Berkeley DB: txn_open
Полезная информация

txn_open


#include <db.h>

int txn_open(const char *dir, u_int32_t flags, int mode, DB_ENV *dbenv, DB_TXNMGR **regionp);

Description

The txn_open function copies a pointer, to the "transaction region" identified by the directory dir, into the memory location referenced by regionp.

The dir pathname argument is interpreted as described in Berkeley DB File Naming.

The flags and mode arguments specify how files will be opened and/or created if they do not already exist. The flags value is specified by logically 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 m4_reg(DbTxnMgr) handle returned by txn_open to be useable by multiple threads within a single address space, i.e., to be free-threaded.

DB_TXN_NOSYNC
On transaction commit, do not synchronously flush the log. This means that transactions exhibit the ACI (atomicity, consistency and isolation) properties, but not D (durability), i.e., database integrity will be maintained but it is possible that some number of the most recently committed transactions may be undone during recovery instead of being redone.

The number of transactions that are potentially at risk is governed by how often the log is checkpointed (see db_checkpoint for more information) and how many log updates can fit on a single log page.

All files created by the transaction 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 Berkeley DB.

The transaction subsystem is configured based on the dbenv argument to txn_open which is a pointer to a structure of type DB_ENV. Applications normally use the same DB_ENV structure (initialized by db_appinit) as an argument to all of the subsystems in the Berkeley DB package.

References to the DB_ENV structure are maintained by Berkeley 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. To ensure compatibility with future releases of Berkeley 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 one of the C library routines bzero(3) or memset(3).

The fields of the DB_ENV structure used by txn_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 txn_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.

DB_LOG *lg_info;
The logging region that is being used for this transaction environment. The lg_info field contains a return value from the log_open function. Logging is required for transaction environments, and it is an error to not specify a logging region.

DB_LOCKTAB *lk_info;
The locking region that is being used for this transaction environment. The lk_info field contains a return value from the lock_open function. If lk_info is NULL, no locking is done in this transaction environment.

u_int32_t tx_max;
The maximum number of simultaneous transactions that are supported. This bounds the size of backing files and is used to derive limits for the size of the lock region and logfiles. When there are more than tx_max concurrent transactions, calls to txn_begin may cause backing files to grow. If tx_max is 0, a default value is used.

int (*tx_recover)(DB_LOG *logp, DBT *log_rec, DB_LSN *lsnp, int redo, void *info);
A function that is called by txn_abort during transaction abort. This function takes five arguments:

logp
A pointer to the transaction log (DB_LOG *).

log_rec
A log record.

lsnp
A pointer to a log sequence number (DB_LSN *).

redo
An integer value that is set to one of the following values:

DB_TXN_BACKWARD_ROLL
The log is being read backward to determine which transactions have been committed and which transactions were not (and should therefore be aborted during recovery).

DB_TXN_FORWARD_ROLL
The log is being played forward, any transaction ids encountered that have not been entered into the list referenced by info should be ignored.

DB_TXN_OPENFILES
The log is being read to open all the files required to perform recovery.

DB_TXN_REDO
Redo the operation described by the log record.

DB_TXN_UNDO
Undo the operation described by the log record.

info
An opaque pointer used to reference the list of transaction IDs encountered during recovery.

If tx_recover is NULL, the default is that only Berkeley DB access method operations are transaction protected, and the default recover function will be used.

The txn_open function returns the value of errno on failure, and 0 on success.

Environment Variables

DB_HOME
If the dbenv argument to txn_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.

TMPDIR
If the dbenv argument to txn_open was NULL or not initialized using db_appinit the environment variable TMPDIR may be used as the directory in which to create the transaction region, as described in txn_open.

Errors

If a fatal error occurs in Berkeley DB, the txn_open function may fail and return DB_RUNRECOVERY, at which point all subsequent database calls will also return DB_RUNRECOVERY.

The txn_open function may fail and return errno for any of the errors specified for the following Berkeley DB and C library functions: abort(3), close(3), db_version, fcntl(3), fflush(3), fprintf(3), free(3), fstat(3), fsync(3), getenv(3), getpid(3), getuid(3), isdigit(3), lseek(3), malloc(3), memcpy(3), memset(3), mmap(3), munmap(3), open(3), pstat_getdynamic(3), read(3), shmat(3), shmctl(3), shmdt(3), sigfillset(3), sigprocmask(3), stat(3), strerror(3), strlen(3), sysconf(3), time(3), txn_unlink, unlink(3), vfprintf(3), vsnprintf(3), and write(3).

In addition, the txn_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 dbenv parameter was NULL.

See Also

txn_abort, txn_begin, txn_checkpoint, txn_close, txn_commit, txn_id, txn_open, txn_prepare, txn_stat and txn_unlink.