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This is gdbm.info, produced by makeinfo version 6.7 from gdbm.texi.

Published by the Free Software Foundation, 51 Franklin Street, Fifth
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Permission is granted to copy, distribute and/or modify this document
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License."
INFO-DIR-SECTION Programming & development tools
START-INFO-DIR-ENTRY
* GDBM: (gdbm). The GNU database manager.
* gdbm_dump: (gdbm) gdbm_dump. Dump the GDBM database into a flat file.
* gdbm_load: (gdbm) gdbm_load. Load the database from a flat file.
* gdbmtool: (gdbm) gdbmtool. Examine and modify a GDBM database.
END-INFO-DIR-ENTRY

File: gdbm.info, Node: Top, Next: Copying, Up: (dir)

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 1.22.
The software was originally written by Philip A. Nelson. This document
was originally written by Pierre Gaumond from texts written by Phil.

* Menu:

* Copying:: Your rights.
* Intro:: Introduction to GNU dbm.

Functions:

* Open:: Opening the database.
* Close:: Closing the database.
* Count:: Counting records in the database.
* Store:: Inserting and replacing records in the database.
* Fetch:: Searching records in the database.
* Delete:: Removing records from the database.
* Sequential:: Sequential access to records.
* Reorganization:: Database reorganization.
* Sync:: Insure all writes to disk have competed.
* Database format:: GDBM database formats.
* Flat files:: Export and import to Flat file format.
* Errors:: Error handling.
* Database consistency:: Structural and logical consistency.
* Recovery:: Recovery from fatal errors.
* Crash Tolerance:: Ensuring recovery to a consistent state.
* Options:: Setting internal options.
* Locking:: File locking.
* Variables:: Useful global variables.
* Additional functions:: Functions for verifying internal structures.
* Error codes:: Error codes returned by GDBM calls.
* Compatibility:: Compatibility with UNIX dbm and ndbm.

Programs

* gdbmtool:: Examine and modify a GDBM database.
* gdbm_dump:: Dump the database into a flat file.
* gdbm_load:: Load the database from a flat file.
* Exit codes:: Exit codes returned by GDBM utilities.

Other topics:

* Bugs:: Problems and bugs.
* Resources:: Additional resources,

* GNU Free Documentation License:: Document license.
* Index:: Index

-- The Detailed Node Listing --

Compatibility with standard 'dbm' and 'ndbm'

* ndbm:: NDBM interface functions.
* dbm:: DBM interface functions.

Examine and modify a GDBM database

* invocation::
* shell::

gdbmtool interactive mode

* variables:: shell variables.
* commands:: shell commands.
* definitions:: how to define structured data.
* startup files::

File: gdbm.info, Node: Copying, Next: Intro, Prev: Top, Up: Top

1 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'.

File: gdbm.info, Node: Intro, Next: Open, Prev: Copying, Up: Top

2 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 (*note 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

File: gdbm.info, Node: Open, Next: Close, Prev: Intro, Up: Top

3 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
(*note (open(2))O_CLOEXEC::).

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

*Note 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 (*note Sync::). *Note 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. *Note Numsync::, for a detailed
discussion of this format, and *note Crash Tolerance::,
for a discussion of crash recovery.

MODE
File mode(1), 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 1.22, 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 (*note gdbm_errno: Variables.). Available
error codes are discussed in *note 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.

---------- Footnotes ----------

(1) *Note (chmod(2))chmod::, and *Note open a file: (open(2))open.

File: gdbm.info, Node: Close, Next: Count, Prev: Open, Up: Top

4 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.

File: gdbm.info, Node: Count, Next: Store, Prev: Close, Up: Top

5 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.

File: gdbm.info, Node: Store, Next: Fetch, Prev: Count, Up: Top

6 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. *Note 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'. *Note Recovery::, for a
discussion of database recovery process.

'GDBM_BAD_DIR_ENTRY'
Database directory entry is corrupted. The database is marked
as needing recovery. *Note Recovery::.

'GDBM_BAD_BUCKET'
Database bucket is corrupted. The database is marked as
needing recovery. *Note Recovery::.

'GDBM_BAD_AVAIL'
Database available storage index is corrupted. The database
is marked as needing recovery. *Note 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').

File: gdbm.info, Node: Fetch, Next: Delete, Prev: Store, Up: Top

7 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 (*note gdbm_errno: Variables.). 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.

File: gdbm.info, Node: Delete, Next: Sequential, Prev: Fetch, Up: Top

8 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.

File: gdbm.info, Node: Sequential, Next: Reorganization, Prev: Delete, Up: Top

9 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
(*note gdbm_errno: Variables.). 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 (*note gdbm_errno: Variables.). 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;
}

File: gdbm.info, Node: Reorganization, Next: Sync, Prev: Sequential, Up: Top

10 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.

File: gdbm.info, Node: Sync, Next: Database format, Prev: Reorganization, Up: Top

11 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". *Note
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 *note Crash Tolerance::.

File: gdbm.info, Node: Database format, Next: Flat files, Prev: Sync, Up: Top

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

As of version 1.22, '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 (*note
Crash Tolerance::).

Depending on the value of the FLAGS parameter in a call to
'gdbm_open' (*note GDBM_NUMSYNC: Open.), 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 (*note
Numsync::).

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

On error, returns -1 and sets the 'gdbm_errno' variable (*note
gdbm_errno: Variables.).

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

File: gdbm.info, Node: Flat files, Next: Errors, Prev: Database format, Up: Top

13 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 (*note 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 1.22 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 (*note
open a file: (open(2))open.).

-- 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 (*note 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. *Note
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);

File: gdbm.info, Node: Errors, Next: Database consistency, Prev: Flat files, Up: Top

14 Error handling
*****************

The global variable 'gdbm_errno' (*note gdbm_errno: Variables.) 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 (*note 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 (*note Recovery::).

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

File: gdbm.info, Node: Database consistency, Next: Recovery, Prev: Errors, Up: Top

15 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. *Note 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.

File: gdbm.info, Node: Recovery, Next: Crash Tolerance, Prev: Database consistency, Up: Top

16 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 (*note
(printf(3))Format of the format string::), 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.

File: gdbm.info, Node: Crash Tolerance, Next: Options, Prev: Recovery, Up: Top

17 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 1.22, 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 (*note 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>.

* Menu:

* Filesystems supporting crash tolerance::
* Enabling crash tolerance::
* Synchronizing the Database::
* Crash recovery::
* Manual crash recovery::
* Performance Impact::
* Availability::
* Numsync::
* Crash Tolerance API::

File: gdbm.info, Node: Filesystems supporting crash tolerance, Next: Enabling crash tolerance, Up: Crash Tolerance

17.1 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

*Note reflink: (coreutils)cp invocation.

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

File: gdbm.info, Node: Enabling crash tolerance, Next: Synchronizing the Database, Prev: Filesystems supporting crash tolerance, Up: Crash Tolerance

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

Open a GNU dbm database with 'gdbm_open'. Whenever possible, use the
extended 'GDBM' format (*note 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 (*note 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'. *Note Crash recovery::, for discussion of crash recovery.

File: gdbm.info, Node: Synchronizing the Database, Next: Crash recovery, Prev: Enabling crash tolerance, Up: Crash Tolerance

17.3 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?).

File: gdbm.info, Node: Crash recovery, Next: Manual crash recovery, Prev: Synchronizing the Database, Up: Crash Tolerance

17.4 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(1). 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. *Note 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 (*note 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.
*Note (stat(2))ERRORS::, 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
(*note Numsync::).

-- gdbm_latest_snapshot: GDBM_SNAPSHOT_SUSPICIOUS
For the database in extended "numsync" format (*note 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".

---------- Footnotes ----------

(1) 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.

File: gdbm.info, Node: Manual crash recovery, Next: Performance Impact, Prev: Crash recovery, Up: Crash Tolerance

17.5 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

File: gdbm.info, Node: Performance Impact, Next: Availability, Prev: Manual crash recovery, Up: Crash Tolerance

17.6 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'.

File: gdbm.info, Node: Availability, Next: Numsync, Prev: Performance Impact, Up: Crash Tolerance

17.7 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'.

File: gdbm.info, Node: Numsync, Next: Crash Tolerance API, Prev: Availability, Up: Crash Tolerance

17.8 Numsync Extension
======================

In *note 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 (*note 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'
(*note 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 (*note Database format::):

rc = gdbm_convert(dbf, GDBM_NUMSYNC);

You can do the same using the 'gdbmtool' utility (*note upgrade:
commands.):

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

File: gdbm.info, Node: Crash Tolerance API, Prev: Numsync, Up: Crash Tolerance

17.9 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. *Note 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' (*note Recovery::) and 'gdbm_reorganize' (*note
Reorganization::). In case of 'gdbm_recover', it should not
be a problem, because if you enabled crash tolerance, the
procedure described in *note 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 (*note 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.

*Note 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. *note
Manual crash recovery::, for details.

File: gdbm.info, Node: Options, Next: Locking, Prev: Crash Tolerance, Up: Top

18 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:

GDBM_SETCACHESIZE
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 set the best cache size automatically.

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

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 set the best cache size, use the constant 'GDBM_CACHE_AUTO':

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

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

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

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

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

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

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

The 'GDBM_SYNCMODE' option is provided for compatibility with
earlier versions.

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

GDBM_SETCENTFREE
GDBM_CENTFREE
_NOTICE: This feature is still under study._

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

The 'GDBM_CENTFREE' option is provided for compatibility with
earlier versions.

GDBM_SETCOALESCEBLKS
GDBM_COALESCEBLKS
_NOTICE: This feature is still under study._

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

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

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

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

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

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

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

char *name;

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

GDBM_GETBLOCKSIZE
Return the block size in bytes. The VALUE should point to 'int'.

File: gdbm.info, Node: Locking, Next: Variables, Prev: Options, Up: Top

19 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.

File: gdbm.info, Node: Variables, Next: Additional functions, Prev: Locking, Up: Top

20 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.
*Note Error codes::, for a list of available error codes and their
descriptions.

Use 'gdbm_strerror' (*note 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
(*note 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. *Note 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

File: gdbm.info, Node: Additional functions, Next: Error codes, Prev: Variables, Up: Top

21 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 (*note Recovery::) and return -1.

File: gdbm.info, Node: Error codes, Next: Compatibility, Prev: Additional functions, Up: Top

22 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 (*note 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' (*note Open::), 'gdbm_dump' ('gdbm_export') and
'gdbm_load' ('gdbm_import') functions (*note 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'
(*note 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'
(*note 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'
(*note 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 (*note
GDBM_READER: Open.).

-- 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 (*note Open::).

-- Error Code: GDBM_READER_CANT_DELETE
Set by the 'gdbm_delete' (*note Delete::) if it attempted to
operate on a database that is open in read-only mode (*note
GDBM_READER: Open.).

-- Error Code: GDBM_READER_CANT_STORE
Set by the 'gdbm_store' (*note Store::) if it attempted to operate
on a database that is open in read-only mode (*note GDBM_READER:
Open.).

-- Error Code: GDBM_READER_CANT_REORGANIZE
Set by the 'gdbm_reorganize' (*note Reorganization::) if it
attempted to operate on a database that is open in read-only mode
(*note GDBM_READER: Open.).

-- Error Code: GDBM_ITEM_NOT_FOUND
Requested item was not found. This error is set by 'gdbm_delete'
(*note Delete::) and 'gdbm_fetch' (*note 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. *Note 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'. *Note 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 (*note gdbm_load function::). When returned by
'gdbm_store', this means that either KEY or CONTENT parameter had
its 'dptr' field set to 'NULL' (*note 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 1.22, 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. *Note
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 (*note 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 (*note 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. *Note 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' (*note gdbm_load: gdbm_load function.) 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. *Note 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 (*note 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 (*note Additional functions::).

Database recovery is needed (*note 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 (*note Recovery::).

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

-- Error Code: GDBM_BAD_HASH_ENTRY
This error is set during sequential access (*note 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 (*note 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.

*Note 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.

*Note Crash Tolerance API::.

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

File: gdbm.info, Node: Compatibility, Next: gdbmtool, Prev: Error codes, Up: Top

23 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
(*note ndbm::) or present in the traditional DBM implementation (*note
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.

* Menu:

* ndbm:: NDBM interface functions.
* dbm:: DBM interface functions.

File: gdbm.info, Node: ndbm, Next: dbm, Up: Compatibility

23.1 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' (*note
(open(2))open::), 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' (*note
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. *Note 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.

File: gdbm.info, Node: dbm, Prev: ndbm, Up: Compatibility

23.2 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.

-- 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.

-- 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.

File: gdbm.info, Node: gdbmtool, Next: gdbm_dump, Prev: Compatibility, Up: Top

24 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.

* Menu:

* invocation::
* shell::

File: gdbm.info, Node: invocation, Next: shell, Up: gdbmtool

24.1 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 (*note 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'
(*note open a file: (open(2))open.), 'creat' or a similar function.
The database will be opened using 'gdbm_fd_open' (*note
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 (*note 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. *Note timing: variables.

'-t'
'--trace'
Enable command tracing. This is equivalent to setting the 'trace'
variable. *Note trace: variables.

'-q'
'--quiet'
Don't print the usual welcome banner at startup. This is the same
as setting the variable 'quiet' in the startup file. *Note
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 (*note Numsync::).
This option sets the 'format' variable to 'numsync'. *Note format
variable::.

File: gdbm.info, Node: shell, Prev: invocation, Up: gdbmtool

24.2 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:

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

Table 24.1: Backslash escapes

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 (*note Command Line Editing: (readline)Command Line Editing.).

* Menu:

* variables:: shell variables.
* commands:: shell commands.
* definitions:: how to define structured data.
* startup files::

File: gdbm.info, Node: variables, Next: commands, Up: shell

24.2.1 Shell Variables
----------------------

A number of 'gdbmtool' parameters is kept in its internal variables. To
examine or modify variables, use the 'set' command (*note 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
(*note 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 (*note 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
(*note 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
(*note startup files::). *Note -q option::.

The following variables control how the database is opened:

-- gdbmtool variable: numeric blocksize
Sets the block size. *Note block_size: Open. Unset by default.

-- gdbmtool variable: numeric cachesize
Sets the cache size. *Note GDBM_SETCACHESIZE: Options.

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 (*note 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 *note
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. *Note
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'. *Note GDBM_NEWDB: Open.
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'. *Note GDBM_WRCREAT: Open.
reader
readonly
Open the database in read-only mode. Signal an error if it
does not exist.

This sets the 'GDBM_READER' flag (*note GDBM_READER: Open.).

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' (*note GDBM_NOLOCK: Open.).

-- 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' (*note GDBM_NOMMAP: Open.).

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

-- 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'. *Note
GDBM_SETCOALESCEBLKS: Options.

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. *Note GDBM_SETCENTFREE: Options.

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.

File: gdbm.info, Node: commands, Next: definitions, Prev: variables, Up: shell

24.2.2 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. *Note Numsync::.

-- command verb: export FILE-NAME [truncate] [binary|ascii]
Export the database to the flat file FILE-NAME. *Note 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 (*note 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). *Note
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 (*note 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.

*Note 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' (*note 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. *Note The OPEN variable: openvar,
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'.

*Note 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. *Note
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 (*note 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.

*Note 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. *Note 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 (*note Sync::).

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

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

File: gdbm.info, Node: definitions, Next: startup files, Prev: commands, Up: shell

24.2.3 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 (*note 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" }

File: gdbm.info, Node: startup files, Prev: definitions, Up: shell

24.2.4 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
}

File: gdbm.info, Node: gdbm_dump, Next: gdbm_load, Prev: gdbmtool, Up: Top

25 The 'gdbm_dump' utility
**************************

The 'gdbm_dump' utility creates a flat file dump of a 'GDBM' database
(*note 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 (*note ASCII:
Flat files.). 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.

File: gdbm.info, Node: gdbm_load, Next: Exit codes, Prev: gdbm_dump, Up: Top

26 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. *Note block_size: Open.

'-c NUM'
'--cache-size=NUM'
Sets cache size. *Note GDBM_SETCACHESIZE: Options.

'-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.

File: gdbm.info, Node: Exit codes, Next: Bugs, Prev: gdbm_load, Up: Top

27 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.

File: gdbm.info, Node: Bugs, Next: Resources, Prev: Exit codes, Up: Top

28 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' (*note
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>.

File: gdbm.info, Node: Resources, Next: GNU Free Documentation License, Prev: Bugs, Up: Top

29 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>.

File: gdbm.info, Node: GNU Free Documentation License, Next: Index, Prev: Resources, Up: Top

Appendix A GNU Free Documentation License
*****************************************

Version 1.3, 3 November 2008

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

0. PREAMBLE

The purpose of this License is to make a manual, textbook, or other
functional and useful document "free" in the sense of freedom: to
assure everyone the effective freedom to copy and redistribute it,
with or without modifying it, either commercially or
noncommercially. Secondarily, this License preserves for the
author and publisher a way to get credit for their work, while not
being considered responsible for modifications made by others.

This License is a kind of "copyleft", which means that derivative
works of the document must themselves be free in the same sense.
It complements the GNU General Public License, which is a copyleft
license designed for free software.

We have designed this License in order to use it for manuals for
free software, because free software needs free documentation: a
free program should come with manuals providing the same freedoms
that the software does. But this License is not limited to
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of subject matter or whether it is published as a printed book. We
recommend this License principally for works whose purpose is
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A "Modified Version" of the Document means any work containing the
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10. FUTURE REVISIONS OF THIS LICENSE

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the GNU Free Documentation License from time to time. Such new
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differ in detail to address new problems or concerns. See
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11. RELICENSING

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in part, as part of another Document.

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to November 1, 2008.

The operator of an MMC Site may republish an MMC contained in the
site under CC-BY-SA on the same site at any time before August 1,
2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents
====================================================

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

Copyright (C) YEAR YOUR NAME.
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 Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled ``GNU
Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:

with the Invariant Sections being LIST THEIR TITLES, with
the Front-Cover Texts being LIST, and with the Back-Cover Texts
being LIST.

If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
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If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
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their use in free software.

File: gdbm.info, Node: Index, Prev: GNU Free Documentation License, Up: Top

Index
*****

[index]
* Menu:

* !: commands. (line 204)
* (*errfun) of gdbm_recovery: Recovery. (line 52)
* --newdb, gdbmtool option: gdbmtool. (line 20)
* --read-only, gdbmtool option: gdbmtool. (line 16)
* -n, gdbmtool option: gdbmtool. (line 20)
* -r, gdbmtool option: gdbmtool. (line 16)
* .gdbmtoolrc: startup files. (line 6)
* ?: commands. (line 66)
* _GDBM_MAX_ERRNO: Variables. (line 31)
* _GDBM_MIN_ERRNO: Variables. (line 28)
* avail: commands. (line 6)
* backup_name of gdbm_recovery: Recovery. (line 99)
* blocksize: variables. (line 104)
* bucket: commands. (line 9)
* cache: commands. (line 12)
* cachesize: variables. (line 107)
* centfree: variables. (line 205)
* close: commands. (line 15)
* close-on-exec: Open. (line 49)
* closing database: Close. (line 6)
* coalesce: variables. (line 195)
* command line options, gdbmtool: invocation. (line 6)
* compatibility layer: Compatibility. (line 6)
* confirm: variables. (line 9)
* consistency, database: Database consistency.
(line 6)
* count: commands. (line 18)
* creating a database, gdbmtool: gdbmtool. (line 20)
* current: commands. (line 21)
* data of gdbm_recovery: Recovery. (line 62)
* database options: Options. (line 6)
* database reorganization: Reorganization. (line 6)
* database synchronization: Sync. (line 6)
* database, closing: Close. (line 6)
* database, opening or creating: Open. (line 6)
* datum: Intro. (line 12)
* DBM functions: dbm. (line 6)
* dbm.h: Compatibility. (line 10)
* dbmclose: dbm. (line 22)
* dbminit: dbm. (line 10)
* dbm_clearerr: ndbm. (line 95)
* dbm_close: ndbm. (line 25)
* dbm_delete: ndbm. (line 56)
* dbm_dirfno: ndbm. (line 98)
* dbm_error: ndbm. (line 90)
* dbm_fetch: ndbm. (line 29)
* dbm_firstkey: ndbm. (line 61)
* DBM_INSERT: ndbm. (line 49)
* dbm_nextkey: ndbm. (line 71)
* dbm_open: ndbm. (line 8)
* dbm_pagfno: ndbm. (line 106)
* dbm_rdonly: ndbm. (line 110)
* DBM_REPLACE: ndbm. (line 46)
* dbm_store: ndbm. (line 38)
* debug: commands. (line 24)
* default database, gdbmtool: gdbmtool. (line 9)
* delete: dbm. (line 41)
* delete <1>: commands. (line 30)
* deleting records: Delete. (line 6)
* deletion in iteration loops: Sequential. (line 60)
* delim1: variables. (line 15)
* delim2: variables. (line 21)
* dir: commands. (line 33)
* dir file: Compatibility. (line 21)
* downgrade: commands. (line 36)
* error code, most recent: Errors. (line 21)
* error codes: Error codes. (line 6)
* error strings: Errors. (line 6)
* errorexit: variables. (line 27)
* errorexit <1>: variables. (line 28)
* errormask: variables. (line 41)
* errormask <1>: variables. (line 42)
* exit code: Exit codes. (line 6)
* export: Flat files. (line 6)
* export <1>: commands. (line 40)
* failed_buckets of gdbm_recovery: Recovery. (line 96)
* failed_keys of gdbm_recovery: Recovery. (line 93)
* fd: variables. (line 125)
* fetch: dbm. (line 25)
* fetch <1>: commands. (line 51)
* fetching records: Fetch. (line 6)
* filemode: variables. (line 175)
* filename: variables. (line 116)
* first: commands. (line 54)
* firstkey: dbm. (line 47)
* Flat file format: Flat files. (line 6)
* format: variables. (line 136)
* gdbm.h: Intro. (line 45)
* gdbmtool: gdbmtool. (line 6)
* gdbm_avail_verify: Additional functions.
(line 6)
* GDBM_BACKUP_FAILED: Error codes. (line 180)
* GDBM_BAD_AVAIL: Error codes. (line 196)
* GDBM_BAD_BUCKET: Error codes. (line 187)
* GDBM_BAD_DIR_ENTRY: Error codes. (line 211)
* GDBM_BAD_FILE_OFFSET: Error codes. (line 129)
* GDBM_BAD_HASH_ENTRY: Error codes. (line 237)
* GDBM_BAD_HASH_TABLE: Error codes. (line 204)
* GDBM_BAD_HEADER: Error codes. (line 191)
* GDBM_BAD_MAGIC_NUMBER: Error codes. (line 51)
* GDBM_BAD_OPEN_FLAGS: Error codes. (line 133)
* GDBM_BLOCK_SIZE_ERROR: Error codes. (line 15)
* GDBM_BSEXACT: Open. (line 109)
* GDBM_BSEXACT <1>: Error codes. (line 16)
* GDBM_BUCKET_CACHE_CORRUPTED: Error codes. (line 233)
* gdbm_bucket_count: Count. (line 12)
* GDBM_BYTE_SWAPPED: Error codes. (line 125)
* GDBM_CACHESIZE: Options. (line 30)
* GDBM_CACHE_AUTO: Options. (line 32)
* GDBM_CANNOT_REPLACE: Error codes. (line 92)
* GDBM_CANT_BE_READER: Error codes. (line 59)
* GDBM_CANT_BE_WRITER: Error codes. (line 64)
* GDBM_CENTFREE: Options. (line 94)
* gdbm_check_syserr: Errors. (line 52)
* gdbm_clear_error: Errors. (line 67)
* GDBM_CLOERROR: Open. (line 162)
* GDBM_CLOEXEC: Open. (line 48)
* gdbm_close: Close. (line 10)
* GDBM_COALESCEBLKS: Options. (line 108)
* gdbm_convert: Database format. (line 18)
* gdbm_copy_meta: Open. (line 165)
* gdbm_count: Count. (line 6)
* gdbm_db_strerror: Errors. (line 59)
* gdbm_delete: Delete. (line 8)
* gdbm_delete and sequential access: Sequential. (line 60)
* GDBM_DIR_OVERFLOW: Error codes. (line 183)
* gdbm_dump: gdbm_dump. (line 6)
* gdbm_dump <1>: Flat files. (line 56)
* gdbm_dump_to_file: Flat files. (line 163)
* GDBM_EMPTY_DATABASE: Error codes. (line 55)
* gdbm_errlist[]: Variables. (line 16)
* gdbm_errno: Variables. (line 8)
* gdbm_errno <1>: Errors. (line 6)
* GDBM_ERR_FILE_MODE: Flat files. (line 151)
* GDBM_ERR_FILE_MODE <1>: Error codes. (line 168)
* GDBM_ERR_FILE_OWNER: Flat files. (line 148)
* GDBM_ERR_FILE_OWNER <1>: Error codes. (line 161)
* GDBM_ERR_REALPATH: Error codes. (line 252)
* GDBM_ERR_SNAPSHOT_CLONE: Error codes. (line 243)
* GDBM_ERR_USAGE: Error codes. (line 260)
* gdbm_exists: Fetch. (line 42)
* gdbm_export: Flat files. (line 186)
* gdbm_export_to_file: Flat files. (line 194)
* gdbm_failure_atomic: Crash Tolerance API. (line 6)
* GDBM_FAST: Open. (line 100)
* GDBM_FASTMODE: Options. (line 68)
* gdbm_fdesc: Locking. (line 13)
* gdbm_fd_open: Open. (line 148)
* gdbm_fetch: Fetch. (line 6)
* GDBM_FILE: Intro. (line 48)
* GDBM_FILE_CLOSE_ERROR: Error codes. (line 215)
* GDBM_FILE_EOF: Error codes. (line 144)
* GDBM_FILE_OPEN_ERROR: Error codes. (line 20)
* GDBM_FILE_READ_ERROR: Error codes. (line 43)
* GDBM_FILE_SEEK_ERROR: Error codes. (line 36)
* GDBM_FILE_STAT_ERROR: Error codes. (line 137)
* GDBM_FILE_SYNC_ERROR: Error codes. (line 220)
* GDBM_FILE_TRUNCATE_ERROR: Error codes. (line 226)
* GDBM_FILE_WRITE_ERROR: Error codes. (line 28)
* gdbm_firstkey: Sequential. (line 13)
* GDBM_GETBLOCKSIZE: Options. (line 163)
* GDBM_GETCACHESIZE: Options. (line 56)
* GDBM_GETCOALESCEBLKS: Options. (line 120)
* GDBM_GETDBNAME: Options. (line 143)
* GDBM_GETFLAGS: Options. (line 60)
* GDBM_GETMAXMAPSIZE: Options. (line 130)
* GDBM_GETMMAP: Options. (line 139)
* GDBM_GETSYNCMODE: Options. (line 90)
* GDBM_ILLEGAL_DATA: Error codes. (line 99)
* gdbm_import: Flat files. (line 198)
* gdbm_import_from_file: Flat files. (line 211)
* GDBM_INSERT: Store. (line 20)
* GDBM_ITEM_NOT_FOUND: Error codes. (line 83)
* gdbm_last_errno: Errors. (line 21)
* gdbm_last_syserr: Errors. (line 32)
* gdbm_latest_snapshot: Crash Tolerance API. (line 66)
* gdbm_load: gdbm_load. (line 6)
* gdbm_load <1>: Flat files. (line 85)
* gdbm_load_from_file: Flat files. (line 179)
* GDBM_MALFORMED_DATA: Error codes. (line 98)
* GDBM_MALLOC_ERROR: Error codes. (line 12)
* gdbm_needs_recovery: Errors. (line 79)
* GDBM_NEED_RECOVERY: Error codes. (line 175)
* GDBM_NEWDB: Open. (line 30)
* gdbm_nextkey: Sequential. (line 25)
* GDBM_NOLOCK: Open. (line 53)
* GDBM_NOLOCK <1>: Locking. (line 6)
* GDBM_NOMMAP: Open. (line 57)
* GDBM_NO_DBNAME: Error codes. (line 155)
* GDBM_NO_ERROR: Error codes. (line 9)
* GDBM_NUMSYNC: Database format. (line 25)
* GDBM_NUMSYNC <1>: Open. (line 116)
* gdbm_open: Open. (line 6)
* GDBM_OPT_ALREADY_SET: Error codes. (line 110)
* GDBM_OPT_BADVAL: Error codes. (line 116)
* GDBM_OPT_ILLEGAL: Error codes. (line 117)
* GDBM_PREREAD: Open. (line 61)
* GDBM_RCVR_BACKUP: Recovery. (line 100)
* GDBM_RCVR_ERRFUN: Recovery. (line 54)
* GDBM_RCVR_FORCE: Recovery. (line 106)
* GDBM_RCVR_MAX_FAILED_BUCKETS: Recovery. (line 72)
* GDBM_RCVR_MAX_FAILED_KEYS: Recovery. (line 66)
* GDBM_RCVR_MAX_FAILURES: Recovery. (line 79)
* GDBM_READER: Open. (line 30)
* GDBM_READER_CANT_DELETE: Error codes. (line 68)
* GDBM_READER_CANT_REORGANIZE: Error codes. (line 78)
* GDBM_READER_CANT_STORE: Error codes. (line 73)
* gdbm_recover: Recovery. (line 16)
* gdbm_reorganize: Reorganization. (line 8)
* GDBM_REORGANIZE_FAILED: Error codes. (line 88)
* GDBM_REPLACE: Store. (line 20)
* GDBM_SETCACHESIZE: Options. (line 30)
* GDBM_SETCENTFREE: Options. (line 94)
* GDBM_SETCOALESCEBLKS: Options. (line 108)
* GDBM_SETMAXMAPSIZE: Options. (line 124)
* GDBM_SETMMAP: Options. (line 134)
* gdbm_setopt: Options. (line 9)
* GDBM_SETSYNCMODE: Options. (line 77)
* GDBM_SNAPSHOT_BAD: Crash Tolerance API. (line 68)
* GDBM_SNAPSHOT_BAD <1>: Crash recovery. (line 38)
* GDBM_SNAPSHOT_ERR: Crash Tolerance API. (line 68)
* GDBM_SNAPSHOT_ERR <1>: Crash recovery. (line 43)
* GDBM_SNAPSHOT_OK: Crash Tolerance API. (line 68)
* GDBM_SNAPSHOT_SAME: Crash Tolerance API. (line 68)
* GDBM_SNAPSHOT_SAME <1>: Crash recovery. (line 78)
* GDBM_SNAPSHOT_SUSPICIOUS: Crash Tolerance API. (line 79)
* GDBM_SNAPSHOT_SUSPICIOUS <1>: Crash recovery. (line 83)
* gdbm_store: Store. (line 6)
* gdbm_strerror: Errors. (line 12)
* GDBM_SYNC: Open. (line 91)
* GDBM_SYNC <1>: Sync. (line 36)
* gdbm_sync: Sync. (line 13)
* GDBM_SYNCMODE: Options. (line 77)
* gdbm_syserr[]: Variables. (line 23)
* gdbm_version: Variables. (line 34)
* gdbm_version_cmp: Variables. (line 63)
* GDBM_VERSION_MAJOR: Variables. (line 49)
* GDBM_VERSION_MINOR: Variables. (line 52)
* gdbm_version_number[3]: Variables. (line 37)
* GDBM_VERSION_PATCH: Variables. (line 55)
* GDBM_WRCREAT: Open. (line 30)
* GDBM_WRITER: Open. (line 30)
* GDBM_XVERIFY: Open. (line 78)
* global error state: Errors. (line 6)
* GNU Readline: shell. (line 65)
* hash: commands. (line 59)
* header: commands. (line 62)
* help: commands. (line 65)
* history: commands. (line 77)
* history <1>: commands. (line 78)
* history <2>: commands. (line 79)
* import: Flat files. (line 6)
* import <1>: commands. (line 71)
* init file, gdbmtool: startup files. (line 6)
* interactive mode, gdbmtool: shell. (line 6)
* iterating over records: Sequential. (line 6)
* iteration and gdbm_delete: Sequential. (line 60)
* iteration loop: Sequential. (line 40)
* iteration loop, using NDBM: ndbm. (line 79)
* junk.gdbm: gdbmtool. (line 9)
* libgdbm_compat: Compatibility. (line 10)
* list: commands. (line 92)
* lock: variables. (line 179)
* locking: Locking. (line 6)
* logical consistency: Database consistency.
(line 17)
* looking up records: Fetch. (line 6)
* max_failed_buckets of gdbm_recovery: Recovery. (line 71)
* max_failed_keys of gdbm_recovery: Recovery. (line 65)
* max_failures of gdbm_recovery: Recovery. (line 78)
* mmap: variables. (line 185)
* most recent error code: Errors. (line 21)
* NDBM functions: ndbm. (line 6)
* ndbm.h: Compatibility. (line 10)
* next: commands. (line 95)
* nextkey: dbm. (line 57)
* number of records: Count. (line 6)
* open: commands. (line 109)
* open <1>: commands. (line 110)
* open <2>: variables. (line 149)
* opening the database: Open. (line 6)
* options, database: Options. (line 6)
* pag file: Compatibility. (line 21)
* pager: variables. (line 47)
* perror: commands. (line 151)
* ps1: variables. (line 55)
* ps2: variables. (line 73)
* quiet: variables. (line 97)
* quit: commands. (line 168)
* read-only mode, gdbmtool: gdbmtool. (line 16)
* readline: shell. (line 65)
* record, deleting: Delete. (line 6)
* record, fetching: Fetch. (line 6)
* records, iterating over: Sequential. (line 6)
* records, storing: Store. (line 6)
* records, testing existence: Fetch. (line 40)
* recover: commands. (line 171)
* recovered_buckets of gdbm_recovery: Recovery. (line 90)
* recovered_keys of gdbm_recovery: Recovery. (line 87)
* reorganization, database: Reorganization. (line 6)
* reorganize: commands. (line 200)
* sequential access: Sequential. (line 6)
* sequential access, using NDBM: ndbm. (line 79)
* set: variables. (line 214)
* shell: commands. (line 203)
* snapshot: commands. (line 218)
* source: commands. (line 225)
* startup file, gdbmtool: startup files. (line 6)
* status: commands. (line 228)
* store: dbm. (line 34)
* store <1>: commands. (line 241)
* storing records: Store. (line 6)
* structural consistency: Database consistency.
(line 11)
* sync: commands. (line 245)
* sync <1>: variables. (line 191)
* synchronization, database: Sync. (line 6)
* timing: variables. (line 80)
* trace: variables. (line 91)
* unset: variables. (line 257)
* upgrade: commands. (line 248)
* variables, gdbmtool: variables. (line 6)
* version: commands. (line 252)
* version number: Variables. (line 34)

Tag Table:
Node: Top969
Node: Copying3675
Node: Intro5462
Node: Open10910
Ref: gdbm_fd_open18080
Ref: Open-Footnote-118991
Node: Close19062
Node: Count19720
Node: Store20439
Node: Fetch23720
Node: Delete25524
Node: Sequential26277
Node: Reorganization29592
Node: Sync30600
Node: Database format32904
Node: Flat files34267
Ref: gdbm_load function38391
Node: Errors44092
Ref: gdbm_check_syserr46113
Node: Database consistency47957
Node: Recovery49608
Node: Crash Tolerance54611
Node: Filesystems supporting crash tolerance56763
Node: Enabling crash tolerance58425
Node: Synchronizing the Database60063
Node: Crash recovery61348
Ref: Crash recovery-Footnote-165813
Node: Manual crash recovery66122
Node: Performance Impact69063
Node: Availability69795
Node: Numsync70470
Node: Crash Tolerance API72939
Node: Options77070
Node: Locking83398
Node: Variables83990
Node: Additional functions86652
Node: Error codes87108
Node: Compatibility97672
Node: ndbm99470
Node: dbm104267
Node: gdbmtool106925
Node: invocation107895
Ref: -q option110150
Node: shell110836
Ref: backslash-interpretation112089
Node: variables113838
Ref: quiet117879
Ref: open parameters118098
Ref: format variable119488
Ref: openvar119942
Ref: filemode120804
Ref: set122340
Node: commands124296
Ref: gdbmtool export125400
Ref: gdbmtool import126621
Node: definitions133117
Node: startup files137927
Node: gdbm_dump138729
Node: gdbm_load139972
Node: Exit codes141852
Node: Bugs142401
Node: Resources143938
Node: GNU Free Documentation License144707
Node: Index169863

End Tag Table

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