CONTENTS

• NAME
• SYNOPSIS
• DESCRIPTION
  • Tie
• STATIC METHODS
  • GDBM_version
• ERROR HANDLING
  • $GDBM_File::gdbm_errno
  • gdbm_check_syserr
• DATABASE METHODS
  • close
  • errno
  • syserrno
  • strerror
  • clear_error
  • needs_recovery
  • reorganize
  • sync
  • count
  • flags
  • dbname
  • cache_size
  • block_size
  • sync_mode
  • centfree
  • coalesce
  • mmap
  • mmapsize
  • recover
  • convert
  • dump
  • load
• CRASH TOLERANCE
  • crash_tolerance_status
  • failure_atomic
  • latest_snapshot
• AVAILABILITY
• SECURITY AND PORTABILITY
• SEE ALSO

#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 theGDBM manual at hand. The manual is avaialble online athttps://www.gnu.org.ua/software/gdbm/manual.

Most of thegdbm functions are available through theGDBM_File interface.

Unlike Perl's built-in hashes, it is not safe todelete the current item from a GDBM_File tied hash while iterating over it witheach. This is a limitation of the gdbm library.

#Tie

Use the Perl built-intie to associate aGDBM 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 ofaccess mode and optionalmodifiers. 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 (seehttps://www.gnu.org.ua/software/gdbm/manual/Open.html for a complete list), but one is worth mentioning. TheGDBM_NUMSYNC modifier, when used withGDBM_NEWDB, instructsGDBM to create the database inextended (so callednumsync) format. This format is best suited for crash-tolerant implementations. SeeCRASH TOLERANCE below for more information.

The$mode parameter is the file mode for creating new database file. Use an octal constant or a combination ofS_I* constants from theFcntl module. This parameter is used if$flags isGDBM_NEWDB orGDBM_WRCREAT.

On success,tie returns an object of classGDBM_File. On failure, it returnsundef. It is recommended to always check the return value, to make sure your hash is successfully associated with the database file. SeeERROR HANDLING below for examples.

#STATIC METHODS

#GDBM_version

$str = GDBM_File->GDBM_version;@ar = GDBM_File->GDBM_version;

Returns the version number of the underlyinglibgdbm library. In scalar context, returns the library version formatted as string:

MINOR.MAJOR[.PATCH][ (GUESS)]

whereMINOR,MAJOR, andPATCH are version numbers, andGUESS is a guess level (see below).

In list context, returns a list:

( MINOR, MAJOR, PATCH [, GUESS] )

TheGUESS component is present only iflibgdbm version is 1.8.3 or earlier. This is because earlier releases oflibgdbm did not include information about their version and theGDBM_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 thanMAJOR.MINOR.

#ERROR HANDLING

#$GDBM_File::gdbm_errno

When referenced in numeric context, retrieves the current value of thegdbm_errno variable, i.e. a numeric code describing the state of the most recent operation on anygdbm database. Each numeric code has a symbolic name associated with it. For a comprehensive list of these, seehttps://www.gnu.org.ua/software/gdbm/manual/Error-codes.html. Notice, that this list includes all error codes defined for the most recent version ofgdbm. Depending on the actual version of the libraryGDBM_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 dountie. However, you will need to use this function if you have explicitly assigned the result oftie 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, doinguntie 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 (Cerrno 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 totie.

#dbname

$db->dbname;

Returns the database name (i.e. 3rd argument totie.

#cache_size

$db->cache_size;$db->cache_size($newsize);

Returns the size of the internalGDBM 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 istrue orfalse.

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 willcroak if thelibgdbm 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 willcroak if thelibgdbm 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 andextended. The former is the traditional database format, used by previousgdbm versions. Theextended 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 isGDBM_NUMSYNC to convert the database from standard to extended format, and0 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 theload method.

GDBM supports two dump formats: oldbinary and newascii. 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 thegdbm version 1.11 and is the preferred dump format. Thedump 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 mode0666 modified by the currentumask.

These defaults can be altered using the following%options:

#binary => 1

Create dump inbinary format.

#mode =>MODE

Set file mode toMODE.

#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 thedump 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

If0, don't try to restore the mode of the database file to that stored in the dump.

#restore_owner => 0 | 1

If0, 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 ingdbm 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 thegdbm implementation is available in theGDBM 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 useextended database format for crash tolerant databases. To create a database in extended format, use theGDBM_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 theconvert 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 withsnapshots 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 thereflink operation (https://www.gnu.org.ua/software/gdbm/manual/Filesystems-supporting-crash-tolerance.html>.

After a successful call tofailure_atomic, every call to$db-sync> method will make an efficient reflink snapshot of the database file in one of these files; consecutive calls tosync 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 thelatest_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 tofailure_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 orundef 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 isGDBM_SNAPSHOT_OK. On error, the file name isundef, 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 tofailure_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

Thenumsync 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 isftp.gnu.org, but you are strongly urged to use one of the many mirrors. You can obtain a list of mirror sites fromhttp://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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