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3.9 KiB
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<!--$Id: error.so,v 11.13 2001/01/09 18:48:06 bostic Exp $-->
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<!--Copyright 1997, 1998, 1999, 2000 by Sleepycat Software, Inc.-->
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<!--All rights reserved.-->
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<html>
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<head>
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<title>Berkeley DB Reference Guide: Tcl error handling</title>
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<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
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<meta name="keywords" content="embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,java,C,C++">
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</head>
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<body bgcolor=white>
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<table><tr valign=top>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Tcl</dl></h3></td>
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<td width="1%"><a href="../../ref/tcl/program.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../../ref/toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../../ref/tcl/faq.html"><img src="../../images/next.gif" alt="Next"></a>
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</td></tr></table>
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<p>
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<h1 align=center>Tcl error handling</h1>
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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 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.
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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, to detect that an attempt to put a record into
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the database failed because the key already existed:
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<p><blockquote><pre>% 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></blockquote>
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<p>To simplify parsing, it is recommended that the initial Berkeley DB error name
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be checked, e.g., DB_KEYEXIST in the above example. These values will
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not change in future releases of Berkeley DB to ensure that Tcl scripts are not
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broken by upgrading to new releases of Berkeley DB. There are currently only
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three such "expected" error returns. They are:
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<p><blockquote><pre>DB_NOTFOUND: No matching key/data pair found
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DB_KEYEMPTY: Non-existent key/data pair
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DB_KEYEXIST: Key/data pair already exists</pre></blockquote>
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<p>Finally, in some cases, when a Berkeley DB error occurs Berkeley DB will output
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additional error information. By default, all Berkeley DB error messages will
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be prefixed with the created command in whose context the error occurred
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(e.g., "env0", "db2", etc.). There are several ways to capture and
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access this information.
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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
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command and in the errorInfo backtrace variable in Tcl.
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<p>Also the two calls to open an environment and
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open a database take an option, <b>-errfile filename</b>, which sets an
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output file to which these additional error messages should be written.
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<p>Additionally the two calls to open an environment and
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open a database take an option, <b>-errpfx string</b>, which sets the
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error prefix to the given string. This option may be useful
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in circumstances where a more descriptive prefix is desired or
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where a constant prefix indicating an error is desired.
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<table><tr><td><br></td><td width="1%"><a href="../../ref/tcl/program.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../../ref/toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../../ref/tcl/faq.html"><img src="../../images/next.gif" alt="Next"></a>
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</td></tr></table>
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<p><font size=1><a href="http://www.sleepycat.com">Copyright Sleepycat Software</a></font>
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</body>
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</html>
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