamically
created when their key is used on a read if they did not previously
exist. The values for C<$ENV{HOME}>, C<$ENV{TERM}>, C<$ENV{PATH}>, and
C<$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 L<C<%ENV>|perlvar/%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 L<C<%SIG>|perlvar/%SIG> for anything.
Don't count on filename globbing. Use
L<C<opendir>|perlfunc/opendir DIRHANDLE,EXPR>,
L<C<readdir>|perlfunc/readdir DIRHANDLE>, and
L<C<closedir>|perlfunc/closedir DIRHANDLE> instead.
Don't count on per-program environment variables, or per-program current
directories.
Don't count on specific values of L<C<$!>|perlvar/$!>, 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 L<C<Errno>|Errno> module, like C<ENOENT>. And don't trust on the
values of L<C<$!>|perlvar/$!> at all except immediately after a failed
system call.
=head2 Command names versus file pathnames
Don't assume that the name used to invoke a command or program with
L<C<system>|perlfunc/system LIST> or L<C<exec>|perlfunc/exec LIST> 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,
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 C<perl> might exist in a file named
F<perl>, F<perl.exe>, or F<perl.pm>, depending on the operating system.
The variable L<C<$Config{_exe}>|Config/C<_exe>> in the
L<C<Config>|Config> module holds the executable suffix, if any. Third,
the VMS port carefully sets up L<C<$^X>|perlvar/$^X> and
L<C<$Config{perlpath}>|Config/C<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 L<C<$^X>|perlvar/$^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 $thisperl =~ m/\Q$Config{_exe}\E$/i;
}
To convert L<C<$Config{perlpath}>|Config/C<perlpath>> to a file pathname, say:
use Config;
my $thisperl = $Config{perlpath};
if ($^O ne 'VMS') {
$thisperl .= $Config{_exe}
unless $thisperl =~ m/\Q$Config{_exe}\E$/i;
}
=head2 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
L<C<ioctl>|perlfunc/ioctl FILEHANDLE,FUNCTION,SCALAR>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 L<C<Sys::Hostname>|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 I<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.
=head2 Interprocess Communication (IPC)
In general, don't directly access the system in code meant to be
portable. That means, no L<C<system>|perlfunc/system LIST>,
L<C<exec>|perlfunc/exec LIST>, L<C<fork>|perlfunc/fork>,
L<C<pipe>|perlfunc/pipe READHANDLE,WRITEHANDLE>,
L<C<``> or C<qxE<sol>E<sol>>|perlop/C<qxE<sol>I<STRING>E<sol>>>,
L<C<open>|perlfunc/open FILEHANDLE,MODE,EXPR> with a C<|>, 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
C<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 B<sendmail>:
open(my $mail, '|-', '/usr/lib/sendmail -t')
or die "cannot fork sendmail: $!";
This is fine for systems programming when sendmail is known to be
available. But it is not fine for many non-Unix systems, and even
some Unix systems that may not have sendmail installed. If a portable
solution is needed, see the various distributions on CPAN that deal
with it. L<C<Mail::Mailer>|Mail::Mailer> and L<C<Mail::Send>|Mail::Send>
in the C<MailTools> distribution are commonly used, and provide several
mailing methods, including C<mail>, C<sendmail>, and direct SMTP (via
L<C<Net::SMTP>|Net::SMTP>) if a mail transfer agent is not available.
L<C<Mail::Sendmail>|Mail::Sendmail> is a standalone module that provides
simple, platform-independent mailing.
The Unix System V IPC (C<msg*(), sem*(), shm*()>) is not available
even on all Unix platforms.
Do not use either the bare result of C<pack("N", 10, 20, 30, 40)> or
bare v-strings (such as C<v10.20.30.40>) to represent IPv4 addresses:
both forms just pack the four bytes into network order. That this
would be equal to the C language C<in_addr> struct (which is what the
socket code internally uses) is not guaranteed. To be portable use
the routines of the L<C<Socket>|Socket> module, such as
L<C<inet_aton>|Socket/$ip_address = inet_aton $string>,
L<C<inet_ntoa>|Socket/$string = inet_ntoa $ip_address>, and
L<C<sockaddr_in>|Socket/$sockaddr = sockaddr_in $port, $ip_address>.
The rule of thumb for portable code is: Do it all in portable Perl, or
use a module (that may internally implement it with platform-specific
code, but exposes a common interface).
=head2 External Subroutines (XS)
XS code can usually be made to work with any platform, but dependent
libraries, header files, etc., might not be readily available or
portable, or the XS code itself might be platform-specific, just as Perl
code might be. If the libraries and headers are portable, then it is
normally reasonable to make sure the XS code is portable, too.
A different type of portability issue arises when writing XS code:
availability of a C compiler on the end-user's system. C brings
with it its own portability issues, and writing XS code will expose
you to some of those. Writing purely in Perl is an easier way to
achieve portability.
=head2 Standard Modules
In general, the standard modules work across platforms. Notable
exceptions are the L<C<CPAN>|CPAN> module (which currently makes
connections to external programs that may not be available),
platform-specific modules (like L<C<ExtUtils::MM_VMS>|ExtUtils::MM_VMS>),
and DBM modules.
There is no one DBM module available on all platforms.
L<C<SDBM_File>|SDBM_File> and the others are generally available on all
Unix and DOSish ports, but not in MacPerl, where only
L<C<NDBM_File>|NDBM_File> and L<C<DB_File>|DB_File> are available.
The good news is that at least some DBM module should be available, and
L<C<AnyDBM_File>|AnyDBM_File> will use whichever module it can find. Of
course, then the code needs to be fairly strict, dropping to the greatest
common factor (e.g., not exceeding 1K for each record), so that it will
work with any DBM module. See L<AnyDBM_File> for more details.
=head2 Time and Date
The system's notion of time of day and calendar date is controlled in
widely different ways. Don't assume the timezone is stored in C<$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
L<C<Time::Piece>|Time::Piece> (see L<Time::Piece/Date Parsing>) or
L<C<Date::Parse>|Date::Parse>. An array of values, such as those
returned by L<C<localtime>|perlfunc/localtime EXPR>, can be converted to an OS-specific
representation using L<C<Time::Local>|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.
use Time::Local qw(timegm);
my $offset = timegm(0, 0, 0, 1, 0, 1970);
The value for C<$offset> in Unix will be C<0>, but in Mac OS Classic
will be some large number. C<$offset> can then be added to a Unix time
value to get what should be the proper value on any system.
=head2 Character sets and character encoding
Assume very little about character sets.
Assume nothing about numerical values (L<C<ord>|perlfunc/ord EXPR>,
L<C<chr>|perlfunc/chr NUMBER>) of characters.
Do not use explicit code point ranges (like C<\xHH-\xHH)>. However,
starting in Perl v5.22, regular expression pattern bracketed character
class ranges specified like C<qr/[\N{U+HH}-\N{U+HH}]/> are portable,
and starting in Perl v5.24, the same ranges are portable in
L<C<trE<sol>E<sol>E<sol>>|perlop/C<trE<sol>I<SEARCHLIST>E<sol>I<REPLACEMENTLIST>E<sol>cdsr>>.
You can portably use symbolic character classes like C<[: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 C<qr/[A-Z]/>, C<qr/[a-z]/>, and
C<qr/[0-9]/> behave as expected.
L<C<trE<sol>E<sol>E<sol>>|perlop/C<trE<sol>I<SEARCHLIST>E<sol>I<REPLACEMENTLIST>E<sol>cdsr>>
behaves the same for these ranges. In patterns, any ranges specified with
end points using the C<\N{...}> notations ensures character set
portability, but it is a bug in Perl v5.22 that this isn't true of
L<C<trE<sol>E<sol>E<sol>>|perlop/C<trE<sol>I<SEARCHLIST>E<sol>I<REPLACEMENTLIST>E<sol>cdsr>>,
fixed in v5.24.
Do not assume anything about the ordering of the characters.
The lowercase letters may come before or after the uppercase letters;
the lowercase and uppercase may be interlaced so that both "a" and "A"
come before "b"; the accented and other international characters may
be interlaced so that E<auml> comes before "b".
L<Unicode::Collate> can be used to sort this all out.
=head2 Internationalisation
If you may assume POSIX (a rather large assumption), you may read
more about the POSIX locale system from L<perllocale>. The locale
system at least attempts to make things a little bit more portable,
or at least more convenient and native-friendly for non-English
users. The system affects character sets and encoding, and date
and time formatting--amongst other things.
If you really want to be international, you should consider Unicode.
See L<perluniintro> and L<perlunicode> for more information.
By default Perl assumes your source code is written in an 8-bit ASCII
superset. To embed Unicode characters in your strings and regexes, you can
use the L<C<\x{HH}> or (more portably) C<\N{U+HH}>
notations|perlop/Quote and Quote-like Operators>. You can also use the
L<C<utf8>|utf8> pragma and write your code in UTF-8, which lets you use
Unicode characters directly (not just in quoted constructs but also in
identifiers).
=head2 System Resources
If your code is destined for systems with severely constrained (or
missing!) virtual memory systems then you want to be I<especially> mindful
of avoiding wasteful constructs such as:
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.
=head2 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 C<rwx>. Even if the C<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 L<C<< $< >>|perlvar/$E<lt>> and L<C<< $> >>|perlvar/$E<gt>> (or
L<C<$(>|perlvar/$(> and L<C<$)>|perlvar/$)>) to work for switching
identities (or memberships).
Don't assume set-uid and set-gid semantics. (And even if you do,
think twice: set-uid and set-gid are a known can of security worms.)
=head2 Style
For those times when it is necessary to have platform-specific code,
consider keeping the platform-specific code in one place, making porting
to other platforms easier. Use the L<C<Config>|Config> module and the
special variable L<C<$^O>|perlvar/$^O> to differentiate platforms, as
described in L</"PLATFORMS">.
Beware of the "else syndrome":
if ($^O eq 'MSWin32') {
# code that assumes Windows
} else {
# code that assumes Linux
}
The C<else> branch should be used for the really ultimate fallback,
not for code specific to some platform.
Be careful in the tests you supply with your module or programs.
Module code may be fully portable, but its tests might not be. This
often happens when tests spawn off other processes or call external
programs to aid in the testing, or when (as noted above) the tests
assume certain things about the filesystem and paths. Be careful not
to depend on a specific output style for errors, such as when checking
L<C<$!>|perlvar/$!> after a failed system call. Using
L<C<$!>|perlvar/$!> for anything else than displaying it as output is
doubtful (though see the L<C<Errno>|Errno> module for testing reasonably
portably for error value). Some platforms expect a certain output format,
and Perl on those platforms may have been adjusted accordingly. Most
specifically, don't anchor a regex when testing an error value.
=head1 CPAN Testers
Modules uploaded to CPAN are tested by a variety of volunteers on
different platforms. These CPAN testers are notified by mail of each
new upload, and reply to the list with PASS, FAIL, NA (not applicable to
this platform), or UNKNOWN (unknown), along with any relevant notations.
The purpose of the testing is twofold: one, to help developers fix any
problems in their code that crop up because of lack of testing on other
platforms; two, to provide users with information about whether
a given module works on a given platform.
Also see:
=over 4
=item *
Mailing list: cpan-testers-discuss@perl.org
=item *
Testing results: L<https://www.cpantesters.org/>
=back
=head1 PLATFORMS
Perl is built with a L<C<$^O>|perlvar/$^O> variable that indicates the
operating system it was built on. This was implemented
to help speed up code that would otherwise have to C<use Config>
and use the value of L<C<$Config{osname}>|Config/C<osname>>. Of course,
to get more detailed information about the system, looking into
L<C<%Config>|Config/DESCRIPTION> is certainly recommended.
L<C<%Config>|Config/DESCRIPTION> cannot always be trusted, however,
because it was built at compile time. If perl was built in one place,
then transferred elsewhere, some values may be wrong. The values may
even have been edited after the fact.
=head2 Unix
Perl works on a bewildering variety of Unix and Unix-like platforms (see
e.g. most of the files in the F<hints/> directory in the source code kit).
On most of these systems, the value of L<C<$^O>|perlvar/$^O> (hence
L<C<$Config{osname}>|Config/C<osname>>, too) is determined either by
lowercasing and stripping punctuation from the first field of the string
returned by typing C<uname -a> (or a similar command) at the shell prompt
or by testing the file system for the presence of uniquely named files
such as a kernel or header file. Here, for example, are a few of the
more popular Unix flavors:
uname $^O $Config{archname}
--------------------------------------------
AIX aix aix
BSD/OS bsdos i386-bsdos
Darwin darwin darwin
DYNIX/ptx dynixptx i386-dynixptx
FreeBSD freebsd freebsd-i386
Haiku haiku BePC-haiku
Linux linux arm-linux
Linux linux armv5tel-linux
Linux linux i386-linux
Linux linux i586-linux
Linux linux ppc-linux
HP-UX hpux PA-RISC1.1
IRIX irix irix
Mac OS X darwin darwin
NeXT 3 next next-fat
NeXT 4 next OPENSTEP-Mach
openbsd openbsd i386-openbsd
OSF1 dec_osf alpha-dec_osf
reliantunix-n svr4 RM400-svr4
SCO_SV sco_sv i386-sco_sv
SINIX-N svr4 RM400-svr4
sn4609 unicos CRAY_C90-unicos
sn6521 unicosmk t3e-unicosmk
sn9617 unicos CRAY_J90-unicos
SunOS solaris sun4-solaris
SunOS solaris i86pc-solaris
SunOS4 sunos sun4-sunos
Because the value of L<C<$Config{archname}>|Config/C<archname>> may
depend on the hardware architecture, it can vary more than the value of
L<C<$^O>|perlvar/$^O>.
=head2 DOS and Derivatives
Perl has long been ported to Intel-style microcomputers running under
systems like PC-DOS, MS-DOS, OS/2, and most Windows platforms you can
bring yourself to mention (except for Windows CE, if you count that).
Users familiar with I<COMMAND.COM> or I<CMD.EXE> style shells should
be aware that each of these file specifications may have subtle
differences:
my $filespec0 = "c:/foo/bar/file.txt";
my $filespec1 = "c:\\foo\\bar\\file.txt";
my $filespec2 = 'c:\foo\bar\file.txt';
my $filespec3 = 'c:\\foo\\bar\\file.txt';
System calls accept either C</> or C<\> as the path separator.
However, many command-line utilities of DOS vintage treat C</> as
the option prefix, so may get confused by filenames containing C</>.
Aside from calling any external programs, C</> will work just fine,
and probably better, as it is more consistent with popular usage,
and avoids the problem of remembering what to backwhack and what
not to.
The DOS FAT filesystem can accommodate only "8.3" style filenames. Under
the "case-insensitive, but case-preserving" HPFS (OS/2) and NTFS (NT)
filesystems you may have to be careful about case returned with functions
like L<C<readdir>|perlfunc/readdir DIRHANDLE> or used with functions like
L<C<open>|perlfunc/open FILEHANDLE,MODE,EXPR> or
L<C<opendir>|perlfunc/opendir DIRHANDLE,EXPR>.
DOS also treats several filenames as special, such as F<AUX>, F<PRN>,
F<NUL>, F<CON>, F<COM1>, F<LPT1>, F<LPT2>, etc. Unfortunately, sometimes
these filenames won't even work if you include an explicit directory
prefix. It is best to avoid such filenames, if you want your code to be
portable to DOS and its derivatives. It's hard to know what these all
are, unfortunately.
Users of these operating systems may also wish to make use of
scripts such as F<pl2bat.bat> to put wrappers around your scripts.
Newline (C<\n>) is translated as C<\015\012> by the I/O system when
reading from and writing to files (see L</"Newlines">).
C<binmode($filehandle)> will keep C<\n> translated as C<\012> for that
filehandle.
L<C<binmode>|perlfunc/binmode FILEHANDLE> should always be used for 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 L<C<$^O>|perlvar/$^O> variable and the
L<C<$Config{archname}>|Config/C<archname>> values for various DOSish
perls are as follows:
OS $^O $Config{archname} ID Version
---------------------------------------------------------
MS-DOS dos ?
PC-DOS dos ?
OS/2 os2 ?
Windows 3.1 ? ? 0 3 01
Windows 95 MSWin32 MSWin32-x86 1 4 00
Windows 98 MSWin32 MSWin32-x86 1 4 10
Windows ME MSWin32 MSWin32-x86 1 ?
Windows NT MSWin32 MSWin32-x86 2 4 xx
Windows NT MSWin32 MSWin32-ALPHA 2 4 xx
Windows NT MSWin32 MSWin32-ppc 2 4 xx
Windows 2000 MSWin32 MSWin32-x86 2 5 00
Windows XP MSWin32 MSWin32-x86 2 5 01
Windows 2003 MSWin32 MSWin32-x86 2 5 02
Windows Vista MSWin32 MSWin32-x86 2 6 00
Windows 7 MSWin32 MSWin32-x86 2 6 01
Windows 7 MSWin32 MSWin32-x64 2 6 01
Windows 2008 MSWin32 MSWin32-x86 2 6 01
Windows 2008 MSWin32 MSWin32-x64 2 6 01
Windows CE MSWin32 ? 3
Cygwin cygwin cygwin
The various MSWin32 Perl's can distinguish the OS they are running on
via the value of the fifth element of the list returned from
L<C<Win32::GetOSVersion()>|Win32/Win32::GetOSVersion()>. For example:
if ($^O eq 'MSWin32') {
my @os_version_info = Win32::GetOSVersion();
print +('3.1','95','NT')[$os_version_info[4]],"\n";
}
There are also C<Win32::IsWinNT()|Win32/Win32::IsWinNT()>,
C<Win32::IsWin95()|Win32/Win32::IsWin95()>, and
L<C<Win32::GetOSName()>|Win32/Win32::GetOSName()>; try
L<C<perldoc Win32>|Win32>.
The very portable L<C<POSIX::uname()>|POSIX/C<uname>> will work too:
c:\> perl -MPOSIX -we "print join '|', uname"
Windows NT|moonru|5.0|Build 2195 (Service Pack 2)|x86
Errors set by Winsock functions are now put directly into C<$^E>,
and the relevant C<WSAE*> error codes are now exported from the
L<Errno> and L<POSIX> modules for testing this against.
The previous behavior of putting the errors (converted to POSIX-style
C<E*> error codes since Perl 5.20.0) into C<$!> was buggy due to
the non-equivalence of like-named Winsock and POSIX error constants,
a relationship between which has unfortunately been established
in one way or another since Perl 5.8.0.