perlport.1

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.\" ========================================================================
.\"
.IX Title "PERLPORT 1"
.TH PERLPORT 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.SH "NAME"
perlport \- Writing portable Perl
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Perl runs on numerous operating systems.  While most of them share
much in common, they also have their own unique features.
.PP
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.
.PP
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.
.PP
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.
.PP
Be aware of two important points:
.IP "Not all Perl programs have to be portable" 4
.IX Item "Not all Perl programs have to be portable"
There is no reason you should not use Perl as a language to glue Unix
tools together, or to prototype a Macintosh application, or to manage the
Windows registry.  If it makes no sense to aim for portability for one
reason or another in a given program, then don't bother.
.IP "Nearly all of Perl already \fIis\fR portable" 4
.IX Item "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.
.PP
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\ \s-1OS\s0, \s-1VMS\s0, etc.), consider writing platform-specific code.
.PP
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.
.PP
The material below is separated into three main sections: main issues of
portability (\*(L"\s-1ISSUES\s0\*(R"), platform-specific issues (\*(L"\s-1PLATFORMS\s0\*(R"), and
built-in perl functions that behave differently on various ports
(\*(L"\s-1FUNCTION\s0 \s-1IMPLEMENTATIONS\s0\*(R").
.PP
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
(\f(CW\*(C`<IMG SRC="yellow_sign.gif" ALT="Under Construction">\*(C'\fR).
.SH "ISSUES"
.IX Header "ISSUES"
.Sh "Newlines"
.IX Subsection "Newlines"
In most operating systems, lines in files are terminated by newlines.
Just what is used as a newline may vary from \s-1OS\s0 to \s-1OS\s0.  Unix
traditionally uses \f(CW\*(C`\e012\*(C'\fR, one type of DOSish I/O uses \f(CW\*(C`\e015\e012\*(C'\fR,
and Mac\ \s-1OS\s0 uses \f(CW\*(C`\e015\*(C'\fR.
.PP
Perl uses \f(CW\*(C`\en\*(C'\fR to represent the \*(L"logical\*(R" newline, where what is
logical may depend on the platform in use.  In MacPerl, \f(CW\*(C`\en\*(C'\fR always
means \f(CW\*(C`\e015\*(C'\fR.  In DOSish perls, \f(CW\*(C`\en\*(C'\fR usually means \f(CW\*(C`\e012\*(C'\fR, but
when accessing a file in \*(L"text\*(R" mode, \s-1STDIO\s0 translates it to (or
from) \f(CW\*(C`\e015\e012\*(C'\fR, depending on whether you're reading or writing.
Unix does the same thing on ttys in canonical mode.  \f(CW\*(C`\e015\e012\*(C'\fR
is commonly referred to as \s-1CRLF\s0.
.PP
To trim trailing newlines from text lines use \fIchomp()\fR.  With default 
settings that function looks for a trailing \f(CW\*(C`\en\*(C'\fR character and thus 
trims in a portable way.
.PP
When dealing with binary files (or text files in binary mode) be sure
to explicitly set $/ to the appropriate value for your file format
before using \fIchomp()\fR.
.PP
Because of the \*(L"text\*(R" mode translation, DOSish perls have limitations
in using \f(CW\*(C`seek\*(C'\fR and \f(CW\*(C`tell\*(C'\fR on a file accessed in \*(L"text\*(R" mode.
Stick to \f(CW\*(C`seek\*(C'\fR\-ing to locations you got from \f(CW\*(C`tell\*(C'\fR (and no
others), and you are usually free to use \f(CW\*(C`seek\*(C'\fR and \f(CW\*(C`tell\*(C'\fR even
in \*(L"text\*(R" mode.  Using \f(CW\*(C`seek\*(C'\fR or \f(CW\*(C`tell\*(C'\fR or other file operations
may be non-portable.  If you use \f(CW\*(C`binmode\*(C'\fR on a file, however, you
can usually \f(CW\*(C`seek\*(C'\fR and \f(CW\*(C`tell\*(C'\fR with arbitrary values in safety.
.PP
A common misconception in socket programming is that \f(CW\*(C`\en\*(C'\fR eq \f(CW\*(C`\e012\*(C'\fR
everywhere.  When using protocols such as common Internet protocols,
\&\f(CW\*(C`\e012\*(C'\fR and \f(CW\*(C`\e015\*(C'\fR are called for specifically, and the values of
the logical \f(CW\*(C`\en\*(C'\fR and \f(CW\*(C`\er\*(C'\fR (carriage return) are not reliable.
.PP
.Vb 2
\&    print SOCKET "Hi there, client!\er\en";      # WRONG
\&    print SOCKET "Hi there, client!\e015\e012";  # RIGHT
.Ve
.PP
However, using \f(CW\*(C`\e015\e012\*(C'\fR (or \f(CW\*(C`\ecM\ecJ\*(C'\fR, or \f(CW\*(C`\ex0D\ex0A\*(C'\fR) can be tedious
and unsightly, as well as confusing to those maintaining the code.  As
such, the Socket module supplies the Right Thing for those who want it.
.PP
.Vb 2
\&    use Socket qw(:DEFAULT :crlf);
\&    print SOCKET "Hi there, client!$CRLF"      # RIGHT
.Ve
.PP
When reading from a socket, remember that the default input record
separator \f(CW$/\fR is \f(CW\*(C`\en\*(C'\fR, but robust socket code will recognize as
either \f(CW\*(C`\e012\*(C'\fR or \f(CW\*(C`\e015\e012\*(C'\fR as end of line:
.PP
.Vb 3
\&    while (<SOCKET>) {
\&        # ...
\&    }
.Ve
.PP
Because both \s-1CRLF\s0 and \s-1LF\s0 end in \s-1LF\s0, the input record separator can
be set to \s-1LF\s0 and any \s-1CR\s0 stripped later.  Better to write:
.PP
.Vb 2
\&    use Socket qw(:DEFAULT :crlf);
\&    local($/) = LF;      # not needed if $/ is already \e012
\&
\&    while (<SOCKET>) {
\&        s/$CR?$LF/\en/;   # not sure if socket uses LF or CRLF, OK
\&    #   s/\e015?\e012/\en/; # same thing
\&    }
.Ve