DbLockTab(3db)

NAME

DbLockTab - lock manager

SYNOPSIS

#include <db2/db_cxx.h>
static int
DbLockTab::open(const char *dir,
u_int32_t flags, int mode, DbEnv *dbenv, DbLockTab **regionp);
int
DbLockTab::id(u_int32_t *idp);
int
DbLockTab::vec(u_int32_t locker, u_int32_t flags,
DB_LOCKREQ list[], int nlist, DB_LOCKREQ **elistp);
int
DbLockTab::get(u_int32_t locker, u_int32_t flags,
const Dbt *obj, const db_lockmode_t lock_mode, DB_LOCK *lock);
int
DbLockTab::close();
static int
DbLockTab::unlink(const char *dir, int force, DbEnv *dbenv);
int
DbLockTab::detect(u_int32_t flags, u_int32_t atype);

DESCRIPTION

The DB library is a family of classes 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 classes (e.g., the file page caching class) are useful independent of the other DB classes, although some classes are explicitly based on other classes (e.g., transactions and logging). For a general description of the DB package, see db_intro(3).

This manual page describes the specific details of the locking interface.

The DbLockTab class is intended to provide general-purpose locking. While designed to work with the other Db classes, this class is also useful for more general locking purposes. Locks can be shared between processes. In most cases, when multiple treads or processes are using locking, the deadlock detector, db_deadlock(1), should be run.

DbLockTab::open The DbLockTab::open method copies a pointer, to the lock table identified by the directory dir, into the memory location referenced by regionp.

If the dbenv argument to DbLockTab::open was initialized using DbEnv::appinit, dir is interpreted as described by DbEnv(3).

Otherwise, if dir is not NULL, it is interpreted relative to the current working directory of the process. If dir 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: /var/tmp, /usr/tmp, /temp, /tmp, C:/temp and C:/tmp.

All files associated with the lock table are created in this directory. This directory must already exist when DbLockTab::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 flags and mode arguments specify how files will be opened and/or created when they don't already exist. The flags value is specified by or'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 DbLockTab handle returned by the DbLockTab::open method 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 mode (as described in chmod(2)) and modified by the process' umask value at the time of creation (see umask(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 which set methods have been used. It is expected that applications will use a single DbEnv object as the argument to all of the subsystems in the DB package. The fields of the DbEnv object used by DbLockTab::open are described below. As references to the DbEnv object may be maintained by DbLockTab::open, it is necessary that the DbEnv object and memory it references be valid until the object is destroyed. Any of the DbEnv fields that are not explicitly set will default to appropriate values.

The following fields in the DbEnv object may be initialized, using the appropriate set method, before calling DbLockTab::open:

void *(*db_errcall)(char *db_errpfx, char *buffer);

FILE *db_errfile;

const char *db_errpfx;

class ostream *db_error_stream;

int db_verbose;: The error fields of the DbEnv behave as described for DbEnv(3).

const u_int8_t lk_conflicts[][];

A lk_modes by lk_modes 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 lk_conflicts is NULL, the conflicts array db_rw_conflicts is used; see the section below entitled ``STANDARD LOCK MODES'' for a description of that array.

u_int32_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 deadlock. The lk_detect field must be set to one of the following values.

DB_LOCK_DEFAULT: Use the default policy as specified in the db_deadlock(1) man page.

DB_LOCK_OLDEST: Abort the oldest transaction.

DB_LOCK_RANDOM: Abort a random transaction involved in the deadlock.

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 DbLockTab::open to estimate how much space to allocate for various lock-table data structures. If lk_max is not explicitly set, 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 lk_modes is 0, the value DB_LOCK_RW_N is used; see below for a description of that value.

The DbLockTab::open method throws a DbException(3) or returns the value of errno on failure and 0 on success.

DbLockTab::id The DbLockTab::id method copies a locker ID, which is guaranteed to be unique in the specified lock table, into the memory location referenced by idp.

The access methods (see Db::open in Db(3)), generate a unique locker ID for each file that is opened with locking. During 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 DbLockTab::id method throws a DbException(3) or returns the value of errno on failure and 0 on success.

DbLockTab::vec The DbLockTab::vec method atomically obtains and releases one or more locks from the specified table. The DbLockTab::vec method 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 locker argument specified to DbLockTab::vec is an unsigned 32-bit integer quantity. It represents the entity requesting or releasing the lock.

The flags 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 immediately instead of waiting for the lock to become available.

The list array provided to DbLockTab::vec is typedef'd in <db2/db_cxx.h> as DB_LOCKREQ. A DB_LOCKREQ structure has at least the following fields, which must be initialized before calling DbLockTab::vec:

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 locker, obj and mode. Upon return from DbLockTab::vec, if the lock field is non-NULL, a reference to the acquired lock is stored there. (This reference is invalidated by any call to DbLockTab::vec or DbLock::put that releases the lock.) See DbLock(3).

DB_LOCK_PUT: The lock referenced by the contents of the lock field is released.

DB_LOCK_PUT_ALL: All locks held by the locker are released. (Any locks acquired as a part of the current call to DbLockTab::vec that appear after the DB_LOCK_PUT_ALL entry are not considered for this operation).

DB_LOCK_PUT_OBJ: All locks held by the locker, on the object obj, with the mode specified by lock_mode, are released. A lock_mode 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 DbLockTab::vec 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 object's conflict array.

DB_LOCK lock;: A lock reference.

The nlist argument specifies the number of elements in the list array.

If any of the requested locks cannot be acquired, or any of the locks to be released cannot be released, the operations before the failing operation are guaranteed to have completed successfully, and DbLockTab::vec returns a non-zero value. In addition, if elistp 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, DbLockTab::vec may return one of the following values:

DB_LOCK_DEADLOCK: The specified locker 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 locker.

DB_LOCK_NOTGRANTED: A lock was requested that could not be granted and the flag parameter was set to DB_LOCK_NOWAIT. In this case, if non-NULL, elistp identifies the request that was granted.

Otherwise, the DbLockTab::vec method throws a DbException(3) or returns the value of errno on failure and 0 on success.

DbLockTab::get The DbLockTab::get method is a simple interface to the DbLockTab::vec functionality, and is equivalent to calling the DbLockTab::vec method with the locker argument, elistp and conflict arguments, and a single element list array, for which the op field is DB_LOCK_GET, and the obj, lock_mode and lock fields are represented by the arguments of the same name. Note that the type of the obj argument to DbLockTab::get is different from the obj element found in the DB_LOCKREQ structure. The DbLockTab::get method returns success and failure as described for the DbLockTab::vec method.

DbLockTab::close The DbLockTab::close method disassociates the calling process from the lock table. The object should not be used after a call to close. Note that DbLockTab::close does not release any locks still held by the closing process. (This provides functionality for long-lived locks.) Processes that wish to have all their locks released can do so by issuing the appropriate DbLockTab::vec call.

In addition, if the dir argument to DbLockTab::open was NULL and dbenv was not initialized using DbEnv::appinit, all files created for this shared region will be removed, as if DbLockTab::unlink were called.

When multiple threads are using the DbLockTab object concurrently, only a single thread may call the DbLockTab::close method.

The DbLockTab::close method throws a DbException(3) or returns the value of errno on failure and 0 on success.

DbLockTab::unlink The DbLockTab::unlink method destroys the lock table identified by the directory dir, removing all files used to implement the lock table. (The directory dir is not removed.) If there are processes that have called DbLockTab::open without calling DbLockTab::close (i.e., there are processes currently using the lock table), DbLockTab::unlink will fail without further action, unless the force flag is set, in which case DbLockTab::unlink 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. Processes 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 unlink(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 db_recover(1) or the DB_RECOVER and DB_RECOVER_FATAL flags to DbEnv::appinit(3)). Alternatively, if recovery is not required because no database state is maintained across failures, it is possible to clean up a lock table by removing all of the files in the directory specified to the DbLockTab::open method, as lock table files are never created in any directory other than the one specified to DbLockTab::open. Note, however, that this has the potential to remove files created by the other DB subsystems in this database environment.

The DbLockTab::unlink method throws a DbException(3) or returns the value of errno on failure and 0 on success.

DbLockTab::detect The DbLockTab::detect method runs one iteration of the deadlock detector on the current lock table. The deadlock detector traverses the lock table, detects deadlocks, and if it finds one, marks one of the participating transactions for abort and then returns.

The flags value is specified by or'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 detector was run.

The atype parameter specifies which transaction to abort in the case of deadlock. It must be set to one of values described above for the lk_detect field of the DbEnv object.

The DbLockTab::detect method throws a DbException(3) or returns the value of errno on failure and 0 on success.

The DbLockTab::detect method is based on the C lock_detect function, which is the underlying function used by the db_deadlock(1) utility. See the source code for the db_deadlock utility for an example of using lock_detect in a UNIX environment.

ENVIRONMENT VARIABLES

The following environment variables affect the execution of db_lock:

DB_HOME: If the dbenv argument to DbLockTab::open was initialized using db_appinit, the environment variable DB_HOME may be used as the path of the database home for the interpretation of the dir argument to DbLockTab::open, as described in db_appinit(3).

TMPDIR: If the dbenv argument to DbLockTab::open was NULL or not initialized using db_appinit, the environment variable TMPDIR may be used as the directory in which to create the lock table, as described in the DbLockTab::open section above.

STANDARD LOCK MODES

The include file <db2/db_cxx.h> declares two commonly used conflict 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 <db2/db_cxx.h> defines the type db_lockmode_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)

ERRORS

Methods marked as returning errno will, by default, throw an exception that encapsulates the error information. The default error behavior can be changed, see DbException(3).

The DbLockTab::open method may fail and throw a DbException(3) or return errno for any of the errors specified for the following DB and library functions: DbLock::unlink(3), close(2), db_version(3), fcntl(2), fflush(3), lseek(2), malloc(3), memcpy(3), memset(3), mmap(2), munmap(2), open(2), sigfillset(3), sigprocmask(2), stat(2), strcpy(3), strdup(3), strerror(3), strlen(3), unlink(2), and write(2).

In addition, the DbLockTab::open method may fail and throw a DbException(3) or return errno 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 DbLockTab::vec method may fail and throw a DbException(3) or return errno for any of the errors specified for the following DB and library functions: DbLock::detect(3), fcntl(2), fflush(3), lseek(2), memcpy(3), memset(3), mmap(2), munmap(2), strerror(3), and write(2).

In addition, the DbLockTab::vec method may fail and throw a DbException(3) or return errno 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 DbLockTab::get method may fail and throw a DbException(3) or return errno for any of the errors specified for the following DB and library functions: DbLock::detect(3), fcntl(2), fflush(3), lseek(2), memcpy(3), memset(3), mmap(2), munmap(2), strerror(3), and write(2).

In addition, the DbLockTab::get method may fail and throw a DbException(3) or return errno for the following conditions:

[EINVAL]: An invalid flag value or parameter was specified.

The DbLockTab::close method may fail and throw a DbException(3) or return errno 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 DbLockTab::unlink method may fail and throw a DbException(3) or return errno for any of the errors specified for the following DB and library functions: close(2), fcntl(2), fflush(3), malloc(3), memcpy(3), memset(3), mmap(2), munmap(2), open(2), sigfillset(3), sigprocmask(2), stat(2), strcpy(3), strdup(3), strerror(3), strlen(3), and unlink(2).

In addition, the DbLockTab::unlink method may fail and throw a DbException(3) or return errno for the following conditions:

[EBUSY]: The shared memory region was in use and the force flag was not set.

The DbLockTab::detect method may fail and throw a DbException(3) or return errno for any of the errors specified for the following DB and library functions: calloc(3), fcntl(2), fflush(3), lseek(2), malloc(3), memcpy(3), memset(3), mmap(2), munmap(2), strerror(3), and write(2).

BUGS

If a process dies while holding locks, those locks remain held and are never released. In this case, all processes should exit as quickly as possible, so that db_recover can be run.