perlfunc.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 1,629 行 · 第 1/5 页

        }
        else {
            # handle all other possible exceptions
        }
    }

Because perl will stringify uncaught exception messages before displaying
them, you may want to overload stringification operations on such custom
exception objects.  See L<overload> for details about that.

You can arrange for a callback to be run just before the C<die>
does its deed, by setting the C<$SIG{__DIE__}> hook.  The associated
handler will be called with the error text and can change the error
message, if it sees fit, by calling C<die> again.  See
L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
L<"eval BLOCK"> for some examples.  Although this feature was 
to be run only right before your program was to exit, this is not
currently the case--the C<$SIG{__DIE__}> hook is currently called
even inside eval()ed blocks/strings!  If one wants the hook to do
nothing in such situations, put

	die @_ if $^S;

as the first line of the handler (see L<perlvar/$^S>).  Because
this promotes strange action at a distance, this counterintuitive
behavior may be fixed in a future release.

=item do BLOCK
X<do> X<block>

Not really a function.  Returns the value of the last command in the
sequence of commands indicated by BLOCK.  When modified by the C<while> or
C<until> loop modifier, executes the BLOCK once before testing the loop
condition. (On other statements the loop modifiers test the conditional
first.)

C<do BLOCK> does I<not> count as a loop, so the loop control statements
C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
See L<perlsyn> for alternative strategies.

=item do SUBROUTINE(LIST)
X<do>

This form of subroutine call is deprecated.  See L<perlsub>.

=item do EXPR
X<do>

Uses the value of EXPR as a filename and executes the contents of the
file as a Perl script.

    do 'stat.pl';

is just like

    eval `cat stat.pl`;

except that it's more efficient and concise, keeps track of the current
filename for error messages, searches the @INC directories, and updates
C<%INC> if the file is found.  See L<perlvar/Predefined Names> for these
variables.  It also differs in that code evaluated with C<do FILENAME>
cannot see lexicals in the enclosing scope; C<eval STRING> does.  It's the
same, however, in that it does reparse the file every time you call it,
so you probably don't want to do this inside a loop.

If C<do> cannot read the file, it returns undef and sets C<$!> to the
error.  If C<do> can read the file but cannot compile it, it
returns undef and sets an error message in C<$@>.   If the file is
successfully compiled, C<do> returns the value of the last expression
evaluated.

Note that inclusion of library modules is better done with the
C<use> and C<require> operators, which also do automatic error checking
and raise an exception if there's a problem.

You might like to use C<do> to read in a program configuration
file.  Manual error checking can be done this way:

    # read in config files: system first, then user
    for $file ("/share/prog/defaults.rc",
               "$ENV{HOME}/.someprogrc")
   {
	unless ($return = do $file) {
	    warn "couldn't parse $file: $@" if $@;
	    warn "couldn't do $file: $!"    unless defined $return;
	    warn "couldn't run $file"       unless $return;
	}
    }

=item dump LABEL
X<dump> X<core> X<undump>

=item dump

This function causes an immediate core dump.  See also the B<-u>
command-line switch in L<perlrun>, which does the same thing.
Primarily this is so that you can use the B<undump> program (not
supplied) to turn your core dump into an executable binary after
having initialized all your variables at the beginning of the
program.  When the new binary is executed it will begin by executing
a C<goto LABEL> (with all the restrictions that C<goto> suffers).
Think of it as a goto with an intervening core dump and reincarnation.
If C<LABEL> is omitted, restarts the program from the top.

B<WARNING>: Any files opened at the time of the dump will I<not>
be open any more when the program is reincarnated, with possible
resulting confusion on the part of Perl.

This function is now largely obsolete, mostly because it's very hard to
convert a core file into an executable. That's why you should now invoke
it as C<CORE::dump()>, if you don't want to be warned against a possible
typo.

=item each HASH
X<each> X<hash, iterator>

When called in list context, returns a 2-element list consisting of the
key and value for the next element of a hash, so that you can iterate over
it.  When called in scalar context, returns only the key for the next
element in the hash.

Entries are returned in an apparently random order.  The actual random
order is subject to change in future versions of perl, but it is
guaranteed to be in the same order as either the C<keys> or C<values>
function would produce on the same (unmodified) hash.  Since Perl
5.8.1 the ordering is different even between different runs of Perl
for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).

When the hash is entirely read, a null array is returned in list context
(which when assigned produces a false (C<0>) value), and C<undef> in
scalar context.  The next call to C<each> after that will start iterating
again.  There is a single iterator for each hash, shared by all C<each>,
C<keys>, and C<values> function calls in the program; it can be reset by
reading all the elements from the hash, or by evaluating C<keys HASH> or
C<values HASH>.  If you add or delete elements of a hash while you're
iterating over it, you may get entries skipped or duplicated, so
don't.  Exception: It is always safe to delete the item most recently
returned by C<each()>, which means that the following code will work:

        while (($key, $value) = each %hash) {
          print $key, "\n";
          delete $hash{$key};   # This is safe
        }

The following prints out your environment like the printenv(1) program,
only in a different order:

    while (($key,$value) = each %ENV) {
	print "$key=$value\n";
    }

See also C<keys>, C<values> and C<sort>.

=item eof FILEHANDLE
X<eof>
X<end of file>
X<end-of-file>

=item eof ()

=item eof

Returns 1 if the next read on FILEHANDLE will return end of file, or if
FILEHANDLE is not open.  FILEHANDLE may be an expression whose value
gives the real filehandle.  (Note that this function actually
reads a character and then C<ungetc>s it, so isn't very useful in an
interactive context.)  Do not read from a terminal file (or call
C<eof(FILEHANDLE)> on it) after end-of-file is reached.  File types such
as terminals may lose the end-of-file condition if you do.

An C<eof> without an argument uses the last file read.  Using C<eof()>
with empty parentheses is very different.  It refers to the pseudo file
formed from the files listed on the command line and accessed via the
C<< <> >> operator.  Since C<< <> >> isn't explicitly opened,
as a normal filehandle is, an C<eof()> before C<< <> >> has been
used will cause C<@ARGV> to be examined to determine if input is
available.   Similarly, an C<eof()> after C<< <> >> has returned
end-of-file will assume you are processing another C<@ARGV> list,
and if you haven't set C<@ARGV>, will read input from C<STDIN>;
see L<perlop/"I/O Operators">.

In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
detect the end of each file, C<eof()> will only detect the end of the
last file.  Examples:

    # reset line numbering on each input file
    while (<>) {
	next if /^\s*#/;	# skip comments
	print "$.\t$_";
    } continue {
	close ARGV  if eof;	# Not eof()!
    }

    # insert dashes just before last line of last file
    while (<>) {
	if (eof()) {		# check for end of last file
	    print "--------------\n";
	}
	print;
	last if eof();          # needed if we're reading from a terminal
    }

Practical hint: you almost never need to use C<eof> in Perl, because the
input operators typically return C<undef> when they run out of data, or if
there was an error.

=item eval EXPR
X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
X<error, handling> X<exception, handling>

=item eval BLOCK

=item eval

In the first form, the return value of EXPR is parsed and executed as if it
were a little Perl program.  The value of the expression (which is itself
determined within scalar context) is first parsed, and if there weren't any
errors, executed in the lexical context of the current Perl program, so
that any variable settings or subroutine and format definitions remain
afterwards.  Note that the value is parsed every time the C<eval> executes.
If EXPR is omitted, evaluates C<$_>.  This form is typically used to
delay parsing and subsequent execution of the text of EXPR until run time.

In the second form, the code within the BLOCK is parsed only once--at the
same time the code surrounding the C<eval> itself was parsed--and executed
within the context of the current Perl program.  This form is typically
used to trap exceptions more efficiently than the first (see below), while
also providing the benefit of checking the code within BLOCK at compile
time.

The final semicolon, if any, may be omitted from the value of EXPR or within
the BLOCK.

In both forms, the value returned is the value of the last expression
evaluated inside the mini-program; a return statement may be also used, just
as with subroutines.  The expression providing the return value is evaluated
in void, scalar, or list context, depending on the context of the C<eval> 
itself.  See L</wantarray> for more on how the evaluation context can be 
determined.

If there is a syntax error or runtime error, or a C<die> statement is
executed, an undefined value is returned by C<eval>, and C<$@> is set to the
error message.  If there was no error, C<$@> is guaranteed to be a null
string.  Beware that using C<eval> neither silences perl from printing
warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.

Note that, because C<eval> traps otherwise-fatal errors, it is useful for
determining whether a particular feature (such as C<socket> or C<symlink>)
is implemented.  It is also Perl's exception trapping mechanism, where
the die operator is used to raise exceptions.

If the code to be executed doesn't vary, you may use the eval-BLOCK
form to trap run-time errors without incurring the penalty of
recompiling each time.  The error, if any, is still returned in C<$@>.
Examples:

    # make divide-by-zero nonfatal
    eval { $answer = $a / $b; }; warn $@ if $@;

    # same thing, but less efficient
    eval '$answer = $a / $b'; warn $@ if $@;

    # a compile-time error
    eval { $answer = };			# WRONG

    # a run-time error
    eval '$answer =';	# sets $@

Using the C<eval{}> form as an exception trap in libraries does have some
issues.  Due to the current arguably broken state of C<__DIE__> hooks, you
may wish not to trigger any C<__DIE__> hooks that user code may have installed.
You can use the C<local $SIG{__DIE__}> construct for this purpose,
as shown in this example:

    # a very private exception trap for divide-by-zero
    eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
    warn $@ if $@;

This is especially significant, given that C<__DIE__> hooks can call
C<die> again, which has the effect of changing their error messages:

    # __DIE__ hooks may modify error messages
    {
       local $SIG{'__DIE__'} =
              sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
       eval { die "foo lives here" };
       print $@ if $@;                # prints "bar lives here"
    }

Because this promotes action at a distance, this counterintuitive behavior
may be fixed in a future release.

With an C<eval>, you should be especially careful to remember what's
being looked at when:

    eval $x;		# CASE 1
    eval "$x";		# CASE 2

    eval '$x';		# CASE 3
    eval { $x };	# CASE 4

    eval "\$$x++";	# CASE 5
    $$x++;		# CASE 6

Cases 1 and 2 above behave identically: they run the code contained in
the variable $x.  (Although case 2 has misleading double quotes making
the reader wonder what else might be happening (nothing is).)  Cases 3
and 4 likewise behave in the same way: they run the code C<'$x'>, which
does nothing but return the value of $x.  (Case 4 is preferred for
purely visual reasons, but it also has the advantage of compiling at
compile-time instead of at run-time.)  Case 5 is a place where
normally you I<would> like to use double quotes, except that in this
particular situation, you can just use symbolic references instead, as
in case 6.

C<eval BLOCK> does I<not> count as a loop, so the loop control statements
C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.

Note that as a very special case, an C<eval ''> executed within the C<DB>
package doesn't see the usual surrounding lexical scope, but rather the
scope of the first non-DB piece of code that called it. You don't normally
need to worry about this unless you are writing a Perl