gdbm(3) GDBM User Reference gdbm(3)
NAME
GDBM - The GNU database manager. Includes dbm and ndbm compatibility.
SYNOPSIS
#include <gdbm.h>
extern gdbm_error gdbm_errno;
extern char *gdbm_version;
extern int gdbm_version[3];
GDBM_FILE gdbm_open (const char *name, int block_size,
int flags, int mode,
void (*fatal_func)(const char *));
GDBM_FILE gdbm_fd_open (int FD, const char *name, int block_size,
int flags, int mode,
void (*fatal_func)(const char *));
GDBM_FILE gdbm_open_ext (const char *name, int flags,
struct gdbm_open_spec *spec);
void gdbm_open_spec_init (struct gdbm_open_spec *spec);
int gdbm_close (GDBM_FILE dbf);
int gdbm_store (GDBM_FILE dbf, datum key, datum content, int flag);
datum gdbm_fetch (GDBM_FILE dbf, datum key);
int gdbm_delete (GDBM_FILE dbf, datum key);
datum gdbm_firstkey (GDBM_FILE dbf);
datum gdbm_nextkey (GDBM_FILE dbf, datum key);
int gdbm_recover (GDBM_FILE dbf, gdbm_recovery *rcvr, intflags);
int gdbm_reorganize (GDBM_FILE dbf);
int gdbm_sync (GDBM_FILE dbf);
int gdbm_exists (GDBM_FILE dbf, datum key);
const char *gdbm_strerror (gdbm_error errno);
int gdbm_setopt (GDBM_FILE dbf, int option, int value, int size);
int gdbm_fdesc (GDBM_FILE dbf);
int gdbm_count (GDBM_FILE dbf, gdbm_count_t *pcount);
int gdbm_bucket_count (GDBM_FILE dbf, size_t *pcount);
int gdbm_avail_verify (GDBM_FILE dbf);
Crash Tolerance (see below):
int gdbm_failure_atomic (GDBM_FILE dbf, const char *even, const char
*odd);
int gdbm_latest_snapshot (const char *even, const char *odd, const char
**result);
NOTICE
This manpage is a short description of the GDBM library. For a
detailed discussion, including examples and usage recommendations,
refer to the GDBM Manual available in Texinfo format. To access it,
run:
info gdbm
The documentation is also available online at
https://www.gnu.org/software/gdbm/manual
Should any discrepancies occur between this manpage and the GDBM
Manual, the later shall be considered the authoritative source.
DESCRIPTION
GNU dbm 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.
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:
GDBM_FILE gdbm_open (const char *name, int block_size,
int flags, int mode,
void (*fatal_func)(const char *));
Name is the name of the file (the complete name, gdbm does not
append any characters to this name).
Block_size 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 GDBM_BSEXACT flag is used.
The flags parameter is a bitmask, composed of the access mode
and one or more modifier flags. The access mode bit designates
the process as a reader or writer and must be one of the
following:
GDBM_READER
reader
GDBM_WRITER
writer
GDBM_WRCREAT
writer - if database does not exist create new one
GDBM_NEWDB
writer - create new database regardless if one exists
Additional flags (modifiers) can be combined with these values
by bitwise OR. Not all of them are meaningful with all access
modes.
Flags that are valid for any value of access mode are:
GDBM_CLOEXEC
Set the close-on-exec flag on the database file
descriptor.
GDBM_NOLOCK
Prevents the library from performing any locking on the
database file.
GDBM_NOMMAP
Instructs gdbm_open to disable the use of mmap(2).
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 read at once and searches
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 retrieve a couple of
keys.
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_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.
The following additional flags are valid when the database is
opened for writing (GDBM_WRITER, GDBM_WRCREAT, or GDBM_NEWDB):
GDBM_SYNC
Causes all database operations to be synchronized to the
disk.
NOTE: 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 CRASH
TOLERANCE below.
GDBM_FAST
A reverse of GDBM_SYNC: synchronize writes only when
needed. This is the default. This flag is provided only
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_BSEXACT
If this flag is set and the requested block_size value
cannot be used, gdbm_open will refuse to create the
database. In this case it will set the gdbm_errno
variable to GDBM_BLOCK_SIZE_ERROR and return NULL.
Without this flag, gdbm_open will silently adjust the
block_size to a usable value, as described above.
GDBM_NUMSYNC
Create new database in extended database format, a format
best suited for effective crash recovery. For a detailed
discussion, see the CRASH RECOVERY chapter below.
Mode is the file mode (see chmod(2) and open(2)). It is used if
the file is created.
Fatal_func is a function to be called when gdbm if it encounters
a fatal error. This parameter is deprecated and must always be
NULL.
The return value is the pointer needed by all other routines to
access that gdbm file. If the return is the NULL pointer,
gdbm_open was not successful. In this case, the reason of the
failure can be found in the gdbm_errno variable. If the
following call returns true (non-zero value):
gdbm_check_syserr(gdbm_open)
the system errno variable must be examined in order to obtain
more detail about the failure.
GDBM_FILE gdbm_fd_open (int FD, const char *name, int block_size,
int flags, int mode,
void (*fatal_func)(const char *));
This is an alternative entry point to gdbm_open. FD is a valid
file descriptor obtained as a result of a call to open(2) or
creat(2). The function opens (or creates) a GDBM database this
descriptor refers to. 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 flags:
GDBM_CLOERROR
Close FD before exiting on error.
The rest of arguments are the same as for gdbm_open.
GDBM_FILE gdbm_open_ext (const char *name, int flags,
struct gdbm_open_spec *spec);
This is an extended interface for opening and creating database
files. The meaning of name and flags is the same as described
above. Third argument, spec, is a pointer to a structure that
supplies additional information about how to open the database.
It can be NULL, in which case the call is equivalent to
gdbm_open (name, 0, flags, 0600, NULL)
The struct gdbm_open_ext type has at least the following fields:
int fd Unless -1, this is the file descriptor of an already
opened database file. The function will then act as
improved gdbm_fd_open.
int mode
File mode to use if the file needs be created.
int block_size
Block size to be used when creating new database. Zero
stands for default value. See the description of the
block_size argument in gdbm_open, for details.
int lock_wait
This field, together with the two fields described below,
controls how to handle failed attempts to lock the file.
Its possible values are:
GDBM_LOCKWAIT_NONE
Return NULL immediately. This is the default.
GDBM_LOCKWAIT_RETRY
Sleep for the amount of time defined by the
lock_interval field, and retry the attempt. In
case of failure, repeat until total time spent
exceeds lock_timeout.
GDBM_LOCKWAIT_SIGNAL
Wait until locking succeeds or the timeout defined
by lock_timeout expires, whichever happens first.
Signal delivery is used to catch timeout
expiration, therefore this method is not advised
for use in multi-threaded programs.
struct timespec lock_timeout
Overall timeout for file locking operation.
struct timespec lock_interval
Time interval between two successive locking attempts.
Used if lock_wait is set to GDBM_LOCKWAIT_RETRY.
A struct gdbm_open_spec variable can be initialized to default
values by assigning it the GDBM_OPEN_SPEC_INITIALIZER value, or
using the gdbm_open_spec_init function. The latter is defined
as:
void gdbm_open_spec_init (struct gdbm_open_spec *spec);
Calling convention
All GDBM functions take as their first parameter the database handle
(GDBM_FILE), returned from gdbm_open, gdbm_fd_open, or gdbm_open_ext.
Any value stored in the GDBM database is described by datum, an
aggregate type defined as:
typedef struct
{
char *dptr;
int dsize;
} datum;
The dptr field points to the actual data. Its type is char * for
historical reasons. Actually it should have been typed void *.
Programmers are free to store data of arbitrary complexity, both scalar
and aggregate, in this field.
The dsize field contains the number of bytes stored in dptr.
The datum type is used to describe both keys and content (values) in
the database. Values of this type can be passed as arguments or
returned from GDBM function calls.
GDBM functions that return datum indicate failure by setting its dptr
field to NULL.
Functions returning integer value, indicate success by returning 0 and
failure by returning a non-zero value (the only exception to this rule
is gdbm_exists, for which the return value is reversed).
If the returned value indicates failure, the gdbm_errno variable
contains an integer value indicating what went wrong. A similar value
is associated with the dbf handle and can be accessed using the
gdbm_last_errno function. Immediately after return from a function,
both values are exactly equal. Subsequent GDBM calls with another dbf
as argument may alter the value of the global gdbm_errno, but the value
returned by gdbm_last_errno will always indicate the most recent code
of an error that occurred for that particular database. Programmers
are encouraged to use such per-database error codes.
Sometimes the actual reason of the failure can be clarified by
examining the system errno value. To make sure its value is meaningful
for a given GDBM error code, use the gdbm_check_syserr function. The
function takes error code as argument and returns 1 if the errno is
meaningful for that error, or 0 if it is irrelevant.
Similarly to gdbm_errno, the latest errno value associated with a
particular database can be obtained using the gdbm_last_syserr
function.
The gdbm_clear_error clears the error indicator (both GDBM and system
error codes) associated with a database handle.
Some critical errors leave the database in a structurally inconsistent
state. If that happens, all subsequent GDBM calls accessing that
database will fail with the GDBM error code of GDBM_NEED_RECOVERY (a
special function gdbm_needs_recovery 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
gdbm_recover function (see below).
The GDBM_NEED_RECOVERY error cannot be cleared using gdbm_clear_error.
Error functions
This section describes the error handling functions outlined above.
gdbm_error gdbm_last_errno (GDBM_FILE dbf)
Returns the error code of the most recent failure encountered
when operating on dbf.
int gdbm_last_syserr (GDBM_FILE dbf)
Returns the value of the system errno variable associated with
the most recent failure that occurred on dbf.
Notice that not all gdbm_error codes have a relevant system
error code. Use the following function to determine if a given
code has.
int gdbm_check_syserr (gdbm_error err)
Returns 1, if system errno value should be checked to get more
info on the error described by GDBM code err.
void gdbm_clear_error (GDBM_FILE dbf)
Clears the error state for the database dbf. This function is
called implicitly upon entry to any GDBM function that operates
on GDBM_FILE.
The GDBM_NEED_RECOVERY error cannot be cleared.
int gdbm_needs_recovery (GDBM_FILE dbf)
Return 1 if the database file dbf is in inconsistent state and
needs recovery.
const char *gdbm_strerror (gdbm_error err)
Returns a textual description of the error code err.
const char *gdbm_db_strerror (GDBM_FILE dbf)
Returns a textual description of the recent error in database
dbf. This description includes the system errno value, if
relevant.
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:
int gdbm_close (GDBM_FILE dbf);
Database lookups
int gdbm_exists (GDBM_FILE dbf, datum key);
If the key is found within the database, the return value will
be true (1). If nothing appropriate is found, false (0) is
returned and gdbm_errno set to GDBM_NO_ERROR.
On error, returns 0 and sets gdbm_errno.
datum gdbm_fetch (GDBM_FILE dbf, datum key);
Dbf is the pointer returned by gdbm_open. Key is the key data.
If the dptr element of the return value is NULL, the gdbm_errno
variable should be examined. The value of GDBM_ITEM_NOT_FOUND
means no data was found for that key. Other value means an
error occurred.
Otherwise the return value is a pointer to the found data. The
storage space for the dptr element is allocated using malloc(3).
GDBM does not automatically free this data. It is the
programmer's responsibility to free this storage when it is no
longer needed.
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.)
datum gdbm_firstkey (GDBM_FILE dbf);
Returns first key in the database.
datum gdbm_nextkey (GDBM_FILE dbf, datum key);
Given a key, returns the database key that follows it. End of
iteration is marked by returning datum with dptr field set to
NULL and setting the gdbm_errno value to GDBM_ITEM_NOT_FOUND.
After successful return from both functions, dptr points to data
allocated by malloc(3). It is the caller responsibility to free the
data when no longer needed.
A typical iteration loop looks like:
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;
}
These functions are intended to visit the database in read-only
algorithms. Avoid any database modifications within the iteration
loop. File visiting is based on a hash table. The gdbm_delete and, in
most cases, gdbm_store, 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:
key = gdbm_firstkey (dbf);
while (key.dptr)
{
nextkey = gdbm_nextkey (dbf, key);
if (some condition)
gdbm_delete ( dbf, key );
free (key.dptr);
key = nextkey;
}
Updating the database
int gdbm_store (GDBM_FILE dbf, datum key, datum content, int flag);
Dbf is the pointer returned by gdbm_open. Key is the key data.
Content is the data to be associated with the key. Flag can
have one of the following values:
GDBM_INSERT
Insert only, generate an error if key exists;
GDBM_REPLACE
Replace contents if key exists.
The function returns 0 on success and -1 on failure. If the key
already exists in the database and the flag is GDBM_INSERT, the
function does not modify the database. It sets gdbm_errno to
GDBM_CANNOT_REPLACE and returns 1.
int gdbm_delete (GDBM_FILE dbf, datum key);
Looks up and deletes the given key from the database dbf.
The return value is 0 if there was a successful delete or -1 on
error. In the latter case, the gdbm_errno value
GDBM_ITEM_NOT_FOUND indicates that the key is not present in the
database. Other gdbm_errno values indicate failure.
Recovering structural consistency
If a function leaves the database in structurally inconsistent state,
it can be recovered using the gdbm_recover function.
int gdbm_recover (GDBM_FILE dbf, gdbm_recovery * rcvr, int flags)
Check the database file DBF and fix eventual inconsistencies.
The rcvr argument can be used both to control the recovery and
to return additional statistics about the process, as indicated
by flags. For a detailed discussion of these arguments and
their usage, see the GDBM Manual, chapter Recovering structural
consistency.
You can pass NULL as rcvr and 0 as flags, if no such control is
needed.
By default, this function first checks the database for
inconsistencies and attempts recovery only if some were found.
The special flags bit GDBM_RCVR_FORCE instructs gdbm_recovery to
skip this check and to perform database recovery
unconditionally.
Export and import
GDBM database files can be exported (dumped) to so called flat files 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.
As of GDBM version 1.21, there are two flat file formats. The ASCII
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.
Another flat file format is the binary 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 GDBM version 1.9.1 and is retained
mainly for backward compatibility.
The following functions are used to export or import GDBM database
files.
int gdbm_dump (GDBM_FILE dbf, const char *filename,
int format, int open_flag, int mode) Dumps the database
file dbf to the file filename in requested format. Allowed values for
format are: GDBM_DUMP_FMT_ASCII, to create an ASCII dump file, and
GDBM_DUMP_FMT_BINARY, to create a binary dump.
The value of open_flag tells gdbm_dump what to do if filename
already exists. If it is GDBM_NEWDB, the function will create a
new output file, replacing it if it already exists. If its
value is GDBM_WRCREAT, the file will be created if it does not
exist. If it does exist, gdbm_dump will return error.
The file mode to use when creating the output file is defined by
the mode parameter. Its meaning is the same as for open(2).
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.
If pdbf is NULL, the function will try to create a new database. On
success, the new GDBM_FILE object will be stored in the memory location
pointed to by pdbf. 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.
Otherwise, if pdbf points to an already open GDBM_FILE, the
function will load data from filename into that database.
The flag parameter controls the function behavior if a key from
the dump file already exists in the database. See the
gdbm_store function for its possible values.
The meta_mask 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:
GDBM_META_MASK_MODE
Do not restore file mode.
GDBM_META_MASK_OWNER
Do not restore file owner.
Other functions
int gdbm_reorganize (GDBM_FILE dbf);
If you have had a lot of deletions and would like to shrink the
space used by the GDBM file, this routine will reorganize the
database.
int gdbm_sync (GDBM_FILE dbf);
Synchronizes the changes in dbf with its disk file.
It will not return until the disk file state is synchronized
with the in-memory state of the database.
int gdbm_setopt (GDBM_FILE dbf, int option, void *value, int size);
Query or change some parameter of an already opened database.
The option argument defines what parameter to set or retrieve.
If the set operation is requested, value points to the new
value. Its actual data type depends on option. If the get
operation is requested, value points to a memory region where to
store the return value. In both cases, size contains the actual
size of the memory pointed to by value.
Possible values of option 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 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.
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 current 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,
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, similar to the
GDBM_FAST option to gdbm_open.
This option is retained for compatibility with previous
versions of GDBM.
GDBM_SETSYNCMODE
GDBM_SYNCMODE
Turn on or off immediate disk synchronization after
updates. The value should point to an integer: 1 to turn
synchronization on, and 0 to turn it off.
NOTE: 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 CRASH TOLERANCE below.
GDBM_GETSYNCMODE
Return the current synchronization status. The value
should point to an int where the status will be stored.
GDBM_SETCENTFREE
GDBM_CENTFREE
Enable or disable central free block pool. 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 alias is provided for compatibility
with earlier versions.
GDBM_SETCOALESCEBLKS
GDBM_COALESCEBLKS
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.
GDBM_GETBLOCKSIZE
Return the block size in bytes. The value should point
to int.
int gdbm_fdesc (GDBM_FILE dbf);
Returns the file descriptor of the database dbf.
CRASH TOLERANCE
By default GNU dbm 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.
Starting with release 1.21 GNU dbm 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 gdbm 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 Crash Tolerance in the GDBM manual.
GLOBAL VARIABLES
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.
const char *gdbm_version
A string containing the library version number and build date.
int const gdbm_version_number[3]
This variable contains library version numbers: major, minor,
and patchlevel.
VERSIONING
The version information is kept in two places. The version of the
library is kept in the gdbm_version_number variable, described above.
Additionally, the header file gdbm.h defines the following macros:
GDBM_VERSION_MAJOR
Major version number.
GDBM_VERSION_MINOR
Minor version number.
GDBM_VERSION_PATCH
Patchlevel number. 0 means no patchlevel.
You can use this to compare whether your header file corresponds to the
library the program is linked with.
The following function can be used to compare two version numbers:
int gdbm_version_cmp (int const a[3], int const b[3])
Compare two version numbers formatted as gdbm_version_number.
Return negative number if a is older than b, positive number if
a is newer than b, and 0 if they are equal.
ERROR CODES
GDBM_NO_ERROR
No error occurred.
GDBM_MALLOC_ERROR
Memory allocation failed.
GDBM_BLOCK_SIZE_ERROR
This error is set by the gdbm_open function, if the value of its
block_size argument is incorrect and the GDBM_BSEXACT flag is
set.
GDBM_FILE_OPEN_ERROR
The library was not able to open a disk file. This can be set
by gdbm_open, gdbm_fd_open, gdbm_dump and gdbm_load functions.
Inspect the value of the system errno variable to get more
detailed diagnostics.
GDBM_FILE_WRITE_ERROR
Writing to a disk file failed. This can be set by gdbm_open,
gdbm_fd_open, gdbm_dump and gdbm_load functions.
Inspect the value of the system errno variable to get more
detailed diagnostics.
GDBM_FILE_SEEK_ERROR
Positioning in a disk file failed. This can be set by gdbm_open
function.
Inspect the value of the system errno variable to get a more
detailed diagnostics.
GDBM_FILE_READ_ERROR
Reading from a disk file failed. This can be set by gdbm_open,
gdbm_dump and gdbm_load functions.
Inspect the value of the system errno variable to get a more
detailed diagnostics.
GDBM_BAD_MAGIC_NUMBER
The file given as argument to gdbm_open function is not a valid
gdbm file: it has a wrong magic number.
GDBM_EMPTY_DATABASE
The file given as argument to gdbm_open function is not a valid
gdbm file: it has zero length. This error is returned unless
the flags argument has GDBM_NEWDB bit set.
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.
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.
GDBM_READER_CANT_DELETE
Set by the gdbm_delete, if it attempted to operate on a database
that is open in read-only mode.
GDBM_READER_CANT_STORE
Set by the gdbm_store if it attempted to operate on a database
that is open in read-only mode.
GDBM_READER_CANT_REORGANIZE
Set by the gdbm_reorganize if it attempted to operate on a
database that is open in read-only mode.
GDBM_ITEM_NOT_FOUND
Requested item was not found. This error is set by gdbm_delete
and gdbm_fetch when the requested key value is not found in the
database.
GDBM_REORGANIZE_FAILED
The gdbm_reorganize function is not able to create a temporary
database.
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.
GDBM_MALFORMED_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. When returned by gdbm_store, this means that either
key or content parameter had its dptr field set to NULL.
The GDBM_ILLEGAL_DATA is an alias for this error code,
maintained for backward compatibility.
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 gdbm_setopt function
when setting the GDBM_CACHESIZE value.
GDBM_OPT_BADVAL
The option argument is not valid or the value argument points to
an invalid value in a call to gdbm_setopt function.
GDBM_OPT_ILLEGAL is an alias for this error code, maintained for
backward compatibility. Modern applications should not use it.
GDBM_BYTE_SWAPPED
The gdbm_open function attempts to open a database which is
created on a machine with different byte ordering.
GDBM_BAD_FILE_OFFSET
The gdbm_open function sets this error code if the file it tries
to open has a wrong magic number.
GDBM_BAD_OPEN_FLAGS
Set by the gdbm_dump function if supplied an invalid flags
argument.
GDBM_FILE_STAT_ERROR
Getting information about a disk file failed. The system errno
will give more details about the error.
This error can be set by the following functions: gdbm_open,
gdbm_reorganize.
GDBM_FILE_EOF
End of file was encountered where more data was expected to be
present. This error can occur when fetching data from the
database and usually means that the database is truncated or
otherwise corrupted.
This error can be set by any GDBM function that does I/O. Some
of these functions are: gdbm_delete, gdbm_exists, gdbm_fetch,
gdbm_export, gdbm_import, gdbm_reorganize, gdbm_firstkey,
gdbm_nextkey, gdbm_store.
GDBM_NO_DBNAME
Output database name is not specified. This error code is set
by gdbm_load if the first argument points to NULL and the input
file does not specify the database name.
GDBM_ERR_FILE_OWNER
This error code is set by gdbm_load if it is unable to restore
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 errno variable to
get a more detailed diagnostics.
GDBM_ERR_FILE_MODE
This error code is set by gdbm_load if it is unable to restore
database file mode. It is a mild error condition, meaning that
the data have been restored successfully, only changing the
target file owner failed. Inspect the system errno variable to
get a more detailed diagnostics.
GDBM_NEED_RECOVERY
Database is in inconsistent state and needs recovery. Call
gdbm_recover if you get this error.
GDBM_BACKUP_FAILED
The GDBM engine is unable to create backup copy of the file.
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.
GDBM_BAD_BUCKET
Invalid index bucket is encountered in the database. Database
recovery is needed.
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.
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.
The database needs recovery.
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.
The database needs recovery.
GDBM_BAD_DIR_ENTRY
Bad directory entry found in the bucket. The database recovery
is needed.
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.
GDBM_FILE_SYNC_ERROR
Cached content couldn't be synchronized to disk. Examine the
errno variable to get more info,
Database recovery is needed.
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.
GDBM_BUCKET_CACHE_CORRUPTED
The bucket cache structure is corrupted. Database recovery is
needed.
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.
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.
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.
GDBM_ERR_USAGE
Function usage error. That includes invalid argument values,
and the like.
DBM COMPATIBILITY ROUTINES
GDBM includes a compatibility library libgdbm_compat, for use with
programs that expect traditional UNIX dbm or ndbm interfaces, such as,
e.g. Sendmail. The library is optional and thus may be absent in some
binary distributions.
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 GDBM Manual, chapter Compatibility with standard dbm
and ndbm.
DBM compatibility routines
In dbm 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 will free. They should be treated as static pointers (as
standard UNIX dbm does).
The following interfaces are provided:
#include <dbm.h>
int dbminit (const char *name);
int store (datum key, datum content);
datum fetch (datum key);
int delete (datum key);
datum firstkey (void);
datum nextkey (datum key);
int dbmclose (void);
NDBM Compatibility routines:
In this mode, multiple databases can be opened. Each database is
identified by a handle of type DBM *. As in the original NDBM, all
returned pointers in datum structures point to data that will be freed
by the compatibility library. They should be treated as static
pointers.
The following interfaces are provided:
#include <ndbm.h>
DBM *dbm_open (const char *name, int flags, int mode);
void dbm_close (DBM *file);
datum dbm_fetch (DBM *file, datum key);
int dbm_store (DBM *file, datum key, datum content, int flags);
int dbm_delete (DBM *file, datum key);
datum dbm_firstkey (DBM *file);
datum dbm_nextkey (DBM *file, datum key);
int dbm_error (DBM *file);
int dbm_clearerr (DBM *file);
int dbm_pagfno (DBM *file);
int dbm_dirfno (DBM *file);
int dbm_rdonly (DBM *file);
LINKING
This library is accessed by specifying -lgdbm as the last parameter to
the compile line, e.g.:
gcc -o prog prog.c -lgdbm
If you wish to use the dbm or ndbm compatibility routines, you must
link in the gdbm_compat library as well. For example:
gcc -o prog proc.c -lgdbm -lgdbm_compat
BUG REPORTS
Send bug reports to <bug-gdbm@gnu.org>.
SEE ALSO
gdbm_dump(1), gdbm_load(1), gdbmtool(1).
AUTHORS
by Philip A. Nelson, Jason Downs and Sergey Poznyakoff; crash tolerance
by Terence Kelly.
COPYRIGHT
Copyright (C) 1990 - 2025 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>
CONTACTS
You may contact the original author by:
e-mail: phil@cs.wwu.edu
us-mail: Philip A. Nelson
Computer Science Department
Western Washington University
Bellingham, WA 98226
You may contact the current maintainers by:
e-mail: downsj@downsj.com
and
e-mail: gray@gnu.org
For questions and feedback regarding crash tolerance, you may contact
Terence Kelly at:
e-mail: tpkelly @ { acm.org, cs.princeton.edu, eecs.umich.edu }
GDBM March 6, 2025 gdbm(3)
gdbm 1.26 - Generated Sat Aug 2 18:25:32 CDT 2025