MIT/GNU Scheme GDBM Plugin Manual


GDBM Plugin Manual

This manual documents MIT/GNU Scheme GDBM 1.0.

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1 Introduction to GNU dbm.

This plugin is a dynamically loadable wrapper for the GNU dbm (DataBase Management) C library. This manual is a derivative of Edition 1.5 of the GNU dbm Manual, for library version 1.8.3, last updated October 15, 2002.

GNU dbm (gdbm) is a library of database functions that use extendible hashing; it works similarly to the standard UNIX dbm functions. The basic use of gdbm is to store key/data pairs in a data file. Each key must be unique and each key is paired with only one data item. The keys can not be directly accessed in sorted order.

The key/data pairs are stored in a gdbm disk file, called a gdbm database. A program must connect to a gdbm database to be able manipulate the keys and data contained in it. Gdbm allows Scheme to connect to multiple databases at the same time. When Scheme connects to a gdbm database, the connection is designated as a reader or a writer. A gdbm database may be connected to at most one writer at a time. However, many readers may connect to the database simultaneously. Readers and writers may not connect to the database at the same time. (Note that these restrictions are not enforced by the library nor the wrapper.)

Each connection is encapsulated in a Scheme gdbf structure which should be used by one Scheme thread at a time. A mutex is used to block any thread attempting to access the database while an operation is in progress. No file locks are used by gdbm or the Scheme wrapper to ensure exclusive access by a Scheme writer.

2 The exported bindings.

The following is a quick list of the procedures provided by the plugin.


Neither gdbm_errno nor gdbm_strerror are exposed because the plugin automatically tests and calls them to detect errors and convert codes into strings. gdbm_fdesc is also not exposed, treated as an implementation detail the plugin should probably hide, used by tricky code that cooperates with multiple file locking libraries.

There is one global variable, gdbm-version, which is initialized from the library’s gdbm_version string.

And several constants:


You can load these bindings into your global environment with the following expresson.

    (load-option 'gdbm)

And you can include these bindings in your package description (.pkg) file with the following expression.

    (global-definitions gdbm/)

3 Opening the database.

Connect to the file. If the file has a size of zero bytes, a file initialization procedure is performed, setting up the initial structure in the file.

The procedure for opening a gdbm file is:

Procedure: gdbm-open name block-size flags mode

The parameters are:


The name of the file (the complete name, gdbm does not append any characters to this name).


It is used during initialization to determine the size of various constructs. It is the size of a single transfer from disk to memory. This parameter is ignored if the file has been previously initialized. The minimum size is 512. If the value is less than 512, the file system blocksize is used, otherwise the value of block-size is used.


If flags is gdbm_reader, the user wants to just read the database and any call to gdbm-store or gdbm-delete will fail. Many readers can access the database at the same time. If flags is gdbm_writer, the user wants both read and write access to the database and requires exclusive access. If flags is gdbm_wrcreat, the user wants both read and write access to the database and if the database does not exist, create a new one. If flags is gdbm_newdb, the user want a new database created, regardless of whether one existed, and wants read and write access to the new database. The following may also be logically or’d into the database flags: gdbm_sync, which causes all database operations to be synchronized to the disk, and gdbm_nolock, which prevents the library from performing any locking on the database file. gdbm_fast is now obsolete, since gdbm defaults to no-sync mode.


File mode (see chmod(2) and open(2) if the file is created).

The return value is the object needed by all other procedures to access that gdbm file.

4 Closing the database.

It is important that every file opened is also closed. This is needed to update the reader/writer count on the file. Scheme will do this automatically if an open gdbm object is garbage collected, but you can close the file immediately with the gdbm-close procedure.

Procedure: gdbm-close dbf

The parameter is:


The object returned by gdbm-open.

Closes the gdbm file and frees all memory associated with dbf.

5 Inserting and replacing records in the database.

The procedure gdbm-store inserts or replaces records in the database.

Procedure: gdbm-store dbf key content flag

The parameters are:


The object returned by gdbm-open.


A non-empty string, converted to utf-8 bytes for lookup in the database.


Another non-empty string, the content to be stored in the database file, also converted to utf-8.


The action to take when key is already in the database. The value of gdbm_replace indicates that the old content should be replaced by content. The value of gdbm_insert indicates that #f should be returned and no action taken if key already exists.

The values returned are:


Success. content is keyed by key. The file on disk is updated to reflect the structure of the new database before returning from this procedure.


The item was not stored because flag was gdbm_insert and key was already in the database.

An error is signaled if the caller is not a writer.

If you store content for a key that is already in the database, gdbm replaces the old content with the new content if called with gdbm_replace. You do not get two content items for the same key and you do not get an error from gdbm-store.

The size in gdbm is not restricted like dbm or ndbm. Your content can be as large as you want.

6 Searching for records in the database.

Read content associated with a key.

Procedure: gdbm-fetch dbf key

The parameters are:


The object returned by gdbm-open.


A non-empty string, converted to utf-8 bytes for lookup in the database.

The return value is a string created from the utf-8 bytes found in the database, or #f if no content was found.

You may also search for a particular key without retrieving it, using:

Procedure: gdbm-exists? dbf key

The parameters are:


The pointer returned by gdbm-open.


A non-empty string, converted to utf-8 bytes for lookup in the database.

Unlike gdbm-fetch this procedure does not read any content and simply returns true or false depending on whether key exists.

7 Removing records from the database.

To remove some content from the database:

Procedure: gdbm-delete dbg key

The parameters are:


The object returned by gdbm-open.


A non-empty string, converted to utf-8 bytes for lookup in the database.

The return value is #f if the item is not present or the requester is a reader. The return value is #t if there was a successful delete.

The keyed content and the key are removed from the database. The file on disk is updated to reflect the structure of the new database before returning from this procedure.

8 Sequential access to records.

The next two functions allow for accessing all content in a database dbf. This access is not key sequential, but it is guaranteed to visit every key in the database once. (The order has to do with the hash values.)

Procedure: gdbm-firstkey dbf

Starts the visit of all keys in the database dbf. Returns the first key to visit, converting its utf-8 bytes to a string. If there are no keys, returns #f.

Procedure: gdbm-nextkey dbf key

Returns the key to visit after key, converting its utf-8 bytes to a string. If there are no more keys, returns #f.

These functions were intended to visit the database in read-only algorithms, for instance, to validate the database or similar operations.

Visiting keys traverses a hash table which writers may re-arrange. The original key order is not guaranteed to remain unchanged in all instances. It is possible that some key will not be visited if the database is changed while traversing the table.

9 Database reorganization.

The following procedure should be used very seldom.

Procedure: gdbm-reorganize dbf

If you have made a lot of deletions and would like to shrink the space used by the gdbm file, this function will reorganize the database. Gdbm will not shorten a gdbm file (will not reuse deleted space) until this procedure is called.

The reorganization requires creating a new file and inserting all the elements in the old file dbf into the new file. The new file is then renamed to the same name as the old file and dbf is updated to contain all the correct information about the new file.

10 Database Synchronization

Unless you opened your database with the gdbm_sync flag, gdbm does not wait for writes to be flushed to the disk. This allows faster writing of databases at the risk of having a corrupted database if Scheme terminates in an abnormal fashion. The following function allows the programmer to flush all changes to disk.

Procedure: gdbm-sync dbf

This would usually be called after a complete set of changes have been made to the database and before some long waiting time. Gdbm-close always flushes any changes to disk.

11 Setting options.

Gdbm supports the ability to set certain options on an already open database.

Procedure: gdbm-setopt dbf option value

The parameters are:


The pointer returned by gdbm-open.


The option to be set, the value of gdbm_cachesize or gdbm_syncmode.


The value to be set, an integer.

If option is gdbm_cachesize the size of the internal bucket cache is set to the given integer. This option may only be set once on a database, and is set to 100 by default when the database is first accessed.

If option is gdbm_syncmode file system synchronization is turned on or off. By default it is off. Value should 1 to turn it on, or 0 to turn it off.

The obsolete and experimental options GDBM_FASTMODE, GDBM_CENTFREE and GDBM_COALESCEBLKS are not supported by this plugin.