perlglossary.pod

来自「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.」· POD 代码 · 共 1,930 行 · 第 1/5 页

To subtract a value from a variable, as in "decrement C<$x>" (meaning
to remove 1 from its value) or "decrement C<$x> by 3".

=item default

A L</value> chosen for you if you don't supply a value of your own.

=item defined

Having a meaning.  Perl thinks that some of the things people try to
do are devoid of meaning, in particular, making use of variables that
have never been given a L</value> and performing certain operations on
data that isn't there.  For example, if you try to read data past the
end of a file, Perl will hand you back an undefined value.  See also
L</false> and L<perlfunc/defined>.

=item delimiter

A L</character> or L</string> that sets bounds to an arbitrarily-sized
textual object, not to be confused with a L</separator> or
L</terminator>.  "To delimit" really just means "to surround" or "to
enclose" (like these parentheses are doing).

=item deprecated modules and features

Deprecated modules and features are those which were part of a stable
release, but later found to be subtly flawed, and which should be avoided.
They are subject to removal and/or bug-incompatible reimplementation in
the next major release (but they will be preserved through maintenance
releases).  Deprecation warnings are issued under B<-w> or C<use
diagnostics>, and notices are found in L<perldelta>s, as well as various
other PODs. Coding practices that misuse features, such as C<my $foo if
0>, can also be deprecated.

=item dereference

A fancy computer science term meaning "to follow a L</reference> to
what it points to".  The "de" part of it refers to the fact that
you're taking away one level of L</indirection>.

=item derived class

A L</class> that defines some of its L<methods|/method> in terms of a
more generic class, called a L</base class>.  Note that classes aren't
classified exclusively into base classes or derived classes: a class
can function as both a derived class and a base class simultaneously,
which is kind of classy.

=item descriptor

See L</file descriptor>.

=item destroy

To deallocate the memory of a L</referent> (first triggering its
C<DESTROY> method, if it has one).

=item destructor

A special L</method> that is called when an L</object> is thinking
about L<destroying|/destroy> itself.  A Perl program's C<DESTROY>
method doesn't do the actual destruction; Perl just
L<triggers|/trigger> the method in case the L</class> wants to do any
associated cleanup.

=item device

A whiz-bang hardware gizmo (like a disk or tape drive or a modem or a
joystick or a mouse) attached to your computer, that the L</operating
system> tries to make look like a L</file> (or a bunch of files).
Under Unix, these fake files tend to live in the I</dev> directory.

=item directive

A L</pod> directive.  See L<perlpod>.

=item directory

A special file that contains other files.  Some L<operating
systems|/operating system> call these "folders", "drawers", or
"catalogs".

=item directory handle

A name that represents a particular instance of opening a directory to
read it, until you close it.  See the L<opendir|perlfunc/opendir>
function.

=item dispatch

To send something to its correct destination.  Often used
metaphorically to indicate a transfer of programmatic control to a
destination selected algorithmically, often by lookup in a table of
function L<references|/reference> or, in the case of object
L<methods|/method>, by traversing the inheritance tree looking for the
most specific definition for the method.

=item distribution

A standard, bundled release of a system of software.  The default
usage implies source code is included.  If that is not the case, it
will be called a "binary-only" distribution.

=item (to be) dropped modules

When Perl 5 was first released (see L<perlhistory>), several modules were
included, which have now fallen out of common use.  It has been suggested
that these modules should be removed, since the distribution became rather
large, and the common criterion for new module additions is now limited to
modules that help to build, test, and extend perl itself.  Furthermore,
the CPAN (which didn't exist at the time of Perl 5.0) can become the new
home of dropped modules. Dropping modules is currently not an option, but
further developments may clear the last barriers.

=item dweomer

An enchantment, illusion, phantasm, or jugglery.  Said when Perl's
magical L</dwimmer> effects don't do what you expect, but rather seem
to be the product of arcane dweomercraft, sorcery, or wonder working.
[From Old English]

=item dwimmer

DWIM is an acronym for "Do What I Mean", the principle that something
should just do what you want it to do without an undue amount of fuss.
A bit of code that does "dwimming" is a "dwimmer".  Dwimming can
require a great deal of behind-the-scenes magic, which (if it doesn't
stay properly behind the scenes) is called a L</dweomer> instead.

=item dynamic scoping

Dynamic scoping works over a dynamic scope, making variables visible
throughout the rest of the L</block> in which they are first used and
in any L<subroutines|/subroutine> that are called by the rest of the
block.  Dynamically scoped variables can have their values temporarily
changed (and implicitly restored later) by a L<local|perlfunc/local>
operator.  (Compare L</lexical scoping>.)  Used more loosely to mean
how a subroutine that is in the middle of calling another subroutine
"contains" that subroutine at L</run time>.

=back

=head2 E

=over 4

=item eclectic

Derived from many sources.  Some would say I<too> many.

=item element

A basic building block.  When you're talking about an L</array>, it's
one of the items that make up the array.

=item embedding

When something is contained in something else, particularly when that
might be considered surprising: "I've embedded a complete Perl
interpreter in my editor!"

=item empty subclass test

The notion that an empty L</derived class> should behave exactly like
its L</base class>.

=item en passant

When you change a L</value> as it is being copied.  [From French, "in
passing", as in the exotic pawn-capturing maneuver in chess.]

=item encapsulation

The veil of abstraction separating the L</interface> from the
L</implementation> (whether enforced or not), which mandates that all
access to an L</object>'s state be through L<methods|/method> alone.

=item endian

See L</little-endian> and L</big-endian>.

=item environment

The collective set of L<environment variables|/environment variable>
your L</process> inherits from its parent.  Accessed via C<%ENV>.

=item environment variable

A mechanism by which some high-level agent such as a user can pass its
preferences down to its future offspring (child L<processes|/process>,
grandchild processes, great-grandchild processes, and so on).  Each
environment variable is a L</key>/L</value> pair, like one entry in a
L</hash>.

=item EOF

End of File.  Sometimes used metaphorically as the terminating string
of a L</here document>.

=item errno

The error number returned by a L</syscall> when it fails.  Perl refers
to the error by the name C<$!> (or C<$OS_ERROR> if you use the English
module).

=item error

See L</exception> or L</fatal error>.

=item escape sequence

See L</metasymbol>.

=item exception

A fancy term for an error.  See L</fatal error>.

=item exception handling

The way a program responds to an error.  The exception handling
mechanism in Perl is the L<eval|perlfunc/eval> operator.

=item exec

To throw away the current L</process>'s program and replace it with
another without exiting the process or relinquishing any resources
held (apart from the old memory image).

=item executable file

A L</file> that is specially marked to tell the L</operating system>
that it's okay to run this file as a program.  Usually shortened to
"executable".

=item execute

To run a L<program|/executable file> or L</subroutine>.  (Has nothing
to do with the L<kill|perlfunc/kill> built-in, unless you're trying to
run a L</signal handler>.)

=item execute bit

The special mark that tells the operating system it can run this
program.  There are actually three execute bits under Unix, and which
bit gets used depends on whether you own the file singularly,
collectively, or not at all.

=item exit status

See L</status>.

=item export

To make symbols from a L</module> available for L</import> by other modules.

=item expression

Anything you can legally say in a spot where a L</value> is required.
Typically composed of L<literals|/literal>, L<variables|/variable>,
L<operators|/operator>, L<functions|/function>, and L</subroutine>
calls, not necessarily in that order.

=item extension

A Perl module that also pulls in compiled C or C++ code.  More
generally, any experimental option that can be compiled into Perl,
such as multithreading.

=back

=head2 F

=over 4

=item false

In Perl, any value that would look like C<""> or C<"0"> if evaluated
in a string context.  Since undefined values evaluate to C<"">, all
undefined values are false, but not all false values are undefined.

=item FAQ

Frequently Asked Question (although not necessarily frequently
answered, especially if the answer appears in the Perl FAQ shipped
standard with Perl).

=item fatal error

An uncaught L</exception>, which causes termination of the L</process>
after printing a message on your L</standard error> stream.  Errors
that happen inside an L<eval|perlfunc/eval> are not fatal.  Instead,
the L<eval|perlfunc/eval> terminates after placing the exception
message in the C<$@> (C<$EVAL_ERROR>) variable.  You can try to
provoke a fatal error with the L<die|perlfunc/die> operator (known as
throwing or raising an exception), but this may be caught by a
dynamically enclosing L<eval|perlfunc/eval>.  If not caught, the
L<die|perlfunc/die> becomes a fatal error.

=item field

A single piece of numeric or string data that is part of a longer
L</string>, L</record>, or L</line>.  Variable-width fields are usually
split up by L<separators|/separator> (so use L<split|perlfunc/split> to
extract the fields), while fixed-width fields are usually at fixed
positions (so use L<unpack|perlfunc/unpack>).  L<Instance
variables|/instance variable> are also known as fields.

=item FIFO

First In, First Out.  See also L</LIFO>.  Also, a nickname for a
L</named pipe>.

=item file

A named collection of data, usually stored on disk in a L</directory>
in a L</filesystem>.  Roughly like a document, if you're into office
metaphors.  In modern filesystems, you can actually give a file more
than one name.  Some files have special properties, like directories
and devices.

=item file descriptor

The little number the L</operating system> uses to keep track of which
opened L</file> you're talking about.  Perl hides the file descriptor
inside a L</standard IE<sol>O> stream and then attaches the stream to
a L</filehandle>.

=item file test operator

A built-in unary operator that you use to determine whether something
is L</true> about a file, such as C<-o $filename> to test whether
you're the owner of the file.

=item fileglob

A "wildcard" match on L<filenames|/filename>.  See the
L<glob|perlfunc/glob> function.

=item filehandle

An identifier (not necessarily related to the real name of a file)
that represents a particular instance of opening a file until you
close it.  If you're going to open and close several different files
in succession, it's fine to open each of them with the same
filehandle, so you don't have to write out separate code to process
each file.

=item filename

One name for a file.  This name is listed in a L</directory>, and you
can use it in an L<open|perlfunc/open> to tell the L</operating
system> exactly which file you want to open, and associate the file
with a L</filehandle> which will carry the subsequent identity of that
file in your program, until you close it.

=item filesystem

A set of L<directories|/directory> and L<files|/file> residing on a
partition of the disk.  Sometimes known as a "partition".  You can
change the file's name or even move a file around from directory to
directory within a filesystem without actually moving the file itself,
at least under Unix.

=item filter

A program designed to take a L</stream> of input and transform it into
a stream of output.

=item flag

We tend to avoid this term because it means so many things.  It may
mean a command-line L</switch> that takes no argument
itself (such as Perl's B<-n> and B<-p>
flags) or, less frequently, a single-bit indicator (such as the
C<O_CREAT> and C<O_EXCL> flags used in
L<sysopen|perlfunc/sysopen>).

=item floating point

A method of storing numbers in "scientific notation", such that the
precision of the number is independent of its magnitude (the decimal
point "floats").  Perl does its numeric work with floating-point
numbers (sometimes called "floats"), when it can't get away with
using L<integers|/integer>.  Floating-point numbers are mere
approximations of real numbers.