     [file version.texi not included]

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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 (GDBM) is a library of functions implementing a hashed database on a disk file.  This manual documents GNU dbm Version VERSION.  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 of 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 of 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 work 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.  In particular, zero-length keys or data (dsize = 0) are allowed. However, the dptr field is required to point to a valid memory location.  In other words, dptr cannot be NULL.  Note also that its type is char * for purely historic reasons.  You can use any C data type (either scalar or aggregate) both as for key and for data.

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 it. 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.

Speaking about application we usually mean a separate process. However, it is entirely normal for a multi-thread program to operate as a GDBM reader in one thread and writer in another, provided, of course, that the two threads don't operate on the same database simultaneously.

To use the GDBM functions, the programmer must first include the header file gdbm.h.

This file defines, among others, the GDBM_FILE data type, an opaque pointer to the structure that represents the opened GDBM database.  To access the database, the programmer must first open it using the gdbm_open function.  The function takes several arguments, the name of the database file being one of them, and returns a GDBM_FILE object on success.  This object is then passed to other functions in order to manipulate the database.  When the database is no longer needed, the programmer closes it using the gdbm_close call.

These and other functions are discussed in detail in chapters that follow.  Here we show an example illustrating the use of GDBM to look up a key in the database.

     #include <stdio.h>
     #include <string.h>
     #include <gdbm.h>

     int
     main (int argc, char **argv)
     {
       GDBM_FILE gdbf;     /* Database file object pointer */
       datum key, content; /* Key and content data */
       int status = 0;     /* Exit status of the program: 0 - OK, 1 - key
                              not found, 2 - error. */

       /*
        * Validate arguments.
        */
       if (argc != 3)
         {
           fprintf (stderr, "usage: %s DBFILE KEY\n", argv[0]);
           return 2;
         }

       /*
        * Open the database.  The GDBM_READER flag indicates that we only
        * intend to read from it.
        */
       gdbf = gdbm_open (argv[1], 0, GDBM_READER, 0, NULL);
       if (gdbf == NULL)
         {
           fprintf (stderr, "can't open database: %s\n",
                    gdbm_strerror (gdbm_errno));
         }

       /*
        * Prepare the lookup key.  Notice, that the terminating \0 character
        * is not counted in the dsize computation.
        */
       key.dptr = argv[2];
       key.dsize = strlen (argv[2]);

       /*
        * Look up the key in the database.
        */
       content = gdbm_fetch (gdbf, key);

       /*
        * Analyze the return.
        */
       if (content.dptr != NULL)
         {
           /*
            * The key is found.  Print the content on the stdout and
            * indicate success.
            */
           fwrite (content.dptr, content.dsize, 1, stdout);
           putchar ('\n');
           status = 0;
         }
       else if (gdbm_errno == GDBM_ITEM_NOT_FOUND)
         {
           /*
            * There is no such key in the database.
            */
           fprintf (stderr, "no such key\n");
           status = 1;
         }
       else
         {
           /*
            * An error occurred.
            */
           fprintf (stderr, "%s\n", gdbm_db_strerror (gdbf));
           status = 2;
         }

       /*
        * Close the database and return.
        */
       gdbm_close (gdbf);
       return status;
     }

To compile this example, run

     cc -oexample example.c -lgdbm

To run it, you will need an example database.  The easiest way to create it is by using the gdbtool program, which is part of the GDBM package (see gdbmtool):

     $ gdbmtool test.gdbm store foo bar

This creates database file test.gdbm and stores a single record in it.  The record's key is foo, and the value is bar. Now you can run the example program to see how it works:

     $ ./example test.gdbm foo
     bar
     $ ./example test.gdbm baz
     no such key

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

 -- gdbm interface: GDBM_FILE gdbm_open (const char *NAME, int BLOCK_SIZE,
     int FLAGS, int MODE, void (*FATAL_FUNC)(const char *))
     Opens or creates a GDBM database file.

     The arguments are:

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

     block_size
          This parameter is used only when gdbm_open has to create a new database file and represents the size of a single transfer from disk to memory.  If its value is less than 512, the file system block size is used instead.  The size is adjusted so that the block can hold exact number of directory entries, so that the effective block size can be slightly greater than requested.  However, if the GDBM_BSEXACT flag is set and the size needs to be adjusted, the function will return with error status, setting the gdbm_errno variable to GDBM_BLOCK_SIZE_ERROR.

     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.  If an existing database file is opened with the GDBM_NEWDB flag, the existing data are destroyed, and an empty database structure is created in its place.

          The following constants may also be logically or'd into the database flags:

           -- gdbm_open flag: GDBM_CLOEXEC
               Set the close-on-exec flag on the database file descriptor.  The libc must support the O_CLOEXEC flag (see O_CLOEXEC (open(2) man page)).

           -- gdbm_open flag: GDBM_NOLOCK
               Don't lock the database file.  Use this flag if you intend to do locking separately.  See Locking.

           -- gdbm_open flag: GDBM_NOMMAP
               Disable memory mapping mechanism.  Note, that this degrades performance.

           -- gdbm_open flag: GDBM_PREREAD
               When mapping GDBM file to memory, read its contents immediately, instead of when needed (prefault reading).  This can be advantageous if you open a read-only database and are going to do a lot of look-ups on it.  In this case entire database will be pre-read and look-ups will operate on an in-memory copy.  In contrast, GDBM_PREREAD should not be used if you open a database (even in read-only mode) only to do a couple of look-ups. Finally, never use GDBM_PREREAD when opening a database for updates, especially for inserts: this will degrade performance.

               This flag has no effect if GDBM_NOMMAP is given, or if the operating system does not support prefault reading.  It is known to work on Linux and FreeBSD kernels.

           -- gdbm_open flag: GDBM_XVERIFY
               Enable additional consistency checks.  With this flag, eventual corruptions of the database are discovered when opening it, instead of when a corrupted structure is read during normal operation.  However, on large databases, it can slow down the opening process.

               See Additional functions.

          The following additional flags are valid when the database is opened for writing (i.e. together with GDBM_WRITER, GDBM_WRCREAT, or GDBM_NEWDB):

           -- gdbm_open flag: GDBM_SYNC
               Synchronize all database operations to disk immediately.  Notice, that this option entails severe performance degradation and does not necessarily ensure that the resulting database state is consistent. In general, we discourage its use (see Sync). See Crash Tolerance, for a discussion of how to ensure database consistency with minimal performance overhead.

           -- gdbm_open flag: GDBM_FAST
               A reverse of GDBM_SYNC.  Synchronize writes only when needed. This is the default.  The flag is provided for compatibility with previous versions of GDBM.

          The following flags can be used together with GDBM_NEWDB.  They also take effect when used with GDBM_WRCREAT, if the requested database file doesn't exist:

           -- gdbm_open flag: GDBM_BSEXACT
               If this flag is set and the requested BLOCK_SIZE cannot be used without adjustment, gdbm_open will refuse to create the databases.  In this case it will set the gdbm_errno variable to GDBM_BLOCK_SIZE_ERROR and return NULL.

           -- gdbm_open flag: GDBM_NUMSYNC
               Useful only together with GDBM_NEWDB, this bit instructs gdbm_open to create new database in extended database format, a format best suitable for effective crash recovery. See Numsync, for a detailed discussion of this format, and Crash Tolerance, for a discussion of crash recovery.

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

     fatal_func
          This parameter is deprecated and must always be NULL.

          Early versions of GDBM (prior to 1.13) lacked proper error handling and would abort on any ``fatal'' error (such as out of memory condition, disk write error, or the like).  In these versions, fatal_func was provided as a hook, allowing the caller to do proper cleanup before such abnormal exit.  As of version VERSION, this functionality is deprecated, although still supported for backward compatibility.

     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 (or gdbm_fd_open, described below).

 -- gdbm interface: GDBM_FILE gdbm_fd_open (int FD,
     const char *NAME, int BLOCK_SIZE,
     int FLAGS, int MODE, void (*FATAL_FUNC)(const char *))

     Alternative function for opening a GDBM database.  The FD argument is the file descriptor of the database file obtained by a call to open(2), creat(2) or similar functions.  The descriptor is not dup'ed, and will be closed when the returned GDBM_FILE is closed.  Use dup(2) if that is not desirable.

     In case of error, the function behaves like gdbm_open and does not close FD.  This can be altered by the following value passed in the FLAGS argument:

      -- gdbm_open flag: GDBM_CLOERROR
          Close FD before exiting on error.

 -- gdbm interface: int gdbm_copy_meta (GDBM_FILE DST,
     GDBM_FILE SRC)
     Copy file ownership and mode from SRC to DST.

Closing the database
********************

It is important that every file opened is also closed.  This is needed to properly update its disk structure and maintain a consistent locking state on the file.

 -- gdbm interface: int 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.

     Gdbm_close returns 0 on success. On error, it sets gdbm_errno and system errno variables to the codes describing the error and returns -1.

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

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

 -- gdbm interface: int gdbm_bucket_count (GDBM_FILE DBF,
     size_t *PCOUNT)
     Counts the number of buckets 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 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:

     0
          Success.  The value of CONTENT is keyed by KEY in the database.

     -1
          An error occurred which prevented the item from being stored in the database.   Examine the gdbm_errno variable to determine the actual cause of the error.

     +1
          The item was not stored because the argument FLAG was GDBM_INSERT and the KEY was already in the database. The gdbm_errno variable is set to GDBM_CANNOT_REPLACE.

     If the function returns -1, gdbm_errno can have the following values:

     GDBM_READER_CANT_STORE
          Database was open in read-only mode, i.e. with the GDBM_READER flag.  See Open.

     GDBM_MALFORMED_DATA
          Either KEY or CONTENT had their dptr field set to NULL.

          It is OK to have a zero-length key or content, i.e. a datum with dsize set to 0, but the dptr field must always be a non-NULL value.

     GDBM_BAD_HASH_TABLE
          Database hash table is malformed.  This usually means that some error in the application or the library caused memory overrun.  The database is marked as needing recovery.  All further calls on this database will return with gdbm_error set to GDBM_NEED_RECOVERY. See Recovery, for a discussion of database recovery process.

     GDBM_BAD_DIR_ENTRY
          Database directory entry is corrupted.  The database is marked as needing recovery.  See Recovery.

     GDBM_BAD_BUCKET
          Database bucket is corrupted.  The database is marked as needing recovery.  See Recovery.

     GDBM_BAD_AVAIL
          Database available storage index is corrupted.  The database is marked as needing recovery.  See Recovery.

     GDBM_FILE_SEEK_ERROR
          A seek error occurred on the underlying disk file.  Examine the system errno variable for more detail.

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 of datum in GDBM is restricted only by the maximum value for an object of type int (type of the dsize member of datum).

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, inspect the value of the gdbm_errno variable (see gdbm_errno).  If it is GDBM_ITEM_NOT_FOUND, no data was found.  Any other value means an error occurred.  Use gdbm_strerror function to convert gdbm_errno to a human-readable string.

     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)
       {
         if (gdbm_errno == GDBM_ITEM_NOT_FOUND)
           fprintf(stderr, "key not found\n");
         else
           fprintf(stderr, "error: %s\n", gdbm_db_strerror (dbf));
       }
     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)
     Checks whether the KEY exists in the database DBF.

     If KEY is found, returns true (1).  If it is not found, returns false (0) and sets gdbm_errno to GDBM_NO_ERROR (0).

     On error, returns 0 and sets gdbm_errno to a non-0 error code.

     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 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 if an error is encountered.  Examine the gdbm_errno variable or the return from gdbm_last_errno (DBF) to know the reason.

     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, inspect the gdbm_errno variable (see gdbm_errno).  The value of GDBM_ITEM_NOT_FOUND means that the database contains no data.  Other value means an error occurred.

     On success, 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 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 inspect the gdbm_errno variable (see gdbm_errno).  The value of GDBM_ITEM_NOT_FOUND means that all keys in the database has been visited.  Any other value means an error occurred.

     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 are 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;
          }

Don't use gdbm_delete or gdbm_store 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 or will be visited twice, 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
************************

Normally, GDBM functions don't flush changed data to the disk immediately after a change.  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: int 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. This set of changes should preserve application-level invariants.  In other words, call gdbm_sync only when the database is in a consistent state with regard to the application logic, a state from which you are willing and able to recover.  You can think about all database operations between two consecutive gdbm_sync calls as constituting a single transaction.  See Synchronizing the Database, for a detailed discussion about how to properly select the synchronization points.

     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.

     Gdbm_sync returns 0 on success. On error, it sets gdbm_errno and system errno variables to the codes describing the error and returns -1.

Opening the database with GDBM_SYNC flag ensures that gdbm_sync function will be called after each change, thereby flushing the changes to disk immediately.  You are advised against using this flag, however, because it incurs a severe performance penalty, while giving only a moderate guarantee that the structural consistency of the database will be preserved in case of failure, and that only unless the failure occurs while being in the fsync call.  For the ways to ensure proper logical consistency of the database, see Crash Tolerance.

Changing database format
************************

As of version VERSION, GDBM supports databases in two formats: standard and extended.  The standard format is used most often.  The extended database format is used to provide additional crash resistance (see Crash Tolerance).

Depending on the value of the FLAGS parameter in a call to gdbm_open (see GDBM_NUMSYNC), a database can be created in either format.

The format of an existing database can be changed using the gdbm_convert function:

 -- gdbm interface: int gdbm_convert (GDBM_FILE DBF,
     int FLAG)
     Changes the format of the database file DBF.  Allowed values for FLAG are:

     0
          Convert database to the standard format.

     GDBM_NUMSYNC
          Convert database to the extended numsync format (see Numsync).

     On success, the function returns 0.  In this case, it should be followed by a call to gdbm_sync (see Sync) or gdbm_close (see Close) to ensure the changes are written to the disk.

     On error, returns -1 and sets the gdbm_errno variable (see gdbm_errno).

     If the database is already in the requested format, the function returns success (0) without doing anything.

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 have the same format and contain the same set of key/value pairs as the database from which the flat file was created. However, it will not constitute a byte-to-byte equivalent of the latter. Various internal structures in the database can differ.  In particular, ordering of key/value pairs can be different and the table of available file space will most probably differ, too.  For databases in extended format, the numsync counter will be reset to 0 (see Numsync).  These details are not visible to the application programmer, and are mentioned here only for completeness sake.

The fact that the restored database contains the same set of key/value pairs does not necessarily mean, however, that it 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.

GDBM version VERSION supports two flat file formats.  The binary flat file format was first implemented in 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.  This format is supported for backward compatibility.

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 open a file (open(2) man page)).

 -- 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_MALFORMED_DATA
     GDBM_ILLEGAL_DATA
          Input contained malformed data, i.e. it is not a valid GDBM dump file.  This often means that the dump file got corrupted during the transfer.

          The GDBM_ILLEGAL_DATA is an alias for this error code, maintained for backward compatibility.

     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_MALFORMED_DATA.

     Mild errors mean that the function was able to successfully load and restore the data, but was unable to change the database file metadata afterwards.  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_load.  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 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 handling
**************

The global variable gdbm_errno (see gdbm_errno) keeps the error code of the most recent error encountered by GDBM functions.

To convert this code to human-readable string, use the following function:

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

Detailed information about the most recent error that occurred while operating on a GDBM file is stored in the GDBM_FILE object itself.  To retrieve it, the following functions are provided:

 -- gdbm interface: gdbm_error gdbm_last_errno (GDBM_FILE DBF)
     Returns the code of the most recent error encountered when operating on DBF.

     When gdbm_last_errno called immediately after the failed function, its return equals the value of the gdbm_errno variable.  However, gdbm_errno can be changed if any GDBM functions (operating on another databases) were called afterwards, and gdbm_last_errno will always return the code of the last error that occurred while working with that database.

 -- gdbm interface: int gdbm_last_syserr (GDBM_FILE DBF)
     Returns the value of the system errno variable associated with the most recent error.

     Notice, that not all GDBM errors have an associated system error code.  The following are the ones that have:

       * GDBM_FILE_OPEN_ERROR

       * GDBM_FILE_WRITE_ERROR

       * GDBM_FILE_SEEK_ERROR

       * GDBM_FILE_READ_ERROR

       * GDBM_FILE_STAT_ERROR

       * GDBM_BACKUP_FAILED

       * GDBM_BACKUP_FAILED

       * GDBM_FILE_CLOSE_ERROR

       * GDBM_FILE_SYNC_ERROR

       * GDBM_FILE_TRUNCATE_ERROR

     For other errors, gdbm_last_syserr will return 0.

 -- gdbm interface: int gdbm_check_syserr (gdbm_errno ERR)
     Returns 1, if the system errno value should be inspected to get more info on the error described by GDBM error code ERR.

To get a human-readable description of the recent error for a particular database file, use the gdbm_db_strerror function:

 -- gdbm interface: const char * gdbm_db_strerror (GDBM_FILE DBF)
     Returns textual description of the most recent error encountered when operating on the database DBF.  The resulting string is often more informative than what would be returned by gdbm_strerror(gdbm_last_errno(DBF)).  In particular, if there is a system error associated with the recent failure, it will be described as well.

 -- gdbm interface: void gdbm_clear_error (GDBM_FILE DBF)
     Clears the error state for the database DBF.  Normally, this function is called upon the entry to any GDBM function.

Certain errors (such as write error when saving stored key) can leave database file in inconsistent state (see Database consistency). When such a critical error occurs, the database file is marked as needing recovery.  Subsequent calls to any GDBM functions for that database file (except gdbm_recover), will return immediately with GDBM error code GDBM_NEED_RECOVERY.  Additionally, the following function can be used to check the state of the database file:

 -- gdbm interface: int gdbm_needs_recovery (GDBM_FILE DBF)
     Returns 1 if the database file DBF is in inconsistent state and needs recovery.

To restore structural consistency of the database, use the gdbm_recover function (see Recovery).

Crash tolerance provides a better way of recovery, because it restores both structural and logical consistency.  See Crash Tolerance, for a detailed discussion,

Database consistency
********************

In the chapters that follow we will cover different aspects of database consistency and ways to maintain it.  Speaking about consistency, it is important to distinguish between two different aspects of it: structural and logical consistency.

Structural consistency means that all internal structures of the database are in good order, contain valid data and are coherent with one another.  Structural consistency means that the database is in good shape technically, but it does not imply that the data it contains are in any way meaningful.

Logical consistency means that the data stored in the database are coherent with respect to the application logic. Usually this implies that structural consistency is observed as well.

For as long as the program is free from memory management errors and each opened database is properly closed before the program terminates, structural consistency is maintained.  Maintaining logical consistency is more complex task and its maintenance is entirely the responsibility of the application programmer.  See Crash Tolerance, for a detailed discussion.

Both consistency aspects can suffer as a result of both application errors that cause the program to terminate prematurely without properly saving the database, and hardware errors, such as disk failures or power outages.  When such situations occur, it becomes necessary to recover the database.

In the next chapter we will discuss how to recover structural consistency of a database.

Recovering structural consistency
*********************************

Certain errors (such as write error when saving stored key) can leave database file in structurally inconsistent state.  When such a critical error occurs, the database file is marked as needing recovery.  Subsequent calls to any GDBM functions for that database file (except gdbm_recover), will return immediately with GDBM error code GDBM_NEED_RECOVERY.

To escape from this state and bring the database back to operational state, use the following function:

 -- gdbm interface: int gdbm_recover (GDBM_FILE DBF,
     gdbm_recovery *RCVR, int FLAGS)
     Check the database file DBF and fix eventual errors.  The RCVR argument points to a structure that has input members, providing additional information to alter the behavior of gdbm_recover, and output members, which are used to return additional statistics about the recovery process (RCVR can be NULL if no such information is needed).

     Each input member has a corresponding flag bit, which must be set in FLAGS, in order to instruct the function to use it.

     The gdbm_recover type is defined as:

          typedef struct gdbm_recovery_s
          {
            /* Input members.
               These are initialized before call to gdbm_recover.
               The flags argument specifies which of them are initialized. */
            void (*errfun) (void *data, char const *fmt, ...);
            void *data;
            size_t max_failed_keys;
            size_t max_failed_buckets;
            size_t max_failures;

            /* Output members.
               The gdbm_recover function fills these before returning. */
            size_t recovered_keys;
            size_t recovered_buckets;
            size_t failed_keys;
            size_t failed_buckets;
            char *backup_name;
          } gdbm_recovery;

     The input members modify the behavior of gdbm_recover:

      -- input member of gdbm_recovery: void (*errfun) (void *DATA,
          char const *FMT, ...)
          If the GDBM_RCVR_ERRFUN flag bit is set, errfun points to a function that will be called upon each recoverable or non-fatal error that occurred during the recovery.  The data field of gdbm_recovery will be passed to it as its first argument.  The FMT argument is a printf-like (see Format of the format string (printf(3) man page)), format string.  The rest of arguments supply parameters for that format.

      -- input member of gdbm_recovery: void * data
          Supplies first argument for the errfun invocations.

      -- input member of gdbm_recovery: size_t max_failed_keys
          If GDBM_RCVR_MAX_FAILED_KEYS is set, this member sets the limit on the number of keys that cannot be retrieved.  If the number of failed keys becomes equal to max_failed_keys, recovery is aborted and error is returned.

      -- input member of gdbm_recovery: size_t max_failed_buckets
          If GDBM_RCVR_MAX_FAILED_BUCKETS is set, this member sets the limit on the number of buckets that cannot be retrieved or that contain bogus information.  If the number of failed buckets becomes equal to max_failed_buckets, recovery is aborted and error is returned.

      -- output member of gdbm_recovery: size_t max_failures
          If GDBM_RCVR_MAX_FAILURES is set, this member sets the limit of failures that are tolerated during recovery.  If the number of errors becomes equal to max_failures, recovery is aborted and error is returned.

     The following members are filled on output, upon successful return from the function:

      -- output member of gdbm_recovery: size_t recovered_keys
          Number of recovered keys.

      -- output member of gdbm_recovery: size_t recovered_buckets
          Number of recovered buckets.

      -- output member of gdbm_recovery: size_t failed_keys
          Number of key/data pairs that could not be retrieved.

      -- output member of gdbm_recovery: size_t failed_buckets
          Number of buckets that could not be retrieved.

      -- output member of gdbm_recovery: char * backup_name
          Name of the file keeping the copy of the original database, in the state prior to recovery.  It is filled if the GDBM_RCVR_BACKUP flag is set.  The string is allocated using the malloc call. The caller is responsible for freeing that memory when no longer needed.

By default, gdbm_recovery first checks the database for inconsistencies and attempts recovery only if some were found. The special flag bit GDBM_RCVR_FORCE instructs gdbm_recovery to omit this check and to perform database recovery unconditionally.

Crash Tolerance
***************

Crash tolerance is a new (as of release 1.21) feature that can be enabled at compile time, and used in environments with appropriate support from the OS and the filesystem.  As of version VERSION, this means a Linux kernel 5.12.12 or later and a filesystem that supports reflink copying, such as XFS, BtrFS, or OCFS2.  If these prerequisites are met, crash tolerance code will be enabled automatically by the configure script when building the package.

The crash-tolerance mechanism, when used correctly, guarantees that a logically consistent (see Database consistency) recent state of application data can be recovered following a crash.  Specifically, it guarantees that the state of the database file corresponding to the most recent successful gdbm_sync call can be recovered.

If the new mechanism is used correctly, crashes such as power outages, OS kernel panics, and (some) application process crashes will be tolerated.  Non-tolerated failures include physical destruction of storage devices and corruption due to bugs in application logic.  For example, the new mechanism won't help if a pointer bug in your application corrupts GDBM's private in-memory data which in turn corrupts the database file.

In the following sections we will describe how to enable crash tolerance in your application and what to do if a crash occurs.

The design rationale of the crash tolerance mechanism is described in detail in the article, Crashproofing the Original NoSQL Key-Value Store, by Terence Kelly, ACM Queue magazine, July/August 2021, available from the ACM Digital Library (https://queue.acm.org/DrillBits5/). If you have difficulty retrieving this paper, please contact the author at tpkelly@acm.org, tpkelly@cs.princeton.edu, or tpkelly@eecs.umich.edu.

Using Proper Filesystem
=======================

Use a filesystem that supports reflink copying.  Currently XFS, BtrFS, and OCFS2 support reflink.  You can create such a filesystem if you don't have one already.  (Note that reflink support may require that special options be specified at the time of filesystem creation; this is true of XFS.)  The most conventional way to create a filesystem is on a dedicated storage device.  However it is also possible to create a filesystem within an ordinary file on some other filesystem.

For example, the following commands, executed as root, will create a smallish XFS filesystem inside a file on another filesystem:

     mkdir XFS
     cd XFS
     truncate --size 512m XFSfile
     mkfs -t xfs -m crc=1 -m reflink=1 XFSfile
     mkdir XFSmountpoint
     mount -o loop XFSfile XFSmountpoint

The XFS filesystem is now available in directory XFSmountpoint.  Now, create a directory where your unprivileged user account may create and delete files:

     cd XFSmountpoint
     mkdir test
     chown USER:GROUP test

(where USER and GROUP are the user and group names of the unprivileged account the application uses).

Reflink copying via ioctl(FICLONE) should work for files in and below this directory.  You can test reflink copying using the GNU cp program:

     cp --reflink=always file1 file2

See reflink (GNU Coreutils).

Your GNU dbm database file and two snapshot files described below must all reside on the same reflink-capable filesystem.

Enabling crash tolerance
========================

Open a GNU dbm database with gdbm_open.  Whenever possible, use the extended GDBM format (see Numsync).  Generally speaking, this means using the GDBM_NUMSYNC flag when creating the database.  Unless you know what you are doing, do not specify the GDBM_SYNC flag when opening the database.  The reason is that you want your application to explicitly control when gdbm_sync is called; you don't want an implicit sync on every database operation (see Sync).

Request crash tolerance by invoking the following interface:

     int gdbm_failure_atomic (GDBM_FILE DBF, const char *EVEN,
                              const char *ODD);

The EVEN and ODD arguments are the pathnames of two files that will be created and filled with snapshots of the database file. These two files must not exist when gdbm_failure_atomic is called and must reside on the same reflink-capable filesystem as the database file.

After you call gdbm_failure_atomic, every call to gdbm_sync will make an efficient reflink snapshot of the database file in either the EVEN or the ODD snapshot file; consecutive gdbm_sync calls alternate between the two, hence the names.  The permission bits and mtime timestamps on the snapshot files determine which one contains the state of the database file corresponding to the most recent successful gdbm_sync. See Crash recovery, for discussion of crash recovery.

Synchronizing the Database
==========================

When your application knows that the state of the database is consistent (i.e., all relevant application-level invariants hold), you may call gdbm_sync.  For example, if your application manages bank accounts, transferring money from one account to another should maintain the invariant that the sum of the two accounts is the same before and after the transfer: It is correct to decrement account A by $7, increment account B by $7, and then call gdbm_sync.  However it is not correct to call gdbm_sync between the decrement of A and the increment of B, because a crash immediately after that call would destroy money.  The general rule is simple, sensible, and memorable: Call gdbm_sync only when the database is in a state from which you are willing and able to recover following a crash.  (If you think about it you'll realize that there's never any other moment when you'd really want to call gdbm_sync, regardless of whether crash-tolerance is enabled.  Why on earth would you push the state of an inconsistent unrecoverable database down to durable media?).

Crash recovery
==============

If a crash occurs, the snapshot file (EVEN or ODD) containing the database state reflecting the most recent successful gdbm_sync call is the snapshot file whose permission bits are read-only and whose last-modification timestamp is greatest.  If both snapshot files are readable, we choose the one with the most recent last-modification timestamp.  Modern operating systems record timestamps in nanoseconds, which gives sufficient confidence that the timestamps of the two snapshots will differ.  However, one can't rule out the possibility that the two snapshot files will both be readable and have identical timestamps (This can happen, for example, if the storage is very fast and the system clock is low-resolution, or if the system administrator sets the system clock backwards.  In the latter case one can end up with the most recent snapshot file having modification time earlier than that of the obsolete snapshot.).  To cope with this, GDBM version 1.21 introduced the new extended database format, which stores in the database file header the number of synchronizations performed so far.  This number can reliably be used to select the most recent snapshot, independently of its timestamp.  We strongly suggest using this new format when writing crash-tolerant applications.  See Numsync, for a detailed discussion.

The gdbm_latest_snapshot function is provided, that selects the right snapshot among the two.  Invoke it as:

     const char *recovery_file = NULL;
     result = gdbm_latest_snapshot (even, odd, &recovery_file);

where EVEN and ODD are names of the snapshot files.  On success, it stores the pointer to the most recent snapshot file name in RECOVERY_FILE and returns GDBM_SNAPSHOT_OK.  To finalize the recovery, rename this file to the name of your database file and re-open it using gdbm_open.  You should discard the remaining snapshot.

If an error occurs, gdbm_latest_snapshot returns one of the following error codes.

 -- gdbm_latest_snapshot: GDBM_SNAPSHOT_BAD
     Neither snapshot file is readable.  This means that the crash has occurred before gdbm_failure_atomic completed.  In this case, it is best to fall back on a safe backup copy of the data file.

 -- gdbm_latest_snapshot: GDBM_SNAPSHOT_ERR
     System error occurred in gdbm_latest_snapshot.  Examine the system errno variable for details.  Its possible values are:

     EACCES
          The file mode of one of the snapshot files was incorrect.  Each snapshot file can be either readable (0400) or writable (0200), but not both. This probably means that someone touched one or both snapshot files after the crash and before your attempt to recover from it.  This case needs additional investigation.  If you're sure that the only change someone made to the files is altering their modes, and your database is in numsync format (see Numsync), you can reset the modes to 0400 and retry the recovery.

          This error can also be returned by underlying stat call, meaning that search permission was denied for one of the directories in the path prefix of a snapshot file name.  That again means that someone has messed with permissions after the crash.

     EINVAL
          Some arguments passed to gdbm_latest_snapshot were not valid. It is a programmer's error which means that your application needs to be fixed.

     ENOSYS
          Function is not implemented.  This means GDBM was built without crash-tolerance support.

     Other value (EBADF, EFAULT, etc)
          An error occurred when trying to stat the snapshot file. See ERRORS (stat(2) man page), for a discussion of possible errno values.

 -- gdbm_latest_snapshot: GDBM_SNAPSHOT_SAME
     File modes and modification dates of both snapshot files are exactly the same.  This can happen only if numsync is not available (see Numsync).

 -- gdbm_latest_snapshot: GDBM_SNAPSHOT_SUSPICIOUS
     For the database in extended numsync format (see Numsync): the numsync values of the two snapshot differ by more than one.  Check the arguments to the gdbm_latest_snapshot function. The most probably reason of such an error is that the EVEN and ODD parameters point to snapshot files belonging to different database files.

If you get any of these errors, we strongly suggest to undertake manual recovery.

Manual crash recovery
=====================

Manual recovery is usually performed with the help of the gdbmtool utility.  Start gdbmtool in read-only mode (the -r) option.  Once in the command shell, issue the following command:

     snapshot A B

where A and B are names of the two snapshot files you configured using the gdbm_failure_atomic function.  This command investigates both files and prints out detailed diagnostics.

Its output begins with a line listing one of the error codes above, followed by a colon and a textual description of the error.  The lines that follow show details for each snapshot file.

Each snapshot description begins with the snapshot file name followed by a colon and four fields, in this order:

 1. File permission bits in octal.

 2. File permission bits in ls -l notation.

 3. Modification timestamp.

 4. Numsync counter. For databases in standard GDBM format, this field is N/A.  If the counter cannot be obtained because of error, this field is ?.

Any errors or inconsistencies discovered are reported in the lines that follow, one error per line.  Here's an example of the snapshot command output, describing the GDBM_SNAPSHOT_ERR condition:

     gdbmtool> snapshot even.dbf odd.dbf
     GDBM_SNAPSHOT_ERR: Error selecting snapshot.
     even.dbf: 200 -w------- 1627820627.485681330 ?
     odd.dbf: 600 rw------- 1627820627.689503918 301
     odd.dbf: ERROR: bad file mode

Line 2 lists the meta-data of the snapshot even.dbf.  The numsync field contains question mark because the file permissions (write-only) prevented gdbmtool from opening it.

The lines for odd.dbf show the actual reason for the error: bad file mode (read-write).  Apparently, the file mode has been changed manually after the crash.  The timestamp of the file, which is more recent than that of even.dbf, suggests that it might be used for recovery.  To confirm this guess, change the mode of the even.dbf to read-only and repeat the snapshot command:

     gdbmtool> ! chmod 400 even.dbf
     gdbmtool> snapshot even.dbf odd.dbf
     GDBM_SNAPSHOT_ERR: Error selecting snapshot.
     even.dbf: 400 r-------- 1627820627.485681330 300
     odd.dbf: 600 rw------- 1627820627.689503918 301
     odd.dbf: ERROR: bad file mode

This shows the numsync value of the even.dbf file, which is exactly one less than that of odd.dbf.  This means that the latter should be selected for recovery.

For completeness sake, you can change the mode of odd.dbf to read-only as well and repeat the snapshot command.  In this case you will see:

     gdbmtool> ! chmod 400 odd.dbf
     gdbmtool> snapshot even.dbf odd.dbf
     GDBM_SNAPSHOT_OK: Selected the most recent snapshot.
     odd.dbf: 400 r-------- 1627820627.689503918 301

Performance Impact
==================

The purpose of a parachute is not to hasten descent.  Crash tolerance is a safety mechanism, not a performance accelerator.  Reflink copying is designed to be as efficient as possible, but making snapshots of the GNU dbm database file on every gdbm_sync call entails overheads.  The performance impact of GDBM crash tolerance will depend on many factors including the type and configuration of the underlying storage system, how often the application calls gdbm_sync, and the extent of changes to the database file between consecutive calls to gdbm_sync.

Availability
============

To ensure that application data can survive the failure of one or more storage devices, replicated storage (e.g., RAID) may be used beneath the reflink-capable filesystem.  Some cloud providers offer block storage services that mimic the interface of individual storage devices but that are implemented as high-availability fault-tolerant replicated distributed storage systems.  Installing a reflink-capable filesystem atop a high-availability storage system is a good starting point for a high-availability crash-tolerant GDBM.

Numsync Extension
=================

In Crash recovery, we have shown that for database recovery, one should select the snapshot whose permission bits are read-only and whose last-modification timestamp is greatest.  However, there may be cases when a crash occurs at such a time that both snapshot files remain readable.  It may also happen, that their permissions had been reset to read-only and/or modification times inadvertently changed before recovery.  To make it possible to select the right snapshot in such cases, a new extended database format was introduced in GDBM version 1.21.  This format adds to the database header the numsync field, which holds the number of synchronizations the database underwent before being closed or abandoned due to a crash.

A readable snapshot is a consistent copy of the database at a given point of time.  Thus, if both snapshots of a database in extended format are readable, it will suffice to examine their numsync counters and select the one whose numsync is greater.  That's what the gdbm_latest_snapshot function does in this case.

It is worth noticing, that the two counters should differ exactly by one.  If the difference is greater than that, gdbm_latest_snapshot will return a special status code, GDBM_SNAPSHOT_SUSPICIOUS. If, during a recovery attempt, you get this status code, we recommend to proceed with the manual recovery (see Manual crash recovery).

To create a database in extended format, call gdbm_open with both GDBM_NEWDB and GDBM_NUMSYNC flags:

     dbf = gdbm_open(dbfile, 0, GDBM_NEWDB|GDBM_NUMSYNC, 0600, NULL);

Notice, that this flag must always be used together with GDBM_NEWDB (see Open).  It is silently ignored when used together with another opening flag.

A standard GDBM database can be converted to the extended format and vice versa.  To convert an existing database to the extended format, use the gdbm_convert function (see Database format):

       rc = gdbm_convert(dbf, GDBM_NUMSYNC);

You can do the same using the gdbmtool utility (see upgrade):

     gdbmtool DBNAME upgrade

To convert a database from extended format back to the standard GDBM format, do:

       rc = gdbm_convert(dbf, 0);

To do the same from the command line, run:

     gdbmtool DBNAME downgrade

Crash Tolerance API
===================

 -- gdbm interface: int gdbm_failure_atomic (GDBM_FILE DBF,
     const char *EVEN, const char *ODD)
     Enables crash tolerance for the database file DBF.  The EVEN and ODD arguments are the pathnames of two files that will be created and filled with snapshots of the database file. These two files must not exist when gdbm_failure_atomic is called and must reside on the same reflink-capable filesystem as the database file.

     Returns 0 on success.  On failure, returns -1 and sets gdbm_errno to one of the following values:

     GDBM_ERR_USAGE
          Improper function usage.  Either EVEN or ODD is NULL, or they point to the same string.

     GDBM_NEED_RECOVERY
          The database needs recovery.  See Recovery.

     GDBM_ERR_SNAPSHOT_CLONE
          Failed to clone the database file into a snapshot.  Examine the system errno variable for details.

     If one of the following error codes is returned, examine the system errno variable for details:

     GDBM_ERR_REALPATH
          Call to realpath function failed.  realpath is used to determine actual path names of the snapshot files.

     GDBM_FILE_OPEN_ERROR
          Unable to create snapshot file.

     GDBM_FILE_SYNC_ERROR
          Failed to sync a snapshot file or one of directories in its pathname, during initial synchronization.

     GDBM_FILE_CLOSE_ERROR
          Failed to close a snapshot file or one of directories in its pathname, during initial synchronization.

     GDBM_ERR_FILE_MODE
          The fchmod call on one of the snapshot files failed.

     Notes:

       * It is not an error to call gdbm_failure_atomic several times. Each subsequent call closes the previously configured snapshot files and installs new ones instead.

       * Crash tolerance settings are cleared by functions gdbm_recover (see Recovery) and gdbm_reorganize (see Reorganization).  In case of gdbm_recover, it should not be a problem, because if you enabled crash tolerance, the procedure described in Crash recovery is the preferred way of recovering the database.  If, however, you decided to call either function even though you had enabled crash tolerance previously, be sure to call gdbm_failure_atomic again with the same arguments as before (provided that the call returns successfully).

 -- gdbm interface: int gdbm_latest_snapshot (const char *EVEN,
     const char *ODD, const char **RETVAL)
     Selects between two snapshots, EVEN and ODD, the one to be used for crash recovery.  On success, stores a pointer to the selected filename in the memory location pointed to by RETVAL and returns GDBM_SNAPSHOT_OK.  If neither snapshot file is usable, the function returns GDBM_SNAPSHOT_BAD.  If a system error occurs, it returns GDBM_SNAPSHOT_ERR and sets errno to the error code describing the problem.  Finally, in the unlikely case that it cannot select between the two snapshots (this means they are both readable and have exactly the same mtime timestamp), the function returns GDBM_SNAPSHOT_SAME.

     If the numsync extension is enabled (see Numsync), the function can also return the GDBM_SNAPSHOT_SUSPICIOUS status code.  This happens when the numsync counters in the two snapshots differ by more than one.

     See Crash recovery, for a detailed description of possible return codes and their interpretation.

     If any value other than GDBM_SNAPSHOT_OK is returned, it is guaranteed that the function did not touch RETVAL.    In this case it is recommended to switch to manual recovery procedure, letting the user examine the snapshots and take the appropriate action. see Manual crash recovery, for details.

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

The valid options are:

 -- Option: GDBM_SETCACHESIZE
 -- Option: GDBM_CACHESIZE
     Set the size of the internal bucket cache.  The VALUE should point to a size_t holding the desired cache size, or the constant GDBM_CACHE_AUTO, to adjust the cache size automatically.

     By default, a newly open database is configured to dynamically accommodate the cache size to the number of index buckets in the database file.  This provides for the best performance.

     If another VALUE is set, it is adjusted to the nearest larger power of two.

     Use this option if you wish to limit the memory usage at the expense of performance.  If you chose to do so, please bear in mind that cache becomes effective when its size is greater then 2/3 of the number of index bucket counts in the database.  The best performance results are achieved when cache size equals the number of buckets.  For example:

          size_t bn;
          gdbm_bucket_count (dbf, &bn);
          ret = gdbm_setopt (dbf, GDBM_SETCACHESIZE, &bn, sizeof (bn));

     To request the automatically adjustable cache size, use the constant GDBM_CACHE_AUTO:

          size_t bn = GDBM_CACHE_AUTO;
          ret = gdbm_setopt (dbf, GDBM_SETCACHESIZE, &bn, sizeof (bn));

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

 -- Option: GDBM_SETCACHEAUTO
     Controls whether the cache size will be adjusted automatically as needed.  The VALUE should point to an integer: TRUE to enable automatic cache adjustment and FALSE to disable it.

     The following two calls are equivalent:

          int t = TRUE;
          gdbm_setopt (dbf, GDBM_SETCACHEAUTO, &t, sizeof (t));

          size_t n = GDBM_CACHE_AUTO;
          gdbm_setopt (dbf, GDBM_SETCACHESIZE, &n, sizeof (n));

 -- Option: GDBM_GETCACHEAUTO
     Return the state of the automatic cache size adjustment.  The VALUE should point to an integer which, upon successful return, will have the value TRUE if the automatic cache size adjustment is enabled and FALSE otherwise.

 -- Option: GDBM_GETFLAGS
     Return the flags describing the state of the database.  The VALUE should point to an int variable where to store the flags.  On success, its value will be similar to the flags used when opening the database (see gdbm_open), except that it will reflect the current state (which may have been altered by another calls to gdbm_setopt).

 -- Option: GDBM_GETDBFORMAT
     Return the database format.  The VALUE should point to an int variable.  Upon successful return, it will be set to 0 if the database is in standard format and GDBM_NUMSYNC if it is in extended format.  See Database format.

 -- Option: GDBM_GETDIRDEPTH
     Returns the directory depth: the number of initial (most significant) bits in hash value that are interpreted as index to the directory.  The actual directory size can be computed as 1 << VALUE.

     The VALUE argument should point to an int.

 -- Option: GDBM_GETBUCKETSIZE
     Returns the bucket capacity: maximum number of keys per bucket (int).

 -- Option: 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.

 -- Option: GDBM_SETSYNCMODE
 -- Option: 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.

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

 -- Option: GDBM_SETCENTFREE
 -- Option: 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.

 -- Option: GDBM_SETCOALESCEBLKS
 -- Option: 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.

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

 -- Option: 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)).

 -- Option: 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.

 -- Option: 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.

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

 -- Option: 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);
            }

 -- Option: GDBM_GETBLOCKSIZE
     Return the block size in bytes.  The VALUE should point to 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.

 -- Variable: int const gdbm_syserr[]
     Array of boolean values indicating, for each GDBM error code, whether the value of errno(3) variable is meaningful for this error code.  See gdbm_check_syserr.

 -- 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

Additional functions
********************

 -- gdbm interface: int gdbm_avail_verify (GDBM_FILE DBF)
     Verify if the available block stack is in consistent state.  On success, returns 0.  If any errors are encountered, sets the gdbm_errno to GDBM_BAD_AVAIL, marks the database as needing recovery (see Recovery) and return -1.

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

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

 -- Error Code: GDBM_NO_ERROR
     No error occurred.

 -- Error Code: GDBM_MALLOC_ERROR
     Memory allocation failed.  Not enough memory.

 -- Error Code: 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 and the GDBM_BSEXACT flag is set.

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

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

 -- Error Code: GDBM_FILE_WRITE_ERROR
     Writing to a disk file failed.  This can be set by gdbm_open (see Open), gdbm_dump (gdbm_export) and gdbm_load (gdbm_import) functions.

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

 -- Error Code: 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.

 -- Error Code: GDBM_FILE_READ_ERROR
     Reading from a disk file failed.  This can be set by gdbm_open (see Open), gdbm_dump (gdbm_export) and gdbm_load (gdbm_import) functions.

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

 -- Error Code: 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.

 -- Error Code: GDBM_EMPTY_DATABASE
     The file given as argument to gdbm_open function is not a valid GDBM file: it has zero length.

 -- Error Code: 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).

 -- Error Code: 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).

 -- Error Code: 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).

 -- Error Code: 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).

 -- Error Code: 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).

 -- Error Code: 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.

 -- Error Code: GDBM_REORGANIZE_FAILED
     The gdbm_reorganize function is not able to create a temporary database.  See Reorganization.

 -- Error Code: 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.

 -- Error Code: GDBM_MALFORMED_DATA
 -- Error Code: GDBM_ILLEGAL_DATA
     Input data was malformed in some way.  When returned by gdbm_load, this means that the input file was not a valid GDBM dump file (see gdbm_load function).  When returned by gdbm_store, this means that either KEY or CONTENT parameter had its dptr field set to NULL (see Store).

     The GDBM_ILLEGAL_DATA is an alias for this error code, maintained for backward compatibility.  Its use in modern applications is discouraged.

 -- Error Code: GDBM_OPT_ALREADY_SET
     Requested option can be set only once and was already set.  As of version VERSION, this error code is no longer used.  In prior versions it could have been returned by the gdbm_setopt function when setting the GDBM_CACHESIZE value.

 -- Error Code: GDBM_OPT_BADVAL
 -- Error Code: 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_OPT_ILLEGAL is an alias for this error code, maintained for backward compatibility.  Modern applications should not use it.

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

 -- Error Code: 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.

 -- Error Code: GDBM_BAD_OPEN_FLAGS
     Set by the gdbm_dump (gdbm_export) function if supplied an invalid FLAGS argument.  See Flat files.

 -- Error Code: 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.

 -- Error Code: 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_dump, gdbm_load, gdbm_export, gdbm_import, gdbm_reorganize, gdbm_firstkey, gdbm_nextkey, gdbm_store.

 -- Error Code: 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.

 -- Error Code: 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.

 -- Error Code: 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.

 -- Error Code: GDBM_NEED_RECOVERY
     Database is in inconsistent state and needs recovery.  Call gdbm_recover if you get this error.  See Recovery, for a detailed description of recovery functions.

 -- Error Code: GDBM_BACKUP_FAILED
     The GDBM engine is unable to create backup copy of the file.

 -- Error Code: GDBM_DIR_OVERFLOW
     Bucket directory would overflow the size limit during an attempt to split hash bucket.  This error can occur while storing a new key.

 -- Error Code: GDBM_BAD_BUCKET
     Invalid index bucket is encountered in the database.  Database recovery is needed (see Recovery).

 -- Error Code: GDBM_BAD_HEADER
     This error is set by gdbm_open and gdbm_fd_open, if the first block read from the database file does not contain a valid GDBM header.

 -- Error Code: GDBM_BAD_AVAIL
     The available space stack is invalid.  This error can be set by gdbm_open and gdbm_fd_open, if the extended database verification was requested (GDBM_XVERIFY).  It is also set by the gdbm_avail_verify function (see Additional functions).

     Database recovery is needed (see Recovery).

 -- Error Code: GDBM_BAD_HASH_TABLE
     Hash table in a bucket is invalid.  This error can be set by the following functions: gdbm_delete, gdbm_exists, gdbm_fetch, gdbm_firstkey, gdbm_nextkey, and gdbm_store.

     Database recovery is needed (see Recovery).

 -- Error Code: GDBM_BAD_DIR_ENTRY
     Bad directory entry found in the bucket.  The database recovery is needed (see Recovery).

 -- Error Code: GDBM_FILE_CLOSE_ERROR
     The gdbm_close function was unable to close the database file descriptor.  The system errno variable contains the corresponding error code.

 -- Error Code: GDBM_FILE_SYNC_ERROR
     Cached content couldn't be synchronized to disk.  Examine the errno variable to get more info,

     Database recovery is needed (see Recovery).

 -- Error Code: GDBM_FILE_TRUNCATE_ERROR
     File cannot be truncated.  Examine the errno variable to get more info.

     This error is set by gdbm_open and gdbm_fd_open when called with the GDBM_NEWDB flag.

 -- Error Code: GDBM_BUCKET_CACHE_CORRUPTED
     The bucket cache structure is corrupted.  Database recovery is needed (see Recovery).

 -- Error Code: GDBM_BAD_HASH_ENTRY
     This error is set during sequential access (see Sequential), if the next hash table entry does not contain the expected key.  This means that the bucket is malformed or corrupted and the database needs recovery (see Recovery).

 -- Error Code: GDBM_ERR_SNAPSHOT_CLONE
     Set by the gdbm_failure_atomic function if it was unable to clone the database file into a snapshot.  Inspect the system errno variable for the underlying cause of the error.  If errno is EINVAL or ENOSYS, crash tolerance settings will be removed from the database.

     See Crash Tolerance API.

 -- Error Code: GDBM_ERR_REALPATH
     Set by the gdbm_failure_atomic function if the call to realpath function failed.  realpath is used to determine actual path names of the snapshot files.  Examine the system errno variable for details.

     See Crash Tolerance API.

 -- Error Code: GDBM_ERR_USAGE
     Function usage error.  That includes invalid argument values, and the like.

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 ... -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!

Compatibility interface includes only functions required by POSIX (see ndbm) or present in the traditional DBM implementation (see dbm).  Advanced GDBM features, such as crash tolerance, cannot be used with such databases.

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 (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 as 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
===================

When started without additional arguments, gdbmtool operates on the default database junk.gdbm. Otherwise, the first argument supplies the name of the database to operate upon. If neither any additional arguments nor the -f (--file) option are given, gdbmtool opens starts interactive shell and receives commands directly from the human operator.

If more than one argument is given, all arguments past the database name are parsed as gdbmtool commands (see shell, for a description of available commands) and executed in turn. All commands, except the last one, should be terminated with semicolons. Semicolon after the last command is optional. Note, that semicolons should be escaped in order to prevent them from being interpreted by the shell.

Finally, if the -f (--file) option is supplied, its argument specifies the name of the disk file with gdbmtool script. The program will open that file and read commands from it.

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.

-d FD
--db-descriptor=FD
     Use the database referred to by the file descriptor FD.  This must be a valid open file descriptor, obtained by a call to open (see open a file (open(2) man page)), creat or a similar function.  The database will be opened using gdbm_fd_open (see gdbm_fd_open).

     This option is intended for use by automatic test suites.

-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 memory mapping.

-T
--timing
     Print time spent in each command.  This is equivalent to setting the timing variable.  See timing.

-t
--trace
     Enable command tracing.  This is equivalent to setting the trace variable.  See trace.

-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.

-x
--extended
--numsync
     Create new database in extended (numsync) format (see Numsync). This option sets the format variable to numsync. See format variable.

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

After successful startup, gdbmtool starts a loop, in which it reads commands from the standard input, executes them and prints 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 and terminated with a newline or semicolon. 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.

If the package is compiled with GNU Readline, the input line can be edited (see Command Line Editing (GNU Readline Library)).

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

A number of gdbmtool parameters is kept in its internal variables.  To examine or modify variables, use the set command (see set).

 -- 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 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 errorexit
 -- gdbmtool variable: bool errorexit
     Comma-delimited list of GDBM error codes which cause program termination.  Error codes are specified via their canonical names (see Error codes).  The GDBM_ prefix can be omitted.  Code name comparison is case-insensitive.  Each error code can optionally be prefixed with minus sign, to indicate that it should be removed from the resulting list, or with plus sign (which is allowed for symmetry).  A special code all stands for all available error codes.

     In boolean context, the true value is equivalent to all, and false (i.e. variable unset) is equivalent to -all.

 -- gdbmtool variable: string errormask
 -- gdbmtool variable: bool errormask
     Comma-delimited list of GDBM error codes which are masked, i.e. which won't trigger a diagnostic message if they occur.  The syntax is the same as described for errorexit.

 -- 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: 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: bool timing
     When each command terminates, print an additional line listing times spent in that command.  The line is formatted as follows:

          [reorganize r=0.070481 u=0.000200 s=0.000033]

     Here, reorganize is the name of the command that finished, the number after r= is real time spent executing the command, the number after u= is the user CPU time used and the number after s= is the system CPU time used.

 -- gdbmtool variable: bool trace
     Enable command tracing.  This is similar to the shell -t option: before executing each command, gdbmtool will print on standard error a line starting with a plus sign and followed by the command name and its arguments.

 -- gdbmtool variable: bool quiet
     Whether to display a welcome banner at startup.  To affect gdbmtool, 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.

     This variable affects the currently opened database immediately.  It is also used by open command.

     To enable automatic cache size selection, unset this variable.  This is the default.

 -- gdbmtool variable: string filename
     Name of the database file.  If the open command is called without argument (e.g. called implicitly), this variable names the database file to open.  If open is called with file name argument, upon successful opening of the database the filename variable is initialized with its file name.

     This variable cannot be unset.

 -- gdbmtool variable: number fd
     File descriptor of the database file to open.  If this variable is set, its value must be an open file descriptor referring to a GDBM database file.  The open command will use gdbm_fd_open function to use this file (see gdbm_fd_open). When this database is closed, the descriptor will be closed as well and the fd variable will be unset.

     See also the -d (--db-descriptor) command line option in invocation.

 -- gdbmtool variable: string format
     Defines the format in which new databases will be created.  Allowed values are:

     standard
          Databases will be created in standard format.  This is the format used by all GDBM versions prior to 1.21.  This value is the default.

     numsync
          Extended format, best for crash-tolerant applications. See Numsync, for a discussion of this format.

 -- 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 results in 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.

 -- gdbmtool variable: bool coalesce
     Enables the coalesce mode, i.e. merging of the freed blocks of GDBM files with entries in available block lists. This provides for effective memory management at the cost of slight increase in execution time when calling gdbm_delete. See GDBM_SETCOALESCEBLKS.

     This variable affects the currently opened database immediately and will be used by open command, when it is invoked.

 -- gdbmtool variable: bool centfree
     Set to true, enables the use of central free block pool in newly opened databases. See GDBM_SETCENTFREE.

     This variable affects the currently opened database immediately and will be used by open command, when it is invoked.

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:

          # blocksize is unset
          # cachesize is unset
          nocentfree
          nocoalesce
          confirm
          delim1=","
          delim2=","
          # fd is unset
          filemode=644
          filename="junk.gdbm"
          format="standard"
          lock
          mmap
          open="wrcreat"
          pager="less"
          ps1="%p>%_"
          ps2="%_>%_"
          # quiet is unset
          nosync

     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: debug [[+-]TOKEN...]
     If GDBM is configured with additional debugging, this statement queries or sets GDBM internal debugging level. This is intended for debugging and testing purposes and requires good knowledge of GDBM internals. The use of this command is not recommended.

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

 -- command verb: dir
     Print hash directory.

 -- command verb: downgrade
     Downgrade the database from extended to the standard database format. See Numsync.

 -- 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.

 -- 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: history
 -- command verb: history COUNT
 -- command verb: history N COUNT
     Shows the command history list with line numbers.  When used without arguments, shows entire history.  When used with one argument, displays COUNT last commands from the history.  With two arguments, displays COUNT commands starting from Nth command.  Command numbering starts with 1.

     This command is available only if GDBM was compiled with GNU Readline.  The history is saved in file .gdbmtool_history in the user's home directory.  If this file exists upon startup, it is read to populate the history.  Thus, command history is preserved between gdbmtool invocations.

 -- 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.

     Issuing several next commands in row is rather common.  A shortcut is provided to facilitate such use: if the last entered command was next, hitting the Enter key repeats it without arguments.

     See also first, above.

     See Sequential, for more information on sequential access.

 -- command verb: open FILENAME
 -- command verb: open
     Open the database file FILENAME.  If used without arguments, the database name is taken from the variable filename.

     If successful, any previously open database is closed and the filename variable is updated.  Otherwise, if the operation fails, the currently opened database remains unchanged.

     This command takes additional information from the following variables:

     filename
          Name of the database to open, if no argument is given.

     fd
          File descriptor to use.  If set, this must be an open file descriptor referring to a valid database file.  The database will be opened using gdbm_fd_open (see gdbm_fd_open).  The file descriptor will be closed and the variable unset upon closing the database.

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

     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.

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

 -- command verb: perror [CODE]
     Describe the given GDBM error code.

     The description occupies one or two lines.  The second line is present if the system error number should be checked when handling this code. In this case, the second line states Examine errno.

     If CODE is omitted, the latest error that occurred in the current database is described.  Second line of the output (if present), contains description of the latest system error.

     Example:

          gdbmtool> perror 3
          GDBM error code 3: "File open error"
          Examine errno.

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

 -- command verb: recover [OPTIONS]
     Recover the database from structural inconsistencies.  See Database consistency.

     The following OPTIONS are understood:

     backup
          Create a backup copy of the original database.

     max-failed-buckets=N
          Abort recovery process if N buckets could not be recovered.

     max-failed-keys=N
          Abort recovery process if N keys could not be recovered.

     max-failures=N
          Abort recovery process after N failures. A failure in this context is either a key or a bucket that failed to be recovered.

     summary
          Print the recovery statistics at the end of the run. The statistics includes number of successfully recovered, failed and duplicate keys and the number of recovered and failed buckets.

     verbose
          Verbosely list each error encountered.

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

 -- command verb: shell COMMAND
 -- command verb: ! COMMAND
     Execute COMMAND via current shell.  If COMMAND is empty, shell is started without additional arguments.  Otherwise, it is run as $SHELL -c COMMAND.

     For convenience, COMMAND is not parsed as gdbmtool command line.  It is passed to the shell verbatim.  It can include newline characters if these are preceded by a backslash or appear within singly or doubly quoted strings.

     When using ! form, be sure to separate it from COMMAND by whitespace, otherwise it will be treated as readline event specifier.

 -- command verb: snapshot FILENAME FILENAME
     Analyze two snapshot files and select the most recent of them.  In case of error, display a detailed diagnostics and meta-information of both snapshots.

     See Manual crash recovery, for a detailed discussion.

 -- 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: sync
     Synchronize the database with the disk storage (see Sync).

 -- command verb: upgrade
     Upgrade the database from standard to extended database format. See Numsync.

 -- 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 3 bytes id, and an array of bytes 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, e.g.:

     gdbmtool> store newkey { status=2, id={a,u,x}, name="quux" }

The order in which the fields are listed is not significant.  The above command can as well be written as:

     gdbmtool> store newkey { id={a,u,x}, status=2, name="quux" }

You are not required to supply all defined fields.  Any number of them can be omitted, provided that at least one remains.  The omitted fields are filled with 0:

     gdbmtool> store newkey { name="bar" }
     gdbmtool> fetch newkey
     status=0,id={ ,, },name=bar

Yet another way to supply structured data to a command is by listing the value for each field in the order they are defined, without field names:

     gdbmtool> store newkey { 2, {a,u,x}, "quux" }

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.

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: Philip Nelson phil@cs.wwu.edu, Jason Downs downsj@downsj.com, Sergey Poznyakoff gray@gnu.org or gray@gnu.org.ua.

Crash tolerance support written by Terence Kelly tpkelly@acm.org, tpkelly@cs.princeton.edu, or tpkelly@eecs.umich.edu.

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 via HTTPS from https://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]