     [file version.texi not included]

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Copyright © 1989-1999, 2007-2011 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover, and no Back-Cover texts. A copy of the license is included in the section entitled ``GNU Free Documentation License.''

The GNU database manager.
*************************

GNU dbm is a library of functions implementing a hashed database on a disk file.  This manual documents GNU dbm Version VERSION (gdbm).  The software was originally written by Philip A. Nelson.  This document was originally written by Pierre Gaumond from texts written by Phil.

Copying Conditions.
*******************

This library is free; this means that everyone is free to use it and free to redistribute it on a free basis.  GNU dbm (gdbm) is not in the public domain; it is copyrighted and there are restrictions on its distribution, but these restrictions are designed to permit everything that a good cooperating citizen would want to do.  What is not allowed is to try to prevent others from further sharing any version of gdbm that they might get from you.

Specifically, we want to make sure that you have the right to give away copies gdbm, that you receive source code or else can get it if you want it, that you can change these functions or use pieces of them in new free programs, and that you know you can do these things.

To make sure that everyone has such rights, we have to forbid you to deprive anyone else of these rights.  For example, if you distribute copies gdbm, you must give the recipients all the rights that you have.  You must make sure that they, too, receive or can get the source code.  And you must tell them their rights.

Also, for our own protection, we must make certain that everyone finds out that there is no warranty for anything in the gdbm distribution. If these functions are modified by someone else and passed on, we want their recipients to know that what they have is not what we distributed, so that any problems introduced by others will not reflect on our reputation.

Gdbm is currently distributed under the terms of the GNU General Public License, Version 3.  (NOT under the GNU General Library Public License.)  A copy the GNU General Public License is included with the distribution of gdbm.

Introduction to GNU dbm.
************************

GNU dbm (gdbm) is a library of database functions that use extensible hashing and works similar to the standard UNIX dbm functions.  These routines are provided to a programmer needing to create and manipulate a hashed database. (gdbm is NOT a complete database package for an end user.)

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 basic unit of data in gdbm is the structure:

       typedef struct {
                  char *dptr;
                  int  dsize;
               } datum;

This structure allows for arbitrary sized keys and data items.

The key/data pairs are stored in a gdbm disk file, called a gdbm database.  An application must open a gdbm database to be able manipulate the keys and data contained in the database. gdbm allows an application to have multiple databases open at the same time.  When an application opens a gdbm database, it is designated as a reader or a writer.  A gdbm database can be opened by at most one writer at a time.  However, many readers may open the database simultaneously.  Readers and writers can not open the gdbm database at the same time.

List of functions.
******************

The following is a quick list of the functions contained in the gdbm library.  The include file gdbm.h, that can be included by the user, contains a definition of these functions.

     #include <gdbm.h>

     GDBM_FILE gdbm_open(name, block_size, flags, mode, fatal_func);
     void gdbm_close(dbf);
     int gdbm_store(dbf, key, content, flag);
     datum gdbm_fetch(dbf, key);
     int gdbm_delete(dbf, key);
     datum gdbm_firstkey(dbf);
     datum gdbm_nextkey(dbf, key);
     int gdbm_reorganize(dbf);
     void gdbm_sync(dbf);
     int gdbm_exists(dbf, key);
     char *gdbm_strerror(errno);
     int gdbm_setopt(dbf, option, value, size);
     int gdbm_fdesc(dbf);
     int gdbm_export (GDBM_FILE, const char *, int, int);
     int gdbm_export_to_file (GDBM_FILE dbf, FILE *fp);
     int gdbm_import (GDBM_FILE, const char *, int);
     int gdbm_import_from_file (GDBM_FILE dbf, FILE *fp, int flag);
     int gdbm_count (GDBM_FILE dbf, gdbm_count_t *pcount);
     int gdbm_version_cmp (int const a[], int const b[]);

The gdbm.h include file is often in the /usr/local/include directory. (The actual location of gdbm.h depends on your local installation of gdbm.)

Opening the database.
*********************

 -- gdbm interface: GDBM_FILE gdbm_open (const char *NAME, int BLOCK_SIZE,
     int FLAGS, int MODE, void (*fatal_func)(const char *))
     Initializes gdbm system.  If the file has a size of zero bytes, a file initialization procedure is performed, setting up the initial structure in the file.

     The arguments are:

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

     block_size
          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 block size is used, otherwise the value of BLOCK_SIZE is used.

     flags
          If flags is set to 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 set to GDBM_WRITER, the user wants both read and write access to the database and requires exclusive access.  If flags is set to GDBM_WRCREAT, the user wants both read and write access to the database and wants it created if it does not already exist.  If flags is set to 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, GDBM_NOLOCK, which prevents the library from performing any locking on the database file, and GDBM_NOMMAP, which disables the memory mapping mechanism.  The option GDBM_FAST is now obsolete, since gdbm defaults to no-sync mode.

          If the host open call (see open (open(2) man page)) supports the O_CLOEXEC flag, the GDBM_CLOEXEC can be or'd into the flags, to enable the close-on-exec flag for the database file descriptor.

     mode
          File mode (see change permissions of a file (chmod(2) man page), and see open a file (open(2) man page)), which is used if the file is created).

     fatal_func
          A function for gdbm to call if it detects a fatal error.  The only parameter of this function is a string.  If the value of NULL is provided, gdbm will use a default function.

     The return value, is the pointer needed by all other functions to access that gdbm file.  If the return is the NULL pointer, gdbm_open was not successful.  The errors can be found in gdbm_errno variable (see gdbm_errno).  Available error codes are discussed in Error codes.

     In all of the following calls, the parameter DBF refers to the pointer returned from gdbm_open.

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:

 -- gdbm interface: void gdbm_close (GDBM_FILE DBF)
     This function closes the gdbm file and frees all memory associated with it.  The parameter is:

     dbf
          The pointer returned by gdbm_open.

Number of Records
*****************

 -- gdbm interface: int gdbm_count (GDBM_FILE DBF,
     gdbm_count_t *PCOUNT)
     Counts number of records in the database DBF.  On success, stores it in the memory location pointed to by PCOUNT and return 0.  On error, sets gdbm_errno (if relevant, also errno) and returns -1.

Inserting and replacing records in the database.
************************************************

 -- gdbm interface: int gdbm_store (GDBM_FILE DBF, datum KEY,
     datum CONTENT, int FLAG)
     The function gdbm_store inserts or replaces records in the database.

     The parameters are:

     dbf
          The pointer returned by gdbm_open.

     key
          The search key.

     content
          The data to be associated with the key.

     flag
          Defines the action to take when the key is already in the database.  The value GDBM_REPLACE (defined in gdbm.h) asks that the old data be replaced by the new CONTENT.  The value GDBM_INSERT asks that an error be returned and no action taken if the KEY already exists.

     This function can return the following values:

     -1
          The item was not stored in the database because the caller was not an official writer or either KEY or CONTENT have a NULL dptr field.

          Both KEY and CONTENT must have the dptr field be a non-NULL value.  Since a NULL dptr field is used by other functions to indicate an error, it cannot be valid data.

     +1
          The item was not stored because the argument FLAG was GDBM_INSERT and the KEY was already in the database.

     0
          No error.  The value of CONTENT is keyed by KEY.  The file on disk is updated to reflect the structure of the new database before returning from this function.

If you store data for a KEY that is already in the data base, gdbm replaces the old data with the new data if called with GDBM_REPLACE.  You do not get two data 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 data can be as large as you want.

Searching for records in the database.
**************************************

 -- gdbm interface: datum gdbm_fetch (GDBM_FILE DBF, datum KEY)
     Looks up a given KEY and returns the information associated with it. The dptr field in the structure that is returned points to a memory block allocated by malloc.  It is the caller's responsibility to free it when no longer needed.

     If the dptr is NULL, no data was found.

     The parameters are:

     dbf
          The pointer returned by gdbm_open.

     key
          The search key.

An example of using this function:

     content = gdbm_fetch (dbf, key);
     if (content.dptr == NULL)
       {
         fprintf(stderr, "key not found\n");
       }
     else
       {
         /* do something with content.dptr */
       }

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

 -- gdbm interface: int gdbm_exists (GDBM_FILE DBF, datum KEY)
     Returns true (1) if the KEY exists in DBF and false (0) otherwise.

     The parameters are:

     dbf
          The pointer returned by gdbm_open.

     key
          The search key.

Removing records from the database.
***********************************

To remove some data from the database, use the gdbm_delete function.

 -- gdbm interface: int gdbm_delete (GDBM_FILE DBF, datum KEY)
     Deletes the data associated with the given KEY, if it exists in the database DBF.  The file on disk is updated to reflect the structure of the new database before returning from this function.

     The parameters are:

     dbf
          The pointer returned by gdbm_open.

     datum key
          The search key.

     The function returns -1 if the item is not present or the requester is a reader.  The return of 0 marks a successful delete.

Sequential access to records.
*****************************

The next two functions allow for accessing all items in the database.  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. gdbm_firstkey starts the visit of all keys in the database. gdbm_nextkey finds and reads the next entry in the hash structure for dbf.

 -- gdbm interface: datum gdbm_firstkey (GDBM_FILE DBF)
     Initiate sequential access to the database DBF.  The returned value is the first key accessed in the database.  If the dptr field in the returned datum is NULL, the database contains no data.

     Otherwise, dptr points to a memory block obtained from malloc, which holds the key value.  The caller is responsible for freeing this memory block when no longer needed.

 -- gdbm interface: datum gdbm_nextkey (GDBM_FILE DBF, datum PREV)
     This function continues the iteration over the keys in DBF, initiated by gdbm_firstkey.  The parameter PREV holds the value returned from a previous call to gdbm_nextkey or gdbm_firstkey.

     The function returns next key from the database.  If the dptr field in the returned datum is NULL, all keys in the database has been visited.

     Otherwise, dptr points to a memory block obtained from malloc, which holds the key value.  The caller is responsible for freeing this memory block when no longer needed.

These functions were intended to visit the database in read-only algorithms, for instance, to validate the database or similar operations.  The usual algorithm for sequential access is:

        key = gdbm_firstkey (dbf);
        while (key.dptr)
          {
             datum nextkey;

             /* do something with the key */
             ...

             /* Obtain the next key */
             nextkey = gdbm_nextkey (dbf, key);
             /* Reclaim the memory used by the key */
             free (key.dptr);
             /* Use nextkey in the next iteration. */
             key = nextkey;
          }

Care should be taken when the gdbm_delete function is used in such a loop.  File visiting is based on a hash table.  The gdbm_delete function re-arranges the hash table to make sure that any collisions in the table do not leave some item un-findable.  The original key order is not guaranteed to remain unchanged in all instances.  So it is possible that some key will not be visited if a loop like the following is executed:

        key = gdbm_firstkey (dbf);
        while (key.dptr)
          {
             datum nextkey;
             if (some condition)
               {
                  gdbm_delete (dbf, key);
               }
              nextkey = gdbm_nextkey (dbf, key);
              free (key.dptr);
              key = nextkey;
           }

Database reorganization.
************************

The following function should be used very seldom.

 -- gdbm interface: int gdbm_reorganize (GDBM_FILE DBF)
     Reorganizes the database.

     The parameter is:

     dbf
          The pointer returned by gdbm_open.

If you have had a lot of deletions and would like to shrink the space used by the gdbm file, this function will reorganize the database. This results, in particular, in shortening the length of a gdbm file by removing the space occupied by deleted records.

This 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.  If an error is detected, the return value is negative.  The value zero is returned after a successful reorganization.

Database Synchronization
************************

Unless your database was opened with the GDBM_SYNC flag, gdbm does not wait for writes to be flushed to the disk before continuing.  This allows for faster writing of databases at the risk of having a corrupted database if the application terminates in an abnormal fashion.  The following function allows the programmer to make sure the disk version of the database has been completely updated with all changes to the current time.

 -- gdbm interface: void gdbm_sync (GDBM_FILE DBF)
     Synchronizes the changes in DBF with its disk file.  The parameter is a pointer returned by gdbm_open.

     This function would usually be called after a complete set of changes have been made to the database and before some long waiting time. The gdbm_close function automatically calls the equivalent of gdbm_sync so no call is needed if the database is to be closed immediately after the set of changes have been made.

Export and Import
*****************

Gdbm databases can be converted into so-called flat format files.  Such files cannot be used for searching, their sole purpose is to keep the data from the database for restoring it when the need arrives.  There are two flat file formats, which differ in the way they represent the data and in the amount of meta-information stored.  Both formats can be used, for example, to migrate between the different versions of gdbm databases.  Generally speaking, flat files are safe to send over the network, and can be used to recreate the database on another machine.  The recreated database is guaranteed to be a byte-to-byte equivalent of the database from which the flat file was created.  This does not necessarily mean, however, that this file can be used in the same way as the original one.  For example, if the original database contained non-ASCII data (e.g. C structures, integers etc.), the recreated database can be of any use only if the target machine has the same integer size and byte ordering as the source one and if its C compiler uses the same packing conventions as the one which generated C which populated the original database.  In general, such binary databases are not portable between machines, unless you follow some stringent rules on what data is written to them and how it is interpreted.

The GDBM version VERSION supports two flat file formats.  The binary flat file format was first implemented in GDBM version 1.9.1.  This format stores only key/data pairs, it does not keep information about the database file itself.  As its name implies, files in this format are binary files.

The ascii flat file format encodes all data in base64 and stores not only key/data pairs, but also the original database file metadata, such as file name, mode and ownership.  Files in this format can be sent without additional encapsulation over transmission channels that normally allow only ASCII data, such as, e.g. SMTP.  Due to additional metadata they allow for restoring an exact copy of the database, including file ownership and privileges, which is especially important if the database in question contained some security-related data.

We call a process of creating a flat file from a database exporting or dumping this database.  The reverse process, creating the database from a flat file is called importing or loading the database.

 -- gdbm interface: int gdbm_dump (GDBM_FILE DBF,
     const char *FILENAME, int FORMAT,
     int OPEN_FLAGS, int MODE)
     Dumps the database file to the named file in requested format. Arguments are:

     dbf
          A pointer to the source database, returned by a prior call to gdbm_open.

     filename
          Name of the dump file.

     format
          Output file format.  Allowed values are: GDBM_DUMP_FMT_BINARY to create a binary dump and GDBM_DUMP_FMT_ASCII to create an ASCII dump file.

     open_flags
          How to create the output file.  If FLAG is GDBM_WRCREAT the file will be created if it does not exist.  If it does exist, the gdbm_dump will fail.

          If FLAG is GDBM_NEWDB, the function will create a new output file, replacing it if it already exists.

     mode
          The permissions to use when creating the output file. See http://www.manpagez.com/man/2/open,

          open a file (open(2) man page), for a detailed discussion.

 -- gdbm interface: int gdbm_load (GDBM_FILE *PDBF,
     const char *FILENAME, int FLAG,
     int META_MASK,
     unsigned long *ERRLINE)
     Loads data from the dump file FILENAME into the database pointed to by PDBF.  The latter can point to NULL, in which case the function will try to create a new database.  If it succeeds, the function will return, in the memory location pointed to by PDBF, a pointer to the newly created database.  If the dump file carries no information about the original database file name, the function will set gdbm_errno to GDBM_NO_DBNAME and return -1, indicating failure.

     The FLAG has the same meaning as the FLAG argument to the gdbm_store function (see Store).

     The META_MASK argument can be used to disable restoring certain bits of file's meta-data from the information in the input dump file. It is a binary OR of zero or more of the following:

     GDBM_META_MASK_MODE
          Do not restore file mode.

     GDBM_META_MASK_OWNER
          Do not restore file owner.

     The function returns 0 upon successful completion or -1 on fatal errors and 1 on mild (non-fatal) errors.

     If a fatal error occurs, gdbm_errno will be set to one of the following values:

     GDBM_FILE_OPEN_ERROR
          Input file (FILENAME) cannot be opened.  The errno variable can be used to get more detail about the failure.

     GDBM_MALLOC_ERROR
          Not enough memory to load data.

     GDBM_FILE_READ_ERROR
          Reading from FILENAME failed.  The errno variable can be used to get more detail about the failure.

     GDBM_ILLEGAL_DATA
          Input contained some illegal data.

     GDBM_ITEM_NOT_FOUND
          This error can occur only when the input file is in ASCII format.  It indicates that the data part of the record about to be read lacked length specification.  Application developers are advised to treat this error equally as GDBM_ILLEGAL_DATA.

     Mild errors mean that the function was able to successfully load and restore the data, but was unable to change database file metadata afterward.  The table below lists possible values for gdbm_errno in this case.  To get more detail, inspect the system errno variable.

     GDBM_ERR_FILE_OWNER
          The function was unable to restore database file owner.

     GDBM_ERR_FILE_MODE
          The function was unable to restore database file mode (permission bits).

     If an error occurs while loading data from an input file in ASCII format, the number of line in which the error occurred will be stored in the location pointed to by the ERRLINE parameter, unless it is NULL.

     If the line information is not available or applicable, ERRLINE will be set to 0.

 -- gdbm interface: int gdbm_dump_to_file (GDBM_FILE DBF,
     FILE *FP, int FORMAT)
     This is an alternative entry point to gdbm_dump (which see). Arguments are:

     dbf
          A pointer to the source database, returned by a call to gdbm_open.

     fp
          File to write the data to.

     format
          Format of the dump file.  See the FORMAT argument to the gdbm_dump function.

 -- gdbm interface: int gdbm_load_from_file (GDBM_FILE *PDBF,
     FILE *FP, int REPLACE, int META_MASK,
     unsigned long *LINE)
     This is an alternative entry point to gdbm_dump.  It writes the output to FP which must be a file open for writing.  The rest of arguments is the same as for gdbm_load (excepting of course FLAG, which is not needed in this case).

 -- gdbm interface: int gdbm_export (GDBM_FILE DBF,
     const char *EXPORTFILE, int FLAG, int MODE)
     This function is retained for compatibility with GDBM 1.10 and earlier.  It dumps the database to a file in binary dump format and is entirely equivalent to

          gdbm_dump(DBF, EXPORTFILE, GDBM_DUMP_FMT_BINARY,
                    FLAG, MODE)

 -- gdbm interface: int gdbm_export_to_file (GDBM_FILE DBF, FILE *FP)
     This is an alternative entry point to gdbm_export.  This function writes to file FP a binary dump of the database DBF.

 -- gdbm interface: int gdbm_import (GDBM_FILE DBF,
     const char *IMPORTFILE, int FLAG)
     This function is retained for compatibility with GDBM 1.10 and earlier.  It loads the file IMPORTFILE, which must be a binary flat file, into the database DBF and is equivalent to the following construct:

          DBF = gdbm_open (IMPORTFILE, 0,
                                 FLAG == GDBM_REPLACE ?
                                   GDBM_WRCREAT : GDBM_NEWDB,
                                 0600, NULL);
          gdbm_load (&DBF, EXPORTFILE, 0, FLAG, NULL)

 -- gdbm interface: int gdbm_import_from_file (GDBM_FILE DBF,
     FILE *FP, int FLAG)
     An alternative entry point to gdbm_import.  Reads the binary dump from the file FP and stores the key/value pairs to DBF.  See Store, for a description of FLAG.

     This function is equivalent to:

          DBF = gdbm_open (IMPORTFILE, 0,
                                 FLAG == GDBM_REPLACE ?
                                   GDBM_WRCREAT : GDBM_NEWDB,
                                 0600, NULL);
          gdbm_load_from_file (DBF, FP, FLAG, 0, NULL);

Error strings.
**************

To convert a gdbm error code into English text, use this routine:

 -- gdbm interface: const char * gdbm_strerror (gdbm_error ERRNO)
     Converts ERRNO (which is an integer value) into a human-readable descriptive text.  Returns a pointer to a static string.  The caller must not alter or free the returned pointer.

     The ERRNO argument is usually the value of the global variable gdbm_errno.  See gdbm_errno.

Setting options
***************

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

 -- gdbm interface: int gdbm_setopt (GDBM_FILE DBF, int OPTION,
     void *VALUE, int SIZE)
     Sets an option on the database or returns the value of an option.

     The parameters are:

     dbf
          The pointer returned by gdbm_open.

     option
          The option to be set or retrieved.

     value
          A pointer to the value to which OPTION will be set or where to place the option value (depending on the option).

     size
          The length of the data pointed to by VALUE.

The valid options are:

GDBM_SETCACHESIZE
GDBM_CACHESIZE
     Set the size of the internal bucket cache.  This option may only be set once on each GDBM_FILE descriptor, and is set automatically to 100 upon the first access to the database.  The VALUE should point to a size_t holding the desired cache size.

     The GDBM_CACHESIZE option is provided for compatibility with earlier versions.

GDBM_GETCACHESIZE
     Return the size of the internal bucket cache.  The VALUE should point to a size_t variable, where the size will be stored.

GDBM_GETFLAGS
     Return the flags describing the state of the database.  The VALUE should point to a int variable where to store the flags.  The return is the same as the flags used when opening the database (see gdbm_open), except that it reflects the current state (which may have been altered by another calls to gdbm_setopt.

GDBM_FASTMODE
     Enable or disable the fast writes mode, i.e. writes without subsequent synchronization.  The VALUE should point to an integer: TRUE to enable fast mode, and FALSE to disable it.

     This option is retained for compatibility with previous versions of gdbm.  Its effect is the reverse of GDBM_SETSYNCMODE (see below).

GDBM_SETSYNCMODE
GDBM_SYNCMODE
     Turn on or off file system synchronization operations.  This setting defaults to off.  The VALUE should point to an integer: TRUE to turn synchronization on, and FALSE to turn it off.

     Note, that this option is a reverse of GDBM_FASTMODE, i.e. calling GDBM_SETSYNCMODE with TRUE has the same effect as calling GDBM_FASTMODE with FALSE.

     The GDBM_SYNCMODE option is provided for compatibility with earlier versions.

GDBM_GETSYNCMODE
     Return the current synchronization status.  The VALUE should point to an int where the status will be stored.

GDBM_SETCENTFREE
GDBM_CENTFREE
     NOTICE: This feature is still under study.

     Set central free block pool to either on or off.  The default is off, which is how previous versions of gdbm handled free blocks.  If set, this option causes all subsequent free blocks to be placed in the global pool, allowing (in theory) more file space to be reused more quickly.  The VALUE should point to an integer: TRUE to turn central block pool on, and FALSE to turn it off.

     The GDBM_CENTFREE option is provided for compatibility with earlier versions.

GDBM_SETCOALESCEBLKS
GDBM_COALESCEBLKS
     NOTICE: This feature is still under study.

     Set free block merging to either on or off.  The default is off, which is how previous versions of gdbm handled free blocks.  If set, this option causes adjacent free blocks to be merged.  This can become a CPU expensive process with time, though, especially if used in conjunction with GDBM_CENTFREE.  The VALUE should point to an integer: TRUE to turn free block merging on, and FALSE to turn it off.

GDBM_GETCOALESCEBLKS
     Return the current status of free block merging.  The VALUE should point to an int where the status will be stored.

GDBM_SETMAXMAPSIZE
     Sets maximum size of a memory mapped region.  The VALUE should point to a value of type size_t, unsigned long or unsigned.  The actual value is rounded to the nearest page boundary (the page size is obtained from sysconf(_SC_PAGESIZE)).

GDBM_GETMAXMAPSIZE
     Return the maximum size of a memory mapped region.  The VALUE should point to a value of type size_t where to return the data.

GDBM_SETMMAP
     Enable or disable memory mapping mode.  The VALUE should point to an integer: TRUE to enable memory mapping or FALSE to disable it.

GDBM_GETMMAP
     Check whether memory mapping is enabled.  The VALUE should point to an integer where to return the status.

GDBM_GETDBNAME
     Return the name of the database disk file.  The VALUE should point to a variable of type char**.  A pointer to the newly allocated copy of the file name will be placed there.  The caller is responsible for freeing this memory when no longer needed.  For example:

          char *name;

          if (gdbm_setopt (dbf, GDBM_GETDBNAME, &name, sizeof (name)))
            {
               fprintf (stderr, "gdbm_setopt failed: %s\n",
                        gdbm_strerror (gdbm_errno));
            }
          else
            {
              printf ("database name: %s\n", name);
              free (name);
            }

The return value will be -1 upon failure, or 0 upon success.  The global variable gdbm_errno will be set upon failure.

For instance, to set a database to use a cache of 10, after opening it with gdbm_open, but prior to accessing it in any way, the following code could be used:

     int value = 10;
     ret = gdbm_setopt (dbf, GDBM_CACHESIZE, &value, sizeof (int));

File Locking.
*************

With locking disabled (if gdbm_open was called with GDBM_NOLOCK), the user may want to perform their own file locking on the database file in order to prevent multiple writers operating on the same file simultaneously.

In order to support this, the gdbm_fdesc routine is provided.

 -- gdbm interface: int gdbm_fdesc (GDBM_FILE DBF)
     Returns the file descriptor of the database DBF.  This value can be used as an argument to flock, lockf or similar calls.

Useful global variables.
************************

The following global variables and constants are available:

 -- Variable: gdbm_error gdbm_errno
     This variable contains error code from the last failed gdbm call.  See Error codes, for a list of available error codes and their descriptions.

     Use gdbm_strerror (see Errors) to convert it to a descriptive text.

 -- Variable: const char * gdbm_errlist[]
     This variable is an array of error descriptions, which is used by gdbm_strerror to convert error codes to human-readable text (see Errors).  You can access it directly, if you wish so.  It contains _GDBM_MAX_ERRNO + 1 elements and can be directly indexed by the error code to obtain a corresponding descriptive text.

 -- Constant: _GDBM_MIN_ERRNO
     The minimum error code used by gdbm.

 -- Constant: _GDBM_MAX_ERRNO
     The maximum error code used by gdbm.

 -- Variable: const char * gdbm_version
     A string containing the version information.

 -- Variable: int const gdbm_version_number[3]
     This variable contains the gdbm version numbers:

     Index   Meaning
     0           Major number
     1           Minor number
     2           Patchlevel number

     Additionally, the following constants are defined in the gdbm.h file:

     GDBM_VERSION_MAJOR
          Major number.

     GDBM_VERSION_MINOR
          Minor number.

     GDBM_VERSION_PATCH
          Patchlevel number.

     These can be used to verify whether the header file matches the library.

To compare two split-out version numbers, use the following function:

 -- gdbm interface: int gdbm_version_cmp (int const A[3],
     int const B[3])
     Compare two version numbers.  Return -1 if A is less than B, 1 if A is greater than B and 0 if they are equal.

     Comparison is done from left to right, so that:

          a = { 1, 8, 3 };
          b = { 1, 8, 3 };
          gdbm_version_cmp (a, b) => 0

          a = { 1, 8, 3 };
          b = { 1, 8, 2 };
          gdbm_version_cmp (a, b) => 1

          a = { 1, 8, 3 };
          b = { 1, 9. 0 };
          gdbm_version_cmp (a, b) => -1

Error codes
***********

This chapter summarizes error codes which can be set by the functions in gdbm library.

GDBM_NO_ERROR
     No error occurred.

GDBM_MALLOC_ERROR
     Memory allocation failed.  Not enough memory.

GDBM_BLOCK_SIZE_ERROR
     This error is set by the gdbm_open function (see Open), if the value of its BLOCK_SIZE argument is incorrect.

GDBM_FILE_OPEN_ERROR
     The library was not able to open a disk file.  This can be set by gdbm_open (see Open), gdbm_export and gdbm_import functions (see Flat files).

     Inspect the value of the system errno variable to get more detailed diagnostics.

GDBM_FILE_WRITE_ERROR
     Writing to a disk file failed.  This can be set by gdbm_open (see Open), gdbm_export and gdbm_import functions.

     Inspect the value of the system errno variable to get more detailed diagnostics.

GDBM_FILE_SEEK_ERROR
     Positioning in a disk file failed.  This can be set by gdbm_open (see Open) function.

     Inspect the value of the system errno variable to get a more detailed diagnostics.

GDBM_FILE_READ_ERROR
     Reading from a disk file failed.  This can be set by gdbm_open (see Open), gdbm_export and gdbm_import functions.

     Inspect the value of the system errno variable to get a more detailed diagnostics.

GDBM_BAD_MAGIC_NUMBER
     The file given as argument to gdbm_open function is not a valid gdbm file: it has a wrong magic number.

GDBM_EMPTY_DATABASE
     The file given as argument to gdbm_open function is not a valid gdbm file: it has zero length.

GDBM_CANT_BE_READER
     This error code is set by the gdbm_open function if it is not able to lock file when called in GDBM_READER mode (see GDBM_READER).

GDBM_CANT_BE_WRITER
     This error code is set by the gdbm_open function if it is not able to lock file when called in writer mode (see Open).

GDBM_READER_CANT_DELETE
     Set by the gdbm_delete (see Delete) if it attempted to operate on a database that is open in read-only mode (see GDBM_READER).

GDBM_READER_CANT_STORE
     Set by the gdbm_store (see Store) if it attempted to operate on a database that is open in read-only mode (see GDBM_READER).

GDBM_READER_CANT_REORGANIZE
     Set by the gdbm_reorganize (see Reorganization) if it attempted to operate on a database that is open in read-only mode (see GDBM_READER).

GDBM_UNKNOWN_UPDATE
     Currently unused.  Reserved for future uses.

GDBM_ITEM_NOT_FOUND
     Requested item was not found.  This error is set by gdbm_delete (see Delete) and gdbm_fetch (see Fetch) when the requested KEY value is not found in the database.

GDBM_REORGANIZE_FAILED
     The gdbm_reorganize function is not able to create a temporary database.  See Reorganization.

GDBM_CANNOT_REPLACE
     Cannot replace existing item.  This error is set by the gdbm_store if the requested KEY value is found in the database and the FLAG parameter is not GDBM_REPLACE. See Store, for a detailed discussion.

GDBM_ILLEGAL_DATA
     Either KEY or CONTENT parameter was wrong in a call to to gdbm_store (see Store).

GDBM_OPT_ALREADY_SET
     Requested option can be set only once and was already set.  This error is returned by the gdbm_setopt function.  See GDBM_CACHESIZE.

GDBM_OPT_ILLEGAL
     The OPTION argument is not valid or the VALUE argument points to an invalid value in a call to gdbm_setopt function. See Options.

GDBM_BYTE_SWAPPED
     The gdbm_open function (see Open) attempts to open a database which is created on a machine with different byte ordering.

GDBM_BAD_FILE_OFFSET
     The gdbm_open function (see Open) sets this error code if the file it tries to open has a wrong magic number.

GDBM_BAD_OPEN_FLAGS
     Set by the gdbm_export function if supplied an invalid FLAGS argument.  See Flat files.

GDBM_FILE_STAT_ERROR
     Getting information about a disk file failed.  The system errno will give more details about the error.

     This error can be set by the following functions: gdbm_open, gdbm_reorganize.

GDBM_FILE_EOF
     End of file was encountered where more data was expected to be present.  This error can occur when fetching data from the database and usually means that the database is truncated or otherwise corrupted.

     This error can be set by any GDBM function that does I/O.  Some of these functions are: gdbm_delete, gdbm_exists, gdbm_fetch, gdbm_export, gdbm_import, gdbm_reorganize, gdbm_firstkey, gdbm_nextkey, gdbm_store.

GDBM_NO_DBNAME
     Output database name is not specified.  This error code is set by gdbm_load (see gdbm_load) if the first argument points to NULL and the input file does not specify the database name.

GDBM_ERR_FILE_OWNER
     This error code is set by gdbm_load if it is unable to restore database file owner.  It is a mild error condition, meaning that the data have been restored successfully, only changing the target file owner failed.  Inspect the system errno variable to get a more detailed diagnostics.

GDBM_ERR_FILE_MODE
     This error code is set by gdbm_load if it is unable to restore database file mode.  It is a mild error condition, meaning that the data have been restored successfully, only changing the target file owner failed.  Inspect the system errno variable to get a more detailed diagnostics.

Compatibility with standard dbm and ndbm.
*****************************************

Gdbm includes a compatibility layer, which provides traditional ndbm and older dbm functions.  The layer is compiled and installed if the --enable-libgdbm-compat option is used when configuring the package.

The compatibility layer consists of two header files: ndbm.h and dbm.h and the libgdbm_compat library.

Older programs using ndbm or dbm interfaces can use libgdbm_compat without any changes.  To link a program with the compatibility library, add the following two options to the cc invocation: -lgdbm -lgdbm_compat.  The -L option may also be required, depending on where gdbm is installed, e.g.:

     cc ... -L/usr/local/lib -lgdbm -lgdbm_compat

Databases created and manipulated by the compatibility interfaces consist of two different files: FILE.dir and FILE.pag.  This is required by the POSIX specification and corresponds to the traditional usage.  Note, however, that despite the similarity of the naming convention, actual data stored in these files has not the same format as in the databases created by other dbm or ndbm libraries.  In other words, you cannot access a standard UNIX dbm file with GNU dbm!

GNU dbm files are not sparse.  You can copy them with the usual cp command and they will not expand in the copying process.

NDBM interface functions.
=========================

The functions below implement the POSIX ndbm interface:

 -- ndbm: DBM * dbm_open (char *FILE, int FLAGS, int MODE)
     Opens a database.  The FILE argument is the full name of the database file to be opened.  The function opens two files: FILE.pag and FILE.dir.  The FLAGS and MODE arguments have the same meaning as the second and third arguments of open (see open a file (open(2) man page)), except that a database opened for write-only access opens the files for read and write access and the behavior of the O_APPEND flag is unspecified.

     The function returns a pointer to the DBM structure describing the database.  This pointer is used to refer to this database in all operations described below.

     Any error detected will cause a return value of NULL and an appropriate value will be stored in gdbm_errno (see Variables).

 -- ndbm: void dbm_close (DBM *DBF)
     Closes the database.  The DBF argument must be a pointer returned by an earlier call to dbm_open.

 -- ndbm: datum dbm_fetch (DBM *DBF, datum KEY)
     Reads a record from the database with the matching key.  The KEY argument supplies the key that is being looked for.

     If no matching record is found, the dptr member of the returned datum is NULL.  Otherwise, the dptr member of the returned datum points to the memory managed by the compatibility library.  The application should never free it.

 -- ndbm: int dbm_store (DBM *DBF, datum KEY,
     datum CONTENT, int MODE)
     Writes a key/value pair to the database.  The argument DBF is a pointer to the DBM structure returned from a call to dbm_open.  The KEY and CONTENT provide the values for the record key and content.  The MODE argument controls the behavior of dbm_store in case a matching record already exists in the database.  It can have one of the following two values:

     DBM_REPLACE
          Replace existing record with the new one.

     DBM_INSERT
          The existing record is left unchanged, and the function returns 1.

     If no matching record exists in the database, new record will be inserted no matter what the value of the MODE is.

 -- ndbm: int dbm_delete (DBM *DBF, datum KEY)
     Deletes the record with the matching key from the database.  If the function succeeds, 0 is returned.  Otherwise, if no matching record is found or if an error occurs, -1 is returned.

 -- ndbm: datum dbm_firstkey (DBM *DBF)
     Initializes iteration over the keys from the database and returns the first key.  Note, that the word first does not imply any specific ordering of the keys.

     If there are no records in the database, the dptr member of the returned datum is NULL.  Otherwise, the dptr member of the returned datum points to the memory managed by the compatibility library.  The application should never free it.

 -- ndbm: datum dbm_nextkey (DBM *DBF)
     Continues the iteration started by dbm_firstkey.  Returns the next key in the database.  If the iteration covered all keys in the database, the dptr member of the returned datum is NULL. Otherwise, the dptr member of the returned datum points to the memory managed by the compatibility library.  The application should never free it.

     The usual way of iterating over all the records in the database is:

          for (key = dbm_firstkey (dbf);
               key.ptr;
               key = dbm_nextkey (dbf))
            {
              /* do something with the key */
            }

     The loop above should not try to delete any records from the database, otherwise the iteration is not guaranteed to cover all the keys. See Sequential, for a detailed discussion of this.

 -- ndbm: int dbm_error (DBM *DBF)
     Returns the error condition of the database: 0 if no errors occurred so far while manipulating the database, and a non-zero value otherwise.

 -- ndbm: void dbm_clearerr (DBM *DBF)
     Clears the error condition of the database.

 -- ndbm: int dbm_dirfno (DBM *DBF)
     Returns the file descriptor of the dir file of the database. It is guaranteed to be different from the descriptor returned by the dbm_pagfno function (see below).

     The application can lock this descriptor to serialize accesses to the database.

 -- ndbm: int dbm_pagfno (DBM *DBF)
     Returns the file descriptor of the pag file of the database. See also dbm_dirfno.

 -- ndbm: int dbm_rdonly (DBM *DBF)
     Returns 1 if the database DBF is open in a read-only mode and 0 otherwise.

DBM interface functions.
========================

The functions below are provided for compatibility with the old UNIX DBM interface.  Only one database at a time can be manipulated using them.

 -- dbm: int dbminit (char *FILE)
     Opens a database.  The FILE argument is the full name of the database file to be opened.  The function opens two files: FILE.pag and FILE.dir.  If any of them does not exist, the function fails.  It never attempts to create the files.

     The database is opened in the read-write mode, if its disk permissions permit.

     The application must ensure that the functions described below in this section are called only after a successful call to dbminit.

 -- dbm: int dbmclose (void)
     Closes the database opened by an earlier call to dbminit.

 -- dbm: datum fetch (datum KEY)
     Reads a record from the database with the matching key.  The KEY argument supplies the key that is being looked for.

     If no matching record is found, the dptr member of the returned datum is NULL.  Otherwise, the dptr member of the returned datum points to the memory managed by the compatibility library.  The application should never free it.

 -- dbm: int store (datum KEY, datum CONTENT)
     Stores the key/value pair in the database.  If a record with the matching key already exists, its content will be replaced with the new one.

     Returns 0 on success and -1 on error.

 -- dbm: int delete (datum KEY)
     Deletes a record with the matching key.

     If the function succeeds, 0 is returned.  Otherwise, if no matching record is found or if an error occurs, -1 is returned.

 -- dbm: datum firstkey (void)
     Initializes iteration over the keys from the database and returns the first key.  Note, that the word first does not imply any specific ordering of the keys.

     If there are no records in the database, the dptr member of the returned datum is NULL.  Otherwise, the dptr member of the returned datum points to the memory managed by the compatibility library.  The application should never free it.

 -- dbm: datum nextkey (datum KEY)
     Continues the iteration started by a call to firstkey.  Returns the next key in the database.  If the iteration covered all keys in the database, the dptr member of the returned datum is NULL. Otherwise, the dptr member of the returned datum points to the memory managed by the compatibility library.  The application should never free it.

Examine and modify a GDBM database.
***********************************

The gdbmtool utility allows you to view and modify an existing GDBM database or to create a new one.

When invoked without arguments, it tries to open a database file called junk.gdbm, located in the current working directory.  You can change this default by supplying the name of the database to use as an argument to the program, e.g.:

     $ gdbmtool file.db

The database will be opened in read-write mode, unless the -r (--read-only) option is specified, in which case it will be opened only for reading.

If the database does not exist, gdbmtool will create it. There is a special option -n (--newdb, which instructs the utility to create a new database.  If it is used and if the database already exists, it will be deleted, so use it sparingly.

gdbmtool invocation
===================

The following table summarizes all gdbmtool command line options:

-b SIZE
--block-size=SIZE
     Set block size.

-c SIZE
--cache-size=SIZE
     Set cache size.

-f FILE

--file FILE
     Read commands from FILE, instead of the standard input.

-h
--help
     Print a concise help summary.

-N
--norc
     Don't read startup files (see startup files).

-n
--newdb
     Create the database.

-l
--no-lock
     Disable file locking.

-m
--no-mmap
     Disable mmap.

-q
--quiet
     Don't print the usual welcome banner at startup.  This is the same as setting the variable quiet in the startup file.  See quiet.

-r
--read-only
     Open the database in read-only mode.

-s
--synchronize
     Synchronize to the disk after each write.

-V
--version
     Print program version and licensing information and exit.

--usage
     Print a terse invocation syntax summary along with a list of available command line options.

gdbmtool interactive mode
=========================

After successful startup, gdbmtool starts a loop, in which it reads commands from the standard input, executes them and prints the results on the standard output.  If the standard input is attached to a console, gdbmtool runs in interactive mode, which is indicated by its prompt:

     gdbmtool> _

The utility finishes when it reads the quit command (see below) or detects end-of-file on its standard input, whichever occurs first.

A gdbmtool command consists of a command verb, optionally followed by arguments, separated by any amount of white space.  A command verb can be entered either in full or in an abbreviated form, as long as that abbreviation does not match any other verb.  For example, co can be used instead of count and ca instead of cache.

Any sequence of non-whitespace characters appearing after the command verb forms an argument.  If the argument contains whitespace or unprintable characters it must be enclosed in double quotes.  Within double quotes the usual escape sequences are understood, as shown in the table below:

Table: Backslash escapes

Sequence   Replaced with
\a   Audible bell character (ASCII 7)
\b   Backspace character (ASCII 8)
\f   Form-feed character (ASCII 12)
\n   Newline character (ASCII 10)
\r   Carriage return character (ASCII 13)
\t   Horizontal tabulation character (ASCII 9)
\v   Vertical tabulation character (ASCII 11)
\\   Single slash
\"   Double quote

In addition, a backslash immediately followed by the end-of-line character effectively removes that character, allowing to split long arguments over several input lines.

Command parameters may be optional or mandatory.  If the number of actual arguments is less than the number of mandatory parameters, gdbmtool will prompt you to supply missing arguments.  For example, the store command takes two mandatory parameters, so if you invoked it with no arguments, you would be prompted twice to supply the necessary data, as shown in  example below:

     gdbmtool> store
     key? three
     data? 3

However, such prompting is possible only in interactive mode.  In non-interactive mode (e.g. when running a script), all arguments must be supplied with each command, otherwise gdbmtool will report an error and exit immediately.

Shell Variables
---------------

A number of gdbmtool parameters is kept in its internal variables.

 -- gdbmtool variable: bool confirm
     Whether to ask for confirmation before certain destructive operations, such as truncating the existing database.

     Default is true.

 -- gdbmtool variable: string ps1
     Primary prompt string.  Its value can contain conversion specifiers, consisting of the % character followed by another character.  These specifiers are expanded in the resulting prompt as follows:

     Sequence   Expansion
     %f   name of the current database file
     %p   program invocation name
     %P   package name (GDBM)
     %v   program version
     %_   single space character
     %%   %

     The default value is %p>%_, i.e. the program name, followed by a ``greater than'' sign, followed by a single space.

 -- gdbmtool variable: string ps2
     Secondary prompt.  See ps1 for a description of its value. This prompt is displayed before reading the second and subsequent lines of a multi-line command.

     The default value is %_>%_.

 -- gdbmtool variable: string delim1
     A string used to delimit fields of a structured datum on output (see definitions).

     Default is , (a comma).  This variable cannot be unset.

 -- gdbmtool variable: string delim2
     A string used to delimit array items when printing a structured datum (see definitions).

     Default is , (a comma).  This variable cannot be unset.

 -- gdbmtool variable: string pager
     The name and command line of the pager program to pipe output to. This program is used in interactive mode when the estimated number of output lines is greater then the number of lines on your screen.

     The default value is inherited from the environment variable PAGER.  Unsetting this variable disables paging.

 -- gdbmtool variable: bool quiet
     Whether to display a welcome banner at startup.  This variable should be set in a startup script file (see startup files). See -q option.

The following variables control how the database is opened:

 -- gdbmtool variable: numeric blocksize
     Sets the block size.  See block_size.  Unset by default.

 -- gdbmtool variable: numeric cachesize
     Sets the cache size.  See GDBM_SETCACHESIZE. By default this variable is not set.

 -- gdbmtool variable: string open
     Open mode.  The following values are allowed:

     newdb
          Truncate the database if it exists or create a new one.  Open it in read-write mode.

          Technically, this sets the GDBM_NEWDB flag in call to gdbm_open. See GDBM_NEWDB.

     wrcreat
     rw
          Open the database in read-write mode.  Create it if it does not exist.  This is the default.

          Technically speaking, it sets the GDBM_WRCREAT flag in call to gdbm_open.  See GDBM_WRCREAT.

     reader
     readonly
          Open the database in read-only mode.  Signal an error if it does not exist.

          This sets the GDBM_READER flag (see GDBM_READER).

     Attempting to set any other value or to unset this variable produces an error.

 -- gdbmtool variable: number filemode
     File mode (in octal) for creating new database files and database dumps.

 -- gdbmtool variable: bool lock
     Lock the database.  This is the default.

     Setting this variable to false or unsetting it results in passing GDBM_NOLOCK flag to gdbm_open (see GDBM_NOLOCK).

 -- gdbmtool variable: bool mmap
     Use memory mapping.  This is the default.

     Setting this variable to false or unsetting it results in passing GDBM_NOMMAP flag to gdbm_open (see GDBM_NOMMAP).

 -- gdbmtool variable: bool sync
     Flush all database writes on disk immediately.  Default is false. See GDBM_SYNC.

The following commands are used to list or modify the variables:

 -- command verb: set [ASSIGNMENTS]
     When used without arguments, lists all variables and their values. Unset variables are shown after a comment sign (#).  For string and numeric variables, values are shown after an equals sign.  For boolean variables, only the variable name is displayed if the variable is true.  If it is false, its name is prefixed with no.

     For example:

          ps1="%p>%_"
          ps2="%_>%_"
          delim1=","
          delim2=","
          confirm
          # cachesize is unset
          # blocksize is unset
          open="wrcreat"
          lock
          mmap
          nosync
          pager="less"
          # quiet is unset

     If used with arguments, the set command alters the specified variables.  In this case, arguments are variable assignments in the form NAME=VALUE.  For boolean variables, the VALUE is interpreted as follows: if it is numeric, 0 stands for false, any non-zero value stands for true. Otherwise, the values on, true, and yes denote true, and off, false, no stand for false.  Alternatively, only the name of a boolean variable can be supplied to set it to true, and its name prefixed with no can be used to set it to false.  For example, the following command sets the delim2 variable to ; and the confirm variable to false:

          set delim2=";" noconfirm

 -- command verb: unset VARIABLES
     Unsets the listed variables.  The effect of unsetting depends on the variable.  Unless explicitly described in the discussion of the variables above, unsetting a boolean variable is equivalent to setting it to false.  Unsetting a string variable is equivalent to assigning it an empty string.

Gdbmtool Commands
-----------------

 -- command verb: avail
     Print the avail list.

 -- command verb: bucket NUM
     Print the bucket number NUM and set it as the current one.

 -- command verb: cache
     Print the bucket cache.

 -- command verb: close
     Close the currently open database.

 -- command verb: count
     Print the number of entries in the database.

 -- command verb: current
     Print the current bucket.

 -- command verb: delete KEY
     Delete record with the given KEY

 -- command verb: dir
     Print hash directory.

 -- command verb: export FILE-NAME [truncate] [binary|ascii]
     Export the database to the flat file FILE-NAME.  See Flat files, for a description of the flat file format and its purposes.  This command will not overwrite an existing file, unless the truncate parameter is also given.  Another optional argument determines the type of the dump (see Flat files).  By default, ASCII dump is created.

     The global variable filemode specifies the permissions to use for the created output file.

     See also gdbmexport.

 -- command verb: fetch KEY
     Fetch and display the record with the given KEY.

 -- command verb: first
     Fetch and display the first record in the database.  Subsequent records can be fetched using the next command (see below). See Sequential, for more information on sequential access.

 -- command verb: hash KEY
     Compute and display the hash value for the given KEY.

 -- command verb: header
     Print file header.

 -- command verb: help
 -- command verb: ?
     Print a concise command summary, showing each command verb with its parameters and a short description of what it does.  Optional arguments are enclosed in square brackets.

 -- command verb: import FILE-NAME [replace] [nometa]
     Import data from a flat dump file FILE-NAME (see Flat files).  If the word replace is given as an argument, any records with the same keys as the already existing ones will replace them.  The word nometa turns off restoring meta-information from the dump file.

 -- command verb: list
     List the contents of the database.

 -- command verb: next [KEY]
     Sequential access: fetch and display the next record.  If the KEY is given, the record following the one with this key will be fetched.

     See also first, above.

     See Sequential, for more information on sequential access.

 -- command verb: open FILENAME
     Open the database file FILENAME.  If successful, any previously open database is closed.  Otherwise, if the operation fails, the currently opened database remains unchanged.

     This command takes additional information from the following variables:

     open
          The database access mode.  See The OPEN variable, for a list of its values.

     lock
          Whether or not to lock the database.  Default is on.

     mmap
          Use the memory mapping.  Default is on.

     sync
          Synchronize after each write.  Default is off.

     filemode
          Specifies the permissions to use in case a new file is created.

     See open parameters, for a detailed description of these variables.

 -- command verb: quit
     Close the database and quit the utility.

 -- command verb: reorganize
     Reorganize the database (see Reorganization).

 -- command verb: source FILENAME
     Read gdbmtool commands from the file FILENAME.

 -- command verb: status
     Print current program status.  The following example shows the information displayed:

          Database file: junk.gdbm
          Database is open
          define key string
          define content string

     The two define strings show the defined formats for key and content data.  See definitions, for a detailed discussion of their meaning.

 -- command verb: store KEY DATA
     Store the DATA with KEY in the database.  If KEY already exists, its data will be replaced.

 -- command verb: version
     Print the version of gdbm.

Data Definitions
----------------

GDBM databases are able to keep data of any type, both in the key and in the content part of a record.  Quite often these data are structured, i.e. they consist of several fields of various types. Gdbmtool provides a mechanism for handling such kind of records.

The define command defines a record structure.  The general syntax is:

     define WHAT DEFINITION

where WHAT is key to defining the structure of key data and content to define the structure of the content records.

The DEFINITION can be of two distinct formats.  In the simplest case it is a single data type.  For example,

     define content int

defines content records consisting of a single integer field. Supported data types are:

char
     Single byte (signed).

short
     Signed short integer.

ushort
     Unsigned short integer.

int
     Signed integer.

unsigned
uint
     Unsigned integer.

long
     Signed long integer.

ulong
     Unsigned long integer.

llong
     Signed long long integer.

ullong
     Unsigned long long integer.

float
     A floating point number.

double
     Double-precision floating point number.

string
     Array of bytes.

stringz
     Null-terminated string, trailing null being part of the string.

All numeric data types (integer as well as floating point) have the same respective widths as in C language on the host where the database file resides.

The string and stringz are special.  Both define a string of bytes, similar to char x[] in C.  The former defines an array of bytes, the latter - a null-terminated string. This makes a difference, in particular, when the string is the only part of datum.  Consider the following two definitions:

 1. define key string

 2. define key stringz

Now, suppose we want to store the string "ab" in the key.  Using the definition (1), the dptr member of GDBM datum will contain two bytes: a, and b.  Consequently, the dsize member will have the value 2.  Using the definition (2), the dptr member will contain three bytes: a, b, and ASCII 0.  The dsize member will have the value 3.

The definition (1) is the default for both key and content.

The second form of the define statement is similar to the C struct statement and allows for defining structural data.   In this form, the DEFINITION part is a comma-separated list of data types and variables enclosed in curly braces.  In contrast to the rest of gdbm commands, this command is inherently multiline and is terminated with the closing curly brace.  For example:

     define content {
             int status,
             pad 8,
             char id[3],
             string name
     }

This defines a structure consisting of three members: an integer status, an array of 8 bytes id, and a null-terminated string name.  Notice the pad statement: it allows to introduce padding between structure members.  Another useful statement is offset: it specifies that the member following it begins at the given offset in the structure.  Assuming the size of int is 8 bytes, the above definition can also be written as

     define content {
             int status,
             offset 16,
             char id[3],
             string name
     }

NOTE: The string type can reasonably be used only if it is the last or the only member of the data structure.  That's because it provides no information about the number of elements in the array, so it is interpreted to contain all bytes up to the end of the datum.

When displaying the structured data, gdbmtool precedes each value with the corresponding field name and delimits parts of the structure with the string defined in the delim1 variable (see variables).  Array elements are delimited using the string from delim2.  For example:

     gdbmtool> fetch foo
     status=2,id={ a, u, x },name="quux"

To supply a structured datum as an argument to a gdbmtool command, use the same notation, but without field names, e.g.:

     gdbmtool> hash { 2, {a,u,x}, "quux" }
     hash value = 13089969.

Startup Files
-------------

Upon startup gdbmtool looks for a file named .gdbmtoolrc first in the current working directory and, if not found, in the home directory of the user who started the command.

If found, this file is read and interpreted as a list of gdbmtool commands.  This allows you to customize the program behavior.

Following is an example startup file which disables the welcome banner, sets command line prompt to contain the name of the database file in parentheses and defines the structure of the database content records:

     set quiet
     set ps1="(%f) "
     define key stringz
     define content {
             int time,
             pad 4,
             int status
     }

The gdbm_dump utility
*********************

The gdbm_dump utility creates a flat file dump of a GDBM database (see Flat files).  It takes one mandatory argument: the name of the source database file.  The second argument, if given, specifies the name of the output file.  If not given, gdbm_dump will produce the dump on the standard output.

For example, the following invocation creates a dump of the database file.db in the file file.dump:

     $ gdbm_dump file.db file.dump

By default the utility creates dumps in ASCII format (see ASCII).  Another format can be requested using the --format (-H) option.

The gdbm_dump utility understands the following command line options:

-H FMT
--format=FMT
     Select output format.  Valid values for FMT are: binary or 0 to select binary dump format, and ascii or 1 to select ASCII format.

-h
--help
     Print a concise help summary.

-V
--version
     Print program version and licensing information and exit.

--usage
     Print a terse invocation syntax summary along with a list of available command line options.

The gdbm_load utility
*********************

The gdbm_load utility restores a GDBM database from a flat file.  The utility requires at least one argument: the name of the input flat file.  If it is -, the standard input will be read. The format of the input file is detected automatically.

By default the utility attempts to restore the database under its original name, as stored in the input file.  It will fail to do so if the input is in binary format.  In that case, the name of the database must be given as the second argument.

In general, if two arguments are given the second one is treated as the name of the database to create, overriding the file name specified in the flat file.

The utility understands the following command line arguments:

-b NUM
--block-size=NUM
     Sets block size.  See block_size.

-c NUM
--cache-size=NUM
     Sets cache size.  See GDBM_SETCACHESIZE.

-M
--mmap
     Use memory mapping.

-m MODE

--mode=MODE
     Sets the file mode.  The argument is the desired file mode in octal.

-n
--no-meta
     Do not restore file meta-data (ownership and mode) from the flat file.

-r
--replace
     Replace existing keys.

-u USER[:GROUP]
--user=USER[:GROUP]
     Set file owner.  The USER can be either a valid user name or UID.  Similarly, the GROUP is either a valid group name or GID. If GROUP is not given, the main group of USER is used.

     User and group parts can be separated by a dot, instead of the colon.

-h
--help
     Print a concise help summary.

-V
--version
     Print program version and licensing information and exit.

--usage
     Print a terse invocation syntax summary along with a list of available command line options.

Export a database into a portable format.
*****************************************

The gdbmexport utility converts the database of an older GDBM version into a binary flat format.

The utility takes two mandatory arguments: the name of the database file to convert and the output file name, e.g.:

     $ gdbmexport junk.gdbm junk.flat

In addition the following two options are understood:

-h
     Display short usage summary and exit.

-v
     Display program version and licensing information, and exit.

Exit codes
**********

All GDBM utilities return uniform exit codes.  These are summarized in the table below:

Code   Meaning
0   Successful termination.
1   A fatal error occurred.
2   Program was unable to restore file ownership or mode.
3   Command line usage error.

Problems and bugs.
******************

If you have problems with GNU dbm or think you've found a bug, please report it.  Before reporting a bug, make sure you've actually found a real bug.  Carefully reread the documentation and see if it really says you can do what you're trying to do.  If it's not clear whether you should be able to do something or not, report that too; it's a bug in the documentation!

Before reporting a bug or trying to fix it yourself, try to isolate it to the smallest possible input file that reproduces the problem.  Then send us the input file and the exact results gdbm gave you.  Also say what you expected to occur; this will help us decide whether the problem was really in the documentation.

Once you've got a precise problem, send e-mail to bug-gdbm@gnu.org.

Please include the version number of GNU dbm you are using.  You can get this information by printing the variable gdbm_version (see Variables).

Non-bug suggestions are always welcome as well.  If you have questions about things that are unclear in the documentation or are just obscure features, please report them too.

You may contact the authors and maintainers by e-mail:

     phil@cs.wwu.edu, downsj@downsj.com, gray@gnu.org.ua

Additional resources
********************

For the latest updates and pointers to additional resources, visit http://www.gnu.org/software/gdbm.

In particular, a copy of gdbm documentation in various formats is available online at http://www.gnu.org/software/gdbm/manual.html.

Latest versions of gdbm can be downloaded from anonymous FTP: ftp://ftp.gnu.org/gnu/gdbm, or via HTTP from http://ftp.gnu.org/gnu/gdbm, or from any GNU mirror worldwide.  See http://www.gnu.org/order/ftp.html, for a list of mirrors.

To track gdbm development, visit http://puszcza.gnu.org.ua/projects/gdbm.

GNU Free Documentation License
******************************

     [file fdl.texi not included]