NNAAMMEE db_dbt - DB key/data pairs SSYYNNOOPPSSIISS ttyyppeeddeeff ssttrruucctt {{ vvooiidd **ddaattaa;; uu__iinntt3322__tt ssiizzee;; uu__iinntt3322__tt uulleenn;; uu__iinntt3322__tt ddlleenn;; uu__iinntt3322__tt ddooffff;; uu__iinntt3322__tt ffllaaggss;; }} DDBBTT;; KKEEYY//DDAATTAA PPAAIIRRSS Storage and retrieval for the DB access methods are based on key/data pairs. Both key and data items are repre- sented by the DBT data structure. Key and data byte strings may reference strings of essen- tially unlimited length, although any two keys must fit into available memory at the same time so that they may be compared, and any one data item must fit into available memory so that it may be returned. In order to ensure compatibility with future releases of DB, all fields of the DBT structure that are not explic- itly set should be initialized to 0 before the first time the structure is used. Do this by declaring the structure external or static, or by calling the C library routine _b_z_e_r_o(3) or _m_e_m_s_e_t(3). By default, the _f_l_a_g_s structure element is expected to be 0. In this default case, when being provided a key or data item by the application, the DB package expects the _d_a_t_a structure element to point to a byte string of _s_i_z_e bytes. When returning a key/data item to the application, the DB package will store into the _d_a_t_a structure element a pointer to a byte string of _s_i_z_e bytes. BByy ddeeffaauulltt,, tthhee mmeemmoorryy rreeffeerreenncceedd bbyy tthhiiss ssttoorreedd ppooiinntteerr iiss oonnllyy vvaalliidd uunnttiill tthhee nneexxtt ccaallll ttoo tthhee DDBB ppaacckkaaggee uussiinngg tthhee DDBB hhaannddllee rreettuurrnneedd bbyy _d_b___o_p_e_n. TThhee aacccceessss mmeetthhooddss pprroovviiddee nnoo gguuaarraanntteeeess aabboouutt bbyyttee ssttrriinngg aalliiggnnmmeenntt,, aanndd aapppplliiccaattiioonnss aarree rreessppoonnssiibbllee ffoorr mmaaiinnttaaiinn-- iinngg aannyy nneecceessssaarryy aalliiggnnmmeenntt.. Use the DB_DBT_USERMEM flag to cause returned items to be placed in memory of arbi- trary alignment. The elements of the DBT structure are defined as follows: void *data; A pointer to a byte string. u_int32_t size; The length of _d_a_t_a, in bytes. u_int32_t ulen; The size of the user's buffer (referenced by _d_a_t_a), in bytes. This location is not written by the DB functions. See the DB_DBT_USERMEM flag for more information. u_int32_t dlen; The length of the partial record being read or writ- ten by the application, in bytes. See the DB_DBT_PARTIAL flag for more information. u_int32_t doff; The offset of the partial record being read or writ- ten by the application, in bytes. See the DB_DBT_PARTIAL flag for more information. u_int32_t flags; The flags value is specified by oorr'ing together one or more of the following values: DB_DBT_MALLOC Ignored except when retrieving information from a database, e.g., a _D_B_-_>_g_e_t or _D_B_c_u_r_s_o_r_-_>_c___g_e_t call. This flag causes DB to allocate memory for the returned key or data item (using _m_a_l_- _l_o_c(3), or the user-specified malloc function) and return a pointer to it in the _d_a_t_a field of the key or data DBT structure. The allocated memory becomes the responsibility of the calling application. It is an error to specify both DB_DBT_MALLOC and DB_DBT_USERMEM. DB_DBT_USERMEM Ignored except when retrieving information from a database, e.g., a _D_B_-_>_g_e_t or _D_B_c_u_r_s_o_r_-_>_c___g_e_t call. The _d_a_t_a field of the key or data struc- ture must reference memory that is at least _u_l_e_n bytes in length. If the length of the requested item is less than or equal to that number of bytes, the item is copied into the memory refer- enced by the _d_a_t_a field. Otherwise, an error is returned, the _s_i_z_e field is set to the length needed for the requested item, and the _e_r_r_n_o variable is set to ENOMEM. It is an error to specify both DB_DBT_MALLOC and DB_DBT_USERMEM. DB_DBT_PARTIAL Ignored except when specified for a data parame- ter, where this flag causes the partial retrieval or storage of an item. If the calling application is doing a get, the _d_l_e_n bytes starting _d_o_f_f bytes from the beginning of the retrieved data record are returned as if they comprised the entire record. If any or all of the specified bytes do not exist in the record, the get is successful and the existing bytes or 0 bytes are returned. For example, if the data portion of a retrieved record was 100 bytes, and a partial retrieval was done using a DBT having a _d_l_e_n field of 20 and a _d_o_f_f field of 85, the get call would suc- ceed, the _d_a_t_a field would reference the last 15 bytes of the record, and the _s_i_z_e field would be set to 15. If the calling application is doing a put, the _d_l_e_n bytes starting _d_o_f_f bytes from the begin- ning of the specified key's data record are replaced by the data specified by the _d_a_t_a and _s_i_z_e structure elements. If _d_l_e_n is smaller than _s_i_z_e, the record will grow, and if _d_l_e_n is larger than _s_i_z_e, the record will shrink. If the specified bytes do not exist, the record will be extended using nul bytes as necessary, and the put call will succeed. It is an error to attempt a partial put using the _D_B_-_>_p_u_t function in a database that supports duplicate records. Partial puts in databases supporting duplicate records must be done using a _d_b___c_u_r_s_o_r(3) function. It is an error to attempt a partial put with differing _d_l_e_n and _s_i_z_e values in a recno database with fixed- length records. For example, if the data portion of a retrieved record was 100 bytes, and a partial put was done using a DBT having a _d_l_e_n field of 20, a _d_o_f_f field of 85, and a _s_i_z_e field of 30, the result- ing record would be 115 bytes in length, where the last 30 bytes would be those specified by the put call. The default algorithm of associating returned key or data items with the DB handle returned by _d_b___o_p_e_n(3) will obvi- ously not work when DB handles are being used concurrently by multiple threads within a process, i.e, when DB_THREAD was specified to _d_b___o_p_e_n(3). WWhheenn mmuullttiippllee tthhrreeaaddss aarree uussiinngg tthhee rreettuurrnneedd DDBB hhaannddllee ccoonnccuurrrreennttllyy,, eeiitthheerr tthhee DDBB__DDBBTT__MMAALLLLOOCC oorr DDBB__DDBBTT__UUSSEERRMMEEMM ffllaaggss mmuusstt bbee ssppeecciiffiieedd ffoorr aannyy DDBBTT uusseedd ffoorr kkeeyy oorr ddaattaa rreettrriieevvaall.. LLOOGGIICCAALL RREECCOORRDD NNUUMMBBEERRSS In all cases for the recno access method, and when calling the _d_b_-_>_g_e_t and _c_u_r_s_o_r_-_>_c___g_e_t functions with the DB_SET_RECNO flag specified, the _d_a_t_a field of the key must be a pointer to a memory location of type _d_b___r_e_c_n_o___t, as typedef'd in the include file. This type is a 32-bit unsigned type, (which limits the number of logical records in a recno database, and the maximum logical record which may be directly retrieved from a btree database, to 4,294,967,296). The _s_i_z_e field of the key should be the size of that type, e.g., in the C program- ming language, ``sizeof(db_recno_t)''. Logical record numbers are 1-based, not 0-based, i.e., the first record in the database is record number 1. BBUUGGSS The DB access methods provide no guarantees about byte string alignment, and applications are responsible for maintaining any necessary alignment. The name DBT is a mnemonic for ``data base thang'', and was used because noone could think of a reasonable name that wasn't already in use somewhere else. SSEEEE AALLSSOO The DB library is a family of groups of functions that provides a modular programming interface to transactions and record-oriented file access. The library includes support for transactions, locking, logging and file page caching, as well as various indexed access methods. Many of the functional groups (e.g., the file page caching functions) are useful independent of the other DB func- tions, although some functional groups are explicitly based on other functional groups (e.g., transactions and logging). For a general description of the DB package, see _d_b___i_n_t_r_o(3). _d_b___a_r_c_h_i_v_e(1), _d_b___c_h_e_c_k_p_o_i_n_t(1), _d_b___d_e_a_d_l_o_c_k(1), _d_b___d_u_m_p(1), _d_b___l_o_a_d(1), _d_b___r_e_c_o_v_e_r(1), _d_b___s_t_a_t(1), _d_b___i_n_t_r_o(3), _d_b___a_p_p_i_n_i_t(3), _d_b___c_u_r_s_o_r(3), _d_b___d_b_m(3), _d_b___i_n_t_e_r_n_a_l(3), _d_b___l_o_c_k(3), _d_b___l_o_g(3), _d_b___m_p_o_o_l(3), _d_b___o_p_e_n(3), _d_b___t_h_r_e_a_d(3), _d_b___t_x_n(3)