db_file.pm

UNIX下perl实现代码

    $x->seq($key, $value, R_CURSOR) ;

Here is the relevant quote from the dbopen man page where it defines
the use of the R_CURSOR flag with seq:

    Note, for the DB_BTREE access method, the returned key is not
    necessarily an exact match for the specified key. The returned key
    is the smallest key greater than or equal to the specified key,
    permitting partial key matches and range searches.

In the example script below, the C<match> sub uses this feature to find
and print the first matching key/value pair given a partial key.

    use warnings ;
    use strict ;
    use DB_File ;
    use Fcntl ;

    use vars qw($filename $x %h $st $key $value) ;

    sub match
    {
        my $key = shift ;
        my $value = 0;
        my $orig_key = $key ;
        $x->seq($key, $value, R_CURSOR) ;
        print "$orig_key\t-> $key\t-> $value\n" ;
    }

    $filename = "tree" ;
    unlink $filename ;

    $x = tie %h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_BTREE
        or die "Cannot open $filename: $!\n";
 
    # Add some key/value pairs to the file
    $h{'mouse'} = 'mickey' ;
    $h{'Wall'} = 'Larry' ;
    $h{'Walls'} = 'Brick' ; 
    $h{'Smith'} = 'John' ;
 

    $key = $value = 0 ;
    print "IN ORDER\n" ;
    for ($st = $x->seq($key, $value, R_FIRST) ;
	 $st == 0 ;
         $st = $x->seq($key, $value, R_NEXT) )
	
      {  print "$key	-> $value\n" }
 
    print "\nPARTIAL MATCH\n" ;

    match "Wa" ;
    match "A" ;
    match "a" ;

    undef $x ;
    untie %h ;

Here is the output:

    IN ORDER
    Smith -> John
    Wall  -> Larry
    Walls -> Brick
    mouse -> mickey

    PARTIAL MATCH
    Wa -> Wall  -> Larry
    A  -> Smith -> John
    a  -> mouse -> mickey

=head1 DB_RECNO

DB_RECNO provides an interface to flat text files. Both variable and
fixed length records are supported.

In order to make RECNO more compatible with Perl, the array offset for
all RECNO arrays begins at 0 rather than 1 as in Berkeley DB.

As with normal Perl arrays, a RECNO array can be accessed using
negative indexes. The index -1 refers to the last element of the array,
-2 the second last, and so on. Attempting to access an element before
the start of the array will raise a fatal run-time error.

=head2 The 'bval' Option

The operation of the bval option warrants some discussion. Here is the
definition of bval from the Berkeley DB 1.85 recno manual page:

    The delimiting byte to be used to mark  the  end  of  a
    record for variable-length records, and the pad charac-
    ter for fixed-length records.  If no  value  is  speci-
    fied,  newlines  (``\n'')  are  used to mark the end of
    variable-length records and  fixed-length  records  are
    padded with spaces.

The second sentence is wrong. In actual fact bval will only default to
C<"\n"> when the openinfo parameter in dbopen is NULL. If a non-NULL
openinfo parameter is used at all, the value that happens to be in bval
will be used. That means you always have to specify bval when making
use of any of the options in the openinfo parameter. This documentation
error will be fixed in the next release of Berkeley DB.

That clarifies the situation with regards Berkeley DB itself. What
about B<DB_File>? Well, the behavior defined in the quote above is
quite useful, so B<DB_File> conforms to it.

That means that you can specify other options (e.g. cachesize) and
still have bval default to C<"\n"> for variable length records, and
space for fixed length records.

=head2 A Simple Example

Here is a simple example that uses RECNO (if you are using a version 
of Perl earlier than 5.004_57 this example won't work -- see 
L<Extra RECNO Methods> for a workaround).

    use warnings ;
    use strict ;
    use DB_File ;

    my $filename = "text" ;
    unlink $filename ;

    my @h ;
    tie @h, "DB_File", $filename, O_RDWR|O_CREAT, 0640, $DB_RECNO 
        or die "Cannot open file 'text': $!\n" ;

    # Add a few key/value pairs to the file
    $h[0] = "orange" ;
    $h[1] = "blue" ;
    $h[2] = "yellow" ;

    push @h, "green", "black" ;

    my $elements = scalar @h ;
    print "The array contains $elements entries\n" ;

    my $last = pop @h ;
    print "popped $last\n" ;

    unshift @h, "white" ;
    my $first = shift @h ;
    print "shifted $first\n" ;

    # Check for existence of a key
    print "Element 1 Exists with value $h[1]\n" if $h[1] ;

    # use a negative index
    print "The last element is $h[-1]\n" ;
    print "The 2nd last element is $h[-2]\n" ;

    untie @h ;

Here is the output from the script:

    The array contains 5 entries
    popped black
    shifted white
    Element 1 Exists with value blue
    The last element is green
    The 2nd last element is yellow

=head2 Extra RECNO Methods

If you are using a version of Perl earlier than 5.004_57, the tied
array interface is quite limited. In the example script above
C<push>, C<pop>, C<shift>, C<unshift>
or determining the array length will not work with a tied array.

To make the interface more useful for older versions of Perl, a number
of methods are supplied with B<DB_File> to simulate the missing array
operations. All these methods are accessed via the object returned from
the tie call.

Here are the methods:

=over 5

=item B<$X-E<gt>push(list) ;>

Pushes the elements of C<list> to the end of the array.

=item B<$value = $X-E<gt>pop ;>

Removes and returns the last element of the array.

=item B<$X-E<gt>shift>

Removes and returns the first element of the array.

=item B<$X-E<gt>unshift(list) ;>

Pushes the elements of C<list> to the start of the array.

=item B<$X-E<gt>length>

Returns the number of elements in the array.

=back

=head2 Another Example

Here is a more complete example that makes use of some of the methods
described above. It also makes use of the API interface directly (see 
L<THE API INTERFACE>).

    use warnings ;
    use strict ;
    use vars qw(@h $H $file $i) ;
    use DB_File ;
    use Fcntl ;
    
    $file = "text" ;

    unlink $file ;

    $H = tie @h, "DB_File", $file, O_RDWR|O_CREAT, 0640, $DB_RECNO 
        or die "Cannot open file $file: $!\n" ;
    
    # first create a text file to play with
    $h[0] = "zero" ;
    $h[1] = "one" ;
    $h[2] = "two" ;
    $h[3] = "three" ;
    $h[4] = "four" ;

    
    # Print the records in order.
    #
    # The length method is needed here because evaluating a tied
    # array in a scalar context does not return the number of
    # elements in the array.  

    print "\nORIGINAL\n" ;
    foreach $i (0 .. $H->length - 1) {
        print "$i: $h[$i]\n" ;
    }

    # use the push & pop methods
    $a = $H->pop ;
    $H->push("last") ;
    print "\nThe last record was [$a]\n" ;

    # and the shift & unshift methods
    $a = $H->shift ;
    $H->unshift("first") ;
    print "The first record was [$a]\n" ;

    # Use the API to add a new record after record 2.
    $i = 2 ;
    $H->put($i, "Newbie", R_IAFTER) ;

    # and a new record before record 1.
    $i = 1 ;
    $H->put($i, "New One", R_IBEFORE) ;

    # delete record 3
    $H->del(3) ;

    # now print the records in reverse order
    print "\nREVERSE\n" ;
    for ($i = $H->length - 1 ; $i >= 0 ; -- $i)
      { print "$i: $h[$i]\n" }

    # same again, but use the API functions instead
    print "\nREVERSE again\n" ;
    my ($s, $k, $v)  = (0, 0, 0) ;
    for ($s = $H->seq($k, $v, R_LAST) ; 
             $s == 0 ; 
             $s = $H->seq($k, $v, R_PREV))
      { print "$k: $v\n" }

    undef $H ;
    untie @h ;

and this is what it outputs:

    ORIGINAL
    0: zero
    1: one
    2: two
    3: three
    4: four

    The last record was [four]
    The first record was [zero]

    REVERSE
    5: last
    4: three
    3: Newbie
    2: one
    1: New One
    0: first

    REVERSE again
    5: last
    4: three
    3: Newbie
    2: one
    1: New One
    0: first

Notes:

=over 5

=item 1.

Rather than iterating through the array, C<@h> like this:

    foreach $i (@h)

it is necessary to use either this:

    foreach $i (0 .. $H->length - 1) 

or this:

    for ($a = $H->get($k, $v, R_FIRST) ;
         $a == 0 ;
         $a = $H->get($k, $v, R_NEXT) )

=item 2.

Notice that both times the C<put> method was used the record index was
specified using a variable, C<$i>, rather than the literal value
itself. This is because C<put> will return the record number of the
inserted line via that parameter.

=back

=head1 THE API INTERFACE

As well as accessing Berkeley DB using a tied hash or array, it is also
possible to make direct use of most of the API functions defined in the
Berkeley DB documentation.

To do this you need to store a copy of the object returned from the tie.

	$db = tie %hash, "DB_File", "filename" ;

Once you have done that, you can access the Berkeley DB API functions
as B<DB_File> methods directly like this:

	$db->put($key, $value, R_NOOVERWRITE) ;

B<Important:> If you have saved a copy of the object returned from
C<tie>, the underlying database file will I<not> be closed until both
the tied variable is untied and all copies of the saved object are
destroyed. 

    use DB_File ;
    $db = tie %hash, "DB_File", "filename" 
        or die "Cannot tie filename: $!" ;
    ...
    undef $db ;
    untie %hash ;

See L<The untie() Gotcha> for more details.

All the functions defined in L<dbopen> are available except for
close() and dbopen() itself. The B<DB_File> method interface to the
supported functions have been implemented to mirror the way Berkeley DB
works whenever possible. In particular note that:

=over 5

=item *

The methods return a status value. All return 0 on success.
All return -1 to signify an error and set C<$!> to the exact
error code. The return code 1 generally (but not always) means that the
key specified did not exist in the database.

Other return codes are defined. See below and in the Berkeley DB
documentation for details. The Berkeley DB documentation should be used
as the definitive source.

=item *

Whenever a Berkeley DB function returns data via one of its parameters,
the equivalent B<DB_File> method does exactly the same.

=item *

If you are careful, it is possible to mix API calls with the tied
hash/array interface in the same piece of code. Although only a few of
the methods used to implement the tied interface currently make use of
the cursor, you should always assume that the cursor has been changed
any time the tied hash/array interface is used. As an example, this
code will probably not do what you expect:

    $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
        or die "Cannot tie $filename: $!" ;

    # Get the first key/value pair and set  the cursor
    $X->seq($key, $value, R_FIRST) ;

    # this line will modify the cursor
    $count = scalar keys %x ; 

    # Get the second key/value pair.
    # oops, it didn't, it got the last key/value pair!
    $X->seq($key, $value, R_NEXT) ;

The code above can be rearranged to get around the problem, like this:

    $X = tie %x, 'DB_File', $filename, O_RDWR|O_CREAT, 0777, $DB_BTREE
        or die "Cannot tie $filename: $!" ;

    # this line will modify the cursor
    $count = scalar keys %x ; 

    # Get the first key/value pair and set  the cursor
    $X->seq($key, $value, R_FIRST) ;

    # Get the second key/value pair.
    # worked this time.
    $X->seq($key, $value, R_NEXT) ;

=back

All the constants defined in L<dbopen> for use in the flags parameters
in the methods defined below are also available. Refer to the Berkeley
DB documentation for the precise meaning of the flags values.

Below is a list of the methods available.

=over 5

=item B<$status = $X-E<gt>get($key, $value [, $flags]) ;>

Given a key (C<$key>) this method reads the value associated with it
from the database. The value read from the database is returned in the
C<$value> parameter.

If the key does not exist the method returns 1.

No flags are currently defined for this method.

=item B<$status = $X-E<gt>put($key, $value [, $flags]) ;>

Stores the key/value pair in the database.

If you use either the R_IAFTER or R_IBEFORE flags, the C<$key> parameter
will have the record number of the inserted key/value pair set.

Valid flags are R_CURSOR, R_IAFTER, R_IBEFORE, R_NOOVERWRITE and
R_SETCURSOR.

=item B<$status = $X-E<gt>del($key [, $flags]) ;>

Removes all key/value pairs with key C<$key> from the database.

A return code of 1 means that the requested key was not in the
database.

R_CURSOR is the only valid flag at present.

=item B<$status = $X-E<gt>fd ;>

Returns the file descriptor for the underlying database.

See L<Locking: The Trouble with fd> for an explanation for why you should
not use C<fd> to lock your database.

=item B<$status = $X-E<gt>seq($key, $value, $flags) ;>

This interface allows sequential retrieval from the database. See
L<dbopen> for full details.

Both the C<$key> and C<$value> parameters will be set to the key/value
pair read from the database.

The flags parameter is mandatory. The valid flag values are R_CURSOR,
R_FIRST, R_LAST, R_NEXT and R_PREV.

=item B<$status = $X-E<gt>sync([$flags]) ;>

Flushes any cached buffers to disk.

R_RECNOSYNC is the only valid flag at present.

=back

=head1 DBM FILTERS

A DBM Filter is a piece of code that is be used when you I<always>
want to make the same transformation to all keys and/or values in a
DBM database.

There are four methods associated with DBM Filters. All work identically,
and each is used to install (or uninstall) a single DBM Filter. Each
expects a single parameter, namely a reference to a sub. The only
difference between them is the place that the filter is installed.

To summarise:

=over 5

=item B<filter_store_key>

If a filter has been installed with this method, it will be invoked
every time you write a key to a DBM database.

=item B<filter_store_value>

If a filter has been installed with this method, it will be invoked
every time you write a value to a DBM database.


=item B<filter_fetch_key>

If a filter has been installed with this method, it will be invoked
every time you read a key from a DBM database.