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<!--$Id: open.so,v 10.27 2000/12/14 21:42:18 bostic Exp $-->
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<title>Berkeley DB Reference Guide: Opening a database</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Simple Tutorial</dl></h3></td>
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<p>
<h1 align=center>Opening a database</h1>
<p>Opening a database is done in two steps: first, a DB handle is
created using the Berkeley DB <a href="../../api_c/db_create.html">db_create</a> interface, and then the
actual database is opened using the <a href="../../api_c/db_open.html">DB->open</a> function.
<p>The <a href="../../api_c/db_create.html">db_create</a> interface takes three arguments:
<p><dl compact>
<p><dt>dbp<dd>A location to store a reference to the created structure.
<p><dt>environment<dd>A location to specify an enclosing Berkeley DB environment, not used in our
example.
<p><dt>flags<dd>A placeholder for flags, not used in our example.
</dl>
<p>The <a href="../../api_c/db_open.html">DB->open</a> interface takes five arguments:
<p><dl compact>
<p><dt>file<dd>The name of the database file to be opened.
<p><dt>database<dd>The optional database name, not used in this example.
<p><dt>type<dd>The type of database to open. This value will be one of the four access
methods Berkeley DB supports: DB_BTREE, DB_HASH, DB_QUEUE or DB_RECNO, or the
special value DB_UNKNOWN, which allows you to open an existing file
without knowing its type.
<p><dt>flags<dd>Various flags that modify the behavior of <a href="../../api_c/db_open.html">DB->open</a>. In our
simple case, the only interesting flag is <a href="../../api_c/env_open.html#DB_CREATE">DB_CREATE</a>. This flag
behaves similarly to the IEEE/ANSI Std 1003.1 (POSIX) O_CREATE flag to the open system
call, causing Berkeley DB to create the underlying database if it does not
yet exist.
<p><dt>mode<dd>The file mode of any underlying files that <a href="../../api_c/db_open.html">DB->open</a> will create.
The mode behaves as does the IEEE/ANSI Std 1003.1 (POSIX) mode argument to the open
system call, and specifies file read, write and execute permissions.
Of course, only the read and write permissions are relevant to Berkeley DB.
</dl>
<p>Here's what the code to create the handle and then call <a href="../../api_c/db_open.html">DB->open</a>
looks like:
<p><blockquote><pre><b>#include <sys/types.h>
#include <stdio.h>
#include <db.h>
<p>
#define DATABASE "access.db"
<p>
int
main()
{
DB *dbp;
int ret;
<p>
if ((ret = db_create(&dbp, NULL, 0)) != 0) {
fprintf(stderr, "db_create: %s\n", db_strerror(ret));
exit (1);
}
if ((ret = dbp->open(
dbp, DATABASE, NULL, DB_BTREE, DB_CREATE, 0664)) != 0) {
dbp->err(dbp, ret, "%s", DATABASE);
goto err;
}</b>
</pre></blockquote>
<p>If the call to <a href="../../api_c/db_create.html">db_create</a> is successful, the variable <b>dbp</b>
will contain a database handle that will be used to configure and access
an underlying database.
<p>As you see, the program opens a database named <b>access.db</b>. The
underlying database is a Btree. Because the <a href="../../api_c/env_open.html#DB_CREATE">DB_CREATE</a> flag was
specified, the file will be created if it does not already exist. The
mode of any created files will be 0664 (i.e., readable and writeable by
the owner and the group, and readable by everyone else).
<p>One additional function call is used in this code sample, <a href="../../api_c/db_err.html">DB->err</a>.
This method works like the ANSI C printf interface. The second argument
is the error return from a Berkeley DB function, and the rest of the arguments
are a printf-style format string and argument list. The error message
associated with the error return will be appended to a message constructed
from the format string and other arguments. In the above code, if the
<a href="../../api_c/db_open.html">DB->open</a> call were to fail, the message it would display would be
something like
<p><blockquote><pre>access.db: Operation not permitted</pre></blockquote>
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