devdocs/elisp/function-safety.html


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 <h3 class="section">Determining whether a Function is Safe to Call</h3>   <p>Some major modes, such as SES, call functions that are stored in user files. (See <a href="https://www.gnu.org/software/emacs/manual/html_node/ses/index.html#Top">(ses)Simple Emacs Spreadsheet</a>, for more information on SES.) User files sometimes have poor pedigrees—you can get a spreadsheet from someone you’ve just met, or you can get one through email from someone you’ve never met. So it is risky to call a function whose source code is stored in a user file until you have determined that it is safe. </p> <dl> <dt id="unsafep">Function: <strong>unsafep</strong> <em>form &amp;optional unsafep-vars</em>
</dt> <dd><p>Returns <code>nil</code> if <var>form</var> is a <em>safe</em> Lisp expression, or returns a list that describes why it might be unsafe. The argument <var>unsafep-vars</var> is a list of symbols known to have temporary bindings at this point; it is mainly used for internal recursive calls. The current buffer is an implicit argument, which provides a list of buffer-local bindings. </p></dd>
</dl> <p>Being quick and simple, <code>unsafep</code> does a very light analysis and rejects many Lisp expressions that are actually safe. There are no known cases where <code>unsafep</code> returns <code>nil</code> for an unsafe expression. However, a safe Lisp expression can return a string with a <code>display</code> property, containing an associated Lisp expression to be executed after the string is inserted into a buffer. This associated expression can be a virus. In order to be safe, you must delete properties from all strings calculated by user code before inserting them into buffers. </p><div class="_attribution">
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