mirror of
https://github.com/yasuhirokimura/db18
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Yasuhiro KIMURA
131 lines
5.5 KiB
HTML
131 lines
5.5 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Tcl error handling</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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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 22. 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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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 18.1.40</p>
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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 22. Berkeley DB Extensions: Tcl </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>
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The Tcl interfaces to Berkeley DB generally return TCL_OK on
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success and throw a Tcl error on failure, using the
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appropriate Tcl interfaces to provide the user with an
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informative error message. There are some "expected" failures,
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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
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when a searched-for key is not found, a requested key/data
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pair was previously deleted, or a key/data pair cannot be
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written because the key already exists.
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</p>
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<p>
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These failures can be detected by searching the Berkeley DB
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error message that is returned. For example, use the following
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to detect that an attempt to put a record into the database
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failed because the key already existed:
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</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>
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To simplify parsing, it is recommended that the initial
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Berkeley DB error name be checked; for example, <a href="../api_reference/C/dbcput.html#dbcput_DB_KEYEXIST" class="olink">DB_MULTIPLE</a>
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in the previous example. To ensure that Tcl scripts are not
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broken by upgrading to new releases of Berkeley DB, these
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values will not change in future releases of Berkeley DB.
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There are currently only three such "expected" error
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returns:
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</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>
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Finally, sometimes Berkeley DB will output additional error
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information when a Berkeley DB error occurs. By default, all
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Berkeley DB error messages will be prefixed with the created
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command in whose context the error occurred (for example,
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"env0", "db2", and so on). There are several ways to capture
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and access this information.
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</p>
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<p>
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First, if Berkeley DB invokes the error callback function,
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the additional information will be placed in the error result
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returned from the command and in the errorInfo backtrace
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variable in Tcl.
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</p>
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<p>
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Also, the two calls to open an environment and open a
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database take an option, <span class="bold"><strong>-errfile
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filename</strong></span>, which sets an output file to which
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these additional error messages should be written.
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</p>
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<p>
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Additionally, the two calls to open an environment and open
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a database take an option, <span class="bold"><strong>-errpfx
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string</strong></span>, which sets the error prefix to the
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given string. This option may be useful in circumstances where
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a more descriptive prefix is desired or where a constant
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prefix indicating an error is desired.
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</p>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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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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</td>
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<td width="40%" align="right" valign="top"> Tcl FAQ</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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