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### 12.16 Determining whether a Function is Safe to Call

Some major modes, such as SES, call functions that are stored in user files. (See (ses)Top, 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.

Function: unsafep form &optional unsafep-vars

  Returns `nil` if form is a safe Lisp expression, or returns a list that describes why it might be unsafe. The argument unsafep-vars 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.

Being quick and simple, `unsafep` does a very light analysis and rejects many Lisp expressions that are actually safe. There are no known cases where `unsafep` returns `nil` for an unsafe expression. However, a safe Lisp expression can return a string with a `display` 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.