<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> <title>Opening a Transactional Environment and Database </title> <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> <meta name="generator" content="DocBook XSL Stylesheets V1.62.4" /> <link rel="home" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" /> <link rel="up" href="enabletxn.html" title="Chapter 2. Enabling Transactions" /> <link rel="previous" href="enabletxn.html" title="Chapter 2. Enabling Transactions" /> <link rel="next" href="usingtxns.html" title="Chapter 3. Transaction Basics" /> </head> <body> <div class="navheader"> <table width="100%" summary="Navigation header"> <tr> <th colspan="3" align="center">Opening a Transactional Environment and Database </th> </tr> <tr> <td width="20%" align="left"><a accesskey="p" href="enabletxn.html">Prev</a> </td> <th width="60%" align="center">Chapter 2. Enabling Transactions</th> <td width="20%" align="right"> <a accesskey="n" href="usingtxns.html">Next</a></td> </tr> </table> <hr /> </div> <div class="sect1" lang="en" xml:lang="en"> <div class="titlepage"> <div> <div> <h2 class="title" style="clear: both"><a id="envopen"></a>Opening a Transactional Environment and <span>Database</span> </h2> </div> </div> <div></div> </div> <p> To enable transactions for your environment, you must initialize the transactional subsystem. Note that doing this also initializes the logging subsystem. In addition, you must initialize the memory pool (in-memory cache). Frequently, but not always, you will also initialize the locking subsystem. <span> For example: </span> </p> <p> Notice in the following example that you create your environment handle using the <tt class="function">db_env_create()</tt> function before you open the environment: </p> <pre class="programlisting">#include <stdio.h> #include <stdlib.h> #include "db.h" int main(void) { int ret, ret_c; u_int32_t env_flags; DB_ENV *envp; const char *db_home_dir = "/tmp/myEnvironment"; envp = NULL; /* Open the environment */ ret = db_env_create(&envp, 0); if (ret != 0) { fprintf(stderr, "Error creating environment handle: %s\n", db_strerror(ret)); return (EXIT_FAILURE); } env_flags = DB_CREATE | /* Create the environment if it does * not already exist. */ DB_INIT_TXN | /* Initialize transactions */ DB_INIT_LOCK | /* Initialize locking. */ DB_INIT_LOG | /* Initialize logging */ DB_INIT_MPOOL; /* Initialize the in-memory cache. */ ret = envp->open(envp, db_home_dir, env_flags, 0); if (ret != 0) { fprintf(stderr, "Error opening environment: %s\n", db_strerror(ret)); goto err; } err: /* Close the environment */ if (envp != NULL) { ret_c = envp->close(envp, 0); if (ret_c != 0) { fprintf(stderr, "environment close failed: %s\n", db_strerror(ret_c)); ret = ret_c; } } return (ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE); } </pre> <p> You then create and open your database(s) as you would for a non-transactional system. <span> The only difference is that you must pass the environment handle to the <span> <tt class="function">db_create()</tt> function, </span> and you must open the database within a transaction. Typically auto commit is used for this purpose. To do so, pass <tt class="literal">DB_AUTO_COMMIT</tt> to the database open command. Also, make sure you close all your databases before you close your environment. For example: </span> </p> <pre class="programlisting">#include <stdio.h> #include <stdlib.h> #include "db.h" int main(void) { int ret, ret_c; u_int32_t <b class="userinput"><tt>db_flags,</tt></b> env_flags; <b class="userinput"><tt>DB *dbp;</tt></b> DB_ENV *envp; const char *db_home_dir = "/tmp/myEnvironment"; <b class="userinput"><tt>const char *file_name = "mydb.db"; dbp = NULL;</tt></b> envp = NULL; /* Open the environment */ ret = db_env_create(&envp, 0); if (ret != 0) { fprintf(stderr, "Error creating environment handle: %s\n", db_strerror(ret)); return (EXIT_FAILURE); } env_flags = DB_CREATE | /* Create the environment if it does * not already exist. */ DB_INIT_TXN | /* Initialize transactions */ DB_INIT_LOCK | /* Initialize locking. */ DB_INIT_LOG | /* Initialize logging */ DB_INIT_MPOOL; /* Initialize the in-memory cache. */ ret = envp->open(envp, db_home_dir, env_flags, 0); if (ret != 0) { fprintf(stderr, "Error opening environment: %s\n", db_strerror(ret)); goto err; } <b class="userinput"><tt>/* Initialize the DB handle */ ret = db_create(&dbp, envp, 0); if (ret != 0) { envp->err(envp, ret, "Database creation failed"); goto err; } db_flags = DB_CREATE | DB_AUTO_COMMIT; ret = dbp->open(dbp, /* Pointer to the database */ NULL, /* Txn pointer */ file_name, /* File name */ NULL, /* Logical db name */ DB_BTREE, /* Database type (using btree) */ db_flags, /* Open flags */ 0); /* File mode. Using defaults */ if (ret != 0) { envp->err(envp, ret, "Database '%s' open failed", file_name); goto err; }</tt></b> err: <b class="userinput"><tt>/* Close the database */ if (dbp != NULL) { ret_c = dbp->close(dbp, 0); if (ret_c != 0) { envp->err(envp, ret_c, "Database close failed."); ret = ret_c } }</tt></b> /* Close the environment */ if (envp != NULL) { ret_c = envp->close(envp, 0); if (ret_c != 0) { fprintf(stderr, "environment close failed: %s\n", db_strerror(ret_c)); ret = ret_c; } } return (ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE); } </pre> <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"> <h3 class="title">Note</h3> <p> Never close a database that has active transactions. Make sure all transactions are resolved (either committed or aborted) before closing the database. </p> </div> </div> <div class="navfooter"> <hr /> <table width="100%" summary="Navigation footer"> <tr> <td width="40%" align="left"><a accesskey="p" href="enabletxn.html">Prev</a> </td> <td width="20%" align="center"> <a accesskey="u" href="enabletxn.html">Up</a> </td> <td width="40%" align="right"> <a accesskey="n" href="usingtxns.html">Next</a></td> </tr> <tr> <td width="40%" align="left" valign="top">Chapter 2. Enabling Transactions </td> <td width="20%" align="center"> <a accesskey="h" href="index.html">Home</a> </td> <td width="40%" align="right" valign="top"> Chapter 3. Transaction Basics</td> </tr> </table> </div> </body> </html>