version 1, including all changes.
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perry |
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DBOPEN |
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!!!DBOPEN |
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NAME |
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SYNOPSIS |
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DESCRIPTION |
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KEY/DATA PAIRS |
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ERRORS |
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SEE ALSO |
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BUGS |
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---- |
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!!NAME |
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dbopen - database access methods |
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!!SYNOPSIS |
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__#include |
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__ |
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!!DESCRIPTION |
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''Dbopen'' is the library interface to database files. |
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The supported file formats are btree, hashed and UNIX file |
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oriented. The btree format is a representation of a sorted, |
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balanced tree structure. The hashed format is an extensible, |
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dynamic hashing scheme. The flat-file format is a byte |
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stream file with fixed or variable length records. The |
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formats and file format specific information are described |
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in detail in their respective manual pages btree(3), |
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hash(3) and recno(3). |
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Dbopen opens ''file'' for reading and/or writing. Files |
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never intended to be preserved on disk may be created by |
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setting the file parameter to NULL. |
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The ''flags'' and ''mode arguments'' are as specified |
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to the open(2) routine, however, only the O_CREAT, |
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O_EXCL, O_EXLOCK, O_NONBLOCK, O_RDONLY, O_RDWR, O_SHLOCK and |
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O_TRUNC flags are meaningful. (Note, opening a database file |
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O_WRONLY is not possible.) |
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The ''type'' argument is of type DBTYPE (as defined in |
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the |
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'' |
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The ''openinfo'' argument is a pointer to an access |
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method specific structure described in the access method's |
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manual page. If ''openinfo'' is NULL, each access method |
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will use defaults appropriate for the system and the access |
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method. |
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''Dbopen'' returns a pointer to a DB structure on success |
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and NULL on error. The DB structure is defined in the |
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'' |
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typedef struct { |
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DBTYPE type; |
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int (*close)(DB *db); |
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int (*del)(DB *db, DB_TXN txnid, const DBT *key, u_int flags); |
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int (*fd)(DB *db, int *fdp); |
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int (*get)(DB *db, DB_TXN txnid, DBT *key, DBT *data, u_int flags); |
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int (*join)(DB *db, DBC **, u_int, DBC **); |
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int (*put)(DB *db, DB_TXN txnid, DBT *key, const DBT *data, |
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u_int flags); |
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int (*sync)(const DB *db, u_int flags); |
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int (*seq)(const DB *db, DBT *key, DBT *data, u_int flags); |
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} DB; |
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These elements describe a database type and a set of functions performing various actions. These functions take a pointer to a structure as returned by ''dbopen'', and sometimes one or more pointers to key/data structures and a flag value. |
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type |
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The type of the underlying access method (and file |
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format). |
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close |
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A pointer to a routine to flush any cached information to |
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disk, free any allocated resources, and close the underlying |
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file(s). Since key/data pairs may be cached in memory, |
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failing to sync the file with a ''close'' or ''sync'' |
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function may result in inconsistent or lost information. |
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''Close'' routines return -1 on error (setting |
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''errno'') and 0 on success. |
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del |
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A pointer to a routine to remove key/data pairs from the |
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database. |
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The parameter ''flag'' may be set to the following |
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value: |
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R_CURSOR |
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Delete the record referenced by the cursor. The cursor must |
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have previously been initialized. |
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''Delete'' routines return -1 on error (setting |
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''errno''), 0 on success, and 1 if the specified |
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''key'' was not in the file. |
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fd |
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A pointer to a routine which returns a file descriptor |
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representative of the underlying database. A file descriptor |
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referencing the same file will be returned to all processes |
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which call ''dbopen'' with the same ''file'' name. |
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This file descriptor may be safely used as an argument to |
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the fcntl(2) and flock(2) locking functions. |
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The file descriptor is not necessarily associated with any |
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of the underlying files used by the access method. No file |
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descriptor is available for in memory databases. ''Fd'' |
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routines return -1 on error (setting ''errno''), and the |
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file descriptor on success. |
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get |
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A pointer to a routine which is the interface for keyed |
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retrieval from the database. The address and length of the |
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data associated with the specified ''key'' are returned |
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in the structure referenced by ''data''. ''Get'' |
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routines return -1 on error (setting ''errno''), 0 on |
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success, and 1 if the ''key'' was not in the |
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file. |
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put |
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A pointer to a routine to store key/data pairs in the |
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database. |
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The parameter ''flag'' may be set to one of the following |
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values: |
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R_CURSOR |
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Replace the key/data pair referenced by the cursor. The |
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cursor must have previously been initialized. |
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R_IAFTER |
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Append the data immediately after the data referenced by |
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''key'', creating a new key/data pair. The record number |
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of the appended key/data pair is returned in the ''key'' |
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structure. (Applicable only to the DB_RECNO access |
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method.) |
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R_IBEFORE |
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Insert the data immediately before the data referenced by |
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''key'', creating a new key/data pair. The record number |
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of the inserted key/data pair is returned in the ''key'' |
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structure. (Applicable only to the DB_RECNO access |
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method.) |
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R_NOOVERWRITE |
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Enter the new key/data pair only if the key does not |
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previously exist. |
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R_SETCURSOR |
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Store the key/data pair, setting or initializing the |
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position of the cursor to reference it. (Applicable only to |
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the DB_BTREE and DB_RECNO access methods.) |
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R_SETCURSOR is available only for the DB_BTREE and DB_RECNO |
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access methods because it implies that the keys have an |
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inherent order which does not change. |
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R_IAFTER and R_IBEFORE are available only for the DB_RECNO |
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access method because they each imply that the access method |
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is able to create new keys. This is only true if the keys |
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are ordered and independent, record numbers for |
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example. |
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The default behavior of the ''put'' routines is to enter |
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the new key/data pair, replacing any previously existing |
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key. |
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''Put'' routines return -1 on error (setting |
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''errno''), 0 on success, and 1 if the R_NOOVERWRITE |
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''flag'' was set and the key already exists in the |
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file. |
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seq |
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A pointer to a routine which is the interface for sequential |
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retrieval from the database. The address and length of the |
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key are returned in the structure referenced by ''key'', |
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and the address and length of the data are returned in the |
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structure referenced by ''data''. |
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Sequential key/data pair retrieval may begin at any time, |
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and the position of the ``cursor'' is not affected by calls |
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to the ''del'', ''get'', ''put'', or ''sync'' |
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routines. Modifications to the database during a sequential |
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scan will be reflected in the scan, i.e. records inserted |
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behind the cursor will not be returned while records |
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inserted in front of the cursor will be |
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returned. |
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The flag value __must__ be set to one of the following |
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values: |
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R_CURSOR |
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The data associated with the specified key is returned. This |
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differs from the ''get'' routines in that it sets or |
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initializes the cursor to the location of the key as well. |
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(Note, for the DB_BTREE access method, the returned key is |
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not necessarily an exact match for the specified key. The |
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returned key is the smallest key greater than or equal to |
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the specified key, permitting partial key matches and range |
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searches.) |
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R_FIRST |
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The first key/data pair of the database is returned, and the |
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cursor is set or initialized to reference it. |
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R_LAST |
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The last key/data pair of the database is returned, and the |
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cursor is set or initialized to reference it. (Applicable |
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only to the DB_BTREE and DB_RECNO access |
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methods.) |
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R_NEXT |
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Retrieve the key/data pair immediately after the cursor. If |
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the cursor is not yet set, this is the same as the R_FIRST |
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flag. |
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R_PREV |
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Retrieve the key/data pair immediately before the cursor. If |
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the cursor is not yet set, this is the same as the R_LAST |
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flag. (Applicable only to the DB_BTREE and DB_RECNO access |
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methods.) |
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R_LAST and R_PREV are available only for the DB_BTREE and |
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DB_RECNO access methods because they each imply that the |
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keys have an inherent order which does not |
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change. |
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''Seq'' routines return -1 on error (setting |
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''errno''), 0 on success and 1 if there are no key/data |
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pairs less than or greater than the specified or current |
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key. If the DB_RECNO access method is being used, and if the |
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database file is a character special file and no complete |
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key/data pairs are currently available, the ''seq'' |
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routines return 2. |
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sync |
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A pointer to a routine to flush any cached information to |
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disk. If the database is in memory only, the ''sync'' |
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routine has no effect and will always succeed. |
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The flag value may be set to the following |
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value: |
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R_RECNOSYNC |
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If the DB_RECNO access method is being used, this flag |
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causes the sync routine to apply to the btree file which |
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underlies the recno file, not the recno file itself. (See |
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the ''bfname'' field of the recno(3) manual page |
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for more information.) |
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''Sync'' routines return -1 on error (setting |
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''errno'') and 0 on success. |
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!!KEY/DATA PAIRS |
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Access to all file types is based on key/data pairs. Both |
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keys and data are represented by the following data |
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structure: |
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typedef struct { |
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void *data; |
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size_t size; |
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} DBT; |
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The elements of the DBT structure are defined as |
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follows: |
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data |
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A pointer to a byte string. |
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size |
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The length of the byte string. |
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Key and data byte strings may reference strings of |
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essentially unlimited length although any two of them must |
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fit into available memory at the same time. It should be |
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noted that the access methods provide no guarantees about |
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byte string alignment. |
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!!ERRORS |
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The ''dbopen'' routine may fail and set ''errno'' for |
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any of the errors specified for the library routines |
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open(2) and malloc(3) or the |
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following: |
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[[EFTYPE] |
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A file is incorrectly formatted. |
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[[EINVAL] |
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A parameter has been specified (hash function, pad byte |
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etc.) that is incompatible with the current file |
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specification or which is not meaningful for the function |
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(for example, use of the cursor without prior |
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initialization) or there is a mismatch between the version |
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number of file and the software. |
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The ''close'' routines may fail and set ''errno'' for |
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any of the errors specified for the library routines |
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close(2), read(2), write(2), |
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free(3), or fsync(2). |
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The ''del'', ''get'', ''put'' and ''seq'' |
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routines may fail and set ''errno'' for any of the errors |
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specified for the library routines read(2), |
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write(2), free(3) or |
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malloc(3). |
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410 |
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The ''fd'' routines will fail and set ''errno'' to |
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ENOENT for in memory databases. |
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414 |
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The ''sync'' routines may fail and set ''errno'' for |
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any of the errors specified for the library routine |
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fsync(2). |
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!!SEE ALSO |
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419 |
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420 |
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421 |
btree(3), hash(3), mpool(3), |
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recno(3) |
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423 |
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424 |
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''LIBTP: Portable, Modular Transactions for UNIX'', Margo |
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Seltzer, Michael Olson, USENIX proceedings, Winter |
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1992. |
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!!BUGS |
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429 |
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430 |
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431 |
The typedef DBT is a mnemonic for ``data base thang'', and |
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was used because noone could think of a reasonable name that |
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wasn't already used. |
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434 |
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435 |
|
|
|
436 |
The file descriptor interface is a kluge and will be deleted |
|
|
437 |
in a future version of the interface. |
|
|
438 |
|
|
|
439 |
|
|
|
440 |
None of the access methods provide any form of concurrent |
|
|
441 |
access, locking, or transactions. |
|
|
442 |
---- |