<!--$Id: env_open.so,v 1.4 2002/06/24 14:51:07 bostic Exp $--> <!--Copyright (c) 1997,2008 Oracle. All rights reserved.--> <!--See the file LICENSE for redistribution information.--> <html> <head> <title>Berkeley DB Reference Guide: Opening the environment</title> <meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> <meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++"> </head> <body bgcolor=white> <table width="100%"><tr valign=top> <td><b><dl><dt>Berkeley DB Reference Guide:<dd>Berkeley DB Transactional Data Store Applications</dl></b></td> <td align=right><a href="../transapp/app.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../transapp/data_open.html"><img src="../../images/next.gif" alt="Next"></a> </td></tr></table> <p align=center><b>Opening the environment</b></p> <p>Creating transaction-protected applications using the Berkeley DB library is quite easy. Applications first use <a href="../../api_c/env_open.html">DB_ENV->open</a> to initialize the database environment. Transaction-protected applications normally require all four Berkeley DB subsystems, so the <a href="../../api_c/env_open.html#DB_INIT_MPOOL">DB_INIT_MPOOL</a>, <a href="../../api_c/env_open.html#DB_INIT_LOCK">DB_INIT_LOCK</a>, <a href="../../api_c/env_open.html#DB_INIT_LOG">DB_INIT_LOG</a>, and <a href="../../api_c/env_open.html#DB_INIT_TXN">DB_INIT_TXN</a> flags should be specified.</p> <p>Once the application has called <a href="../../api_c/env_open.html">DB_ENV->open</a>, it opens its databases within the environment. Once the databases are opened, the application makes changes to the databases inside of transactions. Each set of changes that entails a unit of work should be surrounded by the appropriate <a href="../../api_c/txn_begin.html">DB_ENV->txn_begin</a>, <a href="../../api_c/txn_commit.html">DB_TXN->commit</a>, and <a href="../../api_c/txn_abort.html">DB_TXN->abort</a> calls. The Berkeley DB access methods will make the appropriate calls into the Lock, Log and Memory Pool subsystems in order to guarantee transaction semantics. When the application is ready to exit, all outstanding transactions should have been committed or aborted.</p> <p>Databases accessed by a transaction must not be closed during the transaction. Once all outstanding transactions are finished, all open Berkeley DB files should be closed. When the Berkeley DB database files have been closed, the environment should be closed by calling <a href="../../api_c/env_close.html">DB_ENV->close</a>.</p> <p>The following code fragment creates the database environment directory then opens the environment, running recovery. Our <a href="../../api_c/env_class.html">DB_ENV</a> database environment handle is declared to be free-threaded using the <a href="../../api_c/env_open.html#DB_THREAD">DB_THREAD</a> flag, and so may be used by any number of threads that we may subsequently create.</p> <blockquote><pre>#include <sys/types.h> #include <sys/stat.h> <p> #include <errno.h> #include <stdarg.h> #include <stdlib.h> #include <string.h> #include <unistd.h> <p> #include <db.h> <p> #define ENV_DIRECTORY "TXNAPP" <p> void env_dir_create(void); void env_open(DB_ENV **); <p> int main(int argc, char *argv) { extern int optind; DB *db_cats, *db_color, *db_fruit; DB_ENV *dbenv; int ch; <p> while ((ch = getopt(argc, argv, "")) != EOF) switch (ch) { case '?': default: usage(); } argc -= optind; argv += optind; <p> env_dir_create(); env_open(&dbenv); <p> return (0); } <p> void env_dir_create() { struct stat sb; <p> /* * If the directory exists, we're done. We do not further check * the type of the file, DB will fail appropriately if it's the * wrong type. */ if (stat(ENV_DIRECTORY, &sb) == 0) return; <p> /* Create the directory, read/write/access owner only. */ if (mkdir(ENV_DIRECTORY, S_IRWXU) != 0) { fprintf(stderr, "txnapp: mkdir: %s: %s\n", ENV_DIRECTORY, strerror(errno)); exit (1); } } <p> void env_open(DB_ENV **dbenvp) { DB_ENV *dbenv; int ret; <p> /* Create the environment handle. */ if ((ret = db_env_create(&dbenv, 0)) != 0) { fprintf(stderr, "txnapp: db_env_create: %s\n", db_strerror(ret)); exit (1); } <p> /* Set up error handling. */ dbenv->set_errpfx(dbenv, "txnapp"); dbenv->set_errfile(dbenv, stderr); <p> /* * Open a transactional environment: * create if it doesn't exist * free-threaded handle * run recovery * read/write owner only */ if ((ret = dbenv->open(dbenv, ENV_DIRECTORY, DB_CREATE | DB_INIT_LOCK | DB_INIT_LOG | DB_INIT_MPOOL | DB_INIT_TXN | DB_RECOVER | DB_THREAD, S_IRUSR | S_IWUSR)) != 0) { (void)dbenv->close(dbenv, 0); fprintf(stderr, "dbenv->open: %s: %s\n", ENV_DIRECTORY, db_strerror(ret)); exit (1); } <p> *dbenvp = dbenv; }</pre></blockquote> <p>After running this initial program, we can use the <a href="../../utility/db_stat.html">db_stat</a> utility to display the contents of the environment directory:</p> <blockquote><pre>prompt> db_stat -e -h TXNAPP 3.2.1 Environment version. 120897 Magic number. 0 Panic value. 1 References. 6 Locks granted without waiting. 0 Locks granted after waiting. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Mpool Region: 4. 264KB Size (270336 bytes). -1 Segment ID. 1 Locks granted without waiting. 0 Locks granted after waiting. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Log Region: 3. 96KB Size (98304 bytes). -1 Segment ID. 3 Locks granted without waiting. 0 Locks granted after waiting. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Lock Region: 2. 240KB Size (245760 bytes). -1 Segment ID. 1 Locks granted without waiting. 0 Locks granted after waiting. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Txn Region: 5. 8KB Size (8192 bytes). -1 Segment ID. 1 Locks granted without waiting. 0 Locks granted after waiting.</pre></blockquote> <table width="100%"><tr><td><br></td><td align=right><a href="../transapp/app.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../transapp/data_open.html"><img src="../../images/next.gif" alt="Next"></a> </td></tr></table> <p><font size=1>Copyright (c) 1996,2008 Oracle. 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