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perlman:perlportby gods (Initiate) |
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perlportCurrent Perl documentation can be found at perldoc.perl.org. Here is our local, out-dated (pre-5.6) version:
NAMEperlport - Writing portable Perl
DESCRIPTIONPerl runs on a variety of operating systems. While most of them share a lot in common, they also have their own very particular and unique features. This document is meant to help you to find out what constitutes portable Perl code, so that once you have made your 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 a particular type of computer, and taking advantage of a full range of them. Naturally, as you make your range bigger (and thus more diverse), the common denominators drop, and you are left with fewer areas 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 which part of the tradeoff curve you want to operate under. Specifically, whether it is important to you that the task that you are coding needs the full generality of being portable, or if it is sufficient to just get the job done. This is the hardest choice to be made. The rest is easy, because Perl provides lots of 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 to do that. Be aware of two important points:
Here's the general rule: When you approach a task that is commonly done using a whole range of platforms, think in terms of 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, then you may only need to consider 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 builtin 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 with newlines. Just what is used as a newline may vary from
OS to
OS. Unix traditionally uses
Perl uses
Due to the ``text'' mode translation, DOSish perls have limitations of
using seek and tell when a file is being accessed in ``text'' mode. Specifically, if you stick
to seek-ing to locations you got from tell (and no others), you are usually free to use seek and
tell even in ``text'' mode. In general, using seek or tell or other file operations that count bytes instead of characters, without
considering the length of
A common misconception in socket programming is that
print SOCKET "Hi there, client!\r\n"; # WRONG print SOCKET "Hi there, client!\015\012"; # RIGHT
[NOTE: this does not necessarily apply to communications that are filtered by another program or module before sending to the socket; the the most popular
EBCDIC webserver, for instance, accepts
However, using
use Socket qw(:DEFAULT :crlf); print SOCKET "Hi there, client!$CRLF" # RIGHT
When reading from a socket, remember that the default input record separator (
while (<SOCKET>) { # ... } Better:
use Socket qw(:DEFAULT :crlf); local($/) = 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 }
And this example is actually better than the previous one even for Unix
platforms, because now any
Numbers endianness and WidthDifferent CPUs store integers and floating point numbers in different orders (called endianness) and widths (32-bit and 64-bit being the most common). This affects your programs if they attempt to transfer numbers in binary format from a CPU architecture to another over some channel: either 'live' via network connections or storing the numbers to secondary storage such as a disk file.
Conflicting storage orders make utter mess out of the numbers: if a little-endian host (Intel, Alpha) stores 0x12345678 (305419896 in decimal), a big-endian host (Motorola,
MIPS, Sparc,
PA) reads it as 0x78563412 (2018915346 in decimal). To avoid this problem in network (socket) connections use the
pack() and unpack() formats Different 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 consider
using modules like
FilesMost platforms these days structure files in a hierarchical fashion. So, it is reasonably safe to assume that any platform supports the notion of a ``path'' to uniquely identify a file on the system. Just how that path is actually written, differs. While they are similar, file path specifications differ between Unix, Windows, Mac OS, OS/2, VMS, RISC OS and probably others. Unix, for example, is one of the few OSes that has the idea of a single root directory.
VMS, Windows, and
OS/2 can work similarly to Unix with
Mac OS uses
As with the newline problem above, there are modules that can help. The
use File::Spec; chdir(File::Spec->updir()); # go up one directory $file = File::Spec->catfile( File::Spec->curdir(), 'temp', 'file.txt' ); # on Unix and Win32, './temp/file.txt' # on Mac OS, ':temp:file.txt' File::Spec is available in the standard distribution, as of version 5.004_05. In general, production code should not have file paths hardcoded; making them user supplied or from a configuration file is better, keeping in mind that file path syntax varies on different machines.
This is especially noticeable in scripts like Makefiles and test suites,
which often assume
Also of use is Even when on a single platform (if you can call UNIX a single platform), remember not to count on the existence or the contents of system-specific files, like /etc/passwd, /etc/sendmail.conf, or /etc/resolv.conf. For example the /etc/passwd may exist but it may not contain the encrypted passwords because the system is using some form of enhanced security-- or it may not contain all the accounts because the system is using NIS. If code does need to rely on such a file, include a description of the file and its format in the code's documentation, and make it easy for the user to override the default location of the file.
Do not have two files of the same name with different case, like
test.pl and <Test.pl>, as many platforms have case-insensitive filenames. Also, try
not to have non-word characters (except for
Likewise, if using
Don't assume
open(FILE, "<$existing_file") or die $!;
System InteractionNot all platforms provide for the notion of a command line, necessarily. These are usually platforms that rely on a Graphical User Interface (GUI) for user interaction. So a program requiring command lines might not work everywhere. But this is probably for the user of the program to deal with. Some platforms can't delete or rename files that are being held open by the system. Remember to close files when you are done with them. Don't unlink or rename an open file. Don't tie to or open a file that is already tied to or opened; untie or close 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 count on a specific environment variable existing in Don't count on signals. 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.
Interprocess Communication (IPC)
In general, don't directly access the system in code that is meant to be
portable. That means, no system, exec, fork, pipe, Commands that launch external processes are generally supported on most platforms (though many of them do not support any type of forking), but the problem with using them arises from what you invoke with them. External tools are often named differently on different platforms, often not available in the same location, often accept different arguments, often behave differently, and often represent their results in a platform-dependent way. Thus you should seldom depend on them to produce consistent results. One especially common bit of Perl code is opening a pipe to sendmail:
open(MAIL, '|/usr/lib/sendmail -t') or die $!;
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 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).
The
UNIX System
V
IPC (
External Subroutines (XS)XS code, in general, can 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. There is a different kind of portability issue with 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 a comparatively easier way to achieve portability.
Standard Modules
In general, the standard modules work across platforms. Notable exceptions
are
There is no one
DBM module that is available on all platforms.
The good news is that at least some
DBM module should be available, and
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
Don't assume that the epoch starts at 00:00:00, January 1, 1970, because that is OS-specific. Better to store a date in an unambiguous representation. The
ISO 8601 standard defines
YYYY-MM-DD as the date format.
A text representation (like
Character sets and character encodingAssume very little about character sets. Do not assume anything about the numerical values (ord(), chr()) of characters. Do not assume that the alphabetic characters are encoded contiguously (in numerical sense). Do no 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 the 'b', the accented and other international characters may be interlaced so that ä comes before the 'b'.
InternationalisationIf you may assume POSIX (a rather large assumption, that: in practise that means UNIX) you may read more about the POSIX locale system from the perllocale manpage. 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, among other things.
System ResourcesIf 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 may appear unintuitive to most people. The first of those two constructs repeatedly grows a string, while the second allocates a large chunk of memory in one go. On some systems, the latter is more efficient that the former.
SecurityMost multi-user platforms provide basic levels of security that is usually felt at the file-system level. Other platforms usually don't (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 operating under, and write code explicitly for that platform (or class of platforms).
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
CPAN TestersModules 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 or not a given module works on a given platform.
PLATFORMS
As of version 5.002, Perl is built with a
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
uname $^O $Config{'archname'} ------------------------------------------- AIX aix aix FreeBSD freebsd freebsd-i386 Linux linux i386-linux HP-UX hpux PA-RISC1.1 IRIX irix irix OSF1 dec_osf alpha-dec_osf SunOS solaris sun4-solaris SunOS solaris i86pc-solaris SunOS4 sunos sun4-sunos
Note that because the
DOS and DerivativesPerl has long been ported to PC 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 and/or CMD.EXE style shells should be aware that each of these file specifications may have subtle differences:
$filespec0 = "c:/foo/bar/file.txt"; $filespec1 = "c:\\foo\\bar\\file.txt"; $filespec2 = 'c:\foo\bar\file.txt'; $filespec3 = 'c:\\foo\\bar\\file.txt';
System calls accept either The DOS FAT filesystem can only accommodate ``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 these filenames won't even work if you include an explicit directory prefix, in some cases. It is best to avoid such filenames, if you want your code to be portable to DOS and its derivatives. Users of these operating systems may also wish to make use of scripts such as pl2bat.bat or pl2cmd as appropriate to put wrappers around your scripts.
Newline (
The
OS $^O $Config{'archname'} -------------------------------------------- MS-DOS dos PC-DOS dos OS/2 os2 Windows 95 MSWin32 MSWin32-x86 Windows NT MSWin32 MSWin32-x86 Windows NT MSWin32 MSWin32-alpha Windows NT MSWin32 MSWin32-ppc Also see:
Mac OSAny 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. See MacPerl: Power and Ease and CPAN Testers for more details. 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 in a directory are stored in alphabetical order. Filenames are
limited to 31 characters, and may include any character except
Instead of flock, see
In the MacPerl application, you can't run a program from the command line;
programs that expect
if (!@ARGV) { @ARGV = split /\s+/, MacPerl::Ask('Arguments?'); }
A MacPerl script saved as a droplet will populate Mac users can use programs on a kind of command line 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
$is_app = $MacPerl::Version =~ /App/; $is_tool = $MacPerl::Version =~ /MPW/; ($version) = $MacPerl::Version =~ /^(\S+)/; $is_ppc = $MacPerl::Architecture eq 'MacPPC'; $is_68k = $MacPerl::Architecture eq 'Mac68K'; Mac OS X, to be based on NeXT's OpenStep OS, will be able to run MacPerl natively (in the Blue Box, and even in the Yellow Box, once some changes to the toolbox calls are made), but Unix perl will also run natively. Also see:
VMSPerl on VMS is discussed in vms/perlvms.pod in the perl distribution. Note that perl on VMS can accept either VMS- or Unix-style file specifications as in either of the following:
$ perl -ne "print if /perl_setup/i" SYS$LOGIN:LOGIN.COM $ perl -ne "print if /perl_setup/i" /sys$login/login.com but not a mixture of both as in:
$ perl -ne "print if /perl_setup/i" sys$login:/login.com Can't open sys$login:/login.com: file specification syntax error Interacting with Perl from the Digital Command Language (DCL) shell often requires a different set of quotation marks than Unix shells do. For example:
$ perl -e "print ""Hello, world.\n""" Hello, world. There are a number of 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
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 VMS' 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
The
What TCP/IP stacks are optional on VMS, so socket routines might not be implemented. UDP sockets may not be supported.
The value of
if (grep(/VMS_AXP/, @INC)) { print "I'm on Alpha!\n"; } elsif (grep(/VMS_VAX/, @INC)) { print "I'm on VAX!\n"; } else { print "I'm not so sure about where $^O is...\n"; } Also see:
EBCDIC PlatformsRecent versions of Perl have been ported to platforms such as OS/400 on AS/400 minicomputers as well as OS/390 for IBM Mainframes. Such computers use EBCDIC character sets internally (usually Character Code Set ID 00819 for OS/400 and IBM-1047 for OS/390). Note that on the mainframe perl currently works under the ``Unix system services for OS/390'' (formerly known as OpenEdition).
As of
R2.5 of
USS for
OS/390 that Unix sub-system did not support the
: # 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";
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
Fortunately, most web servers for the mainframe will correctly translate
the
print "Content-type: text/html\r\n\r\n";
The value of Some simple tricks for determining if you are running on an EBCDIC platform could include any of the following (perhaps all):
if ("\t" eq "\05") { 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"; } Note that 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:
Acorn RISC OS
As Acorns use
ASCII with newlines ( Native filenames are of the form
Filesystem#Special_Field::DiscName.$.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 perlman:perlop
Note that
Logical paths specified by system variables containing comma-separated
search lists are also allowed, hence
Because
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 allows a user defined list
of known suffixes which it will transpose in this fashion. This may appear
transparent, but consider that with these rules
As implied above the environment accessed through
As native operating system filehandles are global and currently are
allocated down from 255, with 0 being a reserved value the Unix emulation
library emulates Unix filehandles. Consequently, you can't rely on passing
The desire of users to express filenames of the form
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 distribution. MakeMaker does run, but no available make currently
copes with MakeMaker's makefiles; even if/when this is fixed, the lack of a
Unix-like shell can cause problems with makefile rules, especially lines of
the form
``RISC OS'' is the proper name for the operating system, but the value in Also see:
Other perlsPerl has been ported to a variety of platforms that do not fit into any of the above categories. Some, such as AmigaOS, BeOS, QNX, and Plan 9, 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, lynxos, riscos, Tandem Guardian, vos, etc. (yes we know that some of these OSes may fall under the Unix category, but we are not a standards body.) See also:
FUNCTION IMPLEMENTATIONSListed below are functions unimplemented or implemented differently on various platforms. Following each description will be, in parentheses, a list of platforms that the description applies to. The list may very well be incomplete, or wrong in some places. When in doubt, consult the platform-specific README files in the Perl source distribution, and other documentation resources for a given port. Be aware, moreover, that even among Unix-ish systems there are variations.
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