DbLockTab
#include <db_cxx.h>
static int
DbLockTab::open(const char *dir,
int flags, int mode, DbEnv *dbenv, DbLockTab **regionp);
int
DbLockTab::id(u_int32_t *idp);
int
DbLockTab::vec(u_int32_t locker, int flags,
DB_LOCKREQ list[], int nlist, DB_LOCKREQ **elistp);
int
DbLockTab::get(u_int32_t locker, int 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(int flags, int 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
functions, 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.
unsigned int 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.
int 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
<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 handle
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 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 table represented by DbLockTab.
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 <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 <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.
SEE ALSO
db_archive(1), db_checkpoint(1), db_deadlock(1), db_dump(1),
db_load(1), db_recover(1), db_stat(1), db_intro(3),
db_internal(3), db_thread(3), Db(3), Dbc(3), DbEnv(3),
DbException(3), DbInfo(3), DbLock(3), DbLockTab(3), DbLog(3),
DbLsn(3), DbMpool(3), DbMpoolFile(3), Dbt(3), DbTxn(3),
DbTxnMgr(3)
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