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<td><b><dl><dt>Berkeley DB Reference Guide:<dd>Programmer Notes</dl></b></td>
<td align=right><a href="../program/appsignals.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../program/environ.html"><img src="../../images/next.gif" alt="Next"></a>
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<p align=center><b>Error returns to applications</b></p>
<p>Except for the historic <a href="../../api_c/dbm.html">dbm</a>, <a href="../../api_c/dbm.html">ndbm</a>, and <a href="../../api_c/hsearch.html">hsearch</a>
interfaces, Berkeley DB does not use the global variable <b>errno</b> to
return error values.  The return values for all Berkeley DB functions are
grouped into the following three categories:</p>
<br>
<b>0</b><ul compact><li>A return value of 0 indicates that the operation was successful.</ul>
<b>&gt; 0</b><ul compact><li>A return value that is greater than 0 indicates that there was a system
error.  The <b>errno</b> value returned by the system is returned by
the function; for example, when a Berkeley DB function is unable to allocate
memory, the return value from the function will be ENOMEM.</ul>
<b>&lt; 0</b><ul compact><li>A return value that is less than 0 indicates a condition that was not
a system failure, but was not an unqualified success, either.  For
example, a routine to retrieve a key/data pair from the database may
return DB_NOTFOUND when the key/data pair does not appear in
the database; as opposed to the value of 0, which would be returned if
the key/data pair were found in the database.
<p><a name="3"><!--meow--></a>
All values returned by Berkeley DB functions are less than 0 in order to avoid
conflict with possible values of <b>errno</b>.  Specifically, Berkeley DB
reserves all values from -30,800 to -30,999 to itself as possible error
values.  There are a few Berkeley DB interfaces where it is possible for an
application function to be called by a Berkeley DB function and subsequently
fail with an application-specific return.  Such failure returns will be
passed back to the function that originally called a Berkeley DB interface.
To avoid ambiguity about the cause of the error, error values separate
from the Berkeley DB error name space should be used.</p></ul>
<br>
<p>Although possible error returns are specified by each individual function's
manual page, there are a few error returns that deserve general mention:</p>
<b><a name="DB_NOTFOUND">DB_NOTFOUND</a> and <a name="DB_KEYEMPTY">DB_KEYEMPTY</a></b>
<p>There are two special return values that are similar in meaning and that
are returned in similar situations, and therefore might be confused:
DB_NOTFOUND and DB_KEYEMPTY.</p>
<p>The DB_NOTFOUND error return indicates that the requested key/data
pair did not exist in the database or that start-of- or end-of-file has
been reached by a cursor.</p>
<p>The DB_KEYEMPTY error return indicates that the requested
key/data pair logically exists but was never explicitly created by the
application (the Recno and Queue access methods will automatically
create key/data pairs under some circumstances; see <a href="../../api_c/db_open.html">DB-&gt;open</a>
for more information), or that the requested key/data pair was deleted
and never re-created.  In addition, the Queue access method will return
DB_KEYEMPTY for records that were created as part of a
transaction that was later aborted and never re-created.</p>
<b><a name="DB_KEYEXIST">DB_KEYEXIST</a></b>
<p>The DB_KEYEXIST error return indicates the <a href="../../api_c/db_put.html#DB_NOOVERWRITE">DB_NOOVERWRITE</a>
option was specified when inserting a key/data pair into the database and
the key already exists in the database, or the <a href="../../api_c/db_put.html#DB_NODUPDATA">DB_NODUPDATA</a>
option was specified and the key/data pair already exists in the data.</p>
<a name="4"><!--meow--></a>
<b><a name="DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a></b>
<p>When multiple threads of control are modifying the database, there is
normally the potential for deadlock. In Berkeley DB, deadlock is signified by
an error return from the Berkeley DB function of the value
DB_LOCK_DEADLOCK.  Whenever a Berkeley DB function returns
DB_LOCK_DEADLOCK, the enclosing transaction should be aborted.</p>
<p>Any Berkeley DB function that attempts to acquire locks can potentially return
DB_LOCK_DEADLOCK.  Practically speaking, the safest way to deal
with applications that can deadlock is to anticipate a
DB_LOCK_DEADLOCK return from any <a href="../../api_c/db_class.html">DB</a> or <a href="../../api_c/dbc_class.html">DBC</a>
handle method call, or any <a href="../../api_c/env_class.html">DB_ENV</a> handle method call that
references a database, including the database's backing physical file.</p>
<b><a name="DB_LOCK_NOTGRANTED">DB_LOCK_NOTGRANTED</a></b>
<p>If a lock is requested from the <a href="../../api_c/lock_get.html">DB_ENV-&gt;lock_get</a> or <a href="../../api_c/lock_vec.html">DB_ENV-&gt;lock_vec</a> methods
with the <a href="../../api_c/lock_vec.html#DB_LOCK_NOWAIT">DB_LOCK_NOWAIT</a> flag specified, the method will return
DB_LOCK_NOTGRANTED if the lock is not immediately available.</p>
<p>If the <a href="../../api_c/env_set_flags.html#DB_TIME_NOTGRANTED">DB_TIME_NOTGRANTED</a> flag is specified to the
<a href="../../api_c/env_set_flags.html">DB_ENV-&gt;set_flags</a> method, database calls timing out based on lock or
transaction timeout values will return DB_LOCK_NOTGRANTED
instead of DB_LOCK_DEADLOCK.</p>
<b><a name="DB_RUNRECOVERY">DB_RUNRECOVERY</a></b>
<p>There exists a class of errors that Berkeley DB considers fatal to an entire
Berkeley DB environment.  An example of this type of error is a corrupted
database page.  The only way to recover from these failures is to have
all threads of control exit the Berkeley DB environment, run recovery of the
environment, and re-enter Berkeley DB. (It is not strictly necessary that the
processes exit, although that is the only way to recover system
resources, such as file descriptors and memory, allocated by Berkeley DB.)</p>
<p>When this type of error is encountered, the error value
DB_RUNRECOVERY is returned. This error can be returned by any
Berkeley DB interface.  Once DB_RUNRECOVERY is returned by any
interface, it will be returned from all subsequent Berkeley DB calls made by
any threads of control participating in the environment.</p>
<p>Applications can handle such fatal errors in one of two ways: first, by
checking for DB_RUNRECOVERY as part of their normal Berkeley DB error
return checking, similarly to DB_LOCK_DEADLOCK or any other
error.  Alternatively, applications can specify a fatal-error callback
function using the <a href="../../api_c/env_event_notify.html">DB_ENV-&gt;set_event_notify</a> method.  Applications with
no cleanup processing of their own should simply exit from the callback
function.</p>
<b><a name="DB_SECONDARY_BAD">DB_SECONDARY_BAD</a></b>
<p>A secondary index has been corrupted.  This is likely the result of an
application operating on related databases without first associating
them.</p>
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