db_lock
#include <db.h>
int
lock_open(const char *dir,
u_int32_t flags, int mode, DB_ENV *dbenv, DB_LOCKTAB **regionp);
int
lock_id(DB_LOCKTAB *lt, u_int32_t *idp);
int
lock_vec(DB_LOCKTAB *lt, u_int32_t locker, u_int32_t flags,
DB_LOCKREQ list[], int nlist, DB_LOCKREQ **elistp);
int
lock_get(DB_LOCKTAB *lt, u_int32_t locker, u_int32_t flags,
const DBT *obj, const db_lockmode_t lock_mode, DB_LOCK *lock);
int
lock_put(DB_LOCKTAB *lt, DB_LOCK lock);
int
lock_close(DB_LOCKTAB *lt);
int
lock_unlink(const char *dir, int force, DB_ENV *dbenv);
int
lock_detect(DB_LOCKTAB *lt, u_int32_t flags, u_int32_t atype);
int
lock_stat(DB_LOCKTAB *lt,
DB_LOCK_STAT **spp, void *(*db_malloc)(size_t));
DESCRIPTION
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
functions, 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 db_intro(3).
This manual page describes the specific details of the
locking interface.
The db_lock 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, db_deadlock(1), should be run.
lock_open
The lock_open function copies a pointer, to the lock table
identified by the directory dir, into the memory location
referenced by regionp.
If the dbenv argument to lock_open was initialized using
db_appinit, dir is interpreted as described by
db_appinit(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
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 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 DB_LOCKTAB handle returned by the lock_open
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 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 the dbenv
argument to lock_open, which is a pointer to a structure
of type DB_ENV (typedef'd in <db.h>). Applications will
normally use the same DB_ENV structure (initialized by
db_appinit(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
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 bzero(3) or
memset(3).
The fields of the DB_ENV structure used by lock_open are
described below. If dbenv 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
initialized before calling lock_open:
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 db_appinit(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.
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
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 lock_open to
estimate how much space to allocate for various lock-
table data structures. If lk_max 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
lk_modes is 0, the value DB_LOCK_RW_N is used; see
below for a description of that value.
The lock_open function returns the value of errno on
failure and 0 on success.
lock_id
The lock_id function 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(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 lock_id function returns the value of errno on failure
and 0 on success.
lock_vec
The lock_vec function atomically obtains and releases one
or more locks from the specified table, lt. The lock_vec
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 locker argument specified to lock_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 lock_vec is typedef'd in <db.h>
as DB_LOCKREQ. A DB_LOCKREQ structure has at least the
following fields, which must be initialized before calling
lock_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 lock_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 lock_vec or
lock_put that releases the lock.)
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
lock_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 lock_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 lt'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 lock_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, lock_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 lock_vec function returns the value of
errno on failure and 0 on success.
lock_get
The lock_get function is a simple interface to the
lock_vec functionality, and is equivalent to calling the
lock_vec function with the lt and locker arguments, 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 lock_get is different from the obj element found in the
DB_LOCKREQ structure. The lock_get function returns
success and failure as described for the lock_vec
function.
lock_put
The lock_put function is a simple interface to the
lock_vec functionality, and is equivalent to calling the
lock_vec function with a single element list array, for
which the op field is DB_LOCK_PUT and the lock field is
represented by the argument of the same name. The
lock_put function returns success and failure as described
for the lock_vec function.
lock_close
The lock_close function disassociates the calling process
from the lock table lt. Note that lock_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 lock_vec call.
In addition, if the dir argument to lock_open was NULL and
dbenv was not initialized using db_appinit, all files
created for this shared region will be removed, as if
lock_unlink were called.
When multiple threads are using the DB_LOCKTAB handle
concurrently, only a single thread may call the lock_close
function.
The lock_close function returns the value of errno on
failure and 0 on success.
lock_unlink
The lock_unlink function 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
lock_open without calling lock_close (i.e., there are
processes currently using the lock table), lock_unlink
will fail without further action, unless the force flag is
set, in which case lock_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 db_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 lock_open
function, as lock table files are never created in any
directory other than the one specified to lock_open.
Note, however, that this has the potential to remove files
created by the other DB subsystems in this database
environment.
The lock_unlink function returns the value of errno on
failure and 0 on success.
lock_detect
The lock_detect function runs one iteration of the
deadlock detector on the specified table, lt. 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 DB_ENV
structure.
The lock_detect function returns the value of errno on
failure and 0 on success.
The lock_detect function 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.
lock_stat
The lock_stat 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
db_malloc is non-NULL, it is called to allocate the
memory, otherwise, the library function malloc(3) is used.
The function db_malloc must match the calling conventions
of the malloc(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 <db.h>). 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.
ENVIRONMENT VARIABLES
The following environment variables affect the execution
of db_lock:
DB_HOME
If the dbenv argument to lock_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 lock_open, as
described in db_appinit(3).
TMPDIR
If the dbenv argument to lock_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
lock_open section above.
STANDARD LOCK MODES
The include file <db.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.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
The lock_open function may fail and return errno 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),
sigprocmask(2), stat(2), strcpy(3), strdup(3),
strerror(3), strlen(3), unlink(2), and write(2).
In addition, the lock_open function may fail and 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 lock_vec function may fail and return errno 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 lock_vec function may fail and 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 lock_get function may fail and return errno 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 lock_get function may fail and return
errno for the following conditions:
[EINVAL]
An invalid flag value or parameter was specified.
The lock_put function may fail and return errno 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 lock_put function may fail and 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 lock_close function may fail and 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 lock_unlink function may fail and 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 lock_unlink function may fail and return
errno for the following conditions:
[EBUSY]
The shared memory region was in use and the force
flag was not set.
The lock_detect function may fail and 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_appinit(3), db_cursor(3), db_dbm(3), db_internal(3),
db_lock(3), db_log(3), db_mpool(3), db_open(3), db_thread(3),
db_txn(3)
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