GDBM_File(3perl) Perl Programmers Reference Guide GDBM_File(3perl)
NAME
GDBM_File - Perl5 access to the gdbm library.
SYNOPSIS
use GDBM_File;
[$db =] tie %hash, 'GDBM_File', $filename, GDBM_WRCREAT, 0640
or die "$GDBM_File::gdbm_errno";
# Use the %hash...
$e = $db->errno;
$e = $db->syserrno;
$str = $db->strerror;
$bool = $db->needs_recovery;
$db->clear_error;
$db->reorganize;
$db->sync;
$n = $db->count;
$n = $db->flags;
$str = $db->dbname;
$db->cache_size;
$db->cache_size($newsize);
$n = $db->block_size;
$bool = $db->sync_mode;
$db->sync_mode($bool);
$bool = $db->centfree;
$db->centfree($bool);
$bool = $db->coalesce;
$db->coalesce($bool);
$bool = $db->mmap;
$size = $db->mmapsize;
$db->mmapsize($newsize);
$db->recover(%args);
untie %hash ;
DESCRIPTION
GDBM_File is a module which allows Perl programs to make use of the
facilities provided by the GNU gdbm library. If you intend to use this
module you should really have a copy of the GDBM manual at hand. The
manual is avaialble online at
<https://www.gnu.org.ua/software/gdbm/manual>.
Most of the gdbm functions are available through the GDBM_File
interface.
Unlike Perl's built-in hashes, it is not safe to "delete" the current
item from a GDBM_File tied hash while iterating over it with "each".
This is a limitation of the gdbm library.
Tie
Use the Perl built-in tie to associate a GDBM database with a Perl hash:
tie %hash, 'GDBM_File', $filename, $flags, $mode;
Here, $filename is the name of the database file to open or create.
$flags is a bitwise OR of access mode and optional modifiers. Access
mode is one of:
GDBM_READER
Open existing database file in read-only mode.
GDBM_WRITER
Open existing database file in read-write mode.
GDBM_WRCREAT
If the database file exists, open it in read-write mode. If it
doesn't, create it first and open read-write.
GDBM_NEWDB
Create new database and open it read-write. If the database already
exists, truncate it first.
A number of modifiers can be OR'd to the access mode. Most of them are
rarely needed (see
<https://www.gnu.org.ua/software/gdbm/manual/Open.html> for a complete
list), but one is worth mentioning. The GDBM_NUMSYNC modifier, when
used with GDBM_NEWDB, instructs GDBM to create the database in extended
(so called numsync) format. This format is best suited for crash-
tolerant implementations. See CRASH TOLERANCE below for more
information.
The $mode parameter is the file mode for creating new database file.
Use an octal constant or a combination of "S_I*" constants from the
Fcntl module. This parameter is used if $flags is GDBM_NEWDB or
GDBM_WRCREAT.
On success, tie returns an object of class GDBM_File. On failure, it
returns undef. It is recommended to always check the return value, to
make sure your hash is successfully associated with the database file.
See ERROR HANDLING below for examples.
STATIC METHODS
GDBM_version
$str = GDBM_File->GDBM_version;
@ar = GDBM_File->GDBM_version;
Returns the version number of the underlying libgdbm library. In scalar
context, returns the library version formatted as string:
MINOR.MAJOR[.PATCH][ (GUESS)]
where MINOR, MAJOR, and PATCH are version numbers, and GUESS is a guess
level (see below).
In list context, returns a list:
( MINOR, MAJOR, PATCH [, GUESS] )
The GUESS component is present only if libgdbm version is 1.8.3 or
earlier. This is because earlier releases of libgdbm did not include
information about their version and the GDBM_File module has to
implement certain guesswork in order to determine it. GUESS is a textual
description in string context, and a positive number indicating how
rough the guess is in list context. Possible values are:
1 - exact guess
The major and minor version numbers are guaranteed to be correct.
The actual patchlevel is most probably guessed right, but can be 1-2
less than indicated.
2 - approximate
The major and minor number are guaranteed to be correct. The
patchlevel is set to the upper bound.
3 - rough guess
The version is guaranteed to be not newer than MAJOR.MINOR.
ERROR HANDLING
$GDBM_File::gdbm_errno
When referenced in numeric context, retrieves the current value of the
gdbm_errno variable, i.e. a numeric code describing the state of the
most recent operation on any gdbm database. Each numeric code has a
symbolic name associated with it. For a comprehensive list of these,
see <https://www.gnu.org.ua/software/gdbm/manual/Error-codes.html>.
Notice, that this list includes all error codes defined for the most
recent version of gdbm. Depending on the actual version of the library
GDBM_File is built with, some of these may be missing.
In string context, $gdbm_errno returns a human-readable description of
the error. If necessary, this description includes the value of $!.
This makes it possible to use it in diagnostic messages. For example,
the usual tying sequence is
tie %hash, 'GDBM_File', $filename, GDBM_WRCREAT, 0640
or die "$GDBM_File::gdbm_errno";
The following, more complex, example illustrates how you can fall back
to read-only mode if the database file permissions forbid read-write
access:
use Errno qw(EACCES);
unless (tie(%hash, 'GDBM_File', $filename, GDBM_WRCREAT, 0640)) {
if ($GDBM_File::gdbm_errno == GDBM_FILE_OPEN_ERROR
&& $!{EACCES}) {
if (tie(%hash, 'GDBM_File', $filename, GDBM_READER, 0640)) {
die "$GDBM_File::gdbm_errno";
}
} else {
die "$GDBM_File::gdbm_errno";
}
}
gdbm_check_syserr
if (gdbm_check_syserr(gdbm_errno)) ...
Returns true if the system error number ($!) gives more information on
the cause of the error.
DATABASE METHODS
close
$db->close;
Closes the database. Normally you would just do untie. However, you
will need to use this function if you have explicitly assigned the
result of tie to a variable, and wish to release the database to another
users. Consider the following code:
$db = tie %hash, 'GDBM_File', $filename, GDBM_WRCREAT, 0640;
# Do something with %hash or $db...
untie %hash;
$db->close;
In this example, doing untie alone is not enough, since the database
would remain referenced by $db, and, as a consequence, the database file
would remain locked. Calling $db->close ensures the database file is
closed and unlocked.
errno
$db->errno
Returns the last error status associated with this database. In string
context, returns a human-readable description of the error. See also
$GDBM_File::gdbm_errno variable above.
syserrno
$db->syserrno
Returns the last system error status (C "errno" variable), associated
with this database,
strerror
$db->strerror
Returns textual description of the last error that occurred in this
database.
clear_error
$db->clear_error
Clear error status.
needs_recovery
$db->needs_recovery
Returns true if the database needs recovery.
reorganize
$db->reorganize;
Reorganizes the database.
sync
$db->sync;
Synchronizes recent changes to the database with its disk copy.
count
$n = $db->count;
Returns number of keys in the database.
flags
$db->flags;
Returns flags passed as 4th argument to tie.
dbname
$db->dbname;
Returns the database name (i.e. 3rd argument to tie.
cache_size
$db->cache_size;
$db->cache_size($newsize);
Returns the size of the internal GDBM cache for that database.
Called with argument, sets the size to $newsize.
block_size
$db->block_size;
Returns the block size of the database.
sync_mode
$db->sync_mode;
$db->sync_mode($bool);
Returns the status of the automatic synchronization mode. Called with
argument, enables or disables the sync mode, depending on whether $bool
is true or false.
When synchronization mode is on (true), any changes to the database are
immediately written to the disk. This ensures database consistency in
case of any unforeseen errors (e.g. power failures), at the expense of
considerable slowdown of operation.
Synchronization mode is off by default.
centfree
$db->centfree;
$db->centfree($bool);
Returns status of the central free block pool (0 - disabled, 1 -
enabled).
With argument, changes its status.
By default, central free block pool is disabled.
coalesce
$db->coalesce;
$db->coalesce($bool);
mmap
$db->mmap;
Returns true if memory mapping is enabled.
This method will croak if the libgdbm library is complied without memory
mapping support.
mmapsize
$db->mmapsize;
$db->mmapsize($newsize);
If memory mapping is enabled, returns the size of memory mapping. With
argument, sets the size to $newsize.
This method will croak if the libgdbm library is complied without memory
mapping support.
recover
$db->recover(%args);
Recovers data from a failed database. %args is optional and can contain
following keys:
err => sub { ... }
Reference to code for detailed error reporting. Upon encountering an
error, recover will call this sub with a single argument - a
description of the error.
backup => \$str
Creates a backup copy of the database before recovery and returns
its filename in $str.
max_failed_keys => $n
Maximum allowed number of failed keys. If the actual number becomes
equal to $n, recover aborts and returns error.
max_failed_buckets => $n
Maximum allowed number of failed buckets. If the actual number
becomes equal to $n, recover aborts and returns error.
max_failures => $n
Maximum allowed number of failures during recovery.
stat => \%hash
Return recovery statistics in %hash. Upon return, the following keys
will be present:
recovered_keys
Number of successfully recovered keys.
recovered_buckets
Number of successfully recovered buckets.
failed_keys
Number of keys that failed to be retrieved.
failed_buckets
Number of buckets that failed to be retrieved.
convert
$db->convert($format);
Changes the format of the database file referred to by $db.
Starting from version 1.20, gdbm supports two database file formats:
standard and extended. The former is the traditional database format,
used by previous gdbm versions. The extended format contains additional
data and is recommended for use in crash tolerant applications.
<https://www.gnu.org.ua/software/gdbm/manual/Numsync.html>, for the
discussion of both formats.
The $format argument sets the new desired database format. It is
GDBM_NUMSYNC to convert the database from standard to extended format,
and 0 to convert it from extended to standard format.
If the database is already in the requested format, the function returns
success without doing anything.
dump
$db->dump($filename, %options)
Creates a dump of the database file in $filename. Such file can be used
as a backup copy or sent over a wire to recreate the database on another
machine. To create a database from the dump file, use the load method.
GDBM supports two dump formats: old binary and new ascii. The binary
format is not portable across architectures and is deprecated. It is
supported for backward compatibility. The ascii format is portable and
stores additional meta-data about the file. It was introduced with the
gdbm version 1.11 and is the preferred dump format. The dump method
creates ascii dumps by default.
If the named file already exists, the function will refuse to overwrite
and will croak an error. If it doesn't exist, it will be created with
the mode 0666 modified by the current umask.
These defaults can be altered using the following %options:
binary => 1
Create dump in binary format.
mode => MODE
Set file mode to MODE.
overwrite => 1
Silently overwrite existing files.
load
$db->load($filename, %options)
Load the data from the dump file $filename into the database $db. The
file must have been previously created using the dump method. File
format is recognized automatically. By default, the function will croak
if the dump contains a key that already exists in the database. It will
silently ignore the failure to restore database mode and/or ownership.
These defaults can be altered using the following %options:
replace => 1
Replace existing keys.
restore_mode => 0 | 1
If 0, don't try to restore the mode of the database file to that
stored in the dump.
restore_owner => 0 | 1
If 0, don't try to restore the owner of the database file to that
stored in the dump.
strict_errors => 1
Croak if failed to restore ownership and/or mode.
The usual sequence to recreate a database from the dump file is:
my %hash;
my $db = tie %hash, 'GDBM_File', 'a.db', GDBM_NEWDB, 0640;
$db->load('a.dump');
CRASH TOLERANCE
Crash tolerance is a new feature that, given appropriate support from
the OS and the filesystem, guarantees that a logically consistent recent
state of the database can be recovered following a crash, such as power
outage, OS kernel panic, or the like.
Crash tolerance support appeared in gdbm version 1.21. The theory
behind it is explained in "Crashproofing the Original NoSQL Key-Value
Store", by Terence Kelly
(<https://queue.acm.org/detail.cfm?id=3487353>). A detailed discussion
of the gdbm implementation is available in the GDBM Manual
(<https://www.gnu.org.ua/software/gdbm/manual/Crash-Tolerance.html>).
The information below describes the Perl interface.
For maximum robustness, we recommend to use extended database format for
crash tolerant databases. To create a database in extended format, use
the GDBM_NEWDB|GDBM_NUMSYNC when opening the database, e.g.:
$db = tie %hash, 'GDBM_File', $filename,
GDBM_NEWDB|GDBM_NUMSYNC, 0640;
To convert existing database to the extended format, use the convert
method, described above, e.g.:
$db->convert(GDBM_NUMSYNC);
crash_tolerance_status
GDBM_File->crash_tolerance_status;
This static method returns the status of crash tolerance support. A
non-zero value means crash tolerance is compiled in and supported by the
operating system.
failure_atomic
$db->failure_atomic($even, $odd)
Enables crash tolerance for the database $db, Arguments are the
pathnames of two files that will be created and filled with snapshots of
the database file. The two files must not exist when this method is
called and must reside on the same filesystem as the database file.
This filesystem must be support the reflink operation
(https://www.gnu.org.ua/software/gdbm/manual/Filesystems-supporting-crash-tolerance.html>.
After a successful call to failure_atomic, every call to $db-sync>
method will make an efficient reflink snapshot of the database file in
one of these files; consecutive calls to sync alternate between the two,
hence the names.
The most recent of these files can be used to recover the database after
a crash. To select the right snapshot, use the latest_snapshot static
method.
latest_snapshot
$file = GDBM_File->latest_snapshot($even, $odd);
($file, $error) = GDBM_File->latest_snapshot($even, $odd);
Given the two snapshot names (the ones used previously in a call to
failure_atomic), this method selects the one suitable for database
recovery, i.e. the file which contains the most recent database
snapshot.
In scalar context, it returns the selected file name or undef in case of
failure.
In array context, the returns a list of two elements: the file name and
status code. On success, the file name is defined and the code is
GDBM_SNAPSHOT_OK. On error, the file name is undef, and the status is
one of the following:
GDBM_SNAPSHOT_BAD
Neither snapshot file is applicable. This means that the crash has
occurred before a call to failure_atomic completed. In this case,
it is best to fall back on a safe backup copy of the data file.
GDBM_SNAPSHOT_ERR
A system error occurred. Examine $! for details. See
<https://www.gnu.org.ua/software/gdbm/manual/Crash-recovery.html>
for a comprehensive list of error codes and their meaning.
GDBM_SNAPSHOT_SAME
The file modes and modification dates of both snapshot files are
exactly the same. This can happen only for databases in standard
format.
GDBM_SNAPSHOT_SUSPICIOUS
The numsync counters of the two snapshots differ by more than one.
The most probable reason is programmer's error: the two parameters
refer to snapshots belonging to different database files.
AVAILABILITY
gdbm is available from any GNU archive. The master site is
"ftp.gnu.org", but you are strongly urged to use one of the many
mirrors. You can obtain a list of mirror sites from
<http://www.gnu.org/order/ftp.html>.
SECURITY AND PORTABILITY
GDBM files are not portable across platforms. If you wish to transfer a
GDBM file over the wire, dump it to a portable format first.
Do not accept GDBM files from untrusted sources.
Robustness of GDBM against corrupted databases depends highly on its
version. Versions prior to 1.15 did not implement any validity
checking, so that a corrupted or maliciously crafted database file could
cause perl to crash or even expose a security vulnerability. Versions
between 1.15 and 1.20 were progressively strengthened against invalid
inputs. Finally, version 1.21 had undergone extensive fuzzy checking
which proved its ability to withstand any kinds of inputs without
crashing.
SEE ALSO
perl(1), DB_File(3), perldbmfilter, gdbm(3),
<https://www.gnu.org.ua/software/gdbm/manual.html>.
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