[inseparable changes from patch from perl5.003_24 to perl5.003_25]

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

=head1 NAME

perldelta - what's new for perl5.004

=head1 DESCRIPTION

This document describes differences between the 5.003 release (as
documented in I<Programming Perl>, second edition--the Camel Book) and
this one.

=head1 Supported Environments

Perl5.004 builds out of the box on Unix, Plan9, LynxOS, VMS, OS/2,
QNX, and AmigaOS.

=head1 Core Changes

Most importantly, many bugs were fixed.  See the F<Changes>
file in the distribution for details.

=head2 Compilation Option: Binary Compatibility With 5.003

There is a new Configure question that asks if you want to maintain
binary compatibility with Perl 5.003.  If you choose binary
compatibility, you do not have to recompile your extensions, but you
might have symbol conflicts if you embed Perl in another application,
just as in the 5.003 release.  By default, binary compatibility
is preserved at the expense of symbol table pollution.

=head2 New Opcode Module and Revised Safe Module

A new Opcode module supports the creation, manipulation and
application of opcode masks.  The revised Safe module has a new API
and is implemented using the new Opcode module.  Please read the new
Opcode and Safe documentation.

=head2 Internal Change: FileHandle Deprecated

Filehandles are now stored internally as type IO::Handle.
Although C<use FileHandle> and C<*STDOUT{FILEHANDLE}>
are still supported for backwards compatibility,
C<use IO::Handle> (or C<IO::Seekable> or C<IO::File>) and
C<*STDOUT{IO}> are the way of the future.

=head2 Internal Change: PerlIO internal IO abstraction interface

It is now possible to build Perl with AT&T's sfio IO package
instead of stdio.  See L<perlapio> for more details, and
the F<INSTALL> file for how to use it.

=head2 New and Changed Built-in Variables

=over

=item $^E

Extended error message on some platforms.  (Also known as
$EXTENDED_OS_ERROR if you C<use English>).

=item $^H

The current set of syntax checks enabled by C<use strict>.  See the
documentation of C<strict> for more details.  Not actually new, but
newly documented.
Because it is intended for internal use by Perl core components,
there is no C<use English> long name for this variable.

=item $^M

By default, running out of memory it is not trappable.  However, if
compiled for this, Perl may use the contents of C<$^M> as an emergency
pool after die()ing with this message.  Suppose that your Perl were
compiled with -DEMERGENCY_SBRK and used Perl's malloc.  Then

    $^M = 'a' x (1<<16);

would allocate a 64K buffer for use when in emergency.
See the F<INSTALL> file for information on how to enable this option.
As a disincentive to casual use of this advanced feature,
there is no C<use English> long name for this variable.

=item $^S

The status returned by the last pipe close, back-tick (C<``>) command, or
system() operator, in the native system format.  On UNIX and UNIX-like
systems, C<$^S> is a synonym for C<$?>.  Elsewhere, C<$^S> can be used to
determine aspects of child status that are system-specific.  Check C<$^O>
before using this variable.  (Mnemonic: System-Specific Subprocess Status.
Also known as $SYSTEM_CHILD_STATUS if you C<use English>.)

=back

=head2 New and Changed Built-in Functions

=over

=item delete on slices

This now works.  (e.g. C<delete @ENV{'PATH', 'MANPATH'}>)

=item flock

is now supported on more platforms, and prefers fcntl
to lockf when emulating.

=item keys as an lvalue

As an lvalue, C<keys> allows you to increase the number of hash buckets
allocated for the given associative array.  This can gain you a measure
of efficiency if you know the hash is going to get big.  (This is
similar to pre-extending an array by assigning a larger number to
$#array.)  If you say

    keys %hash = 200;

then C<%hash> will have at least 200 buckets allocated for it.  These
buckets will be retained even if you do C<%hash = ()>; use C<undef
%hash> if you want to free the storage while C<%hash> is still in scope.
You can't shrink the number of buckets allocated for the hash using
C<keys> in this way (but you needn't worry about doing this by accident,
as trying has no effect).

=item my() in Control Structures

You can now use my() (with or without the parentheses) in the control
expressions of control structures such as:

    while (my $line = <>) {
        $line = lc $line;
    } continue {
        print $line;
    }

    if ((my $answer = <STDIN>) =~ /^y(es)?$/i) {
        user_agrees();
    } elsif ($answer =~ /^n(o)?$/i) {
        user_disagrees();
    } else {
        chomp $answer;
        die "`$answer' is neither `yes' nor `no'";
    }

Also, you can declare a foreach loop control variable as lexical by
preceding it with the word "my".  For example, in:

    foreach my $i (1, 2, 3) {
        some_function();
    }

$i is a lexical variable, and the scope of $i extends to the end of
the loop, but not beyond it.

Note that you still cannot use my() on global punctuation variables
such as $_ and the like.

=item unpack() and pack()

A new format 'w' represents a BER compressed integer (as defined in
ASN.1).  Its format is a sequence of one or more bytes, each of which
provides seven bits of the total value, with the most significant
first.  Bit eight of each byte is set, except for the last byte, in
which bit eight is clear.

=item use VERSION

If the first argument to C<use> is a number, it is treated as a version
number instead of a module name.  If the version of the Perl interpreter
is less than VERSION, then an error message is printed and Perl exits
immediately.  Because C<use> occurs at compile time, this check happens
immediately during the compilation process, unlike C<require VERSION>,
which waits until run-time for the check.  This is often useful if you
need to check the current Perl version before C<use>ing library modules
which have changed in incompatible ways from older versions of Perl.
(We try not to do this more than we have to.)

=item use Module VERSION LIST

If the VERSION argument is present between Module and LIST, then the
C<use> will call the VERSION method in class Module with the given
version as an argument.  The default VERSION method, inherited from
the Universal class, croaks if the given version is larger than the
value of the variable $Module::VERSION.  (Note that there is not a
comma after VERSION!)

This version-checking mechanism is similar to the one currently used
in the Exporter module, but it is faster and can be used with modules
that don't use the Exporter.  It is the recommended method for new
code.

=item prototype(FUNCTION)

Returns the prototype of a function as a string (or C<undef> if the
function has no prototype).  FUNCTION is a reference to or the name of the
function whose prototype you want to retrieve.
(Not actually new; just never documented before.)

=item $_ as Default

Functions documented in the Camel to default to $_ now in
fact do, and all those that do are so documented in L<perlfunc>.

=item C<m//g> does not trigger a pos() reset on failure

The C<m//g> match iteration construct used to reset the iteration
when it failed to match (so that the next C<m//g> match would start at
the beginning of the string).  You now have to explicitly do a
C<pos $str = 0;> to reset the "last match" position, or modify the
string in some way.  This change makes it practical to chain C<m//g>
matches together in conjunction with ordinary matches using the C<\G>
zero-width assertion.  See L<perlop> and L<perlre>.

=item nested C<sub{}> closures work now

Prior to the 5.004 release, nested anonymous functions 
didn't work right.  They do now.

=item formats work right on changing lexicals

Just like anonymous functions that contain lexical variables
that change (like a lexical index variable for a C<foreach> loop),
formats now work properly.  For example, this silently failed
before, and is fine now:

    my $i;
    foreach $i ( 1 .. 10 ) {
        format =
        my i is @#
        $i
    .
        write;
    } 

=back

=head2 New Built-in Methods

The C<UNIVERSAL> package automatically contains the following methods that
are inherited by all other classes:

=over

=item isa(CLASS)

C<isa> returns I<true> if its object is blessed into a sub-class of C<CLASS>

C<isa> is also exportable and can be called as a sub with two arguments. This
allows the ability to check what a reference points to. Example:

    use UNIVERSAL qw(isa);

    if(isa($ref, 'ARRAY')) {
       ...
    }

=item can(METHOD)

C<can> checks to see if its object has a method called C<METHOD>,
if it does then a reference to the sub is returned; if it does not then
I<undef> is returned.

=item VERSION( [NEED] )

C<VERSION> returns the version number of the class (package).  If the
NEED argument is given then it will check that the current version (as
defined by the $VERSION variable in the given package) not less than
NEED; it will die if this is not the case.  This method is normally
called as a class method.  This method is called automatically by the
C<VERSION> form of C<use>.

    use A 1.2 qw(some imported subs);
    # implies:
    A->VERSION(1.2);

=item class()

C<class> returns the class name of its object.

=item is_instance()

C<is_instance> returns true if its object is an instance of some
class, false if its object is the class (package) itself. Example

    A->is_instance();       # False

    $var = 'A';
    $var->is_instance();    # False

    $ref = bless [], 'A';
    $ref->is_instance();    # True

This can be useful for methods that wish to easily distinguish
whether they were invoked as class or as instance methods.

    sub some_meth {
        my $classname = shift;
        if ($classname->is_instance()) {
            die "unexpectedly called as instance not class method";
        } 
        .....
    } 

=back

B<NOTE:> C<can> directly uses Perl's internal code for method lookup, and
C<isa> uses a very similar method and caching strategy. This may cause
strange effects if the Perl code dynamically changes @ISA in any package.

You may add other methods to the UNIVERSAL class via Perl or XS code.
You do not need to C<use UNIVERSAL> in order to make these methods
available to your program.  This is necessary only if you wish to
have C<isa> available as a plain subroutine in the current package.

=head2 TIEHANDLE Now Supported

See L<perltie> for other kinds of tie()s.

=over

=item TIEHANDLE classname, LIST

This is the constructor for the class.  That means it is expected to
return an object of some sort. The reference can be used to
hold some internal information.

    sub TIEHANDLE { 
        print "<shout>\n"; 
        my $i; 
        return bless \$i, shift;
    }

=item PRINT this, LIST

This method will be triggered every time the tied handle is printed to.
Beyond its self reference it also expects the list that was passed to
the print function.

    sub PRINT { 
        $r = shift; 
        $$r++; 
        return print join( $, => map {uc} @_), $\;
    }

=item READLINE this

This method will be called when the handle is read from. The method
should return undef when there is no more data.

    sub READLINE { 
        $r = shift; 
        return "PRINT called $$r times\n"; 
    }

=item DESTROY this

As with the other types of ties, this method will be called when the
tied handle is about to be destroyed. This is useful for debugging and
possibly for cleaning up.

    sub DESTROY { 
        print "</shout>\n";
    }

=back

=item Efficiency Enhancements

All hash keys with the same string are only allocated once, so
even if you have 100 copies of the same hash, the immutable keys
never have to be re-allocated.

Functions that have an empty prototype and that do nothing but return
a fixed value are now inlined (e.g. C<sub PI () { 3.14159 }>).

=head1 Pragmata

Three new pragmatic modules exist:

=over

=item use blib

=item use blib 'dir'

Looks for MakeMaker-like I<'blib'> directory structure starting in
I<dir> (or current directory) and working back up to five levels of
parent directories.

Intended for use on command line with B<-M> option as a way of testing
arbitrary scripts against an uninstalled version of a package.

=item use locale

Tells the compiler to enable (or disable) the use of POSIX locales for
built-in operations.

When C<use locale> is in effect, the current LC_CTYPE locale is used
for regular expressions and case mapping; LC_COLLATE for string
ordering; and LC_NUMERIC for numeric formating in printf and sprintf
(but B<not> in print).  LC_NUMERIC is always used in write, since
lexical scoping of formats is problematic at best.

Each C<use locale> or C<no locale> affects statements to the end of
the enclosing BLOCK or, if not inside a BLOCK, to the end of the
current file.  Locales can be switched and queried with
POSIX::setlocale().

See L<perllocale> for more information.

=item use ops

Disable unsafe opcodes, or any named opcodes, when compiling Perl code.

=back

=head1 Modules

=head2 Installation Directories

The I<installperl> script now places the Perl source files for
extensions in the architecture-specific library directory, which is
where the shared libraries for extensions have always been.  This
change is intended to allow administrators to keep the Perl 5.004
library directory unchanged from a previous version, without running
the risk of binary incompatibility between extensions' Perl source and
shared libraries.

=head2 Fcntl

New constants in the existing Fcntl modules are now supported,
provided that your operating system happens to support them:

    F_GETOWN F_SETOWN
    O_ASYNC O_DEFER O_DSYNC O_FSYNC O_SYNC 
    O_EXLOCK O_SHLOCK

These constants are intended for use with the Perl operators sysopen()
and fcntl() and the basic database modules like SDBM_File.  For the
exact meaning of these and other Fcntl constants please refer to your
operating system's documentation for fcntl() and open().

In addition, the Fcntl module now provides these constants for use
with the Perl operator flock():

        LOCK_SH LOCK_EX LOCK_NB LOCK_UN

These constants are defined in all environments (because where there is
no flock() system call, Perl emulates it).  However, for historical
reasons, these constants are not exported unless they are explicitly
requested with the ":flock" tag (e.g. C<use Fcntl ':flock'>).

=head2 Module Information Summary

Brand new modules, arranged by topic rather than strictly
alphabetically:

    CPAN                 interface to Comprehensive Perl Archive Network
    CPAN::FirstTime      create a CPAN configuration file
    CPAN::Nox            run CPAN while avoiding compiled extensions

    IO.pm                Top-level interface to IO::* classes
    IO/File.pm           IO::File extension Perl module
    IO/Handle.pm         IO::Handle extension Perl module
    IO/Pipe.pm           IO::Pipe extension Perl module
    IO/Seekable.pm       IO::Seekable extension Perl module
    IO/Select.pm         IO::Select extension Perl module
    IO/Socket.pm         IO::Socket extension Perl module

    Opcode.pm            Disable named opcodes when compiling Perl code

    ExtUtils/Embed.pm    Utilities for embedding Perl in C programs
    ExtUtils/testlib.pm  Fixes up @INC to use just-built extension

    Fatal.pm             Make do-or-die equivalents of functions
    FindBin.pm           Find path of currently executing program

    Class/Template.pm    Structure/member template builder
    File/stat.pm         Object-oriented wrapper around CORE::stat
    Net/hostent.pm       Object-oriented wrapper around CORE::gethost*
    Net/netent.pm        Object-oriented wrapper around CORE::getnet*
    Net/protoent.pm      Object-oriented wrapper around CORE::getproto*
    Net/servent.pm       Object-oriented wrapper around CORE::getserv*
    Time/gmtime.pm       Object-oriented wrapper around CORE::gmtime
    Time/localtime.pm    Object-oriented wrapper around CORE::localtime
    Time/tm.pm           Perl implementation of "struct tm" for {gm,local}time
    User/grent.pm        Object-oriented wrapper around CORE::getgr*
    User/pwent.pm        Object-oriented wrapper around CORE::getpw*

    Tie/RefHash.pm       Base class for tied hashes with references as keys

    UNIVERSAL.pm         Base class for *ALL* classes

=head2 IO

The IO module provides a simple mechanism to load all of the IO modules at one
go.  Currently this includes:

     IO::Handle
     IO::Seekable
     IO::File
     IO::Pipe
     IO::Socket

For more information on any of these modules, please see its
respective documentation.

=head2 Math::Complex

The Math::Complex module has been totally rewritten, and now supports
more operations.  These are overloaded:

     + - * / ** <=> neg ~ abs sqrt exp log sin cos atan2 "" (stringify)

And these functions are now exported:

    pi i Re Im arg
    log10 logn cbrt root
    tan cotan asin acos atan acotan
    sinh cosh tanh cotanh asinh acosh atanh acotanh
    cplx cplxe

=head2 DB_File

There have been quite a few changes made to DB_File. Here are a few of
the highlights:

=over

=item *

Fixed a handful of bugs.

=item *

By public demand, added support for the standard hash function exists().

=item *

Made it compatible with Berkeley DB 1.86.

=item *

Made negative subscripts work with RECNO interface.

=item *

Changed the default flags from O_RDWR to O_CREAT|O_RDWR and the default
mode from 0640 to 0666.

=item *

Made DB_File automatically import the open() constants (O_RDWR,
O_CREAT etc.) from Fcntl, if available.

=item *

Updated documentation.

=back

Refer to the HISTORY section in DB_File.pm for a complete list of
changes. Everything after DB_File 1.01 has been added since 5.003.

=head2 Net::Ping

Major rewrite - support added for both udp echo and real icmp pings.

=head2 Overridden Built-ins

Many of the Perl built-ins returning lists now have
object-oriented overrides.  These are:

    File::stat
    Net::hostent
    Net::netent
    Net::protoent
    Net::servent
    Time::gmtime
    Time::localtime
    User::grent
    User::pwent

For example, you can now say

    use File::stat;
    use User::pwent;
    $his = (stat($filename)->st_uid == pwent($whoever)->pw_uid);

=head1 Utility Changes

=head2 xsubpp

=over

=item C<void> XSUBs now default to returning nothing

Due to a documentation/implementation bug in previous versions of
Perl, XSUBs with a return type of C<void> have actually been
returning one value.  Usually that value was the GV for the XSUB,
but sometimes it was some already freed or reused value, which would
sometimes lead to program failure.

In Perl 5.004, if an XSUB is declared as returning C<void>, it
actually returns no value, i.e. an empty list (though there is a
backward-compatibility exception; see below).  If your XSUB really
does return an SV, you should give it a return type of C<SV *>.

For backward compatibility, I<xsubpp> tries to guess whether a
C<void> XSUB is really C<void> or if it wants to return an C<SV *>.
It does so by examining the text of the XSUB: if I<xsubpp> finds
what looks like an assignment to C<ST(0)>, it assumes that the
XSUB's return type is really C<SV *>.

=back

=head1 C Language API Changes

=over

=item C<gv_fetchmethod> and C<perl_call_sv>

The C<gv_fetchmethod> function finds a method for an object, just like
in Perl 5.003.  The GV it returns may be a method cache entry.
However, in Perl 5.004, method cache entries are not visible to users;
therefore, they can no longer be passed directly to C<perl_call_sv>.
Instead, you should use the C<GvCV> macro on the GV to extract its CV,
and pass the CV to C<perl_call_sv>.

The most likely symptom of passing the result of C<gv_fetchmethod> to
C<perl_call_sv> is Perl's producing an "Undefined subroutine called"
error on the I<second> call to a given method (since there is no cache
on the first call).

=back

=head1 Documentation Changes

Many of the base and library pods were updated.  These
new pods are included in section 1:

=over

=item L<perldelta>

This document.

=item L<perllocale>

Locale support (internationalization and localization).

=item L<perltoot>

Tutorial on Perl OO programming.

=item L<perlapio>

Perl internal IO abstraction interface.

=item L<perldebug>

Although not new, this has been massively updated.

=item L<perlsec>

Although not new, this has been massively updated.

=back

=head1 New Diagnostics

Several new conditions will trigger warnings that were
silent before.  Some only affect certain platforms.
The following new warnings and errors outline these.  
These messages are classified as follows (listed in
increasing order of desperation):

   (W) A warning (optional).
   (D) A deprecation (optional).
   (S) A severe warning (mandatory).
   (F) A fatal error (trappable).
   (P) An internal error you should never see (trappable).
   (X) A very fatal error (non-trappable).
   (A) An alien error message (not generated by Perl).

=over

=item "my" variable %s masks earlier declaration in same scope

(S) A lexical variable has been redeclared in the same scope, effectively
eliminating all access to the previous instance.  This is almost always
a typographical error.  Note that the earlier variable will still exist
until the end of the scope or until all closure referents to it are
destroyed.

=item %s argument is not a HASH element or slice

(F) The argument to delete() must be either a hash element, such as

    $foo{$bar}
    $ref->[12]->{"susie"}

or a hash slice, such as

    @foo{$bar, $baz, $xyzzy}
    @{$ref->[12]}{"susie", "queue"}

=item Allocation too large: %lx

(X) You can't allocate more than 64K on an MSDOS machine.

=item Allocation too large

(F) You can't allocate more than 2^31+"small amount" bytes.

=item Attempt to free non-existent shared string

(P) Perl maintains a reference counted internal table of strings to
optimize the storage and access of hash keys and other strings.  This
indicates someone tried to decrement the reference count of a string
that can no longer be found in the table.

=item Attempt to use reference as lvalue in substr

(W) You supplied a reference as the first argument to substr() used
as an lvalue, which is pretty strange.  Perhaps you forgot to
dereference it first.  See L<perlfunc/substr>.

=item Unsupported function fork

(F) Your version of executable does not support forking.

Note that under some systems, like OS/2, there may be different flavors of
Perl executables, some of which may support fork, some not. Try changing
the name you call Perl by to C<perl_>, C<perl__>, and so on.

=item Ill-formed logical name |%s| in prime_env_iter

(W) A warning peculiar to VMS.  A logical name was encountered when preparing
to iterate over %ENV which violates the syntactic rules governing logical
names.  Since it cannot be translated normally, it is skipped, and will not
appear in %ENV.  This may be a benign occurrence, as some software packages
might directly modify logical name tables and introduce non-standard names,
or it may indicate that a logical name table has been corrupted.

=item Can't use bareword ("%s") as %s ref while "strict refs" in use

(F) Only hard references are allowed by "strict refs".  Symbolic references
are disallowed.  See L<perlref>.

=item Constant subroutine %s redefined

(S) You redefined a subroutine which had previously been eligible for
inlining.  See L<perlsub/"Constant Functions"> for commentary and
workarounds.

=item Died

(F) You passed die() an empty string (the equivalent of C<die "">) or
you called it with no args and both C<$@> and C<$_> were empty.

=item Integer overflow in hex number

(S) The literal hex number you have specified is too big for your
architecture. On a 32-bit architecture the largest hex literal is
0xFFFFFFFF.

=item Integer overflow in octal number

(S) The literal octal number you have specified is too big for your
architecture. On a 32-bit architecture the largest octal literal is
037777777777.

=item Name "%s::%s" used only once: possible typo

(W) Typographical errors often show up as unique variable names.
If you had a good reason for having a unique name, then just mention
it again somehow to suppress the message (the C<use vars> pragma is
provided for just this purpose).

=item Null picture in formline

(F) The first argument to formline must be a valid format picture
specification.  It was found to be empty, which probably means you
supplied it an uninitialized value.  See L<perlform>.

=item Offset outside string

(F) You tried to do a read/write/send/recv operation with an offset
pointing outside the buffer.  This is difficult to imagine.
The sole exception to this is that C<sysread()>ing past the buffer
will extend the buffer and zero pad the new area.

=item Stub found while resolving method `%s' overloading `%s' in package `%s'

(P) Overloading resolution over @ISA tree may be broken by importing stubs.
Stubs should never be implicitely created, but explicit calls to C<can>
may break this.

=item Cannot resolve method `%s' overloading `%s' in package `s'

(P) Internal error trying to resolve overloading specified by a method
name (as opposed to a subroutine reference).

=item Out of memory!

(X|F) The malloc() function returned 0, indicating there was insufficient
remaining memory (or virtual memory) to satisfy the request.

The request was judged to be small, so the possibility to trap it
depends on the way Perl was compiled.  By default it is not trappable.
However, if compiled for this, Perl may use the contents of C<$^M> as
an emergency pool after die()ing with this message.  In this case the
error is trappable I<once>.

=item Out of memory during request for %s

(F) The malloc() function returned 0, indicating there was insufficient
remaining memory (or virtual memory) to satisfy the request. However,
the request was judged large enough (compile-time default is 64K), so
a possibility to shut down by trapping this error is granted.

=item Possible attempt to put comments in qw() list

(W) qw() lists contain items separated by whitespace; as with literal
strings, comment characters are not ignored, but are instead treated
as literal data.  (You may have used different delimiters than the
exclamation marks parentheses shown here; braces are also frequently
used.)

You probably wrote something like this:

    @list = qw( 
        a # a comment
        b # another comment
    );

when you should have written this:

    @list = qw(
        a 
        b
    );

If you really want comments, build your list the
old-fashioned way, with quotes and commas:

    @list = (
        'a',    # a comment
        'b',    # another comment
    );

=item Possible attempt to separate words with commas

(W) qw() lists contain items separated by whitespace; therefore commas
aren't needed to separate the items. (You may have used different
delimiters than the parentheses shown here; braces are also frequently
used.)

You probably wrote something like this: 

    qw! a, b, c !;

which puts literal commas into some of the list items.  Write it without
commas if you don't want them to appear in your data:

    qw! a b c !;

=item Scalar value @%s{%s} better written as $%s{%s}

(W) You've used a hash slice (indicated by @) to select a single element of
a hash.  Generally it's better to ask for a scalar value (indicated by $).
The difference is that C<$foo{&bar}> always behaves like a scalar, both when
assigning to it and when evaluating its argument, while C<@foo{&bar}> behaves
like a list when you assign to it, and provides a list context to its
subscript, which can do weird things if you're expecting only one subscript.

=item untie attempted while %d inner references still exist

(W) A copy of the object returned from C<tie> (or C<tied>) was still
valid when C<untie> was called.

=item Value of %s construct can be "0"; test with defined()

(W) In a conditional expression, you used <HANDLE>, <*> (glob), or
C<readdir> as a boolean value.  Each of these constructs can return a
value of "0"; that would make the conditional expression false, which
is probably not what you intended.  When using these constructs in
conditional expressions, test their values with the C<defined> operator.

=item Variable "%s" may be unavailable

(W) An inner (nested) I<anonymous> subroutine is inside a I<named>
subroutine, and outside that is another subroutine; and the anonymous
(innermost) subroutine is referencing a lexical variable defined in
the outermost subroutine.  For example:

   sub outermost { my $a; sub middle { sub { $a } } }

If the anonymous subroutine is called or referenced (directly or
indirectly) from the outermost subroutine, it will share the variable
as you would expect.  But if the anonymous subroutine is called or
referenced when the outermost subroutine is not active, it will see
the value of the shared variable as it was before and during the
*first* call to the outermost subroutine, which is probably not what
you want.

In these circumstances, it is usually best to make the middle
subroutine anonymous, using the C<sub {}> syntax.  Perl has specific
support for shared variables in nested anonymous subroutines; a named
subroutine in between interferes with this feature.

=item Variable "%s" will not stay shared

(W) An inner (nested) I<named> subroutine is referencing a lexical
variable defined in an outer subroutine.

When the inner subroutine is called, it will probably see the value of
the outer subroutine's variable as it was before and during the
*first* call to the outer subroutine; in this case, after the first
call to the outer subroutine is complete, the inner and outer
subroutines will no longer share a common value for the variable.  In
other words, the variable will no longer be shared.

Furthermore, if the outer subroutine is anonymous and references a
lexical variable outside itself, then the outer and inner subroutines
will I<never> share the given variable.

This problem can usually be solved by making the inner subroutine
anonymous, using the C<sub {}> syntax.  When inner anonymous subs that
reference variables in outer subroutines are called or referenced,
they are automatically re-bound to the current values of such
variables.

=item Warning: something's wrong

(W) You passed warn() an empty string (the equivalent of C<warn "">) or
you called it with no args and C<$_> was empty.

=item Got an error from DosAllocMem

(P) An error peculiar to OS/2.  Most probably you're using an obsolete
version of Perl, and this should not happen anyway.

=item Malformed PERLLIB_PREFIX

(F) An error peculiar to OS/2. PERLLIB_PREFIX should be of the form

    prefix1;prefix2

or

    prefix1 prefix2

with non-empty prefix1 and prefix2. If C<prefix1> is indeed a prefix of
a builtin library search path, prefix2 is substituted. The error may appear
if components are not found, or are too long. See L<perlos2/"PERLLIB_PREFIX">.

=item PERL_SH_DIR too long

(F) An error peculiar to OS/2. PERL_SH_DIR is the directory to find the
C<sh>-shell in. See L<perlos2/"PERL_SH_DIR">.

=item Process terminated by SIG%s

(W) This is a standard message issued by OS/2 applications, while *nix
applications die in silence. It is considered a feature of the OS/2
port. One can easily disable this by appropriate sighandlers, see
L<perlipc/"Signals">.  See L<perlos2/"Process terminated by SIGTERM/SIGINT">.

=back

=head1 BUGS

If you find what you think is a bug, you might check the headers of
recently posted articles in the comp.lang.perl.misc newsgroup.
There may also be information at http://www.perl.com/perl/, the Perl
Home Page.

If you believe you have an unreported bug, please run the B<perlbug>
program included with your release.  Make sure you trim your bug
down to a tiny but sufficient test case.  Your bug report, along
with the output of C<perl -V>, will be sent off to perlbug@perl.com
to be analysed by the Perl porting team.

=head1 SEE ALSO

The F<Changes> file for exhaustive details on what changed.

The F<INSTALL> file for how to build Perl.  This file has been
significantly updated for 5.004, so even veteran users should
look through it.

The F<README> file for general stuff.

The F<Copying> file for copyright information.

=head1 HISTORY

Constructed by Tom Christiansen, grabbing material with permission
from innumerable contributors, with kibitzing by more than a few Perl
porters.

Last update: Tue Jan 14 14:03:02 EST 1997