-x should be C<-x>, reported by Gerben Wierda.

[p5sagit/p5-mst-13.2.git] / pod / perlport.pod

=head1 NAME

perlport - Writing portable Perl


=head1 DESCRIPTION

Perl 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 B<a> particular type
of computer, and taking advantage of a full B<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:

=over 4

=item Not all Perl programs have to be portable

There is no reason why 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 The vast majority of Perl B<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 I<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 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,
S<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 (L<"ISSUES">, platform-specific issues (L<"PLATFORMS">, and
builtin 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
(E<lt>IMG SRC="yellow_sign.gif" ALT="Under Construction"E<gt>).




=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 kind of Windows I/O uses C<\015\012>,
and S<Mac OS> uses C<\015>.

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>.  In DOSish perls, C<\n> usually means C<\012>, but
when accessing a file in "text" mode, STDIO translates it to (or from)
C<\015\012>.

Due to the "text" mode translation, DOSish perls have limitations
of using C<seek> and C<tell> when a file is being accessed in "text"
mode.  Specifically, if you stick to C<seek>-ing to locations you got
from C<tell> (and no others), you are usually free to use C<seek> and
C<tell> even in "text" mode.  In general, using C<seek> or C<tell> or
other file operations that count bytes instead of characters, without
considering the length of C<\n>, may be non-portable.  If you use
C<binmode> on a file, however, you can usually use C<seek> and C<tell>
with arbitrary values quite safely.

A common misconception in socket programming is that C<\n> eq C<\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

[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 C<\r\n>,
which translates those characters, along with all other
characters in text streams, from EBCDIC to ASCII.]

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 C<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 I<from> a socket, remember that the default input record
separator (C<$/>) is C<\n>, but code like this should recognize C<$/> as
C<\012> or C<\015\012>:

    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 C<\015>'s (C<\cM>'s) are stripped out
(and there was much rejoicing).


=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).  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 C<pack()> and C<unpack()> formats C<"n">
and C<"N">, the "network" orders, they are guaranteed to be portable.

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 C<Data::Dumper> (included in
the standard distribution as of Perl 5.005) and C<Storable>.

=head2 Files

Most 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, 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 idea of a single
root directory.

VMS, Windows, and OS/2 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> uses C<:> as a path separator instead of C</>.

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.

C<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 filing systems and disc names.

As with the newline problem above, there are modules that can help.  The
C<File::Spec> modules provide methods to do the Right Thing on whatever
platform happens to be running the program.

    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 C</> as a path separator for subdirectories.

Also of use is C<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
system-specific files, like F</etc/passwd>, F</etc/sendmail.conf>, or
F</etc/resolv.conf>.  For example the F</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
F<test.pl> and <Test.pl>, as many platforms have case-insensitive
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.

Likewise, if using C<AutoSplit>, try to keep the split functions to
8.3 naming and case-insensitive conventions; or, at the very least,
make it so the resulting files have a unique (case-insensitively)
first 8 characters.

Don't assume C<E<lt>> won't be the first character of a filename.  Always
use C<E<lt>> explicitly to open a file for reading:

    open(FILE, "<$existing_file") or die $!;


=head2 System Interaction

Not 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 C<close> files when you are done with them.
Don't C<unlink> or C<rename> an open file.  Don't C<tie> to or C<open> a
file that is already tied to or opened; C<untie> or C<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 C<%ENV>.
Don't count on C<%ENV> entries being case-sensitive, or even
case-preserving.

Don't count on signals.

Don't count on filename globbing.  Use C<opendir>, C<readdir>, and
C<closedir> instead.

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


=head2 Interprocess Communication (IPC)

In general, don't directly access the system in code that is meant to be
portable.  That means, no C<system>, C<exec>, C<fork>, C<pipe>, C<``>,
C<qx//>, C<open> with a C<|>, nor any of the other things that makes being
a Unix 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),
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 C<Mail::Send> and C<Mail::Mailer> modules
in the C<MailTools> distribution.  C<Mail::Mailer> provides several
mailing methods, including mail, sendmail, and direct SMTP
(via C<Net::SMTP>) if a mail transfer agent is not available.

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 (C<msg*(), sem*(), shm*()>) is not available
even in all UNIX platforms.

=head2 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.


=head2 Standard Modules

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

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

The good news is that at least some DBM module should be available, and
C<AnyDBM_File> will use whichever module it can find.  Of course, then
the code needs to be fairly strict, dropping to the lowest common
denominator (e.g., not exceeding 1K for each record).


=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 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 C<1 Jan 1970>) can be easily
converted into an OS-specific value using a module like
C<Date::Parse>.  An array of values, such as those returned by
C<localtime>, can be converted to an OS-specific representation using
C<Time::Local>.


=head2 Character sets and character encoding

Assume very little about character sets.  Do not assume anything about
the numerical values (C<ord()>, C<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 E<auml> comes
before the 'b'.


=head2 Internationalisation

If you may assume POSIX (a rather large assumption, that: in practise
that means UNIX) 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, among other things.


=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:

    # 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.


=head2 Security

Most 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).


=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 C<Config> module and the special
variable C<$^O> to differentiate platforms, as described in
L<"PLATFORMS">.


=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 or not
a given module works on a given platform.

=over 4

=item Mailing list: cpan-testers@perl.org

=item Testing results: C<http://www.connect.net/gbarr/cpan-test/>

=back


=head1 PLATFORMS

As of version 5.002, Perl is built with a C<$^O> variable that
indicates the operating system it was built on.  This was implemented
to help speed up code that would otherwise have to C<use Config;> and
use the value of C<$Config{'osname'}>.  Of course, to get
detailed information about the system, looking into C<%Config> is
certainly recommended.

=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 C<$^O> (hence C<$Config{'osname'}>,
too) is determined 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.  Here, for example, are a few of the more popular
Unix flavors:

    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 C<$Config{'archname'}> may depend on the hardware
architecture it may vary quite a lot, much more than the C<$^O>.

=head2 DOS and Derivatives

Perl 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 I<COMMAND.COM> and/or I<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 C</> or C<\> as the path separator.  However,
many command-line utilities of DOS vintage treat C</> as the option
prefix, so they 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 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 C<readdir> or used with functions like C<open> or C<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 I<pl2bat.bat> or I<pl2cmd> as appropriate to
put wrappers around your scripts.

Newline (C<\n>) is translated as C<\015\012> by STDIO when reading from
and writing to files.  C<binmode(FILEHANDLE)> will keep C<\n> translated
as C<\012> for that filehandle.  Since it is a noop on other systems,
C<binmode> should be used for cross-platform code that deals with binary
data.

The C<$^O> variable and the C<$Config{'archname'}> values for various
DOSish perls are as follows:

    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:

=over 4

=item The djgpp environment for DOS, C<http://www.delorie.com/djgpp/>

=item The EMX environment for DOS, OS/2, etc. C<emx@iaehv.nl>,
C<http://www.juge.com/bbs/Hobb.19.html>

=item Build instructions for Win32, L<perlwin32>.

=item The ActiveState Pages, C<http://www.activestate.com/>

=back


=head2 S<Mac OS>

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.  See I<MacPerl: Power and Ease> and L<"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 C<:>,
which is reserved as a path separator.

Instead of C<flock>, see C<FSpSetFLock> and C<FSpRstFLock> in the
C<Mac::Files> module.

In the MacPerl application, you can't run a program from the command line;
programs that expect C<@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 /\s+/, MacPerl::Ask('Arguments?');
    }

A MacPerl script saved as a droplet will populate C<@ARGV> with the full
pathnames of the files dropped onto the script.

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
C<system>, backticks, and piped C<open>.

"S<Mac OS>" is the proper name for the operating system, but the value
in C<$^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 =~ /^(\S+)/;
    $is_ppc    = $MacPerl::Architecture eq 'MacPPC';
    $is_68k    = $MacPerl::Architecture eq 'Mac68K';

S<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:

=over 4

=item The MacPerl Pages, C<http://www.ptf.com/macperl/>.

=item The MacPerl mailing list, C<mac-perl-request@iis.ee.ethz.ch>.

=back


=head2 VMS

Perl on VMS is discussed in F<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 C<$ ASSIGN/nolog/user SYS$COMMAND: SYS$INPUT> if your
perl-in-DCL script expects to do things like C<$read = E<lt>STDINE<gt>;>.

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 C</[A-Z0-9$_-]/>.

VMS' RMS filesystem is case insensitive and does not preserve case.
C<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 C<readdir> with a file named F<A.;5>
will return F<a.> (though that file could be opened with
C<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
C<PERL_ROOT:[LIB.2.3.4.5.6.7.8]> is a valid directory specification but
C<PERL_ROOT:[LIB.2.3.4.5.6.7.8.9]> is not.  F<Makefile.PL> authors might
have to take this into account, but at least they can refer to the former
as C</PERL_ROOT/lib/2/3/4/5/6/7/8/>.

The C<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 C<\n> represents depends on the type of file that is open.  It could
be C<\015>, C<\012>, C<\015\012>, or nothing.  Reading from a file
translates newlines to C<\012>, unless C<binmode> was executed on that
handle, just like DOSish perls.

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

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

    if (grep(/VMS_AXP/, @INC)) {
        print "I'm on Alpha!\n";
    } elsif (grep(/VMS_VAX/, @INC)) {
        print "I'm on VAX!\n";
    } else {
        print "I'm not so sure about where $^O is...\n";
    }

Also see:

=over 4

=item L<perlvms.pod>

=item vmsperl list, C<vmsperl-request@newman.upenn.edu>

Put words C<SUBSCRIBE VMSPERL> in message body.

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

=back


=head2 VOS

Perl on VOS is discussed in F<README.vos> in the perl distribution.
Note that perl on VOS can accept either VOS- or Unix-style file
specifications as in either of the following:

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

or even a mixture of both as in:

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

Note that 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.

The following C functions are unimplemented on VOS, any any attempt by
Perl to use them will result in a fatal error message and an immediate
exit from Perl:  dup, do_aspawn, do_spawn, execlp, execl, execvp, fork,
waitpid.  Once these functions become available in the VOS POSIX.1
implementation, you can either recompile and rebind Perl, or you can
download a newer port from ftp.stratus.com.

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

    if (grep(/VOS/, @INC)) {
        print "I'm on a Stratus box!\n";
    } else {
        print "I'm not on a Stratus box!\n";
        die;
    }

    if (grep(/860/, @INC)) {
        print "This box is a Stratus XA/R!\n";
    } elsif (grep(/7100/, @INC)) {
        print "This box is a Stratus HP 7100 or 8000!\n";
    } elsif (grep(/8000/, @INC)) {
        print "This box is a Stratus HP 8000!\n";
    } else {
        print "This box is a Stratus 68K...\n";
    }

Also see:

=over 4

=item L<README.vos>

=item 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 majordomo@list.stratagy.com.

=item VOS Perl on the web at C<http://ftp.stratus.com/pub/vos/vos.html>

=back


=head2 EBCDIC Platforms

Recent versions of Perl have been ported to platforms such as OS/400 on
AS/400 minicomputers as well as OS/390 & VM/ESA 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 & VM/ESA).  Note that on
the mainframe perl currently works under the "Unix system services
for OS/390" (formerly known as OpenEdition) and VM/ESA OpenEdition.

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

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

    print "Hello from perl!\n";

On these platforms, bear in mind that the EBCDIC character set may have
an effect on what happens with some perl functions (such as C<chr>,
C<pack>, C<print>, C<printf>, C<ord>, C<sort>, C<sprintf>, C<unpack>), as
well as bit-fiddling with ASCII constants using operators like C<^>, C<&>
and C<|>, not to mention dealing with socket interfaces to ASCII computers
(see L<"NEWLINES">).

Fortunately, most web servers for the mainframe will correctly translate
the C<\n> in the following statement to its ASCII equivalent (note that
C<\r> is the same under both Unix and OS/390 & VM/ESA):

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

The value of C<$^O> on OS/390 is "os390".

The value of C<$^O> on VM/ESA is "vmesa".
Some simple tricks for determining if you are running on an EBCDIC
platform could include any of the following (perhaps all):

    if ("\t" eq "\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:

=over 4

=item perl-mvs list

The perl-mvs@perl.org list is for discussion of porting issues as well as
general usage issues for all EBCDIC Perls.  Send a message body of
"subscribe perl-mvs" to majordomo@perl.org.

=item AS/400 Perl information at C<http://as400.rochester.ibm.com/>

=back


=head2 Acorn RISC OS

As Acorns use ASCII with newlines (C<\n>) in text files as C<\012> like
Unix and Unix filename emulation is turned on by default, it is quite
likely that most simple scripts will work "out of the box".  The native
filing system is modular, and individual filing systems are free to be
case-sensitive or insensitive, and are usually case-preserving.  Some
native filing systems have name length limits which file and directory
names are silently truncated to fit - scripts should be aware that the
standard disc filing system currently has a name length limit of B<10>
characters, with up to 77 items in a directory, but other filing systems
may not impose such limitations.

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 C<tr|/.|./|;>

Note that C<"ADFS::HardDisc.$.File" ne 'ADFS::HardDisc.$.File'> and that
the second stage of C<$> interpolation in regular expressions will fall
foul of the C<$.> if scripts are not careful.

Logical paths specified by system variables containing comma-separated
search lists are also allowed, hence C<System:Modules> is a valid
filename, and the filesystem will prefix C<Modules> with each section of
C<System$Path> until a name is made that points to an object on disc.
Writing to a new file C<System:Modules> would only be allowed if
C<System$Path> contains a single item list.  The filesystem will also
expand system variables in filenames if enclosed in angle brackets, so
C<E<lt>System$DirE<gt>.Modules> would look for the file
S<C<$ENV{'System$Dir'} . 'Modules'>>.  The obvious implication of this is
that B<fully qualified filenames can start with C<E<lt>E<gt>> and should
be protected when C<open> is used for input.

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

    foo.h           h.foo
    C:foo.h         C:h.foo        (logical path variable)
    sys/os.h        sys.h.os       (C compiler groks Unix-speak)
    10charname.c    c.10charname
    10charname.o    o.10charname
    11charname_.c   c.11charname   (assuming filesystem truncates at 10)

The Unix emulation library's translation of filenames to native assumes
that this sort of translation is required, and 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 C<foo/bar/baz.h>
and C<foo/bar/h/baz> both map to C<foo.bar.h.baz>, and that C<readdir> and
C<glob> cannot and do not attempt to emulate the reverse mapping.  Other
C<.>s in filenames are translated to C</>.

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

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 C<STDIN>, C<STDOUT>, or C<STDERR> to your children.

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

Extensions and XS are, in theory, buildable by anyone using free tools.
In practice, many don't, as users of the Acorn platform are used to binary
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 C<cd sdbm && make all>, and anything using quoting.

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

Also see:

=over 4

=item perl list

=back


=head2 Other perls

Perl 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 F<ports/> directory on CPAN for information, and possibly
binaries, for the likes of: aos, atari, lynxos, riscos, Tandem Guardian,
vos, I<etc.> (yes we know that some of these OSes may fall under the Unix
category, but we are not a standards body.)

See also:

=over 4

=item Atari, Guido Flohr's page C<http://stud.uni-sb.de/~gufl0000/>

=item HP 300 MPE/iX  C<http://www.cccd.edu/~markb/perlix.html>

=item Novell Netware

A free perl5-based PERL.NLM for Novell Netware is available from
C<http://www.novell.com/>

=back


=head1 FUNCTION IMPLEMENTATIONS

Listed 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.

For many functions, you can also query C<%Config>, exported by default
from C<Config.pm>.  For example, to check if the platform has the C<lstat>
call, check C<$Config{'d_lstat'}>.  See L<Config.pm> for a full
description of available variables.


=head2 Alphabetical Listing of Perl Functions

=over 8

=item -X FILEHANDLE

=item -X EXPR

=item -X

C<-r>, C<-w>, and C<-x> have only a very limited meaning; directories
and applications are executable, and there are no uid/gid
considerations. C<-o> is not supported. (S<Mac OS>)

C<-r>, C<-w>, C<-x>, and C<-o> tell whether or not file is accessible,
which may not reflect UIC-based file protections. (VMS)

C<-s> returns the size of the data fork, not the total size of data fork
plus resource fork.  (S<Mac OS>).

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

C<-R>, C<-W>, C<-X>, C<-O> are indistinguishable from C<-r>, C<-w>,
C<-x>, C<-o>. (S<Mac OS>, Win32, VMS, S<RISC OS>)

C<-b>, C<-c>, C<-k>, C<-g>, C<-p>, C<-u>, C<-A> are not implemented.
(S<Mac OS>)

C<-g>, C<-k>, C<-l>, C<-p>, C<-u>, C<-A> are not particularly meaningful.
(Win32, VMS, S<RISC OS>)

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

C<-T> and C<-B> are implemented, but might misclassify Mac text files
with foreign characters; this is the case will all platforms, but may
affect S<Mac OS> often. (S<Mac OS>)

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

C<-x> (or C<-X>) determine if a file has an executable file type.
(S<RISC OS>)

=item binmode FILEHANDLE

Meaningless. (S<Mac OS>, S<RISC OS>)

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

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

=item chmod LIST

Only limited meaning. Disabling/enabling write permission is mapped to
locking/unlocking the file. (S<Mac OS>)

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

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

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

=item chown LIST

Not implemented. (S<Mac OS>, Win32, Plan9, S<RISC OS>, VOS)

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

=item chroot FILENAME

=item chroot

Not implemented. (S<Mac OS>, Win32, VMS, Plan9, S<RISC OS>, VOS, VM/ESA)

=item crypt PLAINTEXT,SALT

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

Not implemented. (VOS)

=item dbmclose HASH

Not implemented. (VMS, Plan9, VOS)

=item dbmopen HASH,DBNAME,MODE

Not implemented. (VMS, Plan9, VOS)

=item dump LABEL

Not useful. (S<Mac OS>, S<RISC OS>)

Not implemented. (Win32)

Invokes VMS debugger. (VMS)

=item exec LIST

Not implemented. (S<Mac OS>)

Implemented via Spawn. (VM/ESA)
=item fcntl FILEHANDLE,FUNCTION,SCALAR

Not implemented. (Win32, VMS)

=item flock FILEHANDLE,OPERATION

Not implemented (S<Mac OS>, VMS, S<RISC OS>, VOS).

Available only on Windows NT (not on Windows 95). (Win32)

=item fork

Not implemented. (S<Mac OS>, Win32, AmigaOS, S<RISC OS>, VOS, VM/ESA)

=item getlogin

Not implemented. (S<Mac OS>, S<RISC OS>)

=item getpgrp PID

Not implemented. (S<Mac OS>, Win32, VMS, S<RISC OS>, VOS)

=item getppid

Not implemented. (S<Mac OS>, Win32, VMS, S<RISC OS>)

=item getpriority WHICH,WHO

Not implemented. (S<Mac OS>, Win32, VMS, S<RISC OS>, VOS, VM/ESA)

=item getpwnam NAME

Not implemented. (S<Mac OS>, Win32)

Not useful. (S<RISC OS>)

=item getgrnam NAME

Not implemented. (S<Mac OS>, Win32, VMS, S<RISC OS>)

=item getnetbyname NAME

Not implemented. (S<Mac OS>, Win32, Plan9)

=item getpwuid UID

Not implemented. (S<Mac OS>, Win32)

Not useful. (S<RISC OS>)

=item getgrgid GID

Not implemented. (S<Mac OS>, Win32, VMS, S<RISC OS>)

=item getnetbyaddr ADDR,ADDRTYPE

Not implemented. (S<Mac OS>, Win32, Plan9)

=item getprotobynumber NUMBER

Not implemented. (S<Mac OS>)

=item getservbyport PORT,PROTO

Not implemented. (S<Mac OS>)

=item getpwent

Not implemented. (S<Mac OS>, Win32, VM/ESA)

=item getgrent

Not implemented. (S<Mac OS>, Win32, VMS, VM/ESA)

=item gethostent

Not implemented. (S<Mac OS>, Win32)

=item getnetent

Not implemented. (S<Mac OS>, Win32, Plan9)

=item getprotoent

Not implemented. (S<Mac OS>, Win32, Plan9)

=item getservent

Not implemented. (Win32, Plan9)

=item setpwent

Not implemented. (S<Mac OS>, Win32, S<RISC OS>)

=item setgrent

Not implemented. (S<Mac OS>, Win32, VMS, S<RISC OS>)

=item sethostent STAYOPEN

Not implemented. (S<Mac OS>, Win32, Plan9, S<RISC OS>)

=item setnetent STAYOPEN

Not implemented. (S<Mac OS>, Win32, Plan9, S<RISC OS>)

=item setprotoent STAYOPEN

Not implemented. (S<Mac OS>, Win32, Plan9, S<RISC OS>)

=item setservent STAYOPEN

Not implemented. (Plan9, Win32, S<RISC OS>)

=item endpwent

Not implemented. (S<Mac OS>, Win32, VM/ESA)

=item endgrent

Not implemented. (S<Mac OS>, Win32, VMS, S<RISC OS>, VM/ESA)

=item endhostent

Not implemented. (S<Mac OS>, Win32)

=item endnetent

Not implemented. (S<Mac OS>, Win32, Plan9)

=item endprotoent

Not implemented. (S<Mac OS>, Win32, Plan9)

=item endservent

Not implemented. (Plan9, Win32)

=item getsockopt SOCKET,LEVEL,OPTNAME

Not implemented. (S<Mac OS>, Plan9)

=item glob EXPR

=item glob

Globbing built-in, but only C<*> and C<?> metacharacters are supported.
(S<Mac OS>)

Features depend on external perlglob.exe or perlglob.bat. May be
overridden with something like File::DosGlob, which is recommended.
(Win32)

Globbing built-in, but only C<*> and C<?> metacharacters are supported.
Globbing relies on operating system calls, which may return filenames
in any order.  As most filesystems are case-insensitive, even "sorted"
filenames will not be in case-sensitive order. (S<RISC OS>)

=item ioctl FILEHANDLE,FUNCTION,SCALAR

Not implemented. (VMS)

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

Available only for socket handles. (S<RISC OS>)

=item kill LIST

Not implemented, hence not useful for taint checking. (S<Mac OS>,
S<RISC OS>)

Available only for process handles returned by the C<system(1, ...)>
method of spawning a process. (Win32)

=item link OLDFILE,NEWFILE

Not implemented. (S<Mac OS>, Win32, VMS, S<RISC OS>)

=item lstat FILEHANDLE

=item lstat EXPR

=item lstat

Not implemented. (VMS, S<RISC OS>)

Return values may be bogus. (Win32)

=item msgctl ID,CMD,ARG

=item msgget KEY,FLAGS

=item msgsnd ID,MSG,FLAGS

=item msgrcv ID,VAR,SIZE,TYPE,FLAGS

Not implemented. (S<Mac OS>, Win32, VMS, Plan9, S<RISC OS>, VOS)

=item open FILEHANDLE,EXPR

=item open FILEHANDLE

The C<|> variants are only supported if ToolServer is installed.
(S<Mac OS>)

open to C<|-> and C<-|> are unsupported. (S<Mac OS>, Win32, S<RISC OS>)

=item pipe READHANDLE,WRITEHANDLE

Not implemented. (S<Mac OS>)

=item readlink EXPR

=item readlink

Not implemented. (Win32, VMS, S<RISC OS>)

=item select RBITS,WBITS,EBITS,TIMEOUT

Only implemented on sockets. (Win32)

Only reliable on sockets. (S<RISC OS>)

=item semctl ID,SEMNUM,CMD,ARG

=item semget KEY,NSEMS,FLAGS

=item semop KEY,OPSTRING

Not implemented. (S<Mac OS>, Win32, VMS, S<RISC OS>, VOS)

=item setpgrp PID,PGRP

Not implemented. (S<Mac OS>, Win32, VMS, S<RISC OS>, VOS)

=item setpriority WHICH,WHO,PRIORITY

Not implemented. (S<Mac OS>, Win32, VMS, S<RISC OS>, VOS)

=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL

Not implemented. (S<Mac OS>, Plan9)

=item shmctl ID,CMD,ARG

=item shmget KEY,SIZE,FLAGS

=item shmread ID,VAR,POS,SIZE

=item shmwrite ID,STRING,POS,SIZE

Not implemented. (S<Mac OS>, Win32, VMS, S<RISC OS>, VOS)

=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL

Not implemented. (S<Mac OS>, Win32, VMS, S<RISC OS>, VOS, VM/ESA)

=item stat FILEHANDLE

=item stat EXPR

=item stat

mtime and atime are the same thing, and ctime is creation time instead of
inode change time. (S<Mac OS>)

device and inode are not meaningful.  (Win32)

device and inode are not necessarily reliable.  (VMS)

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

=item symlink OLDFILE,NEWFILE

Not implemented. (Win32, VMS, S<RISC OS>)

=item syscall LIST

Not implemented. (S<Mac OS>, Win32, VMS, S<RISC OS>, VOS, VM/ESA)

=item sysopen FILEHANDLE,FILENAME,MODE,PERMS

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

=item system LIST

Only implemented if ToolServer is installed. (S<Mac OS>)

As an optimization, may not call the command shell specified in
C<$ENV{PERL5SHELL}>. C<system(1, @args)> spawns an external
process and immediately returns its process designator, without
waiting for it to terminate.  Return value may be used subsequently
in C<wait> or C<waitpid>.  (Win32)

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

=item times

Only the first entry returned is nonzero. (S<Mac OS>)

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

Not useful. (S<RISC OS>)

=item truncate FILEHANDLE,LENGTH

=item truncate EXPR,LENGTH

Not implemented. (VMS)

Truncation to zero-length only. (VOS)

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

=item umask EXPR

=item umask

Returns undef where unavailable, as of version 5.005.

=item utime LIST

Only the modification time is updated. (S<Mac OS>, VMS, S<RISC OS>)

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

=item wait

=item waitpid PID,FLAGS

Not implemented. (S<Mac OS>, VOS)

Can only be applied to process handles returned for processes spawned
using C<system(1, ...)>. (Win32)

Not useful. (S<RISC OS>)

=back

=head1 CHANGES

=over 4

=item 1.35, 9 September 1998

Updated for Stratus VOS.

=item 1.33, 06 August 1998

Integrate more minor changes.

=item 1.32, 05 August 1998

Integrate more minor changes.

=item 1.30, 03 August 1998

Major update for RISC OS, other minor changes.

=item 1.23, 10 July 1998

First public release with perl5.005.

=back

=head1 AUTHORS / CONTRIBUTORS

Abigail E<lt>abigail@fnx.comE<gt>,
Charles Bailey E<lt>bailey@newman.upenn.eduE<gt>,
Graham Barr E<lt>gbarr@pobox.comE<gt>,
Tom Christiansen E<lt>tchrist@perl.comE<gt>,
Nicholas Clark E<lt>Nicholas.Clark@liverpool.ac.ukE<gt>,
Andy Dougherty E<lt>doughera@lafcol.lafayette.eduE<gt>,
Dominic Dunlop E<lt>domo@vo.luE<gt>,
Neale Ferguson E<lt>neale@mailbox.tabnsw.com.auE<gt>
Paul Green E<lt>Paul_Green@stratus.comE<gt>,
M.J.T. Guy E<lt>mjtg@cus.cam.ac.ukE<gt>,
Jarkko Hietaniemi E<lt>jhi@iki.fi<gt>,
Luther Huffman E<lt>lutherh@stratcom.comE<gt>,
Nick Ing-Simmons E<lt>nick@ni-s.u-net.comE<gt>,
Andreas J. KE<ouml>nig E<lt>koenig@kulturbox.deE<gt>,
Andrew M. Langmead E<lt>aml@world.std.comE<gt>,
Paul Moore E<lt>Paul.Moore@uk.origin-it.comE<gt>,
Chris Nandor E<lt>pudge@pobox.comE<gt>,
Matthias Neeracher E<lt>neeri@iis.ee.ethz.chE<gt>,
Gary Ng E<lt>71564.1743@CompuServe.COME<gt>,
Tom Phoenix E<lt>rootbeer@teleport.comE<gt>,
Peter Prymmer E<lt>pvhp@forte.comE<gt>,
Hugo van der Sanden E<lt>hv@crypt0.demon.co.ukE<gt>,
Gurusamy Sarathy E<lt>gsar@umich.eduE<gt>,
Paul J. Schinder E<lt>schinder@pobox.comE<gt>,
Dan Sugalski E<lt>sugalskd@ous.eduE<gt>,
Nathan Torkington E<lt>gnat@frii.comE<gt>.

This document is maintained by Chris Nandor.

=head1 VERSION

Version 1.35, last modified 09 September 1998.