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<title>GnuTLS-Guile 3.5.4: Exception Handling</title>
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<a name="Exception-Handling"></a>
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<p>
Previous: <a href="Input-and-Output.html#Input-and-Output" accesskey="p" rel="prev">Input and Output</a>, Up: <a href="Guile-API-Conventions.html#Guile-API-Conventions" accesskey="u" rel="up">Guile API Conventions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Procedure-Index.html#Procedure-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Exception-Handling-1"></a>
<h3 class="section">3.5 Exception Handling</h3>
<a name="index-exceptions"></a>
<a name="index-errors"></a>
<a name="index-gnutls_002derror"></a>
<a name="index-error_002d_003estring"></a>
<p>GnuTLS errors are implemented as Scheme exceptions (see <a href="http://www.gnu.org/software/guile/manual/html_node/Exceptions.html#Exceptions">exceptions in Guile</a> in <cite>The GNU Guile Reference Manual</cite>). Each
time a GnuTLS function returns an error, an exception with key
<code>gnutls-error</code> is raised. The additional arguments that are
thrown include an error code and the name of the GnuTLS procedure that
raised the exception. The error code is pretty much like an enumerate
value: it is one of the <code>error/</code> variables exported by the
<code>(gnutls)</code> module (see <a href="Enumerates-and-Constants.html#Enumerates-and-Constants">Enumerates and Constants</a>). Exceptions
can be turned into error messages using the <code>error->string</code>
procedure.
</p>
<p>The following examples illustrates how GnuTLS exceptions can be
handled:
</p>
<div class="example">
<pre class="example">(let ((session (make-session connection-end/server)))
;;
;; ...
;;
(catch 'gnutls-error
(lambda ()
(handshake session))
(lambda (key err function . currently-unused)
(format (current-error-port)
"a GnuTLS error was raised by `~a': ~a~%"
function (error->string err)))))
</pre></div>
<p>Again, error values can be compared using <code>eq?</code>:
</p>
<div class="example">
<pre class="example"> ;; `gnutls-error' handler.
(lambda (key err function . currently-unused)
(if (eq? err error/fatal-alert-received)
(format (current-error-port)
"a fatal alert was caught!~%")
(format (current-error-port)
"something bad happened: ~a~%"
(error->string err))))
</pre></div>
<p>Note that the <code>catch</code> handler is currently passed only 3
arguments but future versions might provide it with additional
arguments. Thus, it must be prepared to handle more than 3 arguments,
as in this example.
</p>
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