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 ;
Please note that this module will only work with version 1.x of Berkeley DB. Once Berkeley DB version 2 is released, DB_File will be upgraded to work with it.
Berkeley DB is a C library which provides a consistent interface to a number of database formats. DB_File provides an interface to all three of the database types currently supported by Berkeley DB.
The file types are:
A default hashing algorithm, which will be adequate for most applications, is built into Berkeley DB. If you do need to use your own hashing algorithm it is possible to write your own in Perl and have DB_File use it instead.
As with the DB_HASH format, it is possible to provide a user defined Perl routine to perform the comparison of keys. By default, though, the keys are stored in lexical order.
tie() mechanism in
Perl 5 (for full details, see tie()). This facility allows DB_File to access Berkeley
DB files using either an associative array (for
DB_HASH &
DB_BTREE file types) or an ordinary array (for the
DB_RECNO file type).
In addition to the tie() interface, it is also possible to access most of the functions provided in the Berkeley
DB
API directly. See
THE API INTERFACE.
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 3 interface 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 the specific interface
method.
This interface is handled slightly differently in DB_File. Here is an equivalent call using DB_File:
tie %array, 'DB_File', $filename, $flags, $mode, $DB_HASH ;
The filename, flags and mode parameters are the direct equivalent of their dbopen()
counterparts. The final parameter $DB_HASH
performs the function of both the type and openinfo parameters in dbopen().
In the example above $DB_HASH is actually a pre-defined reference to a hash 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 to the 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 are usually adequate for most applications. If you do need to create extra instances of these objects, constructors are available for each file type.
Here are examples of the constructors and the valid options available for 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 equivalent of their C counterpart. Like their C counterparts, all are set to a default values - that means you don't have to set all of the values 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, $a ;
A few of the options need extra discussion here. When used, the
C equivalent of the keys
hash, compare and prefix store pointers to
C functions. In DB_File these keys are used to store references to Perl subs. Below are templates
for each of the subs:
sub hash
{
my ($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 Changing the BTREE sort order for an example of using the
compare template.
If you are using the
DB_RECNO interface and you intend making use of
bval, you should check out The 'bval' Option.
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 the call:
tie %A, "DB_File" ;
is equivalent to:
tie %A, "DB_File", undef, O_CREAT|O_RDWR, 0666, $DB_HASH ;
See In Memory Databases for a discussion on the use of undef in place of a filename.
(char *)0 in
C) in place of the filename. DB_File
uses undef instead of
NULL to provide this functionality.
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 -> $v\n" }
untie %h ;
here is the output:
Banana Exists
orange -> orange
tomato -> red
banana -> yellow
Note that the like ordinary associative arrays, the order of the keys retrieved is in an apparently random order.
use strict ;
use DB_File ;
my %h ;
sub Compare
{
my ($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 the ordering in a BTREE database:
There are some difficulties in using the tied hash interface if you want to manipulate a BTREE database with duplicate keys. Consider this code:
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 they
seem to have the same value, namely Larry. The problem is caused by the way that the associative array interface
works. Basically, when the associative array interface is used to fetch the
value associated with a given key, it will only ever retrieve the first
value.
Although it may not be immediately obvious from the code above, the associative array interface can be used to write values with duplicate keys, 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 method called
seq. This method allows sequential access to key/value pairs. See THE API INTERFACE for details of both the seq method and 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 multiple
values associated with the key Wall.
get_dup, to assist in reading duplicate values from
BTREE databases. The method can take the following
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 associated with
the key, $key.
In list context, it returns all the values which match $key. Note that the values will be returned in an apparently random order.
In list context, if the second parameter is present and evaluates TRUE, the method returns an associative array. The keys of the associative array correspond to the values that matched in the BTREE and the values of the array are a count of the number of times that particular value occurred in the BTREE.
So assuming the database created above, we can use get_dup like this:
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 => []
seq method is used along with the
R_CURSOR flag.
$x->seq($key, $value, R_CURSOR) ;
Here is the relevant quote from the dbopen man page where it defines the use of the R_CURSOR flag with seq:
Note, for the DB_BTREE access method, the returned key is not
necessarily an exact match for the specified key. The returned key
is the smallest key greater than or equal to the specified key,
permitting partial key matches and range searches.
In the example script below, the match sub uses this feature to find and print the first matching key/value pair
given a partial key.
use strict ;
use DB_File ;
use Fcntl ;
use vars qw($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
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.
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-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 DB_File? Well, the behavior defined in the quote above is quite useful, so DB_File conforms it.
That means that you can specify other options (e.g. cachesize) and still have bval default to "\n" for variable length records, and space for fixed length records.
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
Here are the methods:
list to the end of the array.
list to the start of the array.
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:
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) )
put method was used the record index was specified using a variable, $i, rather than the literal value itself. This is because put will return the record number of the inserted line via that parameter.
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 DB_File methods directly like this:
$db->put($key, $value, R_NOOVERWRITE) ;
Important: If you have saved a copy of the object returned from tie, the underlying database file will 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 The untie() Gotcha for more details.
All the functions defined in dbopen are available except for close() and dbopen()
itself. The DB_File method interface to the supported functions have been implemented to mirror the way Berkeley
DB works whenever possible. In particular note that:
$! 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.
$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) ;
Below is a list of the methods available.
$key) this method reads the value associated with it from 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.
If you use either the
R_IAFTER or
R_IBEFORE flags, the
$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.
$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.
See Locking Databases for an example of how to make use of the
fd method to lock your database.
Both the $key and $value parameters will be set to the key/value pair read from the database.
The flags parameter is mandatory. The valid flag values are R_CURSOR, R_FIRST, R_LAST, R_NEXT and R_PREV.
R_RECNOSYNC is the only valid flag at present.
open()
to give something Perl will flock() for you. Run this
repeatedly in the background to watch the locks granted in proper order.
use DB_File;
use strict;
sub LOCK_SH { 1 }
sub LOCK_EX { 2 }
sub LOCK_NB { 4 }
sub LOCK_UN { 8 }
my($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";
The vast majority of problems that are reported in this area boil down to the fact that C strings are NULL terminated, whilst Perl strings are not.
Here is a real example. Netscape 2.0 keeps a record of the locations you visit along with the time you last visited them in a DB_HASH database. This is usually stored in the file ~/.netscape/history.db. The key field in the database is the location string and the value field is the time the location was last visited stored as a 4 byte binary value.
If you haven't already guessed, the location string is stored with a terminating NULL. This means you need to be careful when accessing the database.
Here is a snippet of code that is loosely based on Tom Christiansen's ggh script (available from your nearest CPAN archive in authors/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 ;
Even if you don't currently make use of the API interface, it is still worth reading it.
Here is an example which illustrates the problem from a DB_File perspective:
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()
statement in 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 the error 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 the
DB_File destructor,
DESTROY, will not be called until all
references to the tied object are destroyed. Both the tied variable,
%x, and $X above hold a reference to the object. The call to untie() will
destroy the first, but $X still holds a valid reference, so the destructor will not get called and
the database file
tst.fil will remain open. The fact that Berkeley
DB then reports the attempt 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 be modified to fix the original problem by destroying the API object before the untie:
...
$x{123} = 456 ;
undef $X ;
untie %x ;
$X = tie %x, 'DB_File', 'tst.fil' , O_RDWR|O_CREAT
...
This happens because Berkeley DB uses dynamic memory to allocate buffers which will subsequently be written to the database file. Being dynamic, the memory could have been used for anything before DB malloced it. As Berkeley DB doesn't clear the memory once it has been allocated, the unused portions will contain random junk. In the case where a Perl script gets written to the database, the random junk will correspond to an area of dynamic memory that happened to be used during the compilation of the script.
Unless you don't like the possibility of there being part of your Perl scripts embedded in a database file, this is nothing to worry about.
Check out the MLDBM module, available on CPAN in the directory modules/by-module/MLDBM.
Here are a couple of possibilities:
strict '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 or double quotes, like this:
tie %x, "DB_File", "filename" ;
Although it might seem like a real pain, it is really worth the effort of
having a use strict in all your scripts.
Added complete support for multiple concurrent callbacks.
Using the push method on an empty list didn't work properly. This has been fixed.
The return value from TIEHASH wasn't set to NULL when dbopen returned an error.
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 to
sync added. Without it the resultant database file was empty.
Added get_dup method.
DB_File now imports the constants (O_RDWR, O_CREAT etc.) from Fcntl automatically.
The standard hash function exists is now supported.
Modified the behavior of get_dup. When it returns an associative array, the value is the count of the number of matching BTREE values.
Fixed a bug in hash_cb. Patches supplied by Dave Hammen, <hammen@gothamcity.jsc.nasa.gov >.
Fixed a bug with the constructors for DB_File::HASHINFO, DB_File::BTREEINFO and DB_File::RECNOINFO. Also tidied up the constructors to make them -w clean.
Reworked part of the test harness to be more locale friendly.
Added logic to DB_File.xs to allow the module to be built after Perl is installed.
PrintBtree.
Changed default mode to 0666.
I am sure there are bugs in the code. If you do find any, or can suggest any enhancements, I would welcome your comments.
This version of DB_File will only work with version 1.x of Berkeley DB. It is not yet compatible with version 2.
Version 1 of Berkeley DB is available at your nearest CPAN archive (see CPAN for a list) in src/misc/db.1.85.tar.gz, or via the host ftp.cs.berkeley.edu in /ucb/4bsd/db.tar.gz. Alternatively, check out the Berkeley DB home page at http://www.bostic.com/db. It is not under the GPL.
If you are running IRIX, then get Berkeley DB from http://reality.sgi.com/ariel. It has the patches necessary to compile properly on IRIX 5.3.
As of January 1997, version 1.86 of Berkeley DB is available from the Berkeley DB home page. Although this release does fix a number of bugs that were present in 1.85 you should be aware of the following information (taken from the Berkeley DB home page) before you consider using 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.