IRIX Base Documentation 2002 November

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DDDDBBBBOOOOPPPPEEEENNNN((((3333)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((JJJJaaaannnnuuuuaaaarrrryyyy 2222,,,, 1111999999994444)))) DDDDBBBBOOOOPPPPEEEENNNN((((3333)))) NNNNAAAAMMMMEEEE dbopen - database access methods SSSSYYYYNNNNOOOOPPPPSSSSIIIISSSS ####iiiinnnncccclllluuuuddddeeee <<<<ssssyyyyssss////ttttyyyyppppeeeessss....hhhh>>>> ####iiiinnnncccclllluuuuddddeeee <<<<lllliiiimmmmiiiittttssss....hhhh>>>> ####iiiinnnncccclllluuuuddddeeee <<<<ddddbbbb....hhhh>>>> DDDDBBBB **** ddddbbbbooooppppeeeennnn((((ccccoooonnnnsssstttt cccchhhhaaaarrrr ****ffffiiiilllleeee,,,, iiiinnnntttt ffffllllaaaaggggssss,,,, iiiinnnntttt mmmmooooddddeeee,,,, DDDDBBBBTTTTYYYYPPPPEEEE ttttyyyyppppeeee,,,, ccccoooonnnnsssstttt vvvvooooiiiidddd ****ooooppppeeeennnniiiinnnnffffoooo))));;;; DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN _D_b_o_p_e_n is the library interface to database files. The supported file formats are btree, hashed and UNIX file oriented. The btree format is a representation of a sorted, balanced tree structure. The hashed format is an extensible, dynamic hashing scheme. The flat-file format is a byte stream file with fixed or variable length records. The formats and file format specific information are described in detail in their respective manual pages _b_t_r_e_e(3), _h_a_s_h(3) and _r_e_c_n_o(3). Dbopen opens _f_i_l_e for reading and/or writing. Files never intended to be preserved on disk may be created by setting the file parameter to NULL. The _f_l_a_g_s and _m_o_d_e _a_r_g_u_m_e_n_t_s are as specified to the _o_p_e_n(2) routine, however, only the O_CREAT, O_EXCL, O_EXLOCK, O_NONBLOCK, O_RDONLY, O_RDWR, O_SHLOCK and O_TRUNC flags are meaningful. (Note, opening a database file O_WRONLY is not possible.) The _t_y_p_e argument is of type DBTYPE (as defined in the <db.h> include file) and may be set to DB_BTREE, DB_HASH or DB_RECNO. The _o_p_e_n_i_n_f_o argument is a pointer to an access method specific structure described in the access method's manual page. If _o_p_e_n_i_n_f_o is NULL, each access method will use defaults appropriate for the system and the access method. _D_b_o_p_e_n returns a pointer to a DB structure on success and NULL on error. The DB structure is defined in the <db.h> include file, and contains at least the following fields: typedef struct { DBTYPE type; int (*close)(const DB *db); int (*del)(const DB *db, const DBT *key, u_int flags); int (*fd)(const DB *db); int (*get)(const DB *db, DBT *key, DBT *data, u_int flags); Page 1 (printed 4/30/98) DDDDBBBBOOOOPPPPEEEENNNN((((3333)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((JJJJaaaannnnuuuuaaaarrrryyyy 2222,,,, 1111999999994444)))) DDDDBBBBOOOOPPPPEEEENNNN((((3333)))) int (*put)(const DB *db, DBT *key, const DBT *data, u_int flags); int (*sync)(const DB *db, u_int flags); int (*seq)(const DB *db, DBT *key, DBT *data, u_int flags); } DB; 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 _d_b_o_p_e_n, and sometimes one or more pointers to key/data structures and a flag value. type The type of the underlying access method (and file format). close A pointer to a routine to flush any cached information to disk, free any allocated resources, and close the underlying file(s). Since key/data pairs may be cached in memory, failing to sync the file with a _c_l_o_s_e or _s_y_n_c function may result in inconsistent or lost information. _C_l_o_s_e routines return -1 on error (setting _e_r_r_n_o) and 0 on success. del A pointer to a routine to remove key/data pairs from the database. The parameter _f_l_a_g may be set to the following value: R_CURSOR Delete the record referenced by the cursor. The cursor must have previously been initialized. _D_e_l_e_t_e routines return -1 on error (setting _e_r_r_n_o), 0 on success, and 1 if the specified _k_e_y was not in the file. fd A pointer to a routine which returns a file descriptor representative of the underlying database. A file descriptor referencing the same file will be returned to all processes which call _d_b_o_p_e_n with the same _f_i_l_e name. This file descriptor may be safely used as an argument to the _f_c_n_t_l(2) and _f_l_o_c_k(2) locking functions. The file descriptor is not necessarily associated with any of the underlying files used by the access method. No file descriptor is available for in memory databases. _F_d routines return -1 on error (setting _e_r_r_n_o), and the file descriptor on success. get A pointer to a routine which is the interface for keyed retrieval from the database. The address and length of the data associated with the specified _k_e_y are returned Page 2 (printed 4/30/98) DDDDBBBBOOOOPPPPEEEENNNN((((3333)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((JJJJaaaannnnuuuuaaaarrrryyyy 2222,,,, 1111999999994444)))) DDDDBBBBOOOOPPPPEEEENNNN((((3333)))) in the structure referenced by _d_a_t_a. _G_e_t routines return -1 on error (setting _e_r_r_n_o), 0 on success, and 1 if the _k_e_y was not in the file. put A pointer to a routine to store key/data pairs in the database. The parameter _f_l_a_g may be set to one of the following values: R_CURSOR Replace the key/data pair referenced by the cursor. The cursor must have previously been initialized. R_IAFTER Append the data immediately after the data referenced by _k_e_y, creating a new key/data pair. The record number of the appended key/data pair is returned in the _k_e_y structure. (Applicable only to the DB_RECNO access method.) R_IBEFORE Insert the data immediately before the data referenced by _k_e_y, creating a new key/data pair. The record number of the inserted key/data pair is returned in the _k_e_y structure. (Applicable only to the DB_RECNO access method.) R_NOOVERWRITE Enter the new key/data pair only if the key does not previously exist. R_SETCURSOR Store the key/data pair, setting or initializing the position of the cursor to reference it. (Applicable only to the DB_BTREE and DB_RECNO access methods.) R_SETCURSOR is available only for the DB_BTREE and DB_RECNO access methods because it implies that the keys have an inherent order which does not change. R_IAFTER and R_IBEFORE are available only for the DB_RECNO access method because they each imply that the access method is able to create new keys. This is only true if the keys are ordered and independent, record numbers for example. The default behavior of the _p_u_t routines is to enter the new key/data pair, replacing any previously existing key. Page 3 (printed 4/30/98) DDDDBBBBOOOOPPPPEEEENNNN((((3333)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((JJJJaaaannnnuuuuaaaarrrryyyy 2222,,,, 1111999999994444)))) DDDDBBBBOOOOPPPPEEEENNNN((((3333)))) _P_u_t routines return -1 on error (setting _e_r_r_n_o), 0 on success, and 1 if the R_NOOVERWRITE _f_l_a_g was set and the key already exists in the file. seq A pointer to a routine which is the interface for sequential retrieval from the database. The address and length of the key are returned in the structure referenced by _k_e_y, and the address and length of the data are returned in the structure referenced by _d_a_t_a. Sequential key/data pair retrieval may begin at any time, and the position of the ``cursor'' is not affected by calls to the _d_e_l, _g_e_t, _p_u_t, or _s_y_n_c routines. Modifications to the database during a sequential scan will be reflected in the scan, i.e. records inserted behind the cursor will not be returned while records inserted in front of the cursor will be returned. The flag value mmmmuuuusssstttt be set to one of the following values: R_CURSOR The data associated with the specified key is returned. This differs from the _g_e_t routines in that it sets or initializes the cursor to the location of the key as well. (Note, for the DB_BTREE access method, the returned key is not necessarily an exact match for the specified key. The returned key is the smallest key greater than or equal to the specified key, permitting partial key matches and range searches.) R_FIRST The first key/data pair of the database is returned, and the cursor is set or initialized to reference it. R_LAST The last key/data pair of the database is returned, and the cursor is set or initialized to reference it. (Applicable only to the DB_BTREE and DB_RECNO access methods.) R_NEXT Retrieve the key/data pair immediately after the cursor. If the cursor is not yet set, this is the same as the R_FIRST flag. R_PREV Retrieve the key/data pair immediately before the cursor. If the cursor is not yet set, this is the Page 4 (printed 4/30/98) DDDDBBBBOOOOPPPPEEEENNNN((((3333)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((JJJJaaaannnnuuuuaaaarrrryyyy 2222,,,, 1111999999994444)))) DDDDBBBBOOOOPPPPEEEENNNN((((3333)))) same as the R_LAST flag. (Applicable only to the DB_BTREE and DB_RECNO access methods.) R_LAST and R_PREV are available only for the DB_BTREE and DB_RECNO access methods because they each imply that the keys have an inherent order which does not change. _S_e_q routines return -1 on error (setting _e_r_r_n_o), 0 on success and 1 if there are no key/data pairs less than or greater than the specified or current key. If the DB_RECNO access method is being used, and if the database file is a character special file and no complete key/data pairs are currently available, the _s_e_q routines return 2. sync A pointer to a routine to flush any cached information to disk. If the database is in memory only, the _s_y_n_c routine has no effect and will always succeed. The flag value may be set to the following value: R_RECNOSYNC If the DB_RECNO access method is being used, this flag causes the sync routine to apply to the btree file which underlies the recno file, not the recno file itself. (See the _b_f_n_a_m_e field of the _r_e_c_n_o(3) manual page for more information.) _S_y_n_c routines return -1 on error (setting _e_r_r_n_o) and 0 on success. KKKKEEEEYYYY////DDDDAAAATTTTAAAA PPPPAAAAIIIIRRRRSSSS Access to all file types is based on key/data pairs. Both keys and data are represented by the following data structure: typedef struct { void *data; size_t size; } DBT; The elements of the DBT structure are defined as follows: data A pointer to a byte string. size The length of the byte string. Key and data byte strings may reference strings of essentially unlimited length although any two of them must fit into available memory at the same time. It should be noted that the access methods provide no guarantees about Page 5 (printed 4/30/98) DDDDBBBBOOOOPPPPEEEENNNN((((3333)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((JJJJaaaannnnuuuuaaaarrrryyyy 2222,,,, 1111999999994444)))) DDDDBBBBOOOOPPPPEEEENNNN((((3333)))) byte string alignment. EEEERRRRRRRROOOORRRRSSSS The _d_b_o_p_e_n routine may fail and set _e_r_r_n_o for any of the errors specified for the library routines _o_p_e_n(2) and _m_a_l_l_o_c(3) or the following: [EFTYPE] A file is incorrectly formatted. [EINVAL] A parameter has been specified (hash function, pad byte etc.) that is incompatible with the current file specification or which is not meaningful for the function (for example, use of the cursor without prior initialization) or there is a mismatch between the version number of file and the software. The _c_l_o_s_e routines may fail and set _e_r_r_n_o for any of the errors specified for the library routines _c_l_o_s_e(2), _r_e_a_d(2), _w_r_i_t_e(2), _f_r_e_e(3), or _f_s_y_n_c(2). The _d_e_l, _g_e_t, _p_u_t and _s_e_q routines may fail and set _e_r_r_n_o for any of the errors specified for the library routines _r_e_a_d(2), _w_r_i_t_e(2), _f_r_e_e(3) or _m_a_l_l_o_c(3). The _f_d routines will fail and set _e_r_r_n_o to ENOENT for in memory databases. The _s_y_n_c routines may fail and set _e_r_r_n_o for any of the errors specified for the library routine _f_s_y_n_c(2). SSSSEEEEEEEE AAAALLLLSSSSOOOO _b_t_r_e_e(3), _h_a_s_h(3), _m_p_o_o_l(3), _r_e_c_n_o(3) _L_I_B_T_P: _P_o_r_t_a_b_l_e, _M_o_d_u_l_a_r _T_r_a_n_s_a_c_t_i_o_n_s _f_o_r _U_N_I_X, Margo Seltzer, Michael Olson, USENIX proceedings, Winter 1992. BBBBUUUUGGGGSSSS The typedef DBT is a mnemonic for ``data base thang'', and was used because noone could think of a reasonable name that wasn't already used. The file descriptor interface is a kluge and will be deleted in a future version of the interface. None of the access methods provide any form of concurrent access, locking, or transactions. This version of berkeley db (1.85) is free software which is not developed nor maintained by SGI. It is known to have some bugs that are unlikely to get fixed (See NOTES below) Page 6 (printed 4/30/98) DDDDBBBBOOOOPPPPEEEENNNN((((3333)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((JJJJaaaannnnuuuuaaaarrrryyyy 2222,,,, 1111999999994444)))) DDDDBBBBOOOOPPPPEEEENNNN((((3333)))) in particular, certain cursor and certain deletion/overwrite operations are known to have problems, up to corrupting databases, and should be avoided according to http://www.sleepycat.com/db.185.html. See _h_a_s_h(3), and _b_t_r_e_e(3) for details. NNNNOOOOTTTTEEEESSSS The default hash function in this version of db is the Fowler/Vo/Noll hash which gives better distributions (less collisions) on average than the publicly released version. This version of berkeley db is 1.85. A newer, enhanced version db-2.x requires licensing. Check out http://www.sleepycat.com/ for details. Page 7 (printed 4/30/98)