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<h2 class="title" style="clear: both"><a id="tcl_error"></a>Tcl error handling</h2>
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<p>
The Tcl interfaces to Berkeley DB generally return TCL_OK on
success and throw a Tcl error on failure, using the
appropriate Tcl interfaces to provide the user with an
informative error message. There are some "expected" failures,
however, for which no Tcl error will be thrown and for which
Tcl commands will return TCL_OK. These failures include times
when a searched-for key is not found, a requested key/data
pair was previously deleted, or a key/data pair cannot be
written because the key already exists.
</p>
<p>
These failures can be detected by searching the Berkeley DB
error message that is returned. For example, use the following
to detect that an attempt to put a record into the database
failed because the key already existed:
</p>
<pre class="programlisting">% berkdb open -create -btree a.db
db0
% db0 put dog cat
0
% set ret [db0 put -nooverwrite dog newcat]
DB_KEYEXIST: Key/data pair already exists
% if { [string first DB_KEYEXIST $ret] != -1 } {
puts "This was an error; the key existed"
}
This was an error; the key existed
% db0 close
0
% exit</pre>
<p>
To simplify parsing, it is recommended that the initial
Berkeley DB error name be checked; for example, <a href="../api_reference/C/dbcput.html#dbcput_DB_KEYEXIST" class="olink">DB_MULTIPLE</a>
in the previous example. To ensure that Tcl scripts are not
broken by upgrading to new releases of Berkeley DB, these
values will not change in future releases of Berkeley DB.
There are currently only three such "expected" error
returns:
</p>
<pre class="programlisting">DB_NOTFOUND: No matching key/data pair found
DB_KEYEMPTY: Nonexistent key/data pair
DB_KEYEXIST: Key/data pair already exists</pre>
<p>
Finally, sometimes Berkeley DB will output additional error
information when a Berkeley DB error occurs. By default, all
Berkeley DB error messages will be prefixed with the created
command in whose context the error occurred (for example,
"env0", "db2", and so on). There are several ways to capture
and access this information.
</p>
<p>
First, if Berkeley DB invokes the error callback function,
the additional information will be placed in the error result
returned from the command and in the errorInfo backtrace
variable in Tcl.
</p>
<p>
Also, the two calls to open an environment and open a
database take an option, <span class="bold"><strong>-errfile
filename</strong></span>, which sets an output file to which
these additional error messages should be written.
</p>
<p>
Additionally, the two calls to open an environment and open
a database take an option, <span class="bold"><strong>-errpfx
string</strong></span>, which sets the error prefix to the
given string. This option may be useful in circumstances where
a more descriptive prefix is desired or where a constant
prefix indicating an error is desired.
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