NNAAMMEE db_lock - lock manager SSYYNNOOPPSSIISS ##iinncclluuddee <> iinntt lloocckk__ooppeenn((ccoonnsstt cchhaarr **ddiirr,, uu__iinntt3322__tt ffllaaggss,, iinntt mmooddee,, DDBB__EENNVV **ddbbeennvv,, DDBB__LLOOCCKKTTAABB ****rreeggiioonnpp));; iinntt lloocckk__iidd((DDBB__LLOOCCKKTTAABB **lltt,, uu__iinntt3322__tt **iiddpp));; iinntt lloocckk__vveecc((DDBB__LLOOCCKKTTAABB **lltt,, uu__iinntt3322__tt lloocckkeerr,, uu__iinntt3322__tt ffllaaggss,, DDBB__LLOOCCKKRREEQQ lliisstt[[]],, iinntt nnlliisstt,, DDBB__LLOOCCKKRREEQQ ****eelliissttpp));; iinntt lloocckk__ggeett((DDBB__LLOOCCKKTTAABB **lltt,, uu__iinntt3322__tt lloocckkeerr,, uu__iinntt3322__tt ffllaaggss,, ccoonnsstt DDBBTT **oobbjj,, ccoonnsstt ddbb__lloocckkmmooddee__tt lloocckk__mmooddee,, DDBB__LLOOCCKK **lloocckk));; iinntt lloocckk__ppuutt((DDBB__LLOOCCKKTTAABB **lltt,, DDBB__LLOOCCKK lloocckk));; iinntt lloocckk__cclloossee((DDBB__LLOOCCKKTTAABB **lltt));; iinntt lloocckk__uunnlliinnkk((ccoonnsstt cchhaarr **ddiirr,, iinntt ffoorrccee,, DDBB__EENNVV **ddbbeennvv));; iinntt lloocckk__ddeetteecctt((DDBB__LLOOCCKKTTAABB **lltt,, uu__iinntt3322__tt ffllaaggss,, uu__iinntt3322__tt aattyyppee));; iinntt lloocckk__ssttaatt((DDBB__LLOOCCKKTTAABB **lltt,, DDBB__LLOOCCKK__SSTTAATT ****sspppp,, vvooiidd **((**ddbb__mmaalllloocc))((ssiizzee__tt))));; DDEESSCCRRIIPPTTIIOONN 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). This manual page describes the specific details of the locking interface. The _d_b___l_o_c_k functions are the library interface intended to provide general-purpose locking. While designed to work with the other DB functions, these functions are also useful for more general locking purposes. Locks can be shared between processes. In most cases, when multiple threads or processes are using locking, the deadlock detector, _d_b___d_e_a_d_l_o_c_k(1), should be run. _l_o_c_k___o_p_e_n The _l_o_c_k___o_p_e_n function copies a pointer, to the lock table identified by the ddiirreeccttoorryy _d_i_r, into the memory location referenced by _r_e_g_i_o_n_p. If the _d_b_e_n_v argument to _l_o_c_k___o_p_e_n was initialized using _d_b___a_p_p_i_n_i_t, _d_i_r is interpreted as described by _d_b___a_p_p_i_n_i_t(3). Otherwise, if _d_i_r is not NULL, it is interpreted relative to the current working directory of the process. If _d_i_r is NULL, the following environment variables are checked in order: ``TMPDIR'', ``TEMP'', and ``TMP''. If one of them is set, lock table files are created relative to the directory it specifies. If none of them are set, the first possible one of the following directories is used: _/_v_a_r_/_t_m_p, _/_u_s_r_/_t_m_p, _/_t_e_m_p, _/_t_m_p, _C_:_/_t_e_m_p and _C_:_/_t_m_p. All files associated with the lock table are created in this directory. This directory must already exist when lock table_open is called. If the lock table already exists, the process must have permission to read and write the existing files. If the lock table does not already exist, it is optionally created and initialized. The _f_l_a_g_s and _m_o_d_e arguments specify how files will be opened and/or created when they don't already exist. The flags value is specified by oorr'ing together one or more of the following values: DB_CREATE Create any underlying files, as necessary. If the files do not already exist and the DB_CREATE flag is not specified, the call will fail. DB_THREAD Cause the DB_LOCKTAB handle returned by the _l_o_c_k___o_p_e_n function to be useable by multiple threads within a single address space, i.e., to be ``free-threaded''. All files created by the lock subsystem are created with mode _m_o_d_e (as described in _c_h_m_o_d(2)) and modified by the process' umask value at the time of creation (see _u_m_a_s_k(2)). The group ownership of created files is based on the system and directory defaults, and is not further specified by DB. The locking subsystem is configured based on the _d_b_e_n_v argument to _l_o_c_k___o_p_e_n, which is a pointer to a structure of type DB_ENV (typedef'd in ). Applications will normally use the same DB_ENV structure (initialized by _d_b___a_p_p_i_n_i_t(3)), as an argument to all of the subsystems in the DB package. References to the DB_ENV structure are maintained by DB, so it may not be discarded until the last close function, corresponding to an open function for which it was an argument, has returned. In order to ensure compatibility with future releases of DB, all fields of the DB_ENV structure that are not explicitly set should be initial- ized 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). The fields of the DB_ENV structure used by _l_o_c_k___o_p_e_n are described below. If _d_b_e_n_v is NULL or any of its fields are set to 0, defaults appropriate for the system are used where possible. The following fields in the DB_ENV structure may be ini- tialized before calling _l_o_c_k___o_p_e_n: void *(*db_errcall)(char *db_errpfx, char *buffer); FILE *db_errfile; const char *db_errpfx; int db_verbose; The error fields of the DB_ENV behave as described for _d_b___a_p_p_i_n_i_t(3). const u_int8_t lk_conflicts[][]; A _l_k___m_o_d_e_s by _l_k___m_o_d_e_s array. A non-0 value for the array element: lk_conflicts[requested_mode][held_mode] indicates that requested_mode and held_mode conflict. The ``not-granted'' mode must be represented by 0. If _l_k___c_o_n_f_l_i_c_t_s is NULL, the conflicts array _d_b___r_w___c_o_n_f_l_i_c_t_s is used; see the section below enti- tled ``STANDARD LOCK MODES'' for a description of that array. db_detect_t lk_detect; If non-0, specifies that the deadlock detector be run whenever a lock conflict occurs, and specifies which transaction should be aborted in the case of a dead- lock. The _l_k___d_e_t_e_c_t field must be set to one of the following values. DB_LOCK_DEFAULT Use the default policy as specified in the _d_b___d_e_a_d_l_o_c_k(1) man page. DB_LOCK_OLDEST Abort the oldest transaction. DB_LOCK_RANDOM Abort a random transaction involved in the dead- lock. DB_LOCK_YOUNGEST Abort the youngest transaction. u_int32_t lk_max; The maximum number of locks to be held or requested in the table. This value is used by _l_o_c_k___o_p_e_n to estimate how much space to allocate for various lock- table data structures. If _l_k___m_a_x is 0, a default value is used. u_int32_t lk_modes; The number of lock modes to be recognized by the lock table (including the ``not-granted'' mode). If _l_k___m_o_d_e_s is 0, the value DB_LOCK_RW_N is used; see below for a description of that value. The _l_o_c_k___o_p_e_n function returns the value of _e_r_r_n_o on fail- ure and 0 on success. _l_o_c_k___i_d The _l_o_c_k___i_d function copies a locker ID, which is guaran- teed to be unique in the specified lock table, into the memory location referenced by _i_d_p. The access methods (see _d_b___o_p_e_n(3)), generate a unique locker ID for each file that is opened with locking. Dur- ing DB access method operation, this locker ID will be used for all lock calls unless a transaction identifier was specified for the call, in which case the transaction ID specified is used for locking. The _l_o_c_k___i_d function returns the value of _e_r_r_n_o on failure and 0 on success. _l_o_c_k___v_e_c The _l_o_c_k___v_e_c function atomically obtains and releases one or more locks from the specified table, _l_t. The _l_o_c_k___v_e_c function is intended to support acquisition or trading of multiple locks under one lock table semaphore, as is needed for lock coupling or in multigranularity locking for lock escalation. The _l_o_c_k_e_r argument specified to _l_o_c_k___v_e_c is an unsigned 32-bit integer quantity. It represents the entity requesting or releasing the lock. The _f_l_a_g_s value must be set to 0 or the following value: DB_LOCK_NOWAIT If a lock cannot be granted because the requested lock conflicts with an existing lock, return immedi- ately instead of waiting for the lock to become available. The _l_i_s_t array provided to _l_o_c_k___v_e_c is typedef'd in as DB_LOCKREQ. A DB_LOCKREQ structure has at least the following fields, which must be initialized before calling _l_o_c_k___v_e_c: lockop_t op; The operation to be performed, which must be set to one of the following values: DB_LOCK_GET Get a lock, as defined by the values of _l_o_c_k_e_r, _o_b_j and _m_o_d_e. Upon return from _l_o_c_k___v_e_c, if the _l_o_c_k field is non-NULL, a reference to the acquired lock is stored there. (This reference is invalidated by any call to _l_o_c_k___v_e_c or _l_o_c_k___p_u_t that releases the lock.) DB_LOCK_PUT The lock referenced by the contents of the _l_o_c_k field is released. DB_LOCK_PUT_ALL All locks held by the _l_o_c_k_e_r are released. (Any locks acquired as a part of the current call to _l_o_c_k___v_e_c that appear after the DB_LOCK_PUT_ALL entry are not considered for this operation). DB_LOCK_PUT_OBJ All locks held by the _l_o_c_k_e_r, on the object _o_b_j, with the mode specified by _l_o_c_k___m_o_d_e, are released. A _l_o_c_k___m_o_d_e of DB_LOCK_NG indicates that all locks on the object should be released. Note that any locks acquired as a part of the current call to _l_o_c_k___v_e_c that occur before the DB_LOCK_PUT_OBJ will also be released; those acquired afterwards will not be released. const DBT obj; An untyped byte string that specifies the object to be locked or released. const lockmode_t mode; The lock mode, used as an index into _l_t's conflict array. DB_LOCK lock; A lock reference. The _n_l_i_s_t argument specifies the number of elements in the _l_i_s_t array. If any of the requested locks cannot be acquired, or any of the locks to be released cannot be released, the opera- tions before the failing operation are guaranteed to have completed successfully, and _l_o_c_k___v_e_c returns a non-zero value. In addition, if _e_l_i_s_t_p is not NULL, it is set to point to the DB_LOCKREQ entry that was being processed when the error occurred. In the case of an error, _l_o_c_k___v_e_c may return one of the following values: DB_LOCK_DEADLOCK The specified _l_o_c_k_e_r was selected as a victim in order to resolve a deadlock. DB_LOCK_NOTHELD The lock cannot be released, as it was not held by the _l_o_c_k_e_r. DB_LOCK_NOTGRANTED A lock was requested that could not be granted and the _f_l_a_g parameter was set to DB_LOCK_NOWAIT. In this case, if non-NULL, _e_l_i_s_t_p identifies the request that was granted. Otherwise, the _l_o_c_k___v_e_c function returns the value of _e_r_r_n_o on failure and 0 on success. _l_o_c_k___g_e_t The _l_o_c_k___g_e_t function is a simple interface to the _l_o_c_k___v_e_c functionality, and is equivalent to calling the _l_o_c_k___v_e_c function with the _l_t and _l_o_c_k_e_r arguments, _e_l_i_s_t_p and _c_o_n_f_l_i_c_t arguments, and a single element _l_i_s_t array, for which the _o_p field is DB_LOCK_GET, and the _o_b_j, _l_o_c_k___m_o_d_e and _l_o_c_k fields are represented by the arguments of the same name. Note that the type of the _o_b_j argument to _l_o_c_k___g_e_t is different from the _o_b_j element found in the DB_LOCKREQ structure. The _l_o_c_k___g_e_t function returns suc- cess and failure as described for the _l_o_c_k___v_e_c function. _l_o_c_k___p_u_t The _l_o_c_k___p_u_t function is a simple interface to the _l_o_c_k___v_e_c functionality, and is equivalent to calling the _l_o_c_k___v_e_c function with a single element _l_i_s_t array, for which the _o_p field is DB_LOCK_PUT and the _l_o_c_k field is represented by the argument of the same name. The _l_o_c_k___p_u_t function returns success and failure as described for the _l_o_c_k___v_e_c function. _l_o_c_k___c_l_o_s_e The _l_o_c_k___c_l_o_s_e function disassociates the calling process from the lock table _l_t. Note that _l_o_c_k___c_l_o_s_e does not release any locks still held by the closing process. (This provides functionality for long-lived locks.) Pro- cesses that wish to have all their locks released can do so by issuing the appropriate _l_o_c_k___v_e_c call. In addition, if the _d_i_r argument to _l_o_c_k___o_p_e_n was NULL and _d_b_e_n_v was not initialized using _d_b___a_p_p_i_n_i_t, all files cre- ated for this shared region will be removed, as if _l_o_c_k___u_n_l_i_n_k were called. When multiple threads are using the DB_LOCKTAB handle con- currently, only a single thread may call the _l_o_c_k___c_l_o_s_e function. The _l_o_c_k___c_l_o_s_e function returns the value of _e_r_r_n_o on failure and 0 on success. _l_o_c_k___u_n_l_i_n_k The _l_o_c_k___u_n_l_i_n_k function destroys the lock table identi- fied by the directory _d_i_r, removing all files used to implement the lock table. (The directory _d_i_r is not removed.) If there are processes that have called _l_o_c_k___o_p_e_n without calling _l_o_c_k___c_l_o_s_e (i.e., there are pro- cesses currently using the lock table), _l_o_c_k___u_n_l_i_n_k will fail without further action, unless the force flag is set, in which case _l_o_c_k___u_n_l_i_n_k will attempt to remove the lock table files regardless of any processes still using the lock table. The result of attempting to forcibly destroy the region when a process has the region open is unspecified. Pro- cesses using a shared memory region maintain an open file descriptor for it. On UNIX systems, the region removal should succeed and processes that have already joined the region should continue to run in the region without change, however processes attempting to join the lock table will either fail or attempt to create a new region. On other systems, e.g., WNT, where the _u_n_l_i_n_k(2) system call will fail if any process has an open file descriptor for the file, the region removal will fail. In the case of catastrophic or system failure, database recovery must be performed (see _d_b___r_e_c_o_v_e_r(1) or the DB_RECOVER and DB_RECOVER_FATAL flags to _d_b___a_p_p_i_n_i_t(3)). Alternatively, if recovery is not required because no database state is maintained across failures, it is possi- ble to clean up a lock table by removing all of the files in the directory specified to the _l_o_c_k___o_p_e_n function, as lock table files are never created in any directory other than the one specified to _l_o_c_k___o_p_e_n. Note, however, that this has the potential to remove files created by the other DB subsystems in this database environment. The _l_o_c_k___u_n_l_i_n_k function returns the value of _e_r_r_n_o on failure and 0 on success. _l_o_c_k___d_e_t_e_c_t The _l_o_c_k___d_e_t_e_c_t function runs one iteration of the dead- lock detector on the specified table, _l_t. The deadlock detector traverses the lock table, detects deadlocks, and if it finds one, marks one of the participating transac- tions for abort and then returns. The flags value is specified by oorr'ing together one or more of the following values: DB_LOCK_CONFLICT Only run the deadlock detector if a lock conflict has occurred since the last time that the deadlock detec- tor was run. The _a_t_y_p_e parameter specifies which transaction to abort in the case of deadlock. It must be set to one of values described above for the _l_k___d_e_t_e_c_t field of the _D_B___E_N_V structure. The _l_o_c_k___d_e_t_e_c_t function returns the value of _e_r_r_n_o on failure and 0 on success. The _l_o_c_k___d_e_t_e_c_t function is the underlying function used by the _d_b___d_e_a_d_l_o_c_k(1) utility. See the source code for the _d_b___d_e_a_d_l_o_c_k utility for an example of using _l_o_c_k___d_e_t_e_c_t in a UNIX environment. _l_o_c_k___s_t_a_t The _l_o_c_k___s_t_a_t function creates a statistical structure and copies a pointer to it into the user-specified memory location. Statistical structure are created in allocated memory. If _d_b___m_a_l_l_o_c is non-NULL, it is called to allocate the mem- ory, otherwise, the library function _m_a_l_l_o_c(3) is used. The function _d_b___m_a_l_l_o_c must match the calling conventions of the _m_a_l_l_o_c(3) library routine. Regardless, the caller is responsible for deallocating the returned memory. To deallocate the returned memory, free each returned memory pointer; pointers inside the memory do not need to be individually freed. The lock region statistics are stored in a structure of type DB_LOCK_STAT (typedef'd in ). The following DB_LOCK_STAT fields will be filled in: u_int32_t st_magic; The magic number that identifies a file as a lock file. u_int32_t st_version; The version of the lock file type. u_int32_t st_refcnt; The number of references to the region. u_int32_t st_regsize; The size of the region. u_int32_t st_maxlocks; The maximum number of locks possible. u_int32_t st_nmodes; The number of lock modes. u_int32_t st_numobjs; The number of unique objects locked. u_int32_t st_nlockers; The number of unique lockers. u_int32_t st_nconflicts; The total number of locks not immediately available due to conflicts. u_int32_t st_nrequests; The total number of locks requested. u_int32_t st_nreleases; The total number of locks released. u_int32_t st_ndeadlocks The number of deadlocks detected. u_int32_t st_region_wait; The number of times that a thread of control was forced to wait before obtaining the region lock. u_int32_t st_region_nowait; The number of times that a thread of control was able to obtain the region lock without waiting. EENNVVIIRROONNMMEENNTT VVAARRIIAABBLLEESS The following environment variables affect the execution of _d_b___l_o_c_k: DB_HOME If the _d_b_e_n_v argument to _l_o_c_k___o_p_e_n was initialized using _d_b___a_p_p_i_n_i_t, the environment variable DB_HOME may be used as the path of the database home for the interpretation of the _d_i_r argument to _l_o_c_k___o_p_e_n, as described in _d_b___a_p_p_i_n_i_t(3). TMPDIR If the _d_b_e_n_v argument to _l_o_c_k___o_p_e_n was NULL or not initialized using _d_b___a_p_p_i_n_i_t, the environment vari- able TMPDIR may be used as the directory in which to create the lock table, as described in the _l_o_c_k___o_p_e_n section above. SSTTAANNDDAARRDD LLOOCCKK MMOODDEESS The include file declares two commonly used con- flict arrays: const u_int8_t db_lock_rw_conflicts[]; This is a conflict array for a simple scheme using shared and exclusive lock modes. const u_int8_t db_lock_riw_conflicts[]; This is a conflict array that involves various intent lock modes (e.g., intent shared) that are used for multigranularity locking. Their associated sizes are DB_LOCK_RW_N and DB_LOCK_RIW_N. In addition, the include file defines the type _d_b___l_o_c_k_m_o_d_e___t, which is the type of the lock modes used with the standard tables above: DB_LOCK_NG not granted (always 0) DB_LOCK_READ read (shared) DB_LOCK_WRITE write (exclusive) EERRRROORRSS The _l_o_c_k___o_p_e_n function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: close(2), db_version(3), fcntl(2), fflush(3), lock_unlink(3), lseek(2), malloc(3), memcpy(3), memset(3), mmap(2), munmap(2), open(2), sigfillset(3), sigproc- mask(2), stat(2), strcpy(3), strdup(3), strerror(3), strlen(3), unlink(2), and write(2). In addition, the _l_o_c_k___o_p_e_n function may fail and return _e_r_r_n_o for the following conditions: [EAGAIN] The shared memory region was locked and (repeatedly) unavailable. [EINVAL] An invalid flag value or parameter was specified. The DB_THREAD flag was specified and spinlocks are not implemented for this architecture. The _l_o_c_k___v_e_c function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: fcntl(2), fflush(3), lock_detect(3), lseek(2), memcpy(3), memset(3), mmap(2), munmap(2), strerror(3), and write(2). In addition, the _l_o_c_k___v_e_c function may fail and return _e_r_r_n_o for the following conditions: [EACCES] An attempt was made to release lock held by another locker. [EINVAL] An invalid flag value or parameter was specified. The _l_o_c_k___g_e_t function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: fcntl(2), fflush(3), lock_detect(3), lseek(2), memcpy(3), memset(3), mmap(2), munmap(2), strerror(3), and write(2). In addition, the _l_o_c_k___g_e_t function may fail and return _e_r_r_n_o for the following conditions: [EINVAL] An invalid flag value or parameter was specified. The _l_o_c_k___p_u_t function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: fcntl(2), fflush(3), lock_detect(3), lseek(2), memcpy(3), memset(3), mmap(2), munmap(2), strerror(3), and write(2). In addition, the _l_o_c_k___p_u_t function may fail and return _e_r_r_n_o for the following conditions: [EACCES] An attempt was made to release lock held by another locker. [EINVAL] An invalid flag value or parameter was specified. The _l_o_c_k___c_l_o_s_e function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: close(2), fcntl(2), fflush(3), munmap(2), and strerror(3). The _l_o_c_k___u_n_l_i_n_k function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: close(2), fcntl(2), fflush(3), malloc(3), mem- cpy(3), memset(3), mmap(2), munmap(2), open(2), sig- fillset(3), sigprocmask(2), stat(2), strcpy(3), strdup(3), strerror(3), strlen(3), and unlink(2). In addition, the _l_o_c_k___u_n_l_i_n_k function may fail and return _e_r_r_n_o for the following conditions: [EBUSY] The shared memory region was in use and the force flag was not set. The _l_o_c_k___d_e_t_e_c_t function may fail and return _e_r_r_n_o for any of the errors specified for the following DB and library functions: calloc(3), fcntl(2), fflush(3), lseek(2), mal- loc(3), memcpy(3), memset(3), mmap(2), munmap(2), str- error(3), and write(2). BBUUGGSS If a process dies while holding locks, those locks remain held and are nneevveerr released. In this case, all processes should exit as quickly as possible, so that _d_b___r_e_c_o_v_e_r can be run. SSEEEE AALLSSOO _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)