MAN page from Old RedHat 5.X perl-5.004-4.i386.rpm

DB_File

Section: Perl Programmers Reference Guide (3)
Updated: perl 5.004, patch 04
Index

NAME

DB_File - Perl5 access to Berkeley DB

SYNOPSIS

 use DB_File ;  [$X =] tie %hash,  'DB_File', [$filename, $flags, $mode, $DB_HASH] ; [$X =] tie %hash,  'DB_File', $filename, $flags, $mode, $DB_BTREE ; [$X =] tie @array, 'DB_File', $filename, $flags, $mode, $DB_RECNO ;

 $status = $X->del($key [, $flags]) ; $status = $X->put($key, $value [, $flags]) ; $status = $X->get($key, $value [, $flags]) ; $status = $X->seq($key, $value, $flags) ; $status = $X->sync([$flags]) ; $status = $X->fd ;

 # BTREE only $count = $X->get_dup($key) ; @list  = $X->get_dup($key) ; %list  = $X->get_dup($key, 1) ;

 # RECNO only $a = $X->length; $a = $X->pop ; $X->push(list); $a = $X->shift; $X->unshift(list);

 untie %hash ; untie @array ;

DESCRIPTION

DB_File is a module which allows Perl programs to make use of thefacilities provided by Berkeley DB. If you intend to use thismodule you should really have a copy of the Berkeley DB manual pages athand. The interface defined here mirrors the Berkeley DB interfaceclosely.

Please note that this module will only work with version 1.x ofBerkeley DB. Once Berkeley DB version 2 is released, DB_File will beupgraded to work with it.

Berkeley DB is a C library which provides a consistent interface to anumber of database formats. DB_File provides an interface to allthree of the database types currently supported by Berkeley DB.

The file types are:

DB_HASH: This database type allows arbitrary key/value pairs to be stored in datafiles. This is equivalent to the functionality provided by otherhashing packages like DBM, NDBM, ODBM, GDBM, and SDBM. Remember though,the files created using DB_HASH are not compatible with any of theother packages mentioned.
A default hashing algorithm, which will be adequate for mostapplications, is built into Berkeley DB. If you do need to use your ownhashing algorithm it is possible to write your own in Perl and haveDB_File use it instead.
DB_BTREE: The btree format allows arbitrary key/value pairs to be stored in asorted, balanced binary tree.
As with the DB_HASH format, it is possible to provide a user definedPerl routine to perform the comparison of keys. By default, though, thekeys are stored in lexical order.
DB_RECNO: DB_RECNO allows both fixed-length and variable-length flat text filesto be manipulated using the same key/value pair interface as in DB_HASHand DB_BTREE. In this case the key will consist of a record (line)number.

Interface to Berkeley DB

DB_File allows access to Berkeley DB files using the tie() mechanismin Perl 5 (for full details, see the tie() entry in the perlfunc manpage). This facilityallows DB_File to access Berkeley DB files using either anassociative array (for DB_HASH & DB_BTREE file types) or an ordinaryarray (for the DB_RECNO file type).

In addition to the tie() interface, it is also possible to access mostof the functions provided in the Berkeley DB API directly.See the section on THE API INTERFACE.

Opening a Berkeley DB Database File

Berkeley DB uses the function dbopen() to open or create a database.Here is the C prototype for dbopen():

      DB*      dbopen (const char * file, int flags, int mode,               DBTYPE type, const void * openinfo)

The parameter type is an enumeration which specifies which of the 3interface methods (DB_HASH, DB_BTREE or DB_RECNO) is to be used.Depending on which of these is actually chosen, the final parameter,openinfo points to a data structure which allows tailoring of thespecific interface method.

This interface is handled slightly differently in DB_File. Here isan equivalent call using DB_File:

        tie %array, 'DB_File', $filename, $flags, $mode, $DB_HASH ;

The filename, flags and mode parameters are the directequivalent of their dbopen() counterparts. The final parameter $DB_HASHperforms the function of both the type and openinfo parameters indbopen().

In the example above $DB_HASH is actually a pre-defined reference to ahash object. DB_File has three of these pre-defined references.Apart from $DB_HASH, there is also $DB_BTREE and $DB_RECNO.

The keys allowed in each of these pre-defined references is limited tothe names used in the equivalent C structure. So, for example, the$DB_HASH reference will only allow keys called bsize, cachesize,ffactor, hash, lorder and nelem.

To change one of these elements, just assign to it like this:

        $DB_HASH->{'cachesize'} = 10000 ;

The three predefined variables $DB_HASH, $DB_BTREE and $DB_RECNO areusually adequate for most applications. If you do need to create extrainstances of these objects, constructors are available for each filetype.

Here are examples of the constructors and the valid options availablefor DB_HASH, DB_BTREE and DB_RECNO respectively.

     $a = new DB_File::HASHINFO ;     $a->{'bsize'} ;     $a->{'cachesize'} ;     $a->{'ffactor'};     $a->{'hash'} ;     $a->{'lorder'} ;     $a->{'nelem'} ;

     $b = new DB_File::BTREEINFO ;     $b->{'flags'} ;     $b->{'cachesize'} ;     $b->{'maxkeypage'} ;     $b->{'minkeypage'} ;     $b->{'psize'} ;     $b->{'compare'} ;     $b->{'prefix'} ;     $b->{'lorder'} ;

     $c = new DB_File::RECNOINFO ;     $c->{'bval'} ;     $c->{'cachesize'} ;     $c->{'psize'} ;     $c->{'flags'} ;     $c->{'lorder'} ;     $c->{'reclen'} ;     $c->{'bfname'} ;

The values stored in the hashes above are mostly the direct equivalentof their C counterpart. Like their C counterparts, all are set to adefault values - that means you don't have to set all of thevalues when you only want to change one. Here is an example:

     $a = new DB_File::HASHINFO ;     $a->{'cachesize'} =  12345 ;     tie %y, 'DB_File', "filename", $flags, 0777, $a ;

A few of the options need extra discussion here. When used, the Cequivalent of the keys hash, compare and prefix store pointersto C functions. In DB_File these keys are used to store referencesto Perl subs. Below are templates for each of the subs:

    sub hash    {        my ($data) = @_ ;        ...        # return the hash value for $data        return $hash ;    }

    sub compare    {        my ($key, $key2) = @_ ;        ...        # return  0 if $key1 eq $key2        #        -1 if $key1 lt $key2        #         1 if $key1 gt $key2        return (-1 , 0 or 1) ;    }

    sub prefix    {        my ($key, $key2) = @_ ;        ...        # return number of bytes of $key2 which are         # necessary to determine that it is greater than $key1        return $bytes ;    }

See the section on Changing the BTREE sort order for an example of using thecompare template.

If you are using the DB_RECNO interface and you intend making use ofbval, you should check out the section on The `bval' Option.

Default Parameters

It is possible to omit some or all of the final 4 parameters in thecall to tie and let them take default values. As DB_HASH is the mostcommon file format used, the call:

    tie %A, "DB_File", "filename" ;

is equivalent to:

    tie %A, "DB_File", "filename", O_CREAT|O_RDWR, 0666, $DB_HASH ;

It is also possible to omit the filename parameter as well, so thecall:

    tie %A, "DB_File" ;

is equivalent to:

    tie %A, "DB_File", undef, O_CREAT|O_RDWR, 0666, $DB_HASH ;

See the section on In Memory Databases for a discussion on the use of undefin place of a filename.

In Memory Databases

Berkeley DB allows the creation of in-memory databases by using NULL(that is, a (char *)0 in C) in place of the filename. DB_Fileuses undef instead of NULL to provide this functionality.

DB_HASH

The DB_HASH file format is probably the most commonly used of the threefile formats that DB_File supports. It is also very straightforwardto use.

A Simple Example

This example shows how to create a database, add key/value pairs to thedatabase, delete keys/value pairs and finally how to enumerate thecontents of the database.

    use strict ;    use DB_File ;    use vars qw( %h $k $v ) ;

    tie %h, "DB_File", "fruit", O_RDWR|O_CREAT, 0640, $DB_HASH         or die "Cannot open file 'fruit': $!\n";

    # Add a few key/value pairs to the file    $h{"apple"} = "red" ;    $h{"orange"} = "orange" ;    $h{"banana"} = "yellow" ;    $h{"tomato"} = "red" ;

    # Check for existence of a key    print "Banana Exists\n\n" if $h{"banana"} ;

    # Delete a key/value pair.    delete $h{"apple"} ;

    # print the contents of the file    while (($k, $v) = each %h)      { print "$k -> $v\n" }

    untie %h ;

here is the output:

    Banana Exists     orange -> orange    tomato -> red    banana -> yellow

Note that the like ordinary associative arrays, the order of the keysretrieved is in an apparently random order.

DB_BTREE

The DB_BTREE format is useful when you want to store data in a givenorder. By default the keys will be stored in lexical order, but as youwill see from the example shown in the next section, it is very easy todefine your own sorting function.

Changing the BTREE sort order

This script shows how to override the default sorting algorithm thatBTREE uses. Instead of using the normal lexical ordering, a caseinsensitive compare function will be used.

    use strict ;    use DB_File ;

    my %h ;

    sub Compare    {        my ($key1, $key2) = @_ ;        "\L$key1" cmp "\L$key2" ;    }

    # specify the Perl sub that will do the comparison    $DB_BTREE->{'compare'} = \&Compare ;

    tie %h, "DB_File", "tree", O_RDWR|O_CREAT, 0640, $DB_BTREE         or die "Cannot open file 'tree': $!\n" ;

    # Add a key/value pair to the file    $h{'Wall'} = 'Larry' ;    $h{'Smith'} = 'John' ;    $h{'mouse'} = 'mickey' ;    $h{'duck'}  = 'donald' ;

    # Delete    delete $h{"duck"} ;

    # Cycle through the keys printing them in order.    # Note it is not necessary to sort the keys as    # the btree will have kept them in order automatically.    foreach (keys %h)      { print "$_\n" }

    untie %h ;

Here is the output from the code above.

    mouse    Smith    Wall

There are a few point to bear in mind if you want to change theordering in a BTREE database:

1.: The new compare function must be specified when you create the database.
2.: You cannot change the ordering once the database has been created. Thusyou must use the same compare function every time you access thedatabase.

Handling Duplicate Keys

The BTREE file type optionally allows a single key to be associatedwith an arbitrary number of values. This option is enabled by settingthe flags element of $DB_BTREE to R_DUP when creating the database.

There are some difficulties in using the tied hash interface if youwant to manipulate a BTREE database with duplicate keys. Consider thiscode:

    use strict ;    use DB_File ;

    use vars qw($filename %h ) ;

    $filename = "tree" ;    unlink $filename ;     # Enable duplicate records    $DB_BTREE->{'flags'} = R_DUP ;     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{'Wall'} = 'Larry' ;    $h{'Wall'} = 'Brick' ; # Note the duplicate key    $h{'Wall'} = 'Brick' ; # Note the duplicate key and value    $h{'Smith'} = 'John' ;    $h{'mouse'} = 'mickey' ;

    # iterate through the associative array    # and print each key/value pair.    foreach (keys %h)      { print "$_  -> $h{$_}\n" }

    untie %h ;

Here is the output:

    Smith   -> John    Wall    -> Larry    Wall    -> Larry    Wall    -> Larry    mouse   -> mickey

As you can see 3 records have been successfully created with key Wall- the only thing is, when they are retrieved from the database theyseem to have the same value, namely Larry. The problem is causedby the way that the associative array interface works. Basically, whenthe associative array interface is used to fetch the value associatedwith a given key, it will only ever retrieve the first value.

Although it may not be immediately obvious from the code above, theassociative array interface can be used to write values with duplicatekeys, but it cannot be used to read them back from the database.

The way to get around this problem is to use the Berkeley DB API methodcalled seq. This method allows sequential access to key/valuepairs. See the section on THE API INTERFACE for details of both the seq methodand the API in general.

Here is the script above rewritten using the seq API method.

    use strict ;    use DB_File ;     use vars qw($filename $x %h $status $key $value) ;

    $filename = "tree" ;    unlink $filename ;     # Enable duplicate records    $DB_BTREE->{'flags'} = R_DUP ;     $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{'Wall'} = 'Larry' ;    $h{'Wall'} = 'Brick' ; # Note the duplicate key    $h{'Wall'} = 'Brick' ; # Note the duplicate key and value    $h{'Smith'} = 'John' ;    $h{'mouse'} = 'mickey' ;     # iterate through the btree using seq    # and print each key/value pair.    $key = $value = 0 ;    for ($status = $x->seq($key, $value, R_FIRST) ;         $status == 0 ;         $status = $x->seq($key, $value, R_NEXT) )      {  print "$key -> $value\n" }     undef $x ;    untie %h ;

that prints:

    Smith   -> John    Wall    -> Brick    Wall    -> Brick    Wall    -> Larry    mouse   -> mickey

This time we have got all the key/value pairs, including the multiplevalues associated with the key Wall.

The get_dup() Method

DB_File comes with a utility method, called get_dup, to assist inreading duplicate values from BTREE databases. The method can take thefollowing forms:

    $count = $x->get_dup($key) ;    @list  = $x->get_dup($key) ;    %list  = $x->get_dup($key, 1) ;

In a scalar context the method returns the number of values associatedwith the key, $key.

In list context, it returns all the values which match $key. Notethat the values will be returned in an apparently random order.

In list context, if the second parameter is present and evaluatesTRUE, the method returns an associative array. The keys of theassociative array correspond to the values that matched in the BTREEand the values of the array are a count of the number of times thatparticular value occurred in the BTREE.

So assuming the database created above, we can use get_dup likethis:

    my $cnt  = $x->get_dup("Wall") ;    print "Wall occurred $cnt times\n" ;

    my %hash = $x->get_dup("Wall", 1) ;    print "Larry is there\n" if $hash{'Larry'} ;    print "There are $hash{'Brick'} Brick Walls\n" ;

    my @list = $x->get_dup("Wall") ;    print "Wall =>      [@list]\n" ;

    @list = $x->get_dup("Smith") ;    print "Smith =>     [@list]\n" ;     @list = $x->get_dup("Dog") ;    print "Dog =>       [@list]\n" ;

and it will print:

    Wall occurred 3 times    Larry is there    There are 2 Brick Walls    Wall =>     [Brick Brick Larry]    Smith =>    [John]    Dog =>      []

Matching Partial Keys

The BTREE interface has a feature which allows partial keys to bematched. This functionality is only available when the seq methodis used along with the R_CURSOR flag.

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

Here is the relevant quote from the dbopen man page where it definesthe 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 match sub uses this feature to findand print the first matching key/value pair given a partial key.

    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

DB_RECNO

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

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

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

The `bval' Option

The operation of the bval option warrants some discussion. Here is thedefinition 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"\n" when the openinfo parameter in dbopen is NULL. If a non-NULLopeninfo parameter is used at all, the value that happens to be in bvalwill be used. That means you always have to specify bval when makinguse of any of the options in the openinfo parameter. This documentationerror will be fixed in the next release of Berkeley DB.

That clarifies the situation with regards Berkeley DB itself. Whatabout DB_File? Well, the behavior defined in the quote above isquite useful, so DB_File conforms it.

That means that you can specify other options (e.g. cachesize) andstill have bval default to "\n" for variable length records, andspace for fixed length records.

A Simple Example

Here is a simple example that uses RECNO.

    use strict ;    use DB_File ;

    my @h ;    tie @h, "DB_File", "text", 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" ;

    # 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:

    Element 1 Exists with value blue    The last element is yellow    The 2nd last element is blue

Extra Methods

As you can see from the example above, the tied array interface isquite limited. To make the interface more useful, a number of methodsare supplied with DB_File to simulate the standard array operationsthat are not currently implemented in Perl's tied array interface. Allthese methods are accessed via the object returned from the tie call.

Here are the methods:

$X->push(list) ;: Pushes the elements of list to the end of the array.
$value = $X->pop ;: Removes and returns the last element of the array.
$X->shift: Removes and returns the first element of the array.
$X->unshift(list) ;: Pushes the elements of list to the start of the array.
$X->length: Returns the number of elements in the array.

Another Example

Here is a more complete example that makes use of some of the methodsdescribed above. It also makes use of the API interface directly (see the section on THE API INTERFACE).

    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:

1.

Rather than iterating through the array, @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) )

2.

Notice that both times the put method was used the record index wasspecified using a variable, $i, rather than the literal valueitself. This is because put will return the record number of theinserted line via that parameter.

THE API INTERFACE

As well as accessing Berkeley DB using a tied hash or array, it is alsopossible to make direct use of most of the API functions defined in theBerkeley 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 functionsas DB_File methods directly like this:

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

Important: If you have saved a copy of the object returned fromtie, the underlying database file will not be closed until boththe tied variable is untied and all copies of the saved object aredestroyed.

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

See the section on The untie() Gotcha for more details.

All the functions defined in the dbopen manpage are available except forclose() and dbopen() itself. The DB_File method interface to thesupported functions have been implemented to mirror the way Berkeley DBworks whenever possible. In particular note that:

*

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

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

*

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

*

If you are careful, it is possible to mix API calls with the tiedhash/array interface in the same piece of code. Although only a few ofthe methods used to implement the tied interface currently make use ofthe cursor, you should always assume that the cursor has been changedany time the tied hash/array interface is used. As an example, thiscode 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) ;

All the constants defined in the dbopen manpage for use in the flags parametersin the methods defined below are also available. Refer to the BerkeleyDB documentation for the precise meaning of the flags values.

Below is a list of the methods available.

$status = $X->get($key, $value [, $flags]) ;

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

If the key does not exist the method returns 1.

No flags are currently defined for this method.

$status = $X->put($key, $value [, $flags]) ;

Stores the key/value pair in the database.

If you use either the R_IAFTER or R_IBEFORE flags, the $key parameterwill have the record number of the inserted key/value pair set.

Valid flags are R_CURSOR, R_IAFTER, R_IBEFORE, R_NOOVERWRITE andR_SETCURSOR.

$status = $X->del($key [, $flags]) ;

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

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

R_CURSOR is the only valid flag at present.

$status = $X->fd ;

Returns the file descriptor for the underlying database.

See the section on Locking Databases for an example of how to make use of thefd method to lock your database.

$status = $X->seq($key, $value, $flags) ;

This interface allows sequential retrieval from the database. Seethe dbopen manpage for full details.

Both the $key and $value parameters will be set to the key/valuepair 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.

$status = $X->sync([$flags]) ;

Flushes any cached buffers to disk.

R_RECNOSYNC is the only valid flag at present.

HINTS AND TIPS

Locking Databases

Concurrent access of a read-write database by several parties requiresthem all to use some kind of locking. Here's an example of Tom's thatuses the fd method to get the file descriptor, and then a carefulopen() to give something Perl will flock() for you. Run this repeatedlyin the background to watch the locks granted in proper order.

    use DB_File;

    use strict;

    sub LOCK_SH { 1 }    sub LOCK_EX { 2 }    sub LOCK_NB { 4 }    sub LOCK_UN { 8 }

    my($oldval, $fd, $db, %db, $value, $key);

    $key = shift || 'default';    $value = shift || 'magic';

    $value .= " $$";

    $db = tie(%db, 'DB_File', '/tmp/foo.db', O_CREAT|O_RDWR, 0644)             || die "dbcreat /tmp/foo.db $!";    $fd = $db->fd;    print "$$: db fd is $fd\n";    open(DB_FH, "+<&=$fd") || die "dup $!";

    unless (flock (DB_FH, LOCK_SH | LOCK_NB)) {        print "$$: CONTENTION; can't read during write update!                    Waiting for read lock ($!) ....";        unless (flock (DB_FH, LOCK_SH)) { die "flock: $!" }    }     print "$$: Read lock granted\n";

    $oldval = $db{$key};    print "$$: Old value was $oldval\n";    flock(DB_FH, LOCK_UN);

    unless (flock (DB_FH, LOCK_EX | LOCK_NB)) {        print "$$: CONTENTION; must have exclusive lock!                    Waiting for write lock ($!) ....";        unless (flock (DB_FH, LOCK_EX)) { die "flock: $!" }    }

    print "$$: Write lock granted\n";    $db{$key} = $value;    $db->sync;  # to flush    sleep 10;

    flock(DB_FH, LOCK_UN);    undef $db;    untie %db;    close(DB_FH);    print "$$: Updated db to $key=$value\n";

Sharing Databases With C Applications

There is no technical reason why a Berkeley DB database cannot beshared by both a Perl and a C application.

The vast majority of problems that are reported in this area boil downto the fact that C strings are NULL terminated, whilst Perl strings arenot.

Here is a real example. Netscape 2.0 keeps a record of the locations youvisit along with the time you last visited them in a DB_HASH database.This is usually stored in the file ~/.netscape/history.db. The keyfield in the database is the location string and the value field is thetime the location was last visited stored as a 4 byte binary value.

If you haven't already guessed, the location string is stored with aterminating NULL. This means you need to be careful when accessing thedatabase.

Here is a snippet of code that is loosely based on Tom Christiansen'sggh script (available from your nearest CPAN archive inauthors/id/TOMC/scripts/nshist.gz).

    use strict ;    use DB_File ;    use Fcntl ;

    use vars qw( $dotdir $HISTORY %hist_db $href $binary_time $date ) ;    $dotdir = $ENV{HOME} || $ENV{LOGNAME};

    $HISTORY = "$dotdir/.netscape/history.db";

    tie %hist_db, 'DB_File', $HISTORY        or die "Cannot open $HISTORY: $!\n" ;;

    # Dump the complete database    while ( ($href, $binary_time) = each %hist_db ) {

        # remove the terminating NULL        $href =~ s/\x00$// ;

        # convert the binary time into a user friendly string        $date = localtime unpack("V", $binary_time);        print "$date $href\n" ;    }

    # check for the existence of a specific key    # remember to add the NULL    if ( $binary_time = $hist_db{"http://mox.perl.com/\x00"} ) {        $date = localtime unpack("V", $binary_time) ;        print "Last visited mox.perl.com on $date\n" ;    }    else {        print "Never visited mox.perl.com\n"    }

    untie %hist_db ;

The untie() Gotcha

If you make use of the Berkeley DB API, it is very stronglyrecommended that you read the section on The untie Gotcha in the perltie manpage.

Even if you don't currently make use of the API interface, it is stillworth reading it.

Here is an example which illustrates the problem from a DB_Fileperspective:

    use DB_File ;    use Fcntl ;

    my %x ;    my $X ;

    $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_TRUNC        or die "Cannot tie first time: $!" ;

    $x{123} = 456 ;

    untie %x ;

    tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT        or die "Cannot tie second time: $!" ;

    untie %x ;

When run, the script will produce this error message:

    Cannot tie second time: Invalid argument at bad.file line 14.

Although the error message above refers to the second tie() statementin the script, the source of the problem is really with the untie()statement that precedes it.

Having read the perltie manpage you will probably have already guessed that theerror is caused by the extra copy of the tied object stored in $X.If you haven't, then the problem boils down to the fact that theDB_File destructor, DESTROY, will not be called until allreferences to the tied object are destroyed. Both the tied variable,%x, and $X above hold a reference to the object. The call tountie() will destroy the first, but $X still holds a validreference, so the destructor will not get called and the database filetst.fil will remain open. The fact that Berkeley DB then reports theattempt to open a database that is alreday open via the catch-all``Invalid argument'' doesn't help.

If you run the script with the -w flag the error message becomes:

    untie attempted while 1 inner references still exist at bad.file line 12.    Cannot tie second time: Invalid argument at bad.file line 14.

which pinpoints the real problem. Finally the script can now bemodified to fix the original problem by destroying the API objectbefore the untie:

    ...    $x{123} = 456 ;

    undef $X ;    untie %x ;

    $X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT    ...

COMMON QUESTIONS

Why is there Perl source in my database?

If you look at the contents of a database file created by DB_File,there can sometimes be part of a Perl script included in it.

This happens because Berkeley DB uses dynamic memory to allocatebuffers which will subsequently be written to the database file. Beingdynamic, the memory could have been used for anything before DBmalloced it. As Berkeley DB doesn't clear the memory once it has beenallocated, the unused portions will contain random junk. In the casewhere a Perl script gets written to the database, the random junk willcorrespond to an area of dynamic memory that happened to be used duringthe compilation of the script.

Unless you don't like the possibility of there being part of your Perlscripts embedded in a database file, this is nothing to worry about.

How do I store complex data structures with DB_File?

Although DB_File cannot do this directly, there is a module whichcan layer transparently over DB_File to accomplish this feat.

Check out the MLDBM module, available on CPAN in the directorymodules/by-module/MLDBM.

What does Invalid Argument mean?

You will get this error message when one of the parameters in thetie call is wrong. Unfortunately there are quite a few parameters toget wrong, so it can be difficult to figure out which one it is.

Here are a couple of possibilities:

1.: Attempting to reopen a database without closing it.
2.: Using the O_WRONLY flag.

What does Bareword `DB_File' not allowed mean?

You will encounter this particular error message when you have thestrict 'subs' pragma (or the full strict pragma) in your script.Consider this script:

    use strict ;    use DB_File ;    use vars qw(%x) ;    tie %x, DB_File, "filename" ;

Running it produces the error in question:

    Bareword "DB_File" not allowed while "strict subs" in use

To get around the error, place the word DB_File in either single ordouble quotes, like this:

    tie %x, "DB_File", "filename" ;

Although it might seem like a real pain, it is really worth the effortof having a use strict in all your scripts.

HISTORY

0.1

First Release.

0.2

When DB_File is opening a database file it no longer terminates theprocess if dbopen returned an error. This allows file protectionerrors to be caught at run time. Thanks to Judith Grass<grassAATTcybercash.com> for spotting the bug.

0.3

Added prototype support for multiple btree compare callbacks.

1.0

DB_File has been in use for over a year. To reflect that, theversion number has been incremented to 1.0.

Added complete support for multiple concurrent callbacks.

Using the push method on an empty list didn't work properly. Thishas been fixed.

1.01

Fixed a core dump problem with SunOS.

The return value from TIEHASH wasn't set to NULL when dbopen returnedan error.

1.02

Merged OS/2 specific code into DB_File.xs

Removed some redundant code in DB_File.xs.

Documentation update.

Allow negative subscripts with RECNO interface.

Changed the default flags from O_RDWR to O_CREAT|O_RDWR.

The example code which showed how to lock a database needed a call tosync added. Without it the resultant database file was empty.

Added get_dup method.

1.03

Documentation update.

DB_File now imports the constants (O_RDWR, O_CREAT etc.) from Fcntlautomatically.

The standard hash function exists is now supported.

Modified the behavior of get_dup. When it returns an associativearray, the value is the count of the number of matching BTREE values.

1.04

Minor documentation changes.

Fixed a bug in hash_cb. Patches supplied by Dave Hammen,<hammenAATTgothamcity.jsc.nasa.gov>.

Fixed a bug with the constructors for DB_File::HASHINFO,DB_File::BTREEINFO and DB_File::RECNOINFO. Also tidied up theconstructors to make them -w clean.

Reworked part of the test harness to be more locale friendly.

1.05

Made all scripts in the documentation strict and -w clean.

Added logic to DB_File.xs to allow the module to be built after Perlis installed.

1.06

Minor namespace cleanup: Localized PrintBtree.

1.07

Fixed bug with RECNO, where bval wasn't defaulting to ``\n''.

1.08

Documented operation of bval.

1.09

Minor bug fix in DB_File::HASHINFO, DB_File::RECNOINFO andDB_File::BTREEINFO.

Changed default mode to 0666.

1.10

Fixed fd method so that it still returns -1 for in-memory files when db1.86 is used.

1.11

Documented the untie gotcha.

1.12

Documented the incompatibility with version 2 of Berkeley DB.

1.13

Minor changes to DB_FIle.xs and DB_File.pm

1.14

Made it illegal to tie an associative array to a RECNO database and anordinary array to a HASH or BTREE database.

1.15

Patch from Gisle Aas <gisleAATTaas.no> to suppress ``use of undefinedvalue'' warning with db_get and db_seq.

Patch from Gisle Aas <gisleAATTaas.no> to make DB_File export only the O_*constants from Fcntl.

Removed the DESTROY method from the DB_File::HASHINFO module.

Previously DB_File hard-wired the class name of any object that itcreated to ``DB_File''. This makes sub-classing difficult. Now DB_Filecreats objects in the namespace of the package it has been inheritedinto.

BUGS

Some older versions of Berkeley DB had problems with fixed lengthrecords using the RECNO file format. The newest version at the time ofwriting was 1.85 - this seems to have fixed the problems with RECNO.

I am sure there are bugs in the code. If you do find any, or cansuggest any enhancements, I would welcome your comments.

AVAILABILITY

DB_File comes with the standard Perl source distribution. Look inthe directory ext/DB_File.

This version of DB_File will only work with version 1.x of BerkeleyDB. It is not yet compatible with version 2.

Version 1 of Berkeley DB is available at your nearest CPAN archive (seethe section on CPAN in the perlmod manpage for a list) in src/misc/db.1.85.tar.gz, or via thehost ftp.cs.berkeley.edu in /ucb/4bsd/db.tar.gz. Alternatively,check out the Berkeley DB home page at http://www.bostic.com/db. Itis not under the GPL.

If you are running IRIX, then get Berkeley DB fromhttp://reality.sgi.com/ariel. It has the patches necessary tocompile properly on IRIX 5.3.

As of January 1997, version 1.86 of Berkeley DB is available from theBerkeley DB home page. Although this release does fix a number of bugsthat were present in 1.85 you should be aware of the followinginformation (taken from the Berkeley DB home page) before you considerusing it:

    DB version 1.86 includes a new implementation of the hash access    method that fixes a variety of hashing problems found in DB version    1.85. We are making it available as an interim solution until DB    2.0 is available.

    PLEASE NOTE: the underlying file format for the hash access method    changed between version 1.85 and version 1.86, so you will have to    dump and reload all of your databases to convert from version 1.85    to version 1.86. If you do not absolutely require the fixes from    version 1.86, we strongly urge you to wait until DB 2.0 is released    before upgrading from 1.85.

AUTHOR

The DB_File interface was written by Paul Marquess<pmarquessAATTbfsec.bt.co.uk>.Questions about the DB system itself may be addressed to<dbAATTsleepycat.com<gt>.

Index

NAME
SYNOPSIS
DESCRIPTION
Interface to Berkeley DB
Opening a Berkeley DB Database File
Default Parameters
In Memory Databases
DB_HASH
A Simple Example
DB_BTREE
Changing the BTREE sort order
Handling Duplicate Keys
The get_dup() Method
Matching Partial Keys
DB_RECNO
The `bval' Option
A Simple Example
Extra Methods
Another Example
THE API INTERFACE
HINTS AND TIPS
Locking Databases
Sharing Databases With C Applications
The untie() Gotcha
COMMON QUESTIONS
Why is there Perl source in my database?
How do I store complex data structures with DB_File?
What does Invalid Argument mean?
What does Bareword `DB_File' not allowed mean?
HISTORY
BUGS
AVAILABILITY
SEE ALSO
AUTHOR

This document was created byman2html,using the manual pages.

DB_File

NAME

SYNOPSIS

DESCRIPTION

Interface to Berkeley DB

Opening a Berkeley DB Database File

Default Parameters

In Memory Databases

DB_HASH

A Simple Example

DB_BTREE

Changing the BTREE sort order

Handling Duplicate Keys

The get_dup() Method

Matching Partial Keys

DB_RECNO

The `bval' Option

A Simple Example

Extra Methods

Another Example

THE API INTERFACE

HINTS AND TIPS

Locking Databases

Sharing Databases With C Applications

The untie() Gotcha

COMMON QUESTIONS

Why is there Perl source in my database?

How do I store complex data structures with DB_File?

What does Invalid Argument mean?

What does Bareword `DB_File' not allowed mean?

HISTORY

BUGS

AVAILABILITY

SEE ALSO

AUTHOR

Index