2011-09-13 17:44:24 +00:00
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<title>Tcl error handling</title>
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="tcl.html" title="Chapter 21. Berkeley DB Extensions: Tcl" />
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<link rel="prev" href="tcl_program.html" title="Tcl API programming notes" />
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<link rel="next" href="tcl_faq.html" title="Tcl FAQ" />
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<div class="libver">
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2012-11-14 21:35:20 +00:00
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<p>Library Version 11.2.5.3</p>
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2011-09-13 17:44:24 +00:00
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Tcl error handling</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="tcl_program.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 21.
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Berkeley DB Extensions: Tcl
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="tcl_faq.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="tcl_error"></a>Tcl error handling</h2>
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</div>
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</div>
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</div>
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<p>The Tcl interfaces to Berkeley DB generally return TCL_OK on success and throw
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a Tcl error on failure, using the appropriate Tcl interfaces to provide
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the user with an informative error message. There are some "expected"
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failures, however, for which no Tcl error will be thrown and for which
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Tcl commands will return TCL_OK. These failures include times when a
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searched-for key is not found, a requested key/data pair was previously
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deleted, or a key/data pair cannot be written because the key already
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exists.</p>
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<p>These failures can be detected by searching the Berkeley DB error message that
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is returned. For example, use the following to detect that an attempt
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to put a record into the database failed because the key already
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existed:</p>
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<pre class="programlisting">% berkdb open -create -btree a.db
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db0
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% db0 put dog cat
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0
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% set ret [db0 put -nooverwrite dog newcat]
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DB_KEYEXIST: Key/data pair already exists
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% if { [string first DB_KEYEXIST $ret] != -1 } {
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puts "This was an error; the key existed"
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}
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This was an error; the key existed
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% db0 close
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0
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% exit</pre>
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<p>To simplify parsing, it is recommended that the initial Berkeley DB error name
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be checked; for example, <a href="../api_reference/C/dbcput.html#dbcput_DB_KEYEXIST" class="olink">DB_MULTIPLE</a> in the previous example.
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To ensure that Tcl scripts are not broken by upgrading to new releases
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of Berkeley DB, these values will not change in future releases of Berkeley DB.
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There are currently only three such "expected" error returns:</p>
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<pre class="programlisting">DB_NOTFOUND: No matching key/data pair found
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DB_KEYEMPTY: Nonexistent key/data pair
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DB_KEYEXIST: Key/data pair already exists</pre>
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<p>Finally, sometimes Berkeley DB will output additional error information when
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a Berkeley DB error occurs. By default, all Berkeley DB error messages will be
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prefixed with the created command in whose context the error occurred
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(for example, "env0", "db2", and so on). There are several ways to
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capture and access this information.</p>
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<p>First, if Berkeley DB invokes the error callback function, the additional
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information will be placed in the error result returned from the command
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and in the errorInfo backtrace variable in Tcl.</p>
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<p>Also, the two calls to open an environment and open a database take an
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option, <span class="bold"><strong>-errfile filename</strong></span>, which sets an output file to which
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these additional error messages should be written.</p>
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<p>Additionally, the two calls to open an environment and open a database
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take an option, <span class="bold"><strong>-errpfx string</strong></span>, which sets the error prefix to
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the given string. This option may be useful in circumstances where a
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more descriptive prefix is desired or where a constant prefix indicating
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an error is desired.</p>
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<td width="40%" align="left"><a accesskey="p" href="tcl_program.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="tcl.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="tcl_faq.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Tcl API programming notes </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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