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<h4 class="subsection">Entering the Debugger on a Function Call</h4> <p>To investigate a problem that happens in the middle of a program, one useful technique is to enter the debugger whenever a certain function is called. You can do this to the function in which the problem occurs, and then step through the function, or you can do this to a function called shortly before the problem, step quickly over the call to that function, and then step through its caller. </p> <dl> <dt id="debug-on-entry">Command: <strong>debug-on-entry</strong> <em>function-name</em>
</dt> <dd>
<p>This function requests <var>function-name</var> to invoke the debugger each time it is called. </p> <p>Any function or macro defined as Lisp code may be set to break on entry, regardless of whether it is interpreted code or compiled code. If the function is a command, it will enter the debugger when called from Lisp and when called interactively (after the reading of the arguments). You can also set debug-on-entry for primitive functions (i.e., those written in C) this way, but it only takes effect when the primitive is called from Lisp code. Debug-on-entry is not allowed for special forms. </p> <p>When <code>debug-on-entry</code> is called interactively, it prompts for <var>function-name</var> in the minibuffer. If the function is already set up to invoke the debugger on entry, <code>debug-on-entry</code> does nothing. <code>debug-on-entry</code> always returns <var>function-name</var>. </p> <p>Here’s an example to illustrate use of this function: </p> <div class="example"> <pre class="example">(defun fact (n)
(if (zerop n) 1
(* n (fact (1- n)))))
⇒ fact
</pre>
<pre class="example">(debug-on-entry 'fact)
⇒ fact
</pre>
<pre class="example">(fact 3)
</pre>
<pre class="example">------ Buffer: *Backtrace* ------
Debugger entered--entering a function:
* fact(3)
eval((fact 3))
eval-last-sexp-1(nil)
eval-last-sexp(nil)
call-interactively(eval-last-sexp)
------ Buffer: *Backtrace* ------
</pre>
</div> </dd>
</dl> <dl> <dt id="cancel-debug-on-entry">Command: <strong>cancel-debug-on-entry</strong> <em>&optional function-name</em>
</dt> <dd><p>This function undoes the effect of <code>debug-on-entry</code> on <var>function-name</var>. When called interactively, it prompts for <var>function-name</var> in the minibuffer. If <var>function-name</var> is omitted or <code>nil</code>, it cancels break-on-entry for all functions. Calling <code>cancel-debug-on-entry</code> does nothing to a function which is not currently set up to break on entry. </p></dd>
</dl><div class="_attribution">
<p class="_attribution-p">
Copyright © 1990-1996, 1998-2022 Free Software Foundation, Inc. <br>Licensed under the GNU GPL license.<br>
<a href="https://www.gnu.org/software/emacs/manual/html_node/elisp/Function-Debugging.html" class="_attribution-link">https://www.gnu.org/software/emacs/manual/html_node/elisp/Function-Debugging.html</a>
</p>
</div>
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