db_lock



NAME

       db_lock - lock manager


SYNOPSIS

       #include <db.h>

       int
       lock_open(const char *dir,
            int 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, int flags,
            DB_LOCKREQ list[], int nlist, DB_LOCKREQ **elistp);

       int
       lock_get(DB_LOCKTAB *lt, u_int32_t locker, int 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, db_detect_t atype, int flags);


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 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 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
       treads or processes are using locking, the deadlock detec-
       tor, 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_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>).  It is expected that
       applications will use a single  DB_ENV  structure  as  the
       argument  to  all of the subsystems in the DB package.  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  DB_ENV  used  by  lock_open are described
       below.  As references to the DB_ENV structure may be main-
       tained  by  lock_open,  it  is  necessary  that the DB_ENV
       structure and memory it references be  valid  until  after
       the  lock_closed  function is called.  If dbenv is NULL or
       any of its fields are set to 0, defaults  appropriate  for
       the system are used where possible.

       The  following  DB_ENV  fields  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  structure  behave  as
            described for db_appinit(3).

       int (*db_yield)(void);
            The db_yield field of the DB_ENV structure behaves 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  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 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 dead-
                 lock.

            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 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.

       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 lock_open function returns the value of errno on fail-
       ure and 0 on success.

  lock_id
       The lock_id function copies a locker ID, which is  guaran-
       teed  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.  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 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  immedi-
            ately  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 opera-
       tions  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  suc-
       cess 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.  Note that
       the type of the lock argument  to  lock_put  is  different
       from  the  lock element found in the DB_LOCKREQ structure.
       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.)   Pro-
       cesses  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 cre-
       ated for  this  shared  region  will  be  removed,  as  if
       lock_unlink were called.

       When multiple threads are using the DB_LOCKTAB handle con-
       currently, 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  identi-
       fied  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 pro-
       cesses  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.   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 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_recovery(1)  or  the
       DB_RECOVER  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  dead-
       lock  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 transac-
       tions for abort and then returns.

       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  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 detec-
            tor was run.

       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.


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  vari-
            able  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)


COMPILING

       On IRIX, if you are compiling a threaded application,  you
       must compile with the -D_SGI_MP_SOURCE flag:

            cc -D_SGI_MP_SOURCE ...

       On OSF/1, if you are compiling a threaded application, you
       must compile with the -D_REENTRANT flag:

            cc -D_REENTRANT ...

       On Solaris, if you are compiling a  threaded  application,
       you  must compile with the -D_REENTRANT flag and link with
       the -lthread library:

            cc -D_REENTRANT ... -lthread


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),   fcntl(2),   fstat(2),   getpid(2),
       lseek(2),    mmap(2),   munmap(2),   open(2),   unlink(2),
       write(2),  abort(3),  db_version(3),  fflush(3),  free(3),
       getenv(3),  isdigit(3),  llseek(3),  lock_unlink(3),  mal-
       loc(3),  memcpy(3),  memset(3),  sigfillset(3),   sigproc-
       mask(3),   stat(3),   strcpy(3),  strdup(3),  strerror(3),
       strlen(3) and vsnprintf(3).

       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.

            TMPDIR If the dbenv argument to _open was NULL or not
            initialized  using  db_appinit, the environment vari-
            able TMPDIR may be used as the directory in which  to
            create the , as described in the _open section above.

       The lock_vec function may fail and return errno for any of
       the  errors  specified  for  the  following DB and library
       functions:  fcntl(2),   write(2),   getpid(2),   lseek(2),
       mmap(2), munmap(2), fflush(3), isprint(3), llseek(3), mem-
       cpy(3), memset(3), strerror(3) and vsnprintf(3).


       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), getpid(2),  lseek(2),  mmap(2),  mun-
       map(2),  write(2),  fflush(3),  llseek(3), memcpy(3), mem-
       set(3), strerror(3) and vsnprintf(3).


       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),  getpid(2),  lseek(2), mmap(2), mun-
       map(2), write(2), fflush(3),  llseek(3),  memcpy(3),  mem-
       set(3), strerror(3) and vsnprintf(3).


       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),  getpid(2),  munmap(2),
       fflush(3), free(3), strerror(3) and vsnprintf(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),   fstat(2),   getpid(2),
       mmap(2),    munmap(2),   open(2),   unlink(2),   abort(3),
       fflush(3), getenv(3),  isdigit(3),  malloc(3),  memcpy(3),
       memset(3),  sigfillset(3),  sigprocmask(3),  stat(3), str-
       cpy(3),    strdup(3),    strerror(3),    strlen(3)     and
       vsnprintf(3).


       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.


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_intro(3), db_load(1), db_recover(1), db_stat(1),
       db_appinit(3), db_cursor(3), db_dbm(3), db_lock(3), db_log(3),
       db_mpool(3), db_open(3), db_txn(3)