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 © 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 }