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<!DOCTYPE chapter SYSTEM "chapter.dtd">
<chapter>
<header>
<copyright>
<year>2003</year><year>2009</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
The contents of this file are subject to the Erlang Public License,
Version 1.1, (the "License"); you may not use this file except in
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights and limitations
under the License.
</legalnotice>
<title>Errors and Error Handling</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
<file>errors.xml</file>
</header>
<section>
<title>Terminology</title>
<p>Errors can roughly be divided into four different types:</p>
<list type="bulleted">
<item>Compile-time errors</item>
<item>Logical errors</item>
<item>Run-time errors</item>
<item>Generated errors</item>
</list>
<p>A compile-time error, for example a syntax error, should not
cause much trouble as it is caught by the compiler.</p>
<p>A logical error is when a program does not behave as intended,
but does not crash. An example could be that nothing happens when
a button in a graphical user interface is clicked.</p>
<p>A run-time error is when a crash occurs. An example could be
when an operator is applied to arguments of the wrong type.
The Erlang programming language has built-in features for
handling of run-time errors.</p>
<p>A run-time error can also be emulated by calling
<c>erlang:error(Reason)</c>, <c>erlang:error(Reason, Args)</c>
(those appeared in Erlang 5.4/OTP-R10),
<c>erlang:fault(Reason)</c> or <c>erlang:fault(Reason, Args)</c>
(old equivalents).</p>
<p>A run-time error is another name for an exception
of class <c>error</c>.
</p>
<p>A generated error is when the code itself calls
<c>exit/1</c> or <c>throw/1</c>. Note that emulated run-time
errors are not denoted as generated errors here.
</p>
<p>Generated errors are exceptions of classes <c>exit</c> and
<c>throw</c>.
</p>
<p>When a run-time error or generated error occurs in Erlang,
execution for the process which evaluated
the erroneous expression is stopped.
This is referred to as a <em>failure</em>, that execution or
evaluation <em>fails</em>, or that the process <em>fails</em>,
<em>terminates</em> or <em>exits</em>. Note that a process may
terminate/exit for other reasons than a failure.</p>
<p>A process that terminates will emit an <em>exit signal</em> with
an <em>exit reason</em> that says something about which error
has occurred. Normally, some information about the error will
be printed to the terminal.</p>
</section>
<section>
<title>Exceptions</title>
<p>Exceptions are run-time errors or generated errors and
are of three different classes, with different origins. The
<seealso marker="expressions#try">try</seealso> expression
(appeared in Erlang 5.4/OTP-R10B)
can distinguish between the different classes, whereas the
<seealso marker="expressions#catch">catch</seealso>
expression can not. They are described in the Expressions chapter.</p>
<table>
<row>
<cell align="left" valign="middle"><em>Class</em></cell>
<cell align="left" valign="middle"><em>Origin</em></cell>
</row>
<row>
<cell align="left" valign="middle"><c>error</c></cell>
<cell align="left" valign="middle">Run-time error for example <c>1+a</c>, or the process called <c>erlang:error/1,2</c> (appeared in Erlang 5.4/OTP-R10B) or <c>erlang:fault/1,2</c> (old equivalent)</cell>
</row>
<row>
<cell align="left" valign="middle"><c>exit</c></cell>
<cell align="left" valign="middle">The process called <c>exit/1</c></cell>
</row>
<row>
<cell align="left" valign="middle"><c>throw</c></cell>
<cell align="left" valign="middle">The process called <c>throw/1</c></cell>
</row>
<tcaption>Exception Classes.</tcaption>
</table>
<p>An exception consists of its class, an exit reason
(the <seealso marker="#exit_reasons">Exit Reason</seealso>),
and a stack trace (that aids in finding the code location of
the exception).</p>
<p>The stack trace can be retrieved using
<c>erlang:get_stacktrace/0</c> (new in Erlang 5.4/OTP-R10B
from within a <c>try</c> expression, and is returned for
exceptions of class <c>error</c> from a <c>catch</c> expression.</p>
<p>An exception of class <c>error</c> is also known as a run-time
error.</p>
</section>
<section>
<title>Handling of Run-Time Errors in Erlang</title>
<section>
<title>Error Handling Within Processes</title>
<p>It is possible to prevent run-time errors and other
exceptions from causing
the process to terminate by using <c>catch</c> or
<c>try</c>, see the Expressions chapter about
<seealso marker="expressions#catch">Catch</seealso>
and <seealso marker="expressions#try">Try</seealso>.</p>
</section>
<section>
<title>Error Handling Between Processes</title>
<p>Processes can monitor other processes and detect process
terminations, see
the <seealso marker="processes#errors">Processes</seealso>
chapter.</p>
</section>
</section>
<section>
<marker id="exit_reasons"></marker>
<title>Exit Reasons</title>
<p>When a run-time error occurs,
that is an exception of class <c>error</c>,
the exit reason is a tuple <c>{Reason,Stack}</c>.
<c>Reason</c> is a term indicating the type of error:</p>
<table>
<row>
<cell align="left" valign="middle"><em>Reason</em></cell>
<cell align="left" valign="middle"><em>Type of error</em></cell>
</row>
<row>
<cell align="left" valign="middle"><c>badarg</c></cell>
<cell align="left" valign="middle">Bad argument. The argument is of wrong data type, or is otherwise badly formed.</cell>
</row>
<row>
<cell align="left" valign="middle"><c>badarith</c></cell>
<cell align="left" valign="middle">Bad argument in an arithmetic expression.</cell>
</row>
<row>
<cell align="left" valign="middle"><c>{badmatch,V}</c></cell>
<cell align="left" valign="middle">Evaluation of a match expression failed. The value <c>V</c> did not match.</cell>
</row>
<row>
<cell align="left" valign="middle"><c>function_clause</c></cell>
<cell align="left" valign="middle">No matching function clause is found when evaluating a function call.</cell>
</row>
<row>
<cell align="left" valign="middle"><c>{case_clause,V}</c></cell>
<cell align="left" valign="middle">No matching branch is found when evaluating a <c>case</c> expression. The value <c>V</c> did not match.</cell>
</row>
<row>
<cell align="left" valign="middle"><c>if_clause</c></cell>
<cell align="left" valign="middle">No true branch is found when evaluating an <c>if</c> expression.</cell>
</row>
<row>
<cell align="left" valign="middle"><c>{try_clause,V}</c></cell>
<cell align="left" valign="middle">No matching branch is found when evaluating the of-section of a <c>try</c> expression. The value <c>V</c> did not match.</cell>
</row>
<row>
<cell align="left" valign="middle"><c>undef</c></cell>
<cell align="left" valign="middle">The function cannot be found when evaluating a function call.</cell>
</row>
<row>
<cell align="left" valign="middle"><c>{badfun,F}</c></cell>
<cell align="left" valign="middle">There is something wrong with a fun <c>F</c>.</cell>
</row>
<row>
<cell align="left" valign="middle"><c>{badarity,F}</c></cell>
<cell align="left" valign="middle">A fun is applied to the wrong number of arguments. <c>F</c> describes the fun and the arguments.</cell>
</row>
<row>
<cell align="left" valign="middle"><c>timeout_value</c></cell>
<cell align="left" valign="middle">The timeout value in a <c>receive..after</c> expression is evaluated to something else than an integer or <c>infinity</c>.</cell>
</row>
<row>
<cell align="left" valign="middle"><c>noproc</c></cell>
<cell align="left" valign="middle">Trying to link to a non-existing process.</cell>
</row>
<row>
<cell align="left" valign="middle"><c>{nocatch,V}</c></cell>
<cell align="left" valign="middle">Trying to evaluate a <c>throw </c>outside a <c>catch</c>. <c>V</c> is the thrown term.</cell>
</row>
<row>
<cell align="left" valign="middle"><c>system_limit</c></cell>
<cell align="left" valign="middle">A system limit has been reached. See Efficiency Guide for information about system limits.</cell>
</row>
<tcaption>Exit Reasons.</tcaption>
</table>
<p><c>Stack</c> is the stack of function calls being evaluated
when the error occurred, given as a list of tuples
<c>{Module,Name,Arity}</c> with the most recent function call
first. The most recent function call tuple may in some
cases be <c>{Module,Name,[Arg]}</c>.</p>
</section>
</chapter>
