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          <th colspan="3" align="center">Secondary Database Example</th>
        </tr>
        <tr>
          <td width="20%" align="left"><a accesskey="p" href="joins.html">Prev</a> </td>
          <th width="60%" align="center">Chapter 5. Secondary Databases</th>
          <td width="20%" align="right"> <a accesskey="n" href="dbconfig.html">Next</a></td>
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    <div class="sect1" lang="en" xml:lang="en">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title" style="clear: both"><a id="coreindexusage"></a>Secondary Database Example</h2>
          </div>
        </div>
      </div>
      <div class="toc">
        <dl>
          <dt>
            <span class="sect2">
              <a href="coreindexusage.html#edlWIndexes">Secondary Databases with example_database_load</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="coreindexusage.html#edrWIndexes">Secondary Databases with example_database_read</a>
            </span>
          </dt>
        </dl>
      </div>
      <p>
        In previous chapters in this book, we built applications that load
        and display several DB databases. In this example, we will extend those
        examples to use secondary databases. Specifically:
    </p>
      <div class="itemizedlist">
        <ul type="disc">
          <li>
            <p>
            In 
                <a class="xref" href="DbUsage.html" title="Database Usage Example">Database Usage Example</a> 
                 
            we built an application that can open and load data into several databases.
            In <a class="xref" href="coreindexusage.html#edlWIndexes" title="Secondary Databases with example_database_load">Secondary Databases with example_database_load</a> we will extend
            that application to also open a secondary database for the purpose
            of indexing inventory item names.
        </p>
          </li>
          <li>
            <p>
            In <a class="xref" href="CoreCursorUsage.html" title="Cursor Example">Cursor Example</a> we
            built an application to display our inventory database (and related
            vendor information). In 
            <a class="xref" href="coreindexusage.html#edrWIndexes" title="Secondary Databases with example_database_read">Secondary Databases with example_database_read</a>
            we will extend that application to
            show inventory records based on the index we cause to be loaded using
            <code class="function">example_database_load</code>.
        </p>
          </li>
        </ul>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="edlWIndexes"></a>Secondary Databases with example_database_load</h3>
            </div>
          </div>
        </div>
        <p>
            <code class="function">example_database_load</code> uses several utility
            functions to open and close its databases. In order to cause 
            <code class="function">example_database_load</code> to maintain an index of
            inventory item names, all we really need to do is update the utility
            functions to:
        </p>
        <div class="orderedlist">
          <ol type="1">
            <li>
              <p>
                    Create a new database to be used as a secondary database.
                </p>
            </li>
            <li>
              <p>
                    Associate our new database to the inventory primary
                    database.
                </p>
            </li>
            <li>
              <p>
                    Close the secondary database when we close the rest of our
                    databases.
                </p>
            </li>
          </ol>
        </div>
        <p>
            We also need a function that can create our secondary keys for us.
        </p>
        <p>
            Because DB maintains secondary databases for us; once this work
            is done we need not make any other changes to <code class="function">example_database_load</code>. 

            <span>Therefore, we can limit
            all our work to the code found in <code class="filename">gettingstarted_common.h</code>
            and <code class="filename">gettingstarted_common.c</code>.
            </span>
        </p>
        <p>
            Remember that you can find the complete implementation of these functions
            in:
        </p>
        <pre class="programlisting"><span class="emphasis"><em>DB_INSTALL</em></span>/examples_c/getting_started</pre>
        <p>
            where <code class="literal"><span class="emphasis"><em>DB_INSTALL</em></span></code> is the location where you
            placed your DB distribution.
        </p>
        <p>
            To begin, we need to update the <code class="literal">stock_dbs</code>
            structure to accommodate the additional database. We defined this
            structure in <code class="filename">gettingstarted_common.h</code>. We can
            limit our update to this file to just that structure definition:
        </p>
        <p>
            Remember that new code is in <strong class="userinput"><code>bold</code></strong>.
        </p>
        <a id="c_index10"></a>
        <pre class="programlisting">/* file: gettingstarted_common.h */
#include &lt;db.h&gt;

typedef struct stock_dbs {
    DB *inventory_dbp; /* Database containing inventory information */
    DB *vendor_dbp;    /* Database containing vendor information */
    <strong class="userinput"><code>DB *itemname_sdbp; /* Index based on the item name index */</code></strong>

    char *db_home_dir;       /* Directory containing the database files */
    <strong class="userinput"><code>char *itemname_db_name;  /* Itemname secondary database */</code></strong>
    char *inventory_db_name; /* Name of the inventory database */
    char *vendor_db_name;    /* Name of the vendor database */
} STOCK_DBS;

/* Function prototypes */
int databases_setup(STOCK_DBS *, const char *, FILE *);
int databases_close(STOCK_DBS *);
void initialize_stockdbs(STOCK_DBS *);
int open_database(DB **, const char *, const char *, FILE *, <strong class="userinput"><code>int</code></strong>);
void set_db_filenames(STOCK_DBS *my_stock);

</pre>
        <p>
            Because we updated our stock_dbs structure, we need to update our
            stock_dbs utility functions 
            (<a class="xref" href="CoreDbUsage.html#stock-db-functions" title="Example 2.2 The stock_db Utility Functions">The stock_db Utility Functions</a>)
            accordingly. The updates are trivial and so we won't show them here
            in the interest of space. You can find their complete implementation
            in the <code class="filename">gettingstarted_common.c</code> file
            accompanying this example in your DB distribution.
        </p>
        <p>
            More importantly, however, we need to go to 
            <code class="filename">gettingstarted_common.c</code>
            and create our secondary key extractor function. When we
            store our inventory items, we place the item name in the buffer
            immediately after a <code class="literal">float</code> and an
            <code class="literal">int</code>, so retrieving the string from the buffer is
            fairly easy to do:
        </p>
        <a id="c_index11"></a>
        <pre class="programlisting">/* file: gettingstarted_common.c */
#include "gettingstarted_common.h"

<strong class="userinput"><code>/*
 * Used to extract an inventory item's name from an
 * inventory database record. This function is used to create
 * keys for secondary database records.
 */
int
get_item_name(DB *dbp, const DBT *pkey, const DBT *pdata, DBT *skey)
{
    int offset;

    /*
     * First, obtain the buffer location where we placed the
     * item's name. In this example, the item's name is located
     * in the primary data. It is the first string in the
     * buffer after the price (a float) and the quantity (an int).
     *
     * See load_inventory_database() in example_database_load.c
     * for how we marshalled the inventory information into the
     * data DBT.
     */
    offset = sizeof(float) + sizeof(int);

    /* Check to make sure there's data */
    if (pdata-&gt;size &lt; offset)
        return (-1); /* Returning non-zero means that the
                      * secondary record is not created/updated.
                      */

    /* Now set the secondary key's data to be the item name */
    memset(skey, 0, sizeof(DBT));
    skey-&gt;data = pdata-&gt;data + offset;
    skey-&gt;size = strlen(skey-&gt;data) + 1;

    return (0);
}</code></strong> </pre>
        <p>
        Having completed that function, we need to update 
        <code class="function">set_db_filenames()</code> and 
        <code class="function">initialize_stockdbs()</code> to handle the
        new secondary databases that our application will now use.
        These functions were originally introduced in 
        <a class="xref" href="CoreDbUsage.html#stock-db-functions" title="Example 2.2 The stock_db Utility Functions">The stock_db Utility Functions</a>.
    </p>
        <a id="c_index11.1"></a>
        <pre class="programlisting">
/* Initializes the STOCK_DBS struct.*/
void
initialize_stockdbs(STOCK_DBS *my_stock)
{
    my_stock-&gt;db_home_dir = DEFAULT_HOMEDIR;
    my_stock-&gt;inventory_dbp = NULL;
    my_stock-&gt;vendor_dbp = NULL;
    <strong class="userinput"><code>my_stock-&gt;itemname_sdbp = NULL;</code></strong>

    my_stock-&gt;inventory_db_name = NULL;
    my_stock-&gt;vendor_db_name = NULL;
    <strong class="userinput"><code>my_stock-&gt;itemname_db_name = NULL;</code></strong>
}

/* Identify all the files that will hold our databases. */
void
set_db_filenames(STOCK_DBS *my_stock)
{
    size_t size;

    /* Create the Inventory DB file name */
    size = strlen(my_stock-&gt;db_home_dir) + strlen(INVENTORYDB) + 1;
    my_stock-&gt;inventory_db_name = malloc(size);
    snprintf(my_stock-&gt;inventory_db_name, size, "%s%s",
      my_stock-&gt;db_home_dir, INVENTORYDB);
    
    /* Create the Vendor DB file name */
    size = strlen(my_stock-&gt;db_home_dir) + strlen(VENDORDB) + 1;
    my_stock-&gt;vendor_db_name = malloc(size);
    snprintf(my_stock-&gt;vendor_db_name, size, "%s%s",
      my_stock-&gt;db_home_dir, VENDORDB);

    <strong class="userinput"><code>/* Create the itemname DB file name */
    size = strlen(my_stock-&gt;db_home_dir) + strlen(ITEMNAMEDB) + 1;
    my_stock-&gt;itemname_db_name = malloc(size);
    snprintf(my_stock-&gt;itemname_db_name, size, "%s%s",
      my_stock-&gt;db_home_dir, ITEMNAMEDB);</code></strong>
} </pre>
        <p>
        We also need to update the
        <code class="function">open_database()</code> (as described in
        <a class="xref" href="CoreDbUsage.html#open-db" title="Example 2.3 open_database() Function">open_database() Function</a>)
        to take special actions if we are
        opening a secondary database. Unlike our primary databases, we want to
        support sorted duplicates for our secondary database. This is because we
        are indexing based on an item's name, and item names are
        shared by multiple inventory records. As a result every key the secondary
        database (an item name) will be used by multiple records (pointers to
        records in our primary database). We allow this by configuring our
        secondary database to support duplicate records. Further, because
        BTrees perform best when their records are sorted, we go ahead and
        configure our secondary database for sorted duplicates.
    </p>
        <p>
        To do this, we add a parameter to the function that indicates whether we are
        opening a secondary database, and we add in the few lines of code
        necessary to set the sorted duplicates flags.
    </p>
        <a id="c_index12"></a>
        <pre class="programlisting">/* Opens a database */
int
open_database(DB **dbpp,       /* The DB handle that we are opening */
    const char *file_name,     /* The file in which the db lives */
    const char *program_name,  /* Name of the program calling this
                                * function */
    FILE *error_file_pointer,
    <strong class="userinput"><code>int is_secondary</code></strong>)
{
    DB *dbp;    /* For convenience */
    u_int32_t open_flags;
    int ret;

    /* Initialize the DB handle */
    ret = db_create(&amp;dbp, NULL, 0);
    if (ret != 0) {
        fprintf(error_file_pointer, "%s: %s\n", program_name,
                db_strerror(ret));
        return (ret);
    }

    /* Point to the memory malloc'd by db_create() */
    *dbpp = dbp;

    /* Set up error handling for this database */
    dbp-&gt;set_errfile(dbp, error_file_pointer);
    dbp-&gt;set_errpfx(dbp, program_name);

    <strong class="userinput"><code>/*
     * If this is a secondary database, then we want to allow
     * sorted duplicates.
     */
    if (is_secondary) {
        ret = dbp-&gt;set_flags(dbp, DB_DUPSORT);
        if (ret != 0) {
            dbp-&gt;err(dbp, ret, "Attempt to set DUPSORT flag failed.",
              file_name);
            return (ret);
        }
    }</code></strong>

    /* Set the open flags */
    open_flags = DB_CREATE;

    /* Now open the database */
    ret = dbp-&gt;open(dbp,        /* Pointer to the database */
                    NULL,       /* Txn pointer */
                    file_name,  /* File name */
                    NULL,       /* Logical db name (unneeded) */
                    DB_BTREE,   /* Database type (using btree) */
                    open_flags, /* Open flags */
                    0);         /* File mode. Using defaults */
    if (ret != 0) {
        dbp-&gt;err(dbp, ret, "Database '%s' open failed.", file_name);
        return (ret);
    }

    return (ret);
}   </pre>
        <p>
        That done, we can now update <code class="function">databases_setup()</code> 
        (see <a class="xref" href="CoreDbUsage.html#databasesetup" title="Example 2.4 The databases_setup() Function">The databases_setup() Function</a>) to create
        and open our secondary database. To do this, we have to add a flag to
        each call to <code class="function">open_database()</code> that indicates whether
        the database is a secondary. We also have to associate our secondary
        database with the inventory database (the primary). 
    </p>
        <p>
        Note that we do not anywhere in this example show the definition of
        <code class="literal">PRIMARY_DB</code> and <code class="literal">SECONDARY_DB</code>. See
        <code class="filename">gettingstarted_common.h</code> in your DB examples
        directory for those definitions (they are just <code class="literal">0</code> and
        <code class="literal">1</code>, respectively).
    </p>
        <a id="c_index13"></a>
        <pre class="programlisting">/* opens all databases */
int
databases_setup(STOCK_DBS *my_stock, const char *program_name,
  FILE *error_file_pointer)
{
    int ret;

    /* Open the vendor database */
    ret = open_database(&amp;(my_stock-&gt;vendor_dbp),
      my_stock-&gt;vendor_db_name,
      program_name, error_file_pointer,
      <strong class="userinput"><code>PRIMARY_DB</code></strong>);
    if (ret != 0)
        /*
         * Error reporting is handled in open_database() so just return
         * the return code here.
         */
        return (ret);

    /* Open the inventory database */
    ret = open_database(&amp;(my_stock-&gt;inventory_dbp),
      my_stock-&gt;inventory_db_name,
      program_name, error_file_pointer,
      <strong class="userinput"><code>PRIMARY_DB</code></strong>);
    if (ret != 0)
        /*
         * Error reporting is handled in open_database() so just return
         * the return code here.
         */
        return (ret);

    <strong class="userinput"><code>/*
     * Open the itemname secondary database. This is used to
     * index the product names found in the inventory
     * database.
     */
    ret = open_database(&amp;(my_stock-&gt;itemname_sdbp),
      my_stock-&gt;itemname_db_name,
      program_name, error_file_pointer,
      SECONDARY_DB);
    if (ret != 0)
        /*
         * Error reporting is handled in open_database() so just return
         * the return code here.
         */
        return (ret);

    /*
     * Associate the itemname db with its primary db
     * (inventory db).
     */
     my_stock-&gt;inventory_dbp-&gt;associate(
       my_stock-&gt;inventory_dbp,    /* Primary db */
       NULL,                       /* txn id */
       my_stock-&gt;itemname_sdbp,    /* Secondary db */
       get_item_name,              /* Secondary key extractor */
       0);                         /* Flags */
     </code></strong>

    printf("databases opened successfully\n");
    return (0);
}</pre>
        <p>
        Finally, we need to update <code class="function">databases_close()</code> 
        (<a class="xref" href="CoreDbUsage.html#database_close" title="Example 2.5 The databases_close() Function">The databases_close() Function</a>)
        to close our
        new secondary database. Note that we are careful to close the secondary
        before the primary, even though the database close routine is single
        threaded.
    </p>
        <a id="c_index14"></a>
        <pre class="programlisting">/* Closes all the databases and secondary databases. */
int
databases_close(STOCK_DBS *my_stock)
{
    int ret;
    /*
     * Note that closing a database automatically flushes its cached data
     * to disk, so no sync is required here.
     */

    <strong class="userinput"><code>if (my_stock-&gt;itemname_sdbp != NULL) {
        ret = my_stock-&gt;itemname_sdbp-&gt;close(my_stock-&gt;itemname_sdbp, 0);
        if (ret != 0)
            fprintf(stderr, "Itemname database close failed: %s\n",
              db_strerror(ret));
    }</code></strong>

    if (my_stock-&gt;inventory_dbp != NULL) {
        ret = my_stock-&gt;inventory_dbp-&gt;close(my_stock-&gt;inventory_dbp, 0);
        if (ret != 0)
            fprintf(stderr, "Inventory database close failed: %s\n",
              db_strerror(ret));
    }

    if (my_stock-&gt;vendor_dbp != NULL) {
        ret = my_stock-&gt;vendor_dbp-&gt;close(my_stock-&gt;vendor_dbp, 0);
        if (ret != 0)
            fprintf(stderr, "Vendor database close failed: %s\n",
              db_strerror(ret));
    }

    printf("databases closed.\n");
    return (0);
} </pre>
        <p>
        And the implementation changes slightly to take advantage of the new
        boolean. Note that to save space, we just show the constructor where the
        code actually changes:
    </p>
        <p>
        That completes our update to <code class="function">example_database_load</code>.
        Now when this program is called, it will automatically index inventory
        items based on their names. We can then query for those items using the
        new index. We show how to do that in the next section.
    </p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="edrWIndexes"></a>Secondary Databases with example_database_read</h3>
            </div>
          </div>
        </div>
        <p>
            In <a class="xref" href="CoreCursorUsage.html" title="Cursor Example">Cursor Example</a> we
            wrote an application that displays every inventory item in the
            Inventory database. In this section, we will update that example to
            allow us to search for and display an inventory item given a
            specific name. To do this, we will make use of the secondary
            database that <code class="function">example_database_load</code> now
            creates.
        </p>
        <p>
            Because we manage all our database open and close activities in
            <code class="function">databases_setup()</code> and
            <code class="function">databases_close()</code>,
            the update to <code class="function">example_database_read</code> is
            relatively modest. We need only add a command line parameter on
            which we can specify the item name, and we will need a new function
            in which we will perform the query and display the results.
        </p>
        <p>
            To begin, we add a single forward declaration to the application,
            and update our usage function slightly:
        </p>
        <a id="c_index15"></a>
        <pre class="programlisting">/* File: example_database_read.c */
/* gettingstarted_common.h includes db.h for us */
#include "gettingstarted_common.h"

/* Forward declarations */
char * show_inventory_item(void *);
int show_all_records(STOCK_DBS *);
<strong class="userinput"><code>int show_records(STOCK_DBS *, char *);</code></strong>
int show_vendor_record(char *, DB *); </pre>
        <p>
        Next, we update <code class="function">main()</code> to 
        
        accept the new command line switch.
        We also need a new variable to contain the item's name.        
    </p>
        <a id="c_index16"></a>
        <pre class="programlisting">/*
<strong class="userinput"><code> * Searches for a inventory item based on that item's name. The search is
 * performed using the item name secondary database. Displays all
 * inventory items that use the specified name, as well as the vendor
 * associated with that inventory item.
 *
 * If no item name is provided, then all inventory items are displayed.</code></strong>
 */
int
main(int argc, char *argv[])
{
    STOCK_DBS my_stock;
    int ret;
    <strong class="userinput"><code>char *itemname</code></strong>;

    /* Initialize the STOCK_DBS struct */
    initialize_stockdbs(&amp;my_stock);

    <strong class="userinput"><code>itemname = NULL;</code></strong>
    /*
     * Parse the command line arguments here and determine
     * the location of the database files <strong class="userinput"><code>as well as the
     * inventory item we want displayed, if any.</code></strong> This step is
     * omitted for brevity.
     */

    /*
     * Identify the files that will hold our databases
     * This function uses information obtained from the
     * command line to identify the directory in which
     * the database files reside.
     */
    set_db_filenames(&amp;my_stock);

    /* Open all databases */
    ret = databases_setup(&amp;my_stock, "example_database_read", stderr);
    if (ret != 0) {
        fprintf(stderr, "Error opening databases\n");
        databases_close(&amp;my_stock);
        return (ret);
    }</pre>
        <p>
        The final update to the <code class="function">main()</code> entails a little bit
        of logic to determine whether we want to display all available inventory
        items, or just the ones that match a name provided on the
        <code class="literal">-i</code> command line parameter.
    </p>
        <a id="c_index17"></a>
        <pre class="programlisting">    /* 
     * Show either a single item or all items, depending
     * on whether itemname is set to a value.
     */
    <strong class="userinput"><code>if (itemname == NULL)
        ret = show_all_records(&amp;my_stock);
    else
        ret = show_records(&amp;my_stock, itemname);</code></strong>

    /* Close our databases */
    databases_close(&amp;my_stock);
    return (ret);
} </pre>
        <p>
        The only other thing that we need to add to the application is the
        implementation of the 
            <code class="function">show_records()</code> 
             
        function.
    </p>
        <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
          <h3 class="title">Note</h3>
          <p>
            In the interest of space, we refrain from showing the other
            functions used by this application. For their implementation, please
            see <a class="xref" href="CoreCursorUsage.html" title="Cursor Example">Cursor Example</a>.
            Alternatively, you can see the entire implementation of this
            application
            in:
        </p>
          <pre class="programlisting"><span class="emphasis"><em>DB_INSTALL</em></span>/examples_c/getting_started</pre>
          <p>
            where <code class="literal"><span class="emphasis"><em>DB_INSTALL</em></span></code> is the location where you
            placed your DB distribution.
        </p>
        </div>
        <a id="c_index18"></a>
        <pre class="programlisting">/*
 * Search for an inventory item given its name (using the inventory item
 * secondary database) and display that record and any duplicates that may
 * exist.
 */
int
show_records(STOCK_DBS *my_stock, char *itemname)
{
    DBC *itemname_cursorp;
    DBT key, data;
    char *the_vendor;
    int ret, exit_value;

    /* Initialize our DBTs. */
    memset(&amp;key, 0, sizeof(DBT));
    memset(&amp;data, 0, sizeof(DBT));

    /* Get a cursor to the itemname db */
    my_stock-&gt;itemname_sdbp-&gt;cursor(my_stock-&gt;itemname_sdbp, 0,
      &amp;itemname_cursorp, 0);

    /*
     * Get the search key. This is the name on the inventory
     * record that we want to examine.
     */
    key.data = itemname;
    key.size = strlen(itemname) + 1;

    /*
     * Position our cursor to the first record in the secondary
     * database that has the appropriate key.
     */
    exit_value = 0;
    ret = itemname_cursorp-&gt;get(itemname_cursorp, &amp;key, &amp;data, DB_SET);
    if (!ret) {
        do {
            /*
             * Show the inventory record and the vendor responsible
             * for this inventory item.
             */
            the_vendor = show_inventory_item(data.data);
            ret = show_vendor_record(the_vendor, my_stock-&gt;vendor_dbp);
            if (ret) {
                exit_value = ret;
                break;
            }
            /*
             * Our secondary allows duplicates, so we need to loop over
             * the next duplicate records and show them all. This is done
             * because an inventory item's name is not a unique value.
             */
        } while(itemname_cursorp-&gt;get(itemname_cursorp, &amp;key, &amp;data,
            DB_NEXT_DUP) == 0);
    } else {
        printf("No records found for '%s'\n", itemname);
    }

    /* Close the cursor */
    itemname_cursorp-&gt;close(itemname_cursorp);

    return (exit_value);
} </pre>
        <p>
        This completes our update to
        <code class="classname">example_inventory_read</code>. Using this update, you
        can now search for and show all inventory items that match a particular
        name. For example:
    </p>
        <pre class="programlisting">    example_inventory_read -i "Zulu Nut"</pre>
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