perlport - Writing portable Perl
Perl runs on numerous operating systems. While most of
them share much in common, they also have their own unique
features.
This document is meant to help you to find out what constitutes
portable Perl code. That way once you make a
decision to write portably, you know where the lines are
drawn, and you can stay within them.
There is a tradeoff between taking full advantage of one
particular type of computer and taking advantage of a full
range of them. Naturally, as you broaden your range and
become more diverse, the common factors drop, and you are
left with an increasingly smaller area of common ground in
which you can operate to accomplish a particular task.
Thus, when you begin attacking a problem, it is important
to consider under which part of the tradeoff curve you
want to operate. Specifically, you must decide whether it
is important that the task that you are coding have the
full generality of being portable, or whether to just get
the job done right now. This is the hardest choice to be
made. The rest is easy, because Perl provides many
choices, whichever way you want to approach your problem.
Looking at it another way, writing portable code is usually
about willfully limiting your available choices.
Naturally, it takes discipline and sacrifice to do that.
The product of portability and convenience may be a constant.
You have been warned.
Be aware of two important points:
Not all Perl programs have to be portable
There is no reason you should not use Perl as a language
to glue Unix tools together, or to prototype a
Macintosh application, or to manage the Windows registry.
If it makes no sense to aim for portability
for one reason or another in a given program, then
don't bother.
Nearly all of Perl already is portable
Don't be fooled into thinking that it is hard to create
portable Perl code. It isn't. Perl tries its
level-best to bridge the gaps between what's available
on different platforms, and all the means available to
use those features. Thus almost all Perl code runs on
any machine without modification. But there are some
significant issues in writing portable code, and this
document is entirely about those issues.
Here's the general rule: When you approach a task commonly
done using a whole range of platforms, think about writing
portable code. That way, you don't sacrifice much by way
of the implementation choices you can avail yourself of,
and at the same time you can give your users lots of platform
choices. On the other hand, when you have to take
advantage of some unique feature of a particular platform,
as is often the case with systems programming (whether for
Unix, Windows, Mac OS, VMS, etc.), consider writing platform-specific
code.
When the code will run on only two or three operating systems,
you may need to consider only the differences of
those particular systems. The important thing is to
decide where the code will run and to be deliberate in
your decision.
The material below is separated into three main sections:
main issues of portability ("ISSUES", platform-specific
issues ("PLATFORMS", and built-in perl functions that
behave differently on various ports ("FUNCTION IMPLEMENTATIONS".
This information should not be considered complete; it
includes possibly transient information about idiosyncrasies
of some of the ports, almost all of which are in a
state of constant evolution. Thus, this material should
be considered a perpetual work in progress ("<IMG
SRC="yellow_sign.gif" ALT="Under Construction">").
Newlines
In most operating systems, lines in files are terminated
by newlines. Just what is used as a newline may vary from
OS to OS. Unix traditionally uses " 12", one type of
DOSish I/O uses " 15 12", and Mac OS uses " 15".
Perl uses "0 to represent the "logical" newline, where
what is logical may depend on the platform in use. In
MacPerl, "0 always means " 15". In DOSish perls, "0
usually means " 12", but when accessing a file in "text"
mode, STDIO translates it to (or from) " 15 12", depending
on whether you're reading or writing. Unix does the
same thing on ttys in canonical mode. " 15 12" is commonly
referred to as CRLF.
A common cause of unportable programs is the misuse of
chop() to trim newlines:
# XXX UNPORTABLE!
while(<FILE>) {
chop;
@array = split(/:/);
#...
}
You can get away with this on Unix and Mac OS (they have a
single character end-of-line), but the same program will
break under DOSish perls because you're only chop()ing
half the end-of-line. Instead, chomp() should be used to
trim newlines. The Dunce::Files module can help audit
your code for misuses of chop().
When dealing with binary files (or text files in binary
mode) be sure to explicitly set $/ to the appropriate
value for your file format before using chomp().
Because of the "text" mode translation, DOSish perls have
limitations in using "seek" and "tell" on a file accessed
in "text" mode. Stick to "seek"-ing to locations you got
from "tell" (and no others), and you are usually free to
use "seek" and "tell" even in "text" mode. Using "seek"
or "tell" or other file operations may be non-portable.
If you use "binmode" on a file, however, you can usually
"seek" and "tell" with arbitrary values in safety.
A common misconception in socket programming is that "0
eq " 12" everywhere. When using protocols such as common
Internet protocols, " 12" and " 15" are called for "
specifically, and the values of the logical "0 and "
(carriage return) are not reliable.
0; # WRONG
print SOCKET "Hi there, client!
print SOCKET "Hi there, client! 15 12"; # RIGHT
However, using " 15 12" (or " be tedious and unsightly, as well as confusing to those
maintaining the code. As such, the Socket module supplies
the Right Thing for those who want it.
use Socket qw(:DEFAULT :crlf);
print SOCKET "Hi there, client!$CRLF" # RIGHT
When reading from a socket, remember that the default
input record separator $/ is "0, but robust socket code
will recognize as either " 12" or " 15 12" as end of
line:
while (<SOCKET>) {
# ...
}
Because both CRLF and LF end in LF, the input record
separator can be set to LF and any CR stripped later.
Better to write:
use Socket qw(:DEFAULT :crlf);
local($/) = LF; # not needed if $/ is already 12
while (<SOCKET>) {
s/$CR?$LF/0; # not sure if socket uses LF or
CRLF, OK
# s/ 15? 12/0; # same thing
}
This example is preferred over the previous one--even for
Unix platforms--because now any " 15"'s (" stripped
out (and there was much rejoicing).
Similarly, functions that return text data--such as a
function that fetches a web page--should sometimes translate
newlines before returning the data, if they've not
yet been translated to the local newline representation.
A single line of code will often suffice:
$data =~ s/ 15? 12/0g;
return $data;
Some of this may be confusing. Here's a handy reference
to the ASCII CR and LF characters. You can print it out
and stick it in your wallet.
LF eq 12 eq A eq CR eq 15 eq D
eq
| Unix | DOS | Mac |
---------------------------
|| LCR || LCR || CLF ||
**|| LCR ||CRCR || CLF ||
---------------------------
* text-mode STDIO
The Unix column assumes that you are not accessing a
serial line (like a tty) in canonical mode. If you are,
then CR on input becomes "0, and "0 on output becomes
CRLF.
T"ein Perl.usThere maytwellmbe others.ioForoexample, on
"
an EBCDIC implementation such as z/OS (OS/390) or OS/400
(using the ILE, the PASE is ASCII-based) the above material
is similar to "Unix" but the code numbers change:
LF eq 25 eq 5 eq LF eq 45 eq 5
eq chr(37) eq CP-0037 37
CR eq 15 eq D eq CR eq 15 eq D
eq
| z/OS | OS/400 |
----------------------
|| LCR || LCR ||
**|| LCR || LCR ||
----------------------
* text-mode STDIO
Numbers endianness and Width [Toc] [Back]
Different CPUs store integers and floating point numbers
in different orders (called endianness) and widths (32-bit
and 64-bit being the most common today). This affects
your programs when they attempt to transfer numbers in
binary format from one CPU architecture to another, usually
either "live" via network connection, or by storing
the numbers to secondary storage such as a disk file or
tape.
Conflicting storage orders make utter mess out of the numbers.
If a little-endian host (Intel, VAX) stores
0x12345678 (305419896 in decimal), a big-endian host
(Motorola, Sparc, PA) reads it as 0x78563412 (2018915346
in decimal). Alpha and MIPS can be either: Digital/Compaq
used/uses them in little-endian mode; SGI/Cray uses them
in big-endian mode. To avoid this problem in network
(socket) connections use the "pack" and "unpack" formats
"n" and "N", the "network" orders. These are guaranteed
to be portable.
You can explore the endianness of your platform by unpacking
a data structure packed in native format such as:
print unpack("h*", pack("s2", 1, 2)), "0;
# '10002000' on e.g. Intel x86 or Alpha 21064 in little-endian mode
# '00100020' on e.g. Motorola 68040
If you need to distinguish between endian architectures
you could use either of the variables set like so:
$is_big_endian = unpack("h*", pack("s", 1)) =~ /01/;
$is_little_endian = unpack("h*", pack("s", 1)) =~
/^1/;
Differing widths can cause truncation even between platforms
of equal endianness. The platform of shorter width
loses the upper parts of the number. There is no good
solution for this problem except to avoid transferring or
storing raw binary numbers.
One can circumnavigate both these problems in two ways.
Either transfer and store numbers always in text format,
instead of raw binary, or else consider using modules like
Data::Dumper (included in the standard distribution as of
Perl 5.005) and Storable (included as of perl 5.8). Keeping
all data as text significantly simplifies matters.
The v-strings are portable only up to v2147483647
(0x7FFFFFFF), that's how far EBCDIC, or more precisely
UTF-EBCDIC will go.
Files and Filesystems [Toc] [Back]
Most platforms these days structure files in a hierarchical
fashion. So, it is reasonably safe to assume that all
platforms support the notion of a "path" to uniquely identify
a file on the system. How that path is really written,
though, differs considerably.
Although similar, file path specifications differ between
Unix, Windows, Mac OS, OS/2, VMS, VOS, RISC OS, and probably
others. Unix, for example, is one of the few OSes
that has the elegant idea of a single root directory.
DOS, OS/2, VMS, VOS, and Windows can work similarly to
Unix with "/" as path separator, or in their own idiosyncratic
ways (such as having several root directories and
various "unrooted" device files such NIL: and LPT:).
Mac OS uses ":" as a path separator instead of "/".
The filesystem may support neither hard links ("link") nor
symbolic links ("symlink", "readlink", "lstat").
The filesystem may support neither access timestamp nor
change timestamp (meaning that about the only portable
timestamp is the modification timestamp), or one second
granularity of any timestamps (e.g. the FAT filesystem
limits the time granularity to two seconds).
The "inode change timestamp" (the "-C" filetest) may
really be the "creation timestamp" (which it is not in
UNIX).
VOS perl can emulate Unix filenames with "/" as path separator.
The native pathname characters greater-than,
less-than, number-sign, and percent-sign are always
accepted.
RISC OS perl can emulate Unix filenames with "/" as path
separator, or go native and use "." for path separator and
":" to signal filesystems and disk names.
Don't assume UNIX filesystem access semantics: that read,
write, and execute are all the permissions there are, and
even if they exist, that their semantics (for example what
do r, w, and x mean on a directory) are the UNIX ones.
The various UNIX/POSIX compatibility layers usually try to
make interfaces like chmod() work, but sometimes there
simply is no good mapping.
If all this is intimidating, have no (well, maybe only a
little) fear. There are modules that can help. The
File::Spec modules provide methods to do the Right Thing
on whatever platform happens to be running the program.
use File::Spec::Functions;
chdir(updir()); # go up one directory
$file = catfile(curdir(), 'temp', 'file.txt');
# on Unix and Win32, './temp/file.txt'
# on Mac OS, ':temp:file.txt'
# on VMS, '[.temp]file.txt'
File::Spec is available in the standard distribution as of
version 5.004_05. File::Spec::Functions is only in
File::Spec 0.7 and later, and some versions of perl come
with version 0.6. If File::Spec is not updated to 0.7 or
later, you must use the object-oriented interface from
File::Spec (or upgrade File::Spec).
In general, production code should not have file paths
hardcoded. Making them user-supplied or read from a configuration
file is better, keeping in mind that file path
syntax varies on different machines.
This is especially noticeable in scripts like Makefiles
and test suites, which often assume "/" as a path separator
for subdirectories.
Also of use is File::Basename from the standard distribution,
which splits a pathname into pieces (base filename,
full path to directory, and file suffix).
Even when on a single platform (if you can call Unix a
single platform), remember not to count on the existence
or the contents of particular system-specific files or
directories, like /etc/passwd, /etc/sendmail.conf,
/etc/resolv.conf, or even /tmp/. For example, /etc/passwd
may exist but not contain the encrypted passwords, because
the system is using some form of enhanced security. Or it
may not contain all the accounts, because the system is
using NIS. If code does need to rely on such a file,
include a description of the file and its format in the
code's documentation, then make it easy for the user to
override the default location of the file.
Don't assume a text file will end with a newline. They
should, but people forget.
Do not have two files or directories of the same name with
different case, like test.pl and Test.pl, as many platforms
have case-insensitive (or at least case-forgiving)
filenames. Also, try not to have non-word characters
(except for ".") in the names, and keep them to the 8.3
convention, for maximum portability, onerous a burden
though this may appear.
Likewise, when using the AutoSplit module, try to keep
your functions to 8.3 naming and case-insensitive conventions;
or, at the least, make it so the resulting files
have a unique (case-insensitively) first 8 characters.
Whitespace in filenames is tolerated on most systems, but
not all, and even on systems where it might be tolerated,
some utilities might become confused by such whitespace.
Many systems (DOS, VMS) cannot have more than one "." in
their filenames.
Don't assume ">" won't be the first character of a filename.
Always use "<" explicitly to open a file for reading,
or even better, use the three-arg version of open,
unless you want the user to be able to specify a pipe
open.
open(FILE, '<', $existing_file) or die $!;
If filenames might use strange characters, it is safest to
open it with "sysopen" instead of "open". "open" is magic
and can translate characters like ">", "<", and "|", which
may be the wrong thing to do. (Sometimes, though, it's
the right thing.) Three-arg open can also help protect
against this translation in cases where it is undesirable.
Don't use ":" as a part of a filename since many systems
use that for their own semantics (Mac OS Classic for separating
pathname components, many networking schemes and
utilities for separating the nodename and the pathname,
and so on). For the same reasons, avoid "@", ";" and "|".
Don't assume that in pathnames you can collapse two leading
slashes "//" into one: some networking and clustering
filesystems have special semantics for that. Let the
operating system to sort it out.
The portable filename characters as defined by ANSI C are
a b c d e f g h i j k l m n o p q r t u v w x y z
A B C D E F G H I J K L M N O P Q R T U V W X Y Z
0 1 2 3 4 5 6 7 8 9
. _ -
and the "-" shouldn't be the first character. If you want
to be hypercorrect, stay case-insensitive and within the
8.3 naming convention (all the files and directories have
to be unique within one directory if their names are lowercased
and truncated to eight characters before the ".",
if any, and to three characters after the ".", if any).
(And do not use "."s in directory names.)
System Interaction [Toc] [Back]
Not all platforms provide a command line. These are usually
platforms that rely primarily on a Graphical User
Interface (GUI) for user interaction. A program requiring
a command line interface might not work everywhere. This
is probably for the user of the program to deal with, so
don't stay up late worrying about it.
Some platforms can't delete or rename files held open by
the system, this limitation may also apply to changing
filesystem metainformation like file permissions or owners.
Remember to "close" files when you are done with
them. Don't "unlink" or "rename" an open file. Don't
"tie" or "open" a file already tied or opened; "untie" or
"close" it first.
Don't open the same file more than once at a time for
writing, as some operating systems put mandatory locks on
such files.
Don't assume that write/modify permission on a directory
gives the right to add or delete files/directories in that
directory. That is filesystem specific: in some filesystems
you need write/modify permission also (or even just)
in the file/directory itself. In some filesystems (AFS,
DFS) the permission to add/delete directory entries is a
completely separate permission.
Don't assume that a single "unlink" completely gets rid of
the file: some filesystems (most notably the ones in VMS)
have versioned filesystems, and unlink() removes only the
most recent one (it doesn't remove all the versions
because by default the native tools on those platforms
remove just the most recent version, too). The portable
idiom to remove all the versions of a file is
1 while unlink "file";
This will terminate if the file is undeleteable for some
reason (protected, not there, and so on).
Don't count on a specific environment variable existing in
%ENV. Don't count on %ENV entries being case-sensitive,
or even case-preserving. Don't try to clear %ENV by saying
"%ENV = ();", or, if you really have to, make it conditional
on "$^O ne 'VMS'" since in VMS the %ENV table is
much more than a per-process key-value string table.
Don't count on signals or %SIG for anything.
Don't count on filename globbing. Use "opendir", "readdir",
and "closedir" instead.
Don't count on per-program environment variables, or perprogram
current directories.
Don't count on specific values of $!, neither numeric nor
especially the strings values-- users may switch their
locales causing error messages to be translated into their
languages. If you can trust a POSIXish environment, you
can portably use the symbols defined by the Errno module,
like ENOENT. And don't trust on the values of $! at all
except immediately after a failed system call.
Command names versus file pathnames [Toc] [Back]
Don't assume that the name used to invoke a command or
program with "system" or "exec" can also be used to test
for the existence of the file that holds the executable
code for that command or program. First, many systems
have "internal" commands that are built-in to the shell or
OS and while these commands can be invoked, there is no
corresponding file. Second, some operating systems (e.g.,
Cygwin, DJGPP, OS/2, and VOS) have required suffixes for
executable files; these suffixes are generally permitted
on the command name but are not required. Thus, a command
like "perl" might exist in a file named "perl",
"perl.exe", or "perl.pm", depending on the operating system.
The variable "_exe" in the Config module holds the
executable suffix, if any. Third, the VMS port carefully
sets up $^X and $Config{perlpath} so that no further processing
is required. This is just as well, because the
matching regular expression used below would then have to
deal with a possible trailing version number in the VMS
file name.
To convert $^X to a file pathname, taking account of the
requirements of the various operating system possibilities,
say:
use Config;
$thisperl = $^X;
if ($^O ne 'VMS')
{$thisperl .= $Config{_exe} unless $thisperl =~
m/$Config{_exe}$/i;}
To convert $Config{perlpath} to a file pathname, say:
use Config;
$thisperl = $Config{perlpath};
if ($^O ne 'VMS')
{$thisperl .= $Config{_exe} unless $thisperl =~
m/$Config{_exe}$/i;}
Networking [Toc] [Back]
Don't assume that you can reach the public Internet.
Don't assume that there is only one way to get through
firewalls to the public Internet.
Don't assume that you can reach outside world through any
other port than 80, or some web proxy. ftp is blocked by
many firewalls.
Don't assume that you can send email by connecting to the
local SMTP port.
Don't assume that you can reach yourself or any node by
the name 'localhost'. The same goes for '127.0.0.1'. You
will have to try both.
Don't assume that the host has only one network card, or
that it can't bind to many virtual IP addresses.
Don't assume a particular network device name.
Don't assume a particular set of ioctl()s will work.
Don't assume that you can ping hosts and get replies.
Don't assume that any particular port (service) will
respond.
Don't assume that Sys::Hostname() (or any other API or
command) returns either a fully qualified hostname or a
non-qualified hostname: it all depends on how the system
had been configured. Also remember things like DHCP and
NAT-- the hostname you get back might not be very useful.
All the above "don't":s may look daunting, and they are --
but the key is to degrade gracefully if one cannot reach
the particular network service one wants. Croaking or
hanging do not look very professional.
Interprocess Communication (IPC) [Toc] [Back]
In general, don't directly access the system in code meant
to be portable. That means, no "system", "exec", "fork",
"pipe", ``, "qx//", "open" with a "|", nor any of the
other things that makes being a perl hacker worth being.
Commands that launch external processes are generally supported
on most platforms (though many of them do not support
any type of forking). The problem with using them
arises from what you invoke them on. External tools are
often named differently on different platforms, may not be
available in the same location, might accept different
arguments, can behave differently, and often present their
results in a platform-dependent way. Thus, you should
seldom depend on them to produce consistent results. (Then
again, if you're calling netstat -a, you probably don't
expect it to run on both Unix and CP/M.)
One especially common bit of Perl code is opening a pipe
to sendmail:
open(MAIL, '|/usr/lib/sendmail -t')
or die "cannot fork sendmail: $!";
This is fine for systems programming when sendmail is
known to be available. But it is not fine for many nonUnix
systems, and even some Unix systems that may not have
sendmail installed. If a portable solution is needed, see
the various distributions on CPAN that deal with it.
Mail::Mailer and Mail::Send in the MailTools distribution
are commonly used, and provide several mailing methods,
including mail, sendmail, and direct SMTP (via Net::SMTP)
if a mail transfer agent is not available. Mail::Sendmail
is a standalone module that provides simple, platformindependent
mailing.
The Unix System V IPC ("msg*(), sem*(), shm*()") is not
available even on all Unix platforms.
Do not use either the bare result of "pack("N", 10, 20,
30, 40)" or bare v-strings (such as "v10.20.30.40") to
represent IPv4 addresses: both forms just pack the four
bytes into network order. That this would be equal to the
C language "in_addr" struct (which is what the socket code
internally uses) is not guaranteed. To be portable use
the routines of the Socket extension, such as
"inet_aton()", "inet_ntoa()", and "sockaddr_in()".
The rule of thumb for portable code is: Do it all in
portable Perl, or use a module (that may internally implement
it with platform-specific code, but expose a common
interface).
External Subroutines (XS) [Toc] [Back]
XS code can usually be made to work with any platform, but
dependent libraries, header files, etc., might not be
readily available or portable, or the XS code itself might
be platform-specific, just as Perl code might be. If the
libraries and headers are portable, then it is normally
reasonable to make sure the XS code is portable, too.
A different type of portability issue arises when writing
XS code: availability of a C compiler on the end-user's
system. C brings with it its own portability issues, and
writing XS code will expose you to some of those. Writing
purely in Perl is an easier way to achieve portability.
Standard Modules [Toc] [Back]
In general, the standard modules work across platforms.
Notable exceptions are the CPAN module (which currently
makes connections to external programs that may not be
available), platform-specific modules (like ExtUtils::MM_VMS),
and DBM modules.
There is no one DBM module available on all platforms.
SDBM_File and the others are generally available on all
Unix and DOSish ports, but not in MacPerl, where only
NBDM_File and DB_File are available.
The good news is that at least some DBM module should be
available, and AnyDBM_File will use whichever module it
can find. Of course, then the code needs to be fairly
strict, dropping to the greatest common factor (e.g., not
exceeding 1K for each record), so that it will work with
any DBM module. See AnyDBM_File for more details.
Time and Date [Toc] [Back]
The system's notion of time of day and calendar date is
controlled in widely different ways. Don't assume the
timezone is stored in $ENV{TZ}, and even if it is, don't
assume that you can control the timezone through that
variable. Don't assume anything about the three-letter
timezone abbreviations (for example that MST would be the
Mountain Standard Time, it's been known to stand for
Moscow Standard Time). If you need to use timezones,
express them in some unambiguous format like the exact
number of minutes offset from UTC, or the POSIX timezone
format.
Don't assume that the epoch starts at 00:00:00, January 1,
1970, because that is OS- and implementation-specific. It
is better to store a date in an unambiguous representation.
The ISO 8601 standard defines YYYY-MM-DD as the
date format, or YYYY-MM-DDTHH-MM-SS (that's a literal "T"
separating the date from the time). Please do use the ISO
8601 instead of making us to guess what date 02/03/04
might be. ISO 8601 even sorts nicely as-is. A text representation
(like "1987-12-18") can be easily converted
into an OS-specific value using a module like Date::Parse.
An array of values, such as those returned by "localtime",
can be converted to an OS-specific representation using
Time::Local.
When calculating specific times, such as for tests in time
or date modules, it may be appropriate to calculate an
offset for the epoch.
require Time::Local;
$offset = Time::Local::timegm(0, 0, 0, 1, 0, 70);
The value for $offset in Unix will be 0, but in Mac OS
will be some large number. $offset can then be added to a
Unix time value to get what should be the proper value on
any system.
On Windows (at least), you shouldn't pass a negative value
to "gmtime" or "localtime".
Character sets and character encoding [Toc] [Back]
Assume very little about character sets.
Assume nothing about numerical values ("ord", "chr") of
characters. Do not use explicit code point ranges (like
-); use for example symbolic character classes
like "[:print:]".
Do not assume that the alphabetic characters are encoded
contiguously (in the numeric sense). There may be gaps.
Do not assume anything about the ordering of the characters.
The lowercase letters may come before or after the
uppercase letters; the lowercase and uppercase may be
interlaced so that both `a' and `A' come before `b'; the
accented and other international characters may be interlaced
so that ae comes before `b'.
Internationalisation [Toc] [Back]
If you may assume POSIX (a rather large assumption), you
may read more about the POSIX locale system from perllocale.
The locale system at least attempts to make things
a little bit more portable, or at least more convenient
and native-friendly for non-English users. The system
affects character sets and encoding, and date and time
formatting--amongst other things.
If you really want to be international, you should consider
Unicode. See perluniintro and perlunicode for more
information.
If you want to use non-ASCII bytes (outside the bytes
0x00..0x7f) in the "source code" of your code, to be
portable you have to be explicit about what bytes they
are. Someone might for example be using your code under a
UTF-8 locale, in which case random native bytes might be
illegal ("Malformed UTF-8 ...") This means that for example
embedding ISO 8859-1 bytes beyond 0x7f into your
strings might cause trouble later. If the bytes are
native 8-bit bytes, you can use the "bytes" pragma. If
the bytes are in a string (regular expression being a
curious string), you can often also use the "" notation
instead of embedding the bytes as-is. If they are in
some particular legacy encoding (ether single-byte or
something more complicated), you can use the "encoding"
pragma. (If you want to write your code in UTF-8, you can
use either the "utf8" pragma, or the "encoding" pragma.)
The "bytes" and "utf8" pragmata are available since Perl
5.6.0, and the "encoding" pragma since Perl 5.8.0.
System Resources [Toc] [Back]
If your code is destined for systems with severely constrained
(or missing!) virtual memory systems then you
want to be especially mindful of avoiding wasteful constructs
such as:
# NOTE: this is no longer "bad" in perl5.005
for (0..10000000) {} # bad
for (my $x = 0; $x <= 10000000; ++$x) {} # good
@lines = <VERY_LARGE_FILE>; # bad
while (<FILE>) {$file .= $_} # sometimes
bad
$file = join('', <FILE>); # better
The last two constructs may appear unintuitive to most
people. The first repeatedly grows a string, whereas the
second allocates a large chunk of memory in one go. On
some systems, the second is more efficient that the first.
Security [Toc] [Back]
Most multi-user platforms provide basic levels of security,
usually implemented at the filesystem level. Some,
however, do not-- unfortunately. Thus the notion of user
id, or "home" directory, or even the state of being
logged-in, may be unrecognizable on many platforms. If
you write programs that are security-conscious, it is usually
best to know what type of system you will be running
under so that you can write code explicitly for that platform
(or class of platforms).
Don't assume the UNIX filesystem access semantics: the
operating system or the filesystem may be using some ACL
systems, which are richer languages than the usual rwx.
Even if the rwx exist, their semantics might be different.
(From security viewpoint testing for permissions before
attempting to do something is silly anyway: if one tries
this, there is potential for race conditions-- someone or
something might change the permissions between the permissions
check and the actual operation. Just try the
operation.)
Don't assume the UNIX user and group semantics: especially,
don't expect the $< and $> (or the $( and $)) to
work for switching identities (or memberships).
Don't assume set-uid and set-gid semantics. (And even if
you do, think twice: set-uid and set-gid are a known can
of security worms.)
Style [Toc] [Back]
For those times when it is necessary to have platform-specific
code, consider keeping the platform-specific code in
one place, making porting to other platforms easier. Use
the Config module and the special variable $^O to differentiate
platforms, as described in "PLATFORMS".
Be careful in the tests you supply with your module or
programs. Module code may be fully portable, but its
tests might not be. This often happens when tests spawn
off other processes or call external programs to aid in
the testing, or when (as noted above) the tests assume
certain things about the filesystem and paths. Be careful
not to depend on a specific output style for errors, such
as when checking $! after a failed system call. Using $!
for anything else than displaying it as output is doubtful
(though see the Errno module for testing reasonably
portably for error value). Some platforms expect a certain
output format, and Perl on those platforms may have been
adjusted accordingly. Most specifically, don't anchor a
regex when testing an error value.
Modules uploaded to CPAN are tested by a variety of volunteers
on different platforms. These CPAN testers are
notified by mail of each new upload, and reply to the list
with PASS, FAIL, NA (not applicable to this platform), or
UNKNOWN (unknown), along with any relevant notations.
The purpose of the testing is twofold: one, to help developers
fix any problems in their code that crop up because
of lack of testing on other platforms; two, to provide
users with information about whether a given module works
on a given platform.
Mailing list: [email protected]
Testing results: http://testers.cpan.org/
As of version 5.002, Perl is built with a $^O variable
that indicates the operating system it was built on. This
was implemented to help speed up code that would otherwise
have to "use Config" and use the value of $Config{osname}.
Of course, to get more detailed information about the system,
looking into %Config is certainly recommended.
%Config cannot always be trusted, however, because it was
built at compile time. If perl was built in one place,
then transferred elsewhere, some values may be wrong. The
values may even have been edited after the fact.
Unix [Toc] [Back]
Perl works on a bewildering variety of Unix and Unix-like
platforms (see e.g. most of the files in the hints/ directory
in the source code kit). On most of these systems,
the value of $^O (hence $Config{'osname'}, too) is determined
either by lowercasing and stripping punctuation from
the first field of the string returned by typing "uname
-a" (or a similar command) at the shell prompt or by testing
the file system for the presence of uniquely named
files such as a kernel or header file. Here, for example,
are a few of the more popular Unix flavors:
uname $^O $Config{'archname'}
--------------------------------------------
AIX aix aix
BSD/OS bsdos i386-bsdos
Darwin darwin darwin
dgux dgux AViiON-dgux
DYNIX/ptx dynixptx i386-dynixptx
FreeBSD freebsd freebsd-i386
Linux linux arm-linux
Linux linux i386-linux
Linux linux i586-linux
Linux linux ppc-linux
HP-UX hpux PA-RISC1.1
IRIX irix irix
Mac OS X darwin darwin
MachTen PPC machten powerpc-machten
NeXT 3 next next-fat
NeXT 4 next OPENSTEP-Mach
openbsd openbsd i386-openbsd
OSF1 dec_osf alpha-dec_osf
reliantunix-n svr4 RM400-svr4
SCO_SV sco_sv i386-sco_sv
SINIX-N svr4 RM400-svr4
sn4609 unicos CRAY_C90-unicos
sn6521 unicosmk t3e-unicosmk
sn9617 unicos CRAY_J90-unicos
SunOS solaris sun4-solaris
SunOS solaris i86pc-solaris
SunOS4 sunos sun4-sunos
Because the value of $Config{archname} may depend on the
hardware architecture, it can vary more than the value of
$^O.
DOS and Derivatives
Perl has long been ported to Intel-style microcomputers
running under systems like PC-DOS, MS-DOS, OS/2, and most
Windows platforms you can bring yourself to mention
(except for Windows CE, if you count that). Users familiar
with COMMAND.COM or CMD.EXE style shells should be
aware that each of these file specifications may have subtle
differences: r
l
$filespec0 = "c:/eoo/bar/file.txt";
$filespec1 = "c:\.oo\bar\file.txt";
$filespec2 = 'c:ot
$filespec3 = 'c:\foo\bar\file.txt';
t
System calls accept either "/" or "
tor. However, many command-line utilities of DOS vintage
treat "/" as the option prefix, so may get confused by
filenames containing "/". Aside from calling any external
programs, "/" will work just fine, and probably better, as
it is more consistent with popular usage, and avoids the
problem of remembering what to backwhack and what not to.
The DOS FAT filesystem can accommodate only "8.3" style
filenames. Under the "case-insensitive, but case-preserving"
HPFS (OS/2) and NTFS (NT) filesystems you may have to
be careful about case returned with functions like "readdir"
or used with functions like "open" or "opendir".
DOS also treats several filenames as special, such as AUX,
PRN, NUL, CON, COM1, LPT1, LPT2, etc. Unfortunately,
sometimes these filenames won't even work if you include
an explicit directory prefix. It is best to avoid such
filenames, if you want your code to be portable to DOS and
its derivatives. It's hard to know what these all are,
unfortunately.
Users of these operating systems may also wish to make use
of scripts such as pl2bat.bat or pl2cmd to put wrappers
around your scripts.
Newline ("0) is translated as " 15 12" by STDIO when
reading from and writing to files (see "Newlines"). "binmode(FILEHANDLE)"
will keep "0 translated as " 12" for
that filehandle. Since it is a no-op on other systems,
"binmode" should be used for cross-platform code that
deals with binary data. That's assuming you realize in
advance that your data is in binary. General-purpose programs
should often assume nothing about their data.
The $^O variable and the $Config{archname} values for various
DOSish perls are as follows:
OS $^O $Config{archname} ID Version
--------------------------------------------------------
MS-DOS dos ?
PC-DOS dos ?
OS/2 os2 ?
Windows 3.1 ? ? 0 3
01
Windows 95 MSWin32 MSWin32-x86 1 4
00
Windows 98 MSWin32 MSWin32-x86 1 4
10
Windows ME MSWin32 MSWin32-x86 1 ?
Windows NT MSWin32 MSWin32-x86 2 4
xx
Windows NT MSWin32 MSWin32-ALPHA 2 4
xx
Windows NT MSWin32 MSWin32-ppc 2 4
xx
Windows 2000 MSWin32 MSWin32-x86 2 5
xx
Windows XP MSWin32 MSWin32-x86 2 ?
Windows CE MSWin32 ? 3
Cygwin cygwin ?
The various MSWin32 Perl's can distinguish the OS they are
running on via the value of the fifth element of the list
returned from Win32::GetOSVersion(). For example:
if ($^O eq 'MSWin32') {
my @os_version_info = Win32::GetOSVersion();
print +('3.1','95','NT')[$os_version_info[4]],"0;
}
There are also Win32::IsWinNT() and Win32::IsWin95(), try
"perldoc Win32", and as of libwin32 0.19 (not part of the
core Perl distribution) Win32::GetOSName(). The very
portable POSIX::uname() will work too:
c:> perl -MPOSIX -we "print join '|', uname"
Windows NT|moonru|5.0|Build 2195 (Service Pack 2)|x86
Also see:
o The djgpp environment for DOS, http://www.delo-
rie.com/djgpp/ and perldos.
o The EMX environment for DOS, OS/2, etc. [email protected],
http://www.leo.org/pub/comp/os/os2/leo/gnu/emx+gcc/index.html
or ftp://hobbes.nmsu.edu/pub/os2/dev/emx/ Also perlos2.
o Build instructions for Win32 in perlwin32, or under
the Cygnus environment in perlcygwin.
o The "Win32::*" modules in Win32.
o The ActiveState Pages, http://www.activestate.com/
o The Cygwin environment for Win32; README.cygwin
(installed as perlcygwin), http://www.cygwin.com/
o The U/WIN environment for Win32,
http://www.research.att.com/sw/tools/uwin/
o Build instructions for OS/2, perlos2
Mac OS [Toc] [Back]
Any module requiring XS compilation is right out for most
people, because MacPerl is built using non-free (and
non-cheap!) compilers. Some XS modules that can work with
MacPerl are built and distributed in binary form on CPAN.
Directories are specified as:
volume:folder:file for absolute pathnames
volume:folder: for absolute pathnames
:folder:file for relative pathnames
:folder: for relative pathnames
:file for relative pathnames
file for relative pathnames
Files are stored in the directory in alphabetical order.
Filenames are limited to 31 characters, and may include
any character except for null and ":", which is reserved
as the path separator.
Instead of "flock", see "FSpSetFLock" and "FSpRstFLock" in
the Mac::Files module, or "chmod(0444, ...)" and
"chmod(0666, ...)".
In the MacPerl application, you can't run a program from
the command line; programs that expect @ARGV to be populated
can be edited with something like the following,
which brings up a dialog box asking for the command line
arguments.
if (!@ARGV) {
@ARGV = split /, MacPerl::Ask('Arguments?');
}
A MacPerl script saved as a "droplet" will populate @ARGV
with the full pathnames of the files dropped onto the
script.
Mac users can run programs under a type of command line
interface under MPW (Macintosh Programmer's Workshop, a
free development environment from Apple). MacPerl was
first introduced as an MPW tool, and MPW can be used like
a shell:
perl myscript.plx some arguments
ToolServer is another app from Apple that provides access
to MPW tools from MPW and the MacPerl app, which allows
MacPerl programs to use "system", backticks, and piped
"open".
"Mac OS" is the proper name for the operating system, but
the value in $^O is "MacOS". To determine architecture,
version, or whether the application or MPW tool version is
running, check:
$is_app = $MacPerl::Version =~ /App/;
$is_tool = $MacPerl::Version =~ /MPW/;
($version) = $MacPerl::Version =~ /^()/;
$is_ppc = $MacPerl::Architecture eq 'MacPPC';
$is_68k = $MacPerl::Architecture eq 'Mac68K';
Mac OS X, based on NeXT's OpenStep OS, runs MacPerl
natively, under the "Classic" environment. There is no
"Carbon" version of MacPerl to run under the primary Mac
OS X environment. Mac OS X and its Open Source version,
Darwin, both run Unix perl natively.
Also see:
o MacPerl Development, http://dev.macperl.org/ .
o The MacPerl Pages, http://www.macperl.com/ .
o The MacPerl mailing lists, http://lists.perl.org/ .
VMS [Toc] [Back]
Perl on VMS is discussed in perlvms in the perl distribution.
Perl on VMS can accept either VMS- or Unix-style
file specifications as in either of the following:
$ perl -ne "print if /perl_setup/i" SYS$LOGIN:LOGIN.COM
$ perl -ne "print if /perl_setup/i" /sys$login/login.com
but not a mixture of both as in:
$ perl -ne "print if /perl_setup/i" sys$login:/login.com
Can't open sys$login:/login.com: file specification
syntax error
Interacting with Perl from the Digital Command Language
(DCL) shell often requires a different set of quotation
marks than Unix shells do. For example:
$ perl -e "print ""Hello, world.0""
Hello, world.
There are several ways to wrap your perl scripts in DCL
.COM files, if you are so inclined. For example:
$ write sys$output "Hello from DCL!"
$ if p1 .eqs. ""
$ then perl -x 'f$environment("PROCEDURE")
$ else perl -x - 'p1 'p2 'p3 'p4 'p5 'p6 'p7 'p8
$ deck/dollars="__END__"
#!/usr/bin/perl
print "Hello from Perl!0;
__END__
$ endif
Do take care with "$ ASSIGN/nolog/user SYS$COMMAND:
SYS$INPUT" if your perl-in-DCL script expects to do things
like "$read = <STDIN>;".
Filenames are in the format "name.extension;version". The
maximum length for filenames is 39 characters, and the
maximum length for extensions is also 39 characters. Version
is a number from 1 to 32767. Valid characters are
"/[A-Z0-9$_-]/".
VMS's RMS filesystem is case-insensitive and does not preserve
case. "readdir" returns lowercased filenames, but
specifying a file for opening remains case-insensitive.
Files without extensions have a trailing period on them,
so doing a "readdir" with a file named A.;5 will return a.
(though that file could be opened with "open(FH, 'A')").
RMS had an eight level limit on directory depths from any
rooted logical (allowing 16 levels overall) prior to VMS
7.2. Hence "PERL_ROOT:[LIB.2.3.4.5.6.7.8]" is a valid
directory specification but
"PERL_ROOT:[LIB.2.3.4.5.6.7.8.9]" is not. Makefile.PL
authors might have to take this into account, but at least
they can refer to the former as
"/PERL_ROOT/lib/2/3/4/5/6/7/8/".
The VMS::Filespec module, which gets installed as part of
the build process on VMS, is a pure Perl module that can
easily be installed on non-VMS platforms and can be helpful
for conversions to and from RMS native formats.
What "0 represents depends on the type of file opened.
It usually represents " 12" but it could also be " 15",
" 12", " 15 12", " 00", " 40", or nothing depending
on the file organiztion and record format. The VMS::Stdio
module provides access to the special fopen() requirements
of files with unusual attributes on VMS.
TCP/IP stacks are optional on VMS, so socket routines
might not be implemented. UDP sockets may not be supported.
The value of $^O on OpenVMS is "VMS". To determine the
architecture that you are running on without resorting to
loading all of %Config you can examine the content of the
@INC array like so:
if (grep(/VMS_AXP/, @INC)) {
print "I'm on Alpha!0;
} elsif (grep(/VMS_VAX/, @INC)) {
print "I'm on VAX!0;
} else {
print "I'm not so sure about where $^O is...0;
}
On VMS, perl determines the UTC offset from the "SYS$TIMEZONE_DIFFERENTIAL"
logical name. Although the VMS epoch
began at 17-NOV-1858 00:00:00.00, calls to "localtime" are
adjusted to count offsets from 01-JAN-1970 00:00:00.00,
just like Unix.
Also see:
o README.vms (installed as README_vms), perlvms
o vmsperl list, [email protected]
(Put the words "subscribe vmsperl" in message body.)
o vmsperl on the web,
http://www.sidhe.org/vmsperl/index.html
VOS [Toc] [Back]
Perl on VOS is discussed in README.vos in the perl distribution
(installed as perlvos). Perl on VOS can accept
either VOS- or Unix-style file specifications as in either
of the following:
C<< $ perl -ne "print if /perl_setup/i" >system>notices >>
C<< $ perl -ne "print if /perl_setup/i" /system/notices >>
or even a mixture of both as in:
C<< $ perl -ne "print if /perl_setup/i" >system/notices >>
Even though VOS allows the slash character to appear in
object names, because the VOS port of Perl interprets it
as a pathname delimiting character, VOS files, directories,
or links whose names contain a slash character cannot
be processed. Such files must be renamed before they
can be processed by Perl. Note that VOS limits file names
to 32 or fewer characters.
Perl on VOS can be built using two different compilers and
two different versions of the POSIX runtime. The recommended
method for building full Perl is with the GNU C
compiler and the generally-available version of VOS POSIX
support. See README.vos (installed as perlvos) for
restrictions that apply when Perl is built using the VOS
Standard C compiler or the alpha version of VOS POSIX support.
The value of $^O on VOS is "VOS". To determine the architecture
that you are running on without resorting to loading
all of %Config you can examine the content of the @INC
array like so:
if ($^O =~ /VOS/) {
print "I'm on a Stratus box!0;
} else {
print "I'm not on a Stratus box!0;
die;
}
if (grep(/860/, @INC)) {
print "This box is a Stratus XA/R!0;
} elsif (grep(/7100/, @INC)) {
print "This box is a Stratus HP 7100 or 8xxx!0;
} elsif (grep(/8000/, @INC)) {
print "This box is a Stratus HP 8xxx!0;
} else {
print "This box is a Stratus 68K!0;
}
Also see:
o README.vos (installed as perlvos)
o The VOS mailing list.
There is no specific mailing list for Perl on VOS.
You can post comments to the comp.sys.stratus newsgroup,
or subscribe to the general Stratus mailing
list. Send a letter with "subscribe Info-Stratus" in
the message body to [email protected].
o VOS Perl on the web at http://ftp.stra-
tus.com/pub/vos/posix/posix.html
EBCDIC Platforms [Toc] [Back]
Recent versions of Perl have been ported to platforms such
as OS/400
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