path: root/doc/gdbm.3

.\" This file is part of GDBM.
.\" Copyright (C) 2011-2023 Free Software Foundation, Inc.
.\"
.\" GDBM is free software; you can redistribute it and/or modify
.\" it under the terms of the GNU General Public License as published by
.\" the Free Software Foundation; either version 3, or (at your option)
.\" any later version.
.\"
.\" GDBM is distributed in the hope that it will be useful,
.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
.\" GNU General Public License for more details.
.\"
.\" You should have received a copy of the GNU General Public License
.\" along with GDBM. If not, see <http://www.gnu.org/licenses/>. */
.TH GDBM 3 "October 18, 2021" "GDBM" "GDBM User Reference"
.SH NAME
GDBM \- The GNU database manager.  Includes \fBdbm\fR and \fBndbm\fR
compatibility.
.SH SYNOPSIS
.nf
.B #include <gdbm.h>
.sp
.BI "extern gdbm_error"  " gdbm_errno";
.br
.BI "extern char *" gdbm_version ;
.br
.BI "extern int "  gdbm_version[3] ;
.br
.BI "GDBM_FILE gdbm_open (const char *" name ", int " block_size ", "
.ti +21
.BI     "int " flags ", int " mode ", "
.ti +21
.BI "void (*" fatal_func ")(const char *))";
.br
.BI "int gdbm_close (GDBM_FILE " dbf ");"
.br
.BI "int gdbm_store (GDBM_FILE " dbf ", datum " key ", datum " content ", int " flag ");"
.br
.BI "datum gdbm_fetch (GDBM_FILE " dbf ", datum " key ");"
.br
.BI "int gdbm_delete (GDBM_FILE " dbf ", datum " key ");"
.br
.BI "datum gdbm_firstkey (GDBM_FILE " dbf ");"
.br
.BI "datum gdbm_nextkey (GDBM_FILE " dbf ", datum " key ");"
.br
.BI "int gdbm_recover (GDBM_FILE " dbf ", gdbm_recovery *" rcvr ", int" flags ");"
.br
.BI "int gdbm_reorganize (GDBM_FILE " dbf ");"
.br
.BI "int gdbm_sync (GDBM_FILE " dbf ");"
.br
.BI "int gdbm_exists (GDBM_FILE " dbf ", datum " key ");"
.br
.BI "const char *gdbm_strerror (gdbm_error " errno ");"
.br
.BI "int gdbm_setopt (GDBM_FILE " dbf ", int " option ", int " value ", int " size );
.br
.BI "int gdbm_fdesc (GDBM_FILE " dbf );
.br
.BI "int gdbm_count (GDBM_FILE " dbf ", gdbm_count_t *" pcount ");"
.br
.BI "int gdbm_bucket_count (GDBM_FILE " dbf ", size_t *" pcount ");"
.br
.BI "int gdbm_avail_verify (GDBM_FILE " dbf ");"
.PP
.SS Crash Tolerance (see below):
.PP
.BI "int gdbm_failure_atomic (GDBM_FILE " dbf ", const char *" even ", const char *" odd ");"
.br
.BI "int gdbm_latest_snapshot (const char *" even ", const char *" odd ", const char **" result ");"
.SH NOTICE
This manpage is a short description of the \fBGDBM\fR library.
For a detailed discussion, including examples and usage
recommendations, refer to the \fBGDBM Manual\fR available in
Texinfo format.  To access it, run:

  \fBinfo gdbm\fR

The documentation is also available online at

  \fBhttps://www.gnu.org/software/gdbm/manual\fR
  
Should any discrepancies occur between this manpage and the
\fBGDBM Manual\fR, the later shall be considered the authoritative
source.
.SH DESCRIPTION
\fBGNU dbm\fR is a library of routines that manages data files that contain
key/data pairs.  The access provided is that of storing, 
retrieval, and deletion by key and a non-sorted traversal of all
keys.  A process is allowed to use multiple data files at the
same time.
.SS Opening a database
A process that opens a gdbm file is designated as a "reader" or a
"writer".  Only one writer may open a gdbm file and many readers may
open the file.  Readers and writers can not open the gdbm file at the
same time. The procedure for opening a gdbm file is:
.PP
.BI "GDBM_FILE gdbm_open (const char *" name ", int " block_size ", "
.ti +21
.BI     "int " flags ", int " mode ", "
.ti +21
.BI "void (*" fatal_func ")(const char *))";
.PP
\fIName\fR is the name of the file (the complete name,
\fBgdbm\fR does not append any characters to this name).
.PP
\fIBlock_size\fR is the size of a single transfer from disk to
memory.  If the 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.  This adjustment is disabled if the
\fBGDBM_BSEXACT\fR \fIflag\fR is used.
.PP
The \fIflags\fR parameter is a bitmask, composed of the \fIaccess
mode\fR and one or more modifier flags.  The \fIaccess mode\fR bit
designates the process as a reader or writer and must be one of the following:
.TP
.B GDBM_READER
reader
.TP
.B GDBM_WRITER
writer
.TP
.B GDBM_WRCREAT
writer - if database does not exist create new one
.TP
.B GDBM_NEWDB
writer - create new database regardless if one exists
.PP
Additional flags (\fImodifiers\fR) can be combined with these values
by bitwise \fBOR\fR.  Not all of them are meaningful with all access
modes.
.PP
Flags that are valid for any value of access mode are:
.TP
.B GDBM_CLOEXEC
Set the \fIclose-on-exec\fR flag on the database file descriptor.
.TP
.B GDBM_NOLOCK
Prevents the library from performing any locking on the database file.
.TP
.B GDBM_NOMMAP
Instructs \fBgdbm_open\fR to disable the use of
.BR mmap (2).
.TP
.B GDBM_PREREAD
When mapping GDBM file to memory, read its contents immediately,
instead of when needed (\fIprefault reading\fR).  This can be
advantageous if you open a \fIread-only\fR database and are going to
do a lot of look-ups on it.  In this case entire database will be
read at once and searches will operate on an in-memory copy.  In
contrast, \fBGDBM_PREREAD\fR should not be used if you open a database
(even in read-only mode) only to retrieve a couple of keys.
.sp
Finally, never use \fBGDBM_PREREAD\fR when opening a database for
updates, especially for inserts: this will degrade performance.
.sp
This flag has no effect if \fBGDBM_NOMMAP\fR is given, or if the
operating system does not support prefault reading.  It is known to
work on Linux and FreeBSD kernels. 					     
.TP
.B 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.
.PP
The following additional flags are valid when the database is opened
for writing (\fBGDBM_WRITER\fR, \fBGDBM_WRCREAT\fR, or
\fBGDBM_NEWDB\fR):
.TP
.B GDBM_SYNC
Causes all database operations to be synchronized to the disk.
.sp
\fBNOTE\fR: this option entails severe performance degradation and
does not necessarily ensure that the resulting database state is
consistent, therefore we discourage its use.  For a discussion of how
to ensure database consistency with minimal performance overhead, see
.B CRASH TOLERANCE
below.
.TP
.B GDBM_FAST
A reverse of \fBGDBM_SYNC\fR: synchronize writes only when needed.
This is the default.  This flag is provided only for compatibility
with previous versions of GDBM.
.PP
The following flags can be used together with \fBGDBM_NEWDB\fR.  They
also take effect when used with \fBGDBM_WRCREAT\fR, if the requested
database file doesn't exist:
.TP
.B GDBM_BSEXACT
If this flag is set and the requested \fIblock_size\fR value cannot
be used, \fBgdbm_open\fR will refuse to create the database.  In this
case it will set the \fBgdbm_errno\fR variable to
\fBGDBM_BLOCK_SIZE_ERROR\fR and return \fBNULL\fR.
.sp
Without this flag, \fBgdbm_open\fR will silently adjust the
\fIblock_size\fR to a usable value, as described above.
.TP
.B GDBM_NUMSYNC
Create new database in \fIextended database format\fR, a format best
suited for effective crash recovery.  For a detailed discussion, see
the
.B CRASH RECOVERY
chapter below.
.PP
\fIMode\fR is the file mode (see
.BR chmod (2)
and
.BR open (2)).
It is used if the file is created.
.PP
\fIFatal_func\fR is a function to be called when \fBgdbm\fR if
it encounters a fatal error.  This parameter is deprecated and must
always be \fBNULL\fR.
.PP
The return value is the pointer needed by all other routines to
access that gdbm file.  If the return is the \fBNULL\fR pointer,
\fBgdbm_open\fR was not successful.  In this case, the reason of the
failure can be found in the \fIgdbm_errno\fR variable.  If the
following call returns \fItrue\fR (non-zero value):
.sp
.nf
.in +5
  gdbm_check_syserr(gdbm_open)
.in
.fi
.PP
the system \fIerrno\fR variable must be examined in order to obtain more
detail about the failure.
.PP
.BI "GDBM_FILE gdbm_fd_open (int " FD ", const char *" name ", int " block_size ", "
.ti +21
.BI     "int " flags ", int " mode ", "
.ti +21
.BI "void (*" fatal_func ")(const char *))";
.PP
This is an alternative entry point to \fBgdbm_open\fR.  \fIFD\fR is a
valid file descriptor obtained as a result of a call to
.BR open (2)
or
.BR creat (2).
The function opens (or creates) a \fGDBM\fR database this descriptor
refers to.  The descriptor is not \fBdup\fR'ed, and will be closed
when the returned \fBGDBM_FILE\fR is closed.  Use
.B dup (2)
if that is not desirable.
.PP
In case of error, the function behaves like \fBgdbm_open\fR and
\fBdoes not close\fR \fIFD\fR.  This can be altered by the following
value passed in \fIflags\fR:
.TP
.B GDBM_CLOERROR
Close \fIFD\fR before exiting on error.

The rest of arguments are the same as for \fBgdbm_open\fR.
.SS Calling convention
.PP
All \fBGDBM\fR functions take as their first parameter the
\fIdatabase handle\fR (\fBGDBM_FILE\fR), returned from \fBgdbm_open\fR
or \fBgdbm_fd_open\fR.
.PP
Any value stored in the \fBGDBM\fR database is described by
\fIdatum\fR, an aggregate type defined as:
.sp
.nf
.in +5
typedef struct
{
  char *dptr;
  int   dsize;
} datum;
.in
.fi
.PP
The \fIdptr\fR field points to the actual data.  Its type is
\fBchar *\fR for historical reasons.  Actually it should have been
typed
\fBvoid *\fR.  Programmers are free to store data of arbitrary
complexity, both scalar and aggregate, in this field.
.PP
The \fIdsize\fR field contains the number of bytes stored in
\fBdptr\fR.
.PP
The \fBdatum\fR type is used to describe both \fIkeys\fR and
\fIcontent\fR (values) in the database.  Values of this type can
be passed as arguments or returned from \fBGDBM\fR function calls.
.PP
\fBGDBM\fR functions that return \fBdatum\fR indicate failure by setting
its \fIdptr\fR field to \fBNULL\fR.
.PP
Functions returning integer value, indicate success by returning
0 and failure by returning a non-zero value (the only exception to this
rule is \fBgdbm_exists\fR, for which the return value is reversed).
.PP
If the returned value indicates failure, the \fBgdbm_errno\fR variable
contains an integer value indicating what went wrong.  A similar value
is associated with the \fIdbf\fR handle and can be accessed using the
\fBgdbm_last_errno\fR function.  Immediately after return from a
function, both values are exactly equal.  Subsequent \fBGDBM\fR calls
with another \fIdbf\fR as argument may alter the value of the global
\fBgdbm_errno\fR, but the value returned by \fBgdbm_last_errno\fR will
always indicate the most recent code of an error that occurred for
\fIthat particular database\fR.  Programmers are encouraged to use
such per-database error codes.
.PP
Sometimes the actual reason of the failure can be clarified by
examining the system \fBerrno\fR value.  To make sure its value is
meaningful for a given \fBGDBM\fR error code, use the
\fBgdbm_check_syserr\fR function.  The function takes error code as
argument and returns 1 if the \fBerrno\fR is meaningful for that
error, or 0 if it is irrelevant.
.PP
Similarly to \fBgdbm_errno\fR, the latest \fBerrno\fR value associated
with a particular database can be obtained using the
\fBgdbm_last_syserr\fR function.
.PP
The \fBgdbm_clear_error\fR clears the error indicator (both \fBGDBM\fR
and system error codes) associated with a database handle.
.PP
Some critical errors leave the database in a \fIstructurally
inconsistent state\fR.  If that happens, all subsequent \fBGDBM\fR calls
accessing that database will fail with the \fBGDBM\fR error code of
\fBGDBM_NEED_RECOVERY\fR (a special function \fBgdbm_needs_recovery\fR
is also provided, which returns true if the database handle given as
its argument is structurally inconsistent).  To return such
databases to consistent state, use the \fBgdbm_recover\fR function
(see below).
.PP
The \fBGDBM_NEED_RECOVERY\fR error cannot be cleared using
\fBgdbm_clear_error\fR.
.SS Error functions
This section describes the error handling functions outlined above.
.TP
.BI "gdbm_error gdbm_last_errno (GDBM_FILE " dbf ")"
Returns the error code of the most recent failure encountered when operating
on \fIdbf\fR.
.TP
.BI "int gdbm_last_syserr (GDBM_FILE " dbf ")"
Returns the value of the system \fBerrno\fR variable associated with
the most recent failure that occurred on \fIdbf\fR.
.sp
Notice that not all \fBgdbm_error\fR codes have a relevant system
error code.  Use the following function to determine if a given code has.
.TP
.BI "int gdbm_check_syserr (gdbm_error " err ")"
Returns \fB1\fR, if system \fBerrno\fR value should be checked to get more
info on the error described by GDBM code \fIerr\fR.
.TP
.BI "void gdbm_clear_error (GDBM_FILE " dbf ")"
Clears the error state for the database \fIdbf\fR.  This function is
called implicitly upon entry to any GDBM function that operates on
\fBGDBM_FILE\fR.
.sp
The \fBGDBM_NEED_RECOVERY\fR error cannot be cleared.
.TP
.BI "int gdbm_needs_recovery (GDBM_FILE " dbf ")"
Return \fB1\fR if the database file \fIdbf\fR is in inconsistent state
and needs recovery.
.TP
.BI "const char *gdbm_strerror (gdbm_error " err ")"
Returns a textual description of the error code \fIerr\fR.
.TP
.BI "const char *gdbm_db_strerror (GDBM_FILE " dbf ")"
Returns a textual description of the recent error in database
\fIdbf\fR.  This description includes the system \fBerrno\fR value, if
relevant.
.SS Closing the database
It is important that every database file opened is also closed.  This
is needed to update the reader/writer count on the file.  This is done by:
.TP
.BI "int gdbm_close (GDBM_FILE " dbf ");"
.SS Database lookups
.TP
.BI "int gdbm_exists (GDBM_FILE " dbf ", datum " key );
If the \fIkey\fR is found within the database, the return value 
will be \fItrue\fR (\fB1\fR).  If nothing appropriate is found, \fIfalse\fR
(\fB0\fR) is returned and \fBgdbm_errno\fR set to \fBGDBM_NO_ERROR\fR.
.sp
On error, returns 0 and sets \fBgdbm_errno\fR.
.TP
.BI "datum gdbm_fetch (GDBM_FILE " dbf ", datum " key );
\fIDbf\fR is the pointer returned by \fBgdbm_open\fR.  \fIKey\fR is
the key data.
.sp
If the \fIdptr\fR element of the return value is \fBNULL\fR, the
\fBgdbm_errno\fR variable should be examined.  The value of 
\fBGDBM_ITEM_NOT_FOUND\fR means no data was found for that \fIkey\fR.
Other value means an error occurred.
.sp
Otherwise the return value is a pointer to the found data.
The storage space for the \fIdptr\fR element is allocated using
\fBmalloc(3)\fR.  \fBGDBM\fR does not automatically free this data.
It is the programmer's responsibility to free this storage when it is
no longer needed.
.SS Iterating over the database
The following two routines allow for iterating over all items in the
database.  Such iteration is not key sequential, but it is
guaranteed to visit every key in the database exactly once.  (The
order has to do with the hash values.)
.TP
.BI "datum gdbm_firstkey (GDBM_FILE " dbf ");"
Returns first key in the database.
.TP
.BI "datum gdbm_nextkey (GDBM_FILE " dbf ", datum " key );
Given a \fIkey\fR, returns the database key that follows it.  End of
iteration is marked by returning \fIdatum\fR with \fIdptr\fR field set
to \fBNULL\fR and setting the \fBgdbm_errno\fR value to
\fBGDBM_ITEM_NOT_FOUND\fR.
.PP
After successful return from both functions, \fIdptr\fR points to data
allocated by
.BR malloc (3).
It is the caller responsibility to free the data when no longer
needed.
.PP
A typical iteration loop looks like:
.sp
.nf
.in +5
datum key, nextkey, content;
key = gdbm_firstkey (dbf);
while (key.dptr)
  {
    content = gdbm_fetch (dbf, key);
    /* Do something with key and/or content */
    nextkey = gdbm_nextkey (dbf, key);
    free (key.dptr);
    key = nextkey;
  }
.in
.fi
.PP
These functions are intended to visit the database in read-only
algorithms.  Avoid any database modifications within the iteration loop.
File \fIvisiting\fR is based on a hash table.  The \fBgdbm_delete\fR and,
in most cases, \fBgdbm_store\fR, functions rearrange the hash table to
make sure that any collisions in the table do not leave some item
`un-findable'.  Thus, a call to either of these functions changes
the order in which the keys are ordered.  Therefore, these functions
should not be used when iterating over all the keys in the database.
For example, the following loop is wrong: it is possible that some keys
will not be visited or will be visited twice if it is executed: 
.sp
.nf
.in +5
key = gdbm_firstkey (dbf);
while (key.dptr)
  {
    nextkey = gdbm_nextkey (dbf, key);
    if (some condition)
      gdbm_delete ( dbf, key );
    free (key.dptr);
    key = nextkey;
  }
.in
.fi
.SS Updating the database
.TP
.BI "int gdbm_store (GDBM_FILE " dbf ", datum " key ", datum " content ", int " flag );
\fIDbf\fR is the pointer returned by \fBgdbm_open\fR.  \fIKey\fR is the
key data.  \fIContent\fR is the data to be associated with the \fIkey\fR.
\fIFlag\fR can have one of the following values:
.RS 4
.TP
.B GDBM_INSERT
Insert only, generate an error if key exists;
.TP
.B GDBM_REPLACE
Replace contents if key exists.
.RE
.IP
The function returns 0 on success and \-1 on failure.  If the key
already exists in the database and the \fIflag\fR is
\fBGDBM_INSERT\fR, the function does not modify the database.  It sets
\fBgdbm_errno\fR to \fBGDBM_CANNOT_REPLACE\fR and returns 1.
.TP
.BI "int gdbm_delete (GDBM_FILE " dbf ", datum " key );
Looks up and deletes the given \fIkey\fR from the database \fIdbf\fR.
.sp
The return value is 0 if there was a successful delete or \-1 on
error.  In the latter case, the \fBgdbm_errno\fR value
\fBGDBM_ITEM_NOT_FOUND\fR indicates that the key is not present in the
database.  Other \fBgdbm_errno\fR values indicate failure.
.SS Recovering structural consistency
If a function leaves the database in structurally inconsistent state,
it can be recovered using the \fBgdbm_recover\fR function.
.TP
.BI "int gdbm_recover (GDBM_FILE " dbf ", gdbm_recovery * " rcvr ", int " flags ")"
Check the database file DBF and fix eventual inconsistencies.  The
\fIrcvr\fR argument can be used both to control the recovery and to
return additional statistics about the process, as indicated by
\fIflags\fR.  For a detailed discussion of these arguments and their
usage, see the \fBGDBM Manual\fR, chapter \fBRecovering structural
consistency\fR.
.sp
You can pass \fBNULL\fR as \fIrcvr\fR and \fB0\fR as \fIflags\fR, if
no such control is needed.
.sp
By default, this function first checks the database for
inconsistencies and attempts recovery only if some were found.  The
special \fIflags\fR bit \fBGDBM_RCVR_FORCE\fR instructs
\fBgdbm_recovery\fR to skip this check and to perform database
recovery unconditionally.
.SS Export and import
\fBGDBM\fR database files can be exported (dumped) to so called \fIflat
files\fR or imported (loaded) from them.  A flat file contains exactly
the same data as the original database, but it cannot be used for
searches or updates.  Its purpose is to keep the data from the
database for restoring it when the need arrives.  As such, flat files
are used for backup purposes, and for sending databases over the wire.
.PP
As of \fBGDBM\fR version 1.21, there are two flat file formats.  The
\fBASCII\fR 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.
This is the preferred format.
.PP
Another flat file format is the \fBbinary\fR format.  It stores only
key/data pairs and does not keep information about the database file
itself.  It cannot be used to copy databases between different
architectures.  The binary format was introduced in \fBGDBM\fR version
1.9.1 and is retained mainly for backward compatibility.
.PP
The following functions are used to export or import \fBGDBM\fR
database files.
.TP
.BI "int gdbm_dump (GDBM_FILE " dbf ", const char *" filename ","
.PD 0
.TP
.ti +15
.BI "int " format ", int " open_flag ", int " mode ")"
.PD
Dumps the database file \fIdbf\fR to the file \fIfilename\fR in
requested \fIformat\fR.  Allowed values for \fIformat\fR are:
.BR GDBM_DUMP_FMT_ASCII ,
to create an ASCII dump file, and
.BR GDBM_DUMP_FMT_BINARY ,
to create a binary dump.

The value of \fIopen_flag\fR tells \fBgdbm_dump\fR what to do if
\fIfilename\fR already exists.  If it is \fBGDBM_NEWDB\fR, the
function will create a new output file, replacing it if it already
exists.  If its value is \fBGDBM_WRCREAT\fR, the file will be created
if it does not exist.  If it does exist, \fBgdbm_dump\fR will return
error.

The file mode to use when creating the output file is defined by the
\fImode\fR parameter.  Its meaning is the same as for
.BR open (2).
.TP
.BI "int gdbm_load (GDBM_FILE *" pdbf ", const char *" filename ","
.PD 0
.TP
.ti +15
.BI "int " flag ", int " meta_mask ", unsigned long *" errline ")"
.PD
Loads data from the dump file \fIfilename\fR into the database pointed
to by \fIpdbf\fR.  If \fIpdbf\fR is \fBNULL\fR, the function will try
to create a new database.  On success, the new \fBGDBM_FILE\fR object
will be stored in the memory location pointed to by \fIpdbf\fR.  If
the dump file carries no information about the original database file
name, the function will set \fBgdbm_errno\fR to \fBGDBM_NO_DBNAME\fR
and return -1, indicating failure.

Otherwise, if \fIpdbf\fR points to an already open \fBGDBM_FILE\fR,
the function will load data from \fIfilename\fR into that database.

The \fIflag\fR parameter controls the function behavior if a key
from the dump file already exists in the database.  See the
\fBgdbm_store\fR function for its possible values.

The \fImeta_mask\fR parameter 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:
.RS +4
.TP
.B GDBM_META_MASK_MODE
Do not restore file mode.
.TP
.B GDBM_META_MASK_OWNER
Do not restore file owner.
.RE
.SS Other functions
.TP  
.BI "int gdbm_reorganize (GDBM_FILE " dbf ");"
If you have had a lot of deletions and would like to shrink the space
used by the \fBGDBM\fR file, this routine will reorganize the
database.
.TP
.BI "int gdbm_sync (GDBM_FILE " dbf ");"
Synchronizes the changes in \fIdbf\fR with its disk file.
.sp
It will not return until the disk file state is synchronized with the
in-memory state of the database.
.TP
.BI "int gdbm_setopt (GDBM_FILE " dbf ", int " option ", void *" value ", int " size );
Query or change some parameter of an already opened database.  The
\fIoption\fR argument defines what parameter to set or retrieve.  If
the \fIset\fR operation is requested, \fIvalue\fR points to the new
value.  Its actual data type depends on \fIoption\fR.  If the
\fIget\fR operation is requested, \fIvalue\fR points to a memory
region where to store the return value.  In both cases, \fIsize\fR
contains the actual size of the memory pointed to by \fIvalue\fR.
.sp
Possible values of \fIoption\fR are:
.RS +4
.TP
.B GDBM_SETCACHESIZE
.TQ
.B GDBM_CACHESIZE
Set the size of the internal bucket cache.  The \fIvalue\fR should
point to a \fBsize_t\fR holding the desired cache size, or the
constant \fBGDBM_CACHE_AUTO\fR, to select 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.
.TP
.B GDBM_GETCACHESIZE
Return the size of the internal bucket cache.  The \fIvalue\fR should
point to a \fBsize_t\fR variable, where the size will be stored.
.TP
.B GDBM_GETFLAGS
Return the flags describing current state of the database.  The
\fIvalue\fR should point to an \fBint\fR variable where to store the
flags.  On success, its value will be similar to the flags used when
opening the database, except that it will reflect the current state
(which may have been altered by another calls to \fBgdbm_setopt\fR).
.TP
.B GDBM_FASTMODE
Enable or disable the \fIfast writes mode\fR, similar to the
\fBGDBM_FAST\fR option to \fBgdbm_open\fR.

This option is retained for compatibility with previous versions of
\fBGDBM\fR.
.TP
.B GDBM_SETSYNCMODE
.TQ
.B GDBM_SYNCMODE
Turn on or off immediate disk synchronization after updates.  The
\fIvalue\fR should point to an integer: 1 to turn synchronization on,
and 0 to turn it off.

\fBNOTE\fR: setting this option entails severe performance degradation
and does not necessarily ensure that the resulting database state is
consistent, therefore we discourage its use.  For a discussion of how
to ensure database consistency with minimal performance overhead, see
.B CRASH TOLERANCE
below.
.TP
.B GDBM_GETSYNCMODE
Return the current synchronization status.  The \fIvalue\fR should
point to an \fBint\fR where the status will be stored.
.TP
.B GDBM_SETCENTFREE
.TQ
.B GDBM_CENTFREE
Enable or disable central free block pool.  The default is off,
which is how previous versions of \fBGDBM\fR handled free blocks.  If
set, this option causes all subsequent free blocks to be placed in the
\fIglobal\fR pool, allowing (in theory) more file space to be reused
more quickly.  The \fIvalue\fR should point to an integer: \fBTRUE\fR to
turn central block pool on, and \fBFALSE\fR to turn it off.

The \fBGDBM_CENTFREE\fR alias is provided for compatibility with
earlier versions.
.TP
.B GDBM_SETCOALESCEBLKS
.TQ
.B GDBM_COALESCEBLKS
Set free block merging to either on or off.  The default is off, which
is how previous versions of \fBGDBM\fR 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 \fBGDBM_CENTFREE\fR.  The \fIvalue\fR should point 
to an integer: \fBTRUE\fR to turn free block merging on, and \fBFALSE\fR to
turn it off.
.TP
.B GDBM_GETCOALESCEBLKS
Return the current status of free block merging.  The \fIvalue\fR should
point to an \fBint\fR where the status will be stored.
.TP
.B GDBM_SETMAXMAPSIZE
Sets maximum size of a memory mapped region.  The \fIvalue\fR should
point to a value of type \fBsize_t\fR, \fBunsigned long\fR or
\fBunsigned\fR.  The actual value is rounded to the nearest page
boundary (the page size is obtained from \fBsysconf(_SC_PAGESIZE)\fR).
.TP
.B GDBM_GETMAXMAPSIZE
Return the maximum size of a memory mapped region.  The \fIvalue\fR should
point to a value of type \fBsize_t\fR where to return the data.
.TP
.B GDBM_SETMMAP
Enable or disable memory mapping mode.  The \fIvalue\fR should point
to an integer: \fBTRUE\fR to enable memory mapping or \fBFALSE\fR to
disable it.
.TP
.B GDBM_GETMMAP
Check whether memory mapping is enabled.  The \fIvalue\fR should point
to an integer where to return the status.
.TP
.B GDBM_GETDBNAME
Return the name of the database disk file.  The \fIvalue\fR should
point to a variable of type \fBchar**\fR.  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.
.TP
.B GDBM_GETBLOCKSIZE
Return the block size in bytes.  The \fIvalue\fR should point to \fBint\fR.
.RE
.TP
.BI "int gdbm_fdesc (GDBM_FILE " dbf );
Returns the file descriptor of the database \fIdbf\fR.
.SH CRASH TOLERANCE
By default \fBGNU dbm\fR does not protect the integrity of its
databases from corruption or destruction due to failures such as
power outages, operating system kernel panics, or application process
crashes.  Such failures could damage or destroy the underlying
database.
.PP
Starting with release 1.21 \fBGNU dbm\fR includes a mechanism that,
if used correctly, enables post-crash recovery to a consistent state
of the underlying database.  This mechanism requires OS and
filesystem support and must be requested when \fBgdbm\fR is compiled.
The crash-tolerance mechanism is a "pure opt-in" feature, in the
sense that it has no effects whatsoever except on those applications
that explicitly request it.  For details, see the chapter
.B "Crash Tolerance"
in the
.BR "GDBM manual" .
.SH GLOBAL VARIABLES
.TP
.B gdbm_error gdbm_errno
This variable contains code of the most recent error that occurred.
Note, that it is not C variable in the proper sense: you can use its
value, assign any value to it, but taking its address will result in
syntax error.  It is a per-thread memory location.
.TP
.B const char *gdbm_version
A string containing the library version number and build date.
.TP
.B int const gdbm_version_number[3]
This variable contains library version numbers: major, minor, and
patchlevel.
.SH VERSIONING
The version information is kept in two places.  The version of the
library is kept in the \fBgdbm_version_number\fR variable, described
above.  Additionally, the header file \fBgdbm.h\fR defines the
following macros:
.TP
.B GDBM_VERSION_MAJOR
Major version number.
.TP
.B GDBM_VERSION_MINOR
Minor version number.
.TP
.B GDBM_VERSION_PATCH
Patchlevel number.  \fB0\fR means no patchlevel.
.PP
You can use this to compare whether your header file corresponds to
the library the program is linked with.
.PP
The following function can be used to compare two version numbers:
.TP
.BI "int gdbm_version_cmp (int const " a "[3], int const " b "[3])"
Compare two version numbers formatted as \fBgdbm_version_number\fR.
Return negative number if \fBa\fR is older than \fBb\fR, positive
number if \fBa\fR is newer than \fBb\fR, and 0 if they are equal.
.SH ERROR CODES
.TP
.B GDBM_NO_ERROR
No error occurred.
.TP
.B GDBM_MALLOC_ERROR
Memory allocation failed.
.TP
.B GDBM_BLOCK_SIZE_ERROR
This error is set by the \fBgdbm_open\fR function, if
the value of its \fIblock_size\fR argument is incorrect and the
\fBGDBM_BSEXACT\fR flag is set.
.TP
.B GDBM_FILE_OPEN_ERROR
The library was not able to open a disk file.  This can be set by
\fBgdbm_open\fR, \fBgdbm_fd_open\fR, \fBgdbm_dump\fR and
\fBgdbm_load\fR functions.
.sp
Inspect the value of the system \fBerrno\fR variable to get more
detailed diagnostics.
.TP
.B GDBM_FILE_WRITE_ERROR
Writing to a disk file failed.  This can be set by
\fBgdbm_open\fR, \fBgdbm_fd_open\fR, \fBgdbm_dump\fR and
\fBgdbm_load\fR functions.
.sp
Inspect the value of the system \fBerrno\fR variable to get more
detailed diagnostics.
.TP
.B GDBM_FILE_SEEK_ERROR
Positioning in a disk file failed.  This can be set by
\fBgdbm_open\fR function.
.sp
Inspect the value of the system \fBerrno\fR variable to get a more
detailed diagnostics.
.TP
.B GDBM_FILE_READ_ERROR
Reading from a disk file failed.  This can be set by
\fBgdbm_open\fR, \fBgdbm_dump\fR and \fBgdbm_load\fR functions.
.sp
Inspect the value of the system \fBerrno\fR variable to get a more
detailed diagnostics.
.TP
.B GDBM_BAD_MAGIC_NUMBER
The file given as argument to \fBgdbm_open\fR function is not a valid
\fBgdbm\fR file: it has a wrong magic number.
.TP
.B GDBM_EMPTY_DATABASE
The file given as argument to \fBgdbm_open\fR function is not a valid
\fBgdbm\fR file: it has zero length.  This error is returned unless
the \fIflags\fR argument has \fBGDBM_NEWDB\fR bit set.
.TP
.B GDBM_CANT_BE_READER
This error code is set by the \fBgdbm_open\fR function if it is not
able to lock file when called in \fBGDBM_READER\fR mode.
.TP
.B GDBM_CANT_BE_WRITER
This error code is set by the \fBgdbm_open\fR function if it is not
able to lock file when called in writer mode.
.TP
.B GDBM_READER_CANT_DELETE
Set by the \fBgdbm_delete\fR, if it attempted to operate on a database
that is open in read-only mode.
.TP
.B GDBM_READER_CANT_STORE
Set by the \fBgdbm_store\fR if it attempted to operate on a database
that is open in read-only mode.
.TP
.B GDBM_READER_CANT_REORGANIZE
Set by the \fBgdbm_reorganize\fR if it attempted to operate on a
database that is open in read-only mode.
.TP
.B GDBM_ITEM_NOT_FOUND
Requested item was not found.  This error is set by \fBgdbm_delete\fR
and \fBgdbm_fetch\fR when the requested key value is not found in the
database.
.TP
.B GDBM_REORGANIZE_FAILED
The \fBgdbm_reorganize\fR function is not able to create a temporary
database.
.TP
.B GDBM_CANNOT_REPLACE
Cannot replace existing item.  This error is set by the
\fBgdbm_store\fR if the requested key value is found in the 
database and the \fIflag\fR parameter is not \fBGDBM_REPLACE\fR.
.TP
.B GDBM_MALFORMED_DATA
Input data was malformed in some way.  When returned by
\fBgdbm_load\fR, this means that the input file was not a valid
\fBgdbm\fR dump file.  When returned by \fBgdbm_store\fR, this means
that either \fIkey\fR or \fIcontent\fR parameter had its \fBdptr\fR
field set to \fBNULL\fR. 
.sp
The \fBGDBM_ILLEGAL_DATA\fR is an alias for this error code,
maintained for backward compatibility.
.TP
.B GDBM_OPT_ALREADY_SET
Requested option can be set only once and was already set.  As of
version 1.21, this error code is no longer used.  In prior
versions it could have been returned by the \fBgdbm_setopt\fR
function when setting the \fBGDBM_CACHESIZE\fR value.
.TP
.B GDBM_OPT_BADVAL
The \fIoption\fR argument is not valid or the \fIvalue\fR argument
points to an invalid value in a call to \fBgdbm_setopt\fR function.
.sp
\fBGDBM_OPT_ILLEGAL\fR is an alias for this error code, maintained
for backward compatibility.  Modern applications should not use it.
.TP
.B GDBM_BYTE_SWAPPED
The \fBgdbm_open\fR function attempts to open a database which is
created on a machine with different byte ordering.
.TP
.B GDBM_BAD_FILE_OFFSET
The \fBgdbm_open\fR function sets this error code if the file it tries
to open has a wrong magic number.
.TP
.B GDBM_BAD_OPEN_FLAGS
Set by the \fBgdbm_dump\fR function if supplied an invalid
\fIflags\fR argument.
.TP
.B GDBM_FILE_STAT_ERROR
Getting information about a disk file failed.  The system \fBerrno\fR
will give more details about the error.
.sp
This error can be set by the following functions: \fBgdbm_open\fR,
\fBgdbm_reorganize\fR.
.TP
.B 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.
.sp
This error can be set by any GDBM function that does I/O.  Some of
these functions are: \fBgdbm_delete\fR, \fBgdbm_exists\fR,
\fBgdbm_fetch\fR, \fBgdbm_export\fR, \fBgdbm_import\fR,
\fBgdbm_reorganize\fR, \fBgdbm_firstkey\fR, \fBgdbm_nextkey\fR,
\fBgdbm_store\fR.
.TP
.B GDBM_NO_DBNAME
Output database name is not specified.  This error code is set by
\fBgdbm_load\fR if the first argument points to \fBNULL\fR and the
input file does not specify the database name.
.TP
.B GDBM_ERR_FILE_OWNER
This error code is set by \fBgdbm_load\fR if it is unable to restore
the 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 \fBerrno\fR variable to get a more
detailed diagnostics.
.TP
.B GDBM_ERR_FILE_MODE
This error code is set by \fBgdbm_load\fR 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 \fBerrno\fR variable to get a more
detailed diagnostics.
.TP
.B GDBM_NEED_RECOVERY
Database is in inconsistent state and needs recovery.  Call
\fBgdbm_recover\fR if you get this error.
.TP
.B GDBM_BACKUP_FAILED
The GDBM engine is unable to create backup copy of the file.
.TP
.B 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.
.TP
.B GDBM_BAD_BUCKET
Invalid index bucket is encountered in the database.  Database
recovery is needed.
.TP
.B GDBM_BAD_HEADER
This error is set by \fBgdbm_open\fR and \fBgdbm_fd_open\fR, if the
first block read from the database file does not contain a valid GDBM
header.
.TP
.B GDBM_BAD_AVAIL
The available space stack is invalid.  This error can be set by
\fBgdbm_open\fR and \fBgdbm_fd_open\fR, if the extended database
verification was requested (\fBGDBM_XVERIFY\fR).  It is also set
by the \fBgdbm_avail_verify\fR function.
.sp
The database needs recovery.
.TP
.B GDBM_BAD_HASH_TABLE
Hash table in a bucket is invalid.  This error can be set by the
following functions: \fBgdbm_delete\fR, \fBgdbm_exists\fR,
\fBgdbm_fetch\fR, \fBgdbm_firstkey\fR, \fBgdbm_nextkey\fR, and
\fBgdbm_store\fR.
.sp
The database needs recovery.
.TP
.B GDBM_BAD_DIR_ENTRY
Bad directory entry found in the bucket.  The database recovery is
needed.
.TP
.B GDBM_FILE_CLOSE_ERROR
The \fBgdbm_close\fR function was unable to close the database file
descriptor.  The system \fBerrno\fR variable contains the
corresponding error code.
.TP
.B GDBM_FILE_SYNC_ERROR
Cached content couldn't be synchronized to disk.  Examine the
\fBerrno\fR variable to get more info,
.sp
Database recovery is needed.
.TP
.B GDBM_FILE_TRUNCATE_ERROR
File cannot be truncated.  Examine the \fBerrno\fR variable to get
more info.
.sp
This error is set by \fBgdbm_open\fR and \fBgdbm_fd_open\fR when
called with the \fBGDBM_NEWDB\fR flag.
.TP
.B GDBM_BUCKET_CACHE_CORRUPTED
The bucket cache structure is corrupted.  Database recovery is needed.
.TP
.B GDBM_BAD_HASH_ENTRY
This error is set during sequential access (@pxref{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.
.TP
.B GDBM_ERR_SNAPSHOT_CLONE
Set by the \fBgdbm_failure_atomic\fR function if it was unable to
clone the database file into a snapshot.  Inspect the system
\fBerrno\fR variable for the underlying cause of the error.  If
\fBerrno\fR is \fBEINVAL\fR or \fBENOSYS\fR, crash tolerance
settings will be removed from the database.
.TP
.B GDBM_ERR_REALPATH
Set by the \fBgdbm_failure_atomic\fR function if the call to
\fBrealpath\fR function failed.  \fBrealpath\fR is used to
determine actual path names of the snapshot files.  Examine the system
\fBerrno\fR variable for details.
.TP
.B GDBM_ERR_USAGE
Function usage error.  That includes invalid argument values, and the
like.
.SH DBM COMPATIBILITY ROUTINES
\fBGDBM\fR includes a compatibility library \fBlibgdbm_compat\fR, for
use with programs that expect traditional UNIX \fBdbm\fR or
\fBndbm\fR interfaces, such as, e.g. \fBSendmail\fR.  The library is
optional and thus may be absent in some binary distributions.
.PP
As the detailed discussion of the compatibility API is beyond the scope
of this document, below we provide only a short reference.  For
details, see the \fBGDBM Manual\fR, chapter \fBCompatibility with
standard dbm and ndbm\fR.
.SS DBM compatibility routines
In \fBdbm\fR compatibility mode only one file may be opened at a time.
All users are assumed to be writers.  If the database file is read
only, it will fail as a writer, but will be opened as a reader.  All
returned pointers in datum structures point to data that the
compatibility library \fBwill free\fR.  They should be
treated as static pointers (as standard UNIX \fBdbm\fR does).
.PP
The following interfaces are provided:
.PP
.B #include <dbm.h>
.sp
.BI "int dbminit (const char *" name ");"
.br
.BI "int store (datum " key ", datum " content );
.br
.BI "datum fetch (datum " key );
.br
.BI "int delete (datum " key );
.br
.BI "datum firstkey (void);"
.br
.BI "datum nextkey (datum " key );
.br
.BI "int dbmclose (void);"
.SS NDBM Compatibility routines:
In this mode, multiple databases can be opened.  Each database is
identified by a handle of type \fBDBM *\fR.  As in the original
\fBNDBM\fR, all returned pointers in datum structures point to data
that will be freed by the compatibility library.  They should be
treated as static pointers.
.PP
The following interfaces are provided:
.PP
.B #include <ndbm.h>
.sp
.BI "DBM *dbm_open (const char *" name ", int " flags ", int " mode );
.br
.BI "void dbm_close (DBM *" file );
.br
.BI "datum dbm_fetch (DBM *" file ", datum " key );
.br
.BI "int dbm_store (DBM *" file ", datum " key ", datum " content ", int " flags );
.br
.BI "int dbm_delete (DBM *" file ", datum " key );
.br
.BI "datum dbm_firstkey (DBM *" file );
.br
.BI "datum dbm_nextkey (DBM *" file ", datum " key );
.br
.BI "int dbm_error (DBM *" file );
.br
.BI "int dbm_clearerr (DBM *" file );
.br
.BI "int dbm_pagfno (DBM *" file );
.br
.BI "int dbm_dirfno (DBM *" file );
.br
.BI "int dbm_rdonly (DBM *" file );
.SH LINKING
This library is accessed by specifying \fI\-lgdbm\fR as the last
parameter to the compile line, e.g.:
.sp
.nf
.in +5
gcc \-o prog prog.c \-lgdbm
.in
.fi
.PP
If you wish to use the \fBdbm\fR or \fBndbm\fR compatibility routines,
you must link in the \fIgdbm_compat\fR library as well.  For example:
.sp
.nf
.in +5
gcc \-o prog proc.c \-lgdbm \-lgdbm_compat
.in
.fi
.\" .SH BUGS

.SH "BUG REPORTS"
Send bug reports to <bug\-gdbm@gnu.org>.
.SH "SEE ALSO"
.BR gdbm_dump (1),
.BR gdbm_load (1),
.BR gdbmtool (1).
.SH AUTHORS
by Philip A. Nelson, Jason Downs and Sergey Poznyakoff;
crash tolerance by Terence Kelly.
.SH COPYRIGHT
Copyright \(co 1990 - 2021 Free Software Foundation, Inc.

GDBM is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 1, or (at your option)
any later version.

GDBM is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with GDBM.  If not, see <http://gnu.org/licenses/gpl.html>
.SH CONTACTS
You may contact the original author by:
.br
   e-mail:  phil@cs.wwu.edu
.br
  us-mail:  Philip A. Nelson
.br
Computer Science Department
.br
Western Washington University
.br
Bellingham, WA 98226

You may contact the current maintainers by:
.br
   e-mail:  downsj@downsj.com
.br
and
   e-mail:  gray@gnu.org

For questions and feedback regarding crash tolerance, you may contact
Terence Kelly at:
.br
   e-mail:  tpkelly @ { acm.org, cs.princeton.edu, eecs.umich.edu }

.\" Local variables:
.\" eval: (add-hook 'write-file-hooks 'time-stamp)
.\" time-stamp-start: ".TH GDBM 3 \""
.\" time-stamp-format: "%:B %:d, %:y"
.\" time-stamp-end: "\""
.\" time-stamp-line-limit: 20
.\" end: