perlrun.pod

视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.

locations are automatically included if they exist (this lookup
being done at interpreter startup time.)

If PERL5LIB is not defined, PERLLIB is used.  Directories are separated
(like in PATH) by a colon on unixish platforms and by a semicolon on
Windows (the proper path separator being given by the command C<perl
-V:path_sep>).

When running taint checks (either because the program was running setuid
or setgid, or the B<-T> or B<-t> switch was specified), neither variable
is used. The program should instead say:

    use lib "/my/directory";

=item PERL5OPT
X<PERL5OPT>

Command-line options (switches).  Switches in this variable are taken
as if they were on every Perl command line.  Only the B<-[CDIMUdmtw]>
switches are allowed.  When running taint checks (because the program
was running setuid or setgid, or the B<-T> switch was used), this
variable is ignored.  If PERL5OPT begins with B<-T>, tainting will be
enabled, and any subsequent options ignored.

=item PERLIO
X<PERLIO>

A space (or colon) separated list of PerlIO layers. If perl is built
to use PerlIO system for IO (the default) these layers effect perl's IO.

It is conventional to start layer names with a colon e.g. C<:perlio> to
emphasise their similarity to variable "attributes". But the code that parses
layer specification strings (which is also used to decode the PERLIO
environment variable) treats the colon as a separator.

An unset or empty PERLIO is equivalent to the default set of layers for
your platform, for example C<:unix:perlio> on UNIX-like systems
and C<:unix:crlf> on Windows and other DOS-like systems.

The list becomes the default for I<all> perl's IO. Consequently only built-in
layers can appear in this list, as external layers (such as :encoding()) need
IO in  order to load them!. See L<"open pragma"|open> for how to add external
encodings as defaults.

The layers that it makes sense to include in the PERLIO environment
variable are briefly summarised below. For more details see L<PerlIO>.

=over 8

=item :bytes
X<:bytes>

A pseudolayer that turns I<off> the C<:utf8> flag for the layer below.
Unlikely to be useful on its own in the global PERLIO environment variable.
You perhaps were thinking of C<:crlf:bytes> or C<:perlio:bytes>.

=item :crlf
X<:crlf>

A layer which does CRLF to "\n" translation distinguishing "text" and
"binary" files in the manner of MS-DOS and similar operating systems.
(It currently does I<not> mimic MS-DOS as far as treating of Control-Z
as being an end-of-file marker.)

=item :mmap
X<:mmap>

A layer which implements "reading" of files by using C<mmap()> to
make (whole) file appear in the process's address space, and then
using that as PerlIO's "buffer".

=item :perlio
X<:perlio>

This is a re-implementation of "stdio-like" buffering written as a
PerlIO "layer".  As such it will call whatever layer is below it for
its operations (typically C<:unix>).

=item :pop
X<:pop>

An experimental pseudolayer that removes the topmost layer.
Use with the same care as is reserved for nitroglycerin.

=item :raw
X<:raw>

A pseudolayer that manipulates other layers.  Applying the C<:raw>
layer is equivalent to calling C<binmode($fh)>.  It makes the stream
pass each byte as-is without any translation.  In particular CRLF
translation, and/or :utf8 intuited from locale are disabled.

Unlike in the earlier versions of Perl C<:raw> is I<not>
just the inverse of C<:crlf> - other layers which would affect the
binary nature of the stream are also removed or disabled.

=item :stdio
X<:stdio>

This layer provides PerlIO interface by wrapping system's ANSI C "stdio"
library calls. The layer provides both buffering and IO.
Note that C<:stdio> layer does I<not> do CRLF translation even if that
is platforms normal behaviour. You will need a C<:crlf> layer above it
to do that.

=item :unix
X<:unix>

Low level layer which calls C<read>, C<write> and C<lseek> etc.

=item :utf8
X<:utf8>

A pseudolayer that turns on a flag on the layer below to tell perl
that output should be in utf8 and that input should be regarded as
already in valid utf8 form. It does not check for validity and as such
should be handled with caution for input. Generally C<:encoding(utf8)> is
the best option when reading UTF-8 encoded data.

=item :win32
X<:win32>

On Win32 platforms this I<experimental> layer uses native "handle" IO
rather than unix-like numeric file descriptor layer. Known to be
buggy in this release.

=back

On all platforms the default set of layers should give acceptable results.

For UNIX platforms that will equivalent of "unix perlio" or "stdio".
Configure is setup to prefer "stdio" implementation if system's library
provides for fast access to the buffer, otherwise it uses the "unix perlio"
implementation.

On Win32 the default in this release is "unix crlf". Win32's "stdio"
has a number of bugs/mis-features for perl IO which are somewhat
C compiler vendor/version dependent. Using our own C<crlf> layer as
the buffer avoids those issues and makes things more uniform.
The C<crlf> layer provides CRLF to/from "\n" conversion as well as
buffering.

This release uses C<unix> as the bottom layer on Win32 and so still uses C
compiler's numeric file descriptor routines. There is an experimental native
C<win32> layer which is expected to be enhanced and should eventually be
the default under Win32.

=item PERLIO_DEBUG
X<PERLIO_DEBUG>

If set to the name of a file or device then certain operations of PerlIO
sub-system will be logged to that file (opened as append). Typical uses
are UNIX:

   PERLIO_DEBUG=/dev/tty perl script ...

and Win32 approximate equivalent:

   set PERLIO_DEBUG=CON
   perl script ...

This functionality is disabled for setuid scripts and for scripts run
with B<-T>.

=item PERLLIB
X<PERLLIB>

A list of directories in which to look for Perl library
files before looking in the standard library and the current directory.
If PERL5LIB is defined, PERLLIB is not used.

=item PERL5DB
X<PERL5DB>

The command used to load the debugger code.  The default is:

	BEGIN { require 'perl5db.pl' }

=item PERL5DB_THREADED
X<PERL5DB_THREADED>

If set to a true value, indicates to the debugger that the code being
debugged uses threads.

=item PERL5SHELL (specific to the Win32 port)
X<PERL5SHELL>

May be set to an alternative shell that perl must use internally for
executing "backtick" commands or system().  Default is C<cmd.exe /x/d/c>
on WindowsNT and C<command.com /c> on Windows95.  The value is considered
to be space-separated.  Precede any character that needs to be protected
(like a space or backslash) with a backslash.

Note that Perl doesn't use COMSPEC for this purpose because
COMSPEC has a high degree of variability among users, leading to
portability concerns.  Besides, perl can use a shell that may not be
fit for interactive use, and setting COMSPEC to such a shell may
interfere with the proper functioning of other programs (which usually
look in COMSPEC to find a shell fit for interactive use).

=item PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
X<PERL_ALLOW_NON_IFS_LSP>

Set to 1 to allow the use of non-IFS compatible LSP's.
Perl normally searches for an IFS-compatible LSP because this is required
for its emulation of Windows sockets as real filehandles.  However, this may
cause problems if you have a firewall such as McAfee Guardian which requires
all applications to use its LSP which is not IFS-compatible, because clearly
Perl will normally avoid using such an LSP.
Setting this environment variable to 1 means that Perl will simply use the
first suitable LSP enumerated in the catalog, which keeps McAfee Guardian
happy (and in that particular case Perl still works too because McAfee
Guardian's LSP actually plays some other games which allow applications
requiring IFS compatibility to work).

=item PERL_DEBUG_MSTATS
X<PERL_DEBUG_MSTATS>

Relevant only if perl is compiled with the malloc included with the perl
distribution (that is, if C<perl -V:d_mymalloc> is 'define').
If set, this causes memory statistics to be dumped after execution.  If set
to an integer greater than one, also causes memory statistics to be dumped
after compilation.

=item PERL_DESTRUCT_LEVEL
X<PERL_DESTRUCT_LEVEL>

Relevant only if your perl executable was built with B<-DDEBUGGING>,
this controls the behavior of global destruction of objects and other
references.  See L<perlhack/PERL_DESTRUCT_LEVEL> for more information.

=item PERL_DL_NONLAZY
X<PERL_DL_NONLAZY>

Set to one to have perl resolve B<all> undefined symbols when it loads
a dynamic library.  The default behaviour is to resolve symbols when
they are used.  Setting this variable is useful during testing of
extensions as it ensures that you get an error on misspelled function
names even if the test suite doesn't call it.

=item PERL_ENCODING
X<PERL_ENCODING>

If using the C<encoding> pragma without an explicit encoding name, the
PERL_ENCODING environment variable is consulted for an encoding name.

=item PERL_HASH_SEED
X<PERL_HASH_SEED>

(Since Perl 5.8.1.)  Used to randomise perl's internal hash function.
To emulate the pre-5.8.1 behaviour, set to an integer (zero means
exactly the same order as 5.8.0).  "Pre-5.8.1" means, among other
things, that hash keys will always have the same ordering between
different runs of perl.

Most hashes return elements in the same order as Perl 5.8.0 by default.
On a hash by hash basis, if pathological data is detected during a hash
key insertion, then that hash will switch to an alternative random hash
seed.

The default behaviour is to randomise unless the PERL_HASH_SEED is set.
If perl has been compiled with C<-DUSE_HASH_SEED_EXPLICIT>, the default
behaviour is B<not> to randomise unless the PERL_HASH_SEED is set.

If PERL_HASH_SEED is unset or set to a non-numeric string, perl uses
the pseudorandom seed supplied by the operating system and libraries.

B<Please note that the hash seed is sensitive information>. Hashes are
randomized to protect against local and remote attacks against Perl
code. By manually setting a seed this protection may be partially or
completely lost.

See L<perlsec/"Algorithmic Complexity Attacks"> and
L</PERL_HASH_SEED_DEBUG> for more information.

=item PERL_HASH_SEED_DEBUG
X<PERL_HASH_SEED_DEBUG>

(Since Perl 5.8.1.)  Set to one to display (to STDERR) the value of
the hash seed at the beginning of execution.  This, combined with
L</PERL_HASH_SEED> is intended to aid in debugging nondeterministic
behavior caused by hash randomization.

B<Note that the hash seed is sensitive information>: by knowing it one
can craft a denial-of-service attack against Perl code, even remotely,
see L<perlsec/"Algorithmic Complexity Attacks"> for more information.
B<Do not disclose the hash seed> to people who don't need to know it.
See also hash_seed() of L<Hash::Util>.

=item PERL_ROOT (specific to the VMS port)
X<PERL_ROOT>

A translation concealed rooted logical name that contains perl and the
logical device for the @INC path on VMS only.  Other logical names that
affect perl on VMS include PERLSHR, PERL_ENV_TABLES, and
SYS$TIMEZONE_DIFFERENTIAL but are optional and discussed further in
L<perlvms> and in F<README.vms> in the Perl source distribution.

=item PERL_SIGNALS
X<PERL_SIGNALS>

In Perls 5.8.1 and later.  If set to C<unsafe> the pre-Perl-5.8.0
signals behaviour (immediate but unsafe) is restored.  If set to
C<safe> the safe (or deferred) signals are used.
See L<perlipc/"Deferred Signals (Safe Signals)">.

=item PERL_UNICODE
X<PERL_UNICODE>

Equivalent to the B<-C> command-line switch.  Note that this is not
a boolean variable-- setting this to C<"1"> is not the right way to
"enable Unicode" (whatever that would mean).  You can use C<"0"> to
"disable Unicode", though (or alternatively unset PERL_UNICODE in
your shell before starting Perl).  See the description of the C<-C>
switch for more information.

=item SYS$LOGIN (specific to the VMS port)
X<SYS$LOGIN>

Used if chdir has no argument and HOME and LOGDIR are not set.

=back

Perl also has environment variables that control how Perl handles data
specific to particular natural languages.  See L<perllocale>.

Apart from these, Perl uses no other environment variables, except
to make them available to the program being executed, and to child
processes.  However, programs running setuid would do well to execute
the following lines before doing anything else, just to keep people
honest:

    $ENV{PATH}  = '/bin:/usr/bin';    # or whatever you need
    $ENV{SHELL} = '/bin/sh' if exists $ENV{SHELL};
    delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};