* Function Cells:: Accessing or setting the function definition
of a symbol.
* Inline Functions:: Defining functions that the compiler will open code.
-* Function safety:: Determining whether a function is safe to call.
+* Function Safety:: Determining whether a function is safe to call.
* Related Topics:: Cross-references to specific Lisp primitives
that have a special bearing on how functions work.
@end menu
considered primitives.)
Usually the reason we implement a function as a primitive is either
-because it is fundamental, because it provides a low-level interface to
-operating system services, or because it needs to run fast. Primitives
-can be modified or added only by changing the C sources and recompiling
-the editor. See @ref{Writing Emacs Primitives}.
+because it is fundamental, because it provides a low-level interface
+to operating system services, or because it needs to run fast.
+Primitives can be modified or added only by changing the C sources and
+recompiling the editor. See @ref{Writing Emacs Primitives}.
@item lambda expression
A @dfn{lambda expression} is a function written in Lisp.
records.
@end defun
+ You cannot create a new primitive function with @code{defun} or
+@code{defalias}, but you use them to change the function definition of
+any symbol, even one such as @code{car} or @code{x-popup-menu} whose
+normal definition is a primitive. However, this is risky: for
+instance, it is next to impossible to redefine @code{car} without
+breaking Lisp completely. Redefining an obscure function such as
+@code{x-popup-menu} is less dangerous, but it still may not work as
+you expect. If there are calls to the primitive from C code, they
+call the primitive's C definition directly, so changing the symbol's
+definition will have no effect on them.
+
See also @code{defsubst}, which defines a function like @code{defun}
and tells the Lisp compiler to open-code it. @xref{Inline Functions}.
Inline functions can be used and open-coded later on in the same file,
following the definition, just like macros.
-@node Function safety
+@node Function Safety
@section Determining whether a function is safe to call
@cindex function safety
@cindex safety of functions
-@cindex virus detection
-@cindex Trojan-horse detection
-@cindex DDoS attacks
-
-Some major modes such as SES (see @pxref{Top,,,ses}) will call
-functions that are stored in user files. 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.
-Such files can contain viruses and other Trojan horses that could
-corrupt your operating system environment, delete your files, or even
-turn your computer into a DDoS zombie! To avoid this terrible fate,
-you should not call a function whose source code is stored in a user
-file until you have determined that it is safe.
+
+Some major modes such as SES (@pxref{Top,,,ses}) call functions that
+are stored in user files. 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.
@defun unsafep form &optional unsafep-vars
-Returns nil if @var{form} is a @dfn{safe} lisp expression, or returns
-a list that describes why it might be unsafe. The argument
+Returns @code{nil} if @var{form} is a @dfn{safe} lisp expression, or
+returns a list that describes why it might be unsafe. The argument
@var{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
Being quick and simple, @code{unsafep} does a very light analysis and
rejects many Lisp expressions that are actually safe. There are no
-known cases where @code{unsafep} returns nil for an unsafe expression.
-However, a ``safe'' Lisp expression can return a string with a
-@code{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
+known cases where @code{unsafep} returns @code{nil} for an unsafe
+expression. However, a ``safe'' Lisp expression can return a string
+with a @code{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.
+@ignore
What is a safe Lisp expression? Basically, it's an expression that
calls only built-in functions with no side effects (or only innocuous
ones). Innocuous side effects include displaying messages and
A call to a safe function (see below), if all its arguments are
safe expressions.
@item
-One of the special forms [and, catch, cond, if, or, prog1, prog2,
-progn, while, unwind-protect], if all its arguments are safe.
+One of the special forms @code{and}, @code{catch}, @code{cond},
+@code{if}, @code{or}, @code{prog1}, @code{prog2}, @code{progn},
+@code{while}, and @code{unwind-protect}], if all its arguments are
+safe.
@item
-A form that creates temporary bindings [condition-case, dolist,
-dotimes, lambda, let, let*], if all args are safe and the symbols to
-be bound are not explicitly risky (see @pxref{File Local Variables}).
+A form that creates temporary bindings (@code{condition-case},
+@code{dolist}, @code{dotimes}, @code{lambda}, @code{let}, or
+@code{let*}), if all args are safe and the symbols to be bound are not
+explicitly risky (see @pxref{File Local Variables}).
@item
-An assignment [add-to-list, setq, push, pop], if all args are safe and
-the symbols to be assigned are not explicitly risky and they already
-have temporary or buffer-local bindings.
+An assignment using @code{add-to-list}, @code{setq}, @code{push}, or
+@code{pop}, if all args are safe and the symbols to be assigned are
+not explicitly risky and they already have temporary or buffer-local
+bindings.
@item
One of [apply, mapc, mapcar, mapconcat] if the first argument is a
safe explicit lambda and the other args are safe expressions.
@item
A symbol on the list @code{safe-functions}, so the user says it's safe.
@item
-A symbol with a non-nil @code{side-effect-free} property.
+A symbol with a non-@code{nil} @code{side-effect-free} property.
@item
-A symbol with a non-nil @code{safe-function} property. Value t
+A symbol with a non-@code{nil} @code{safe-function} property. Value t
indicates a function that is safe but has innocuous side effects.
Other values will someday indicate functions with classes of side
effects that are not always safe.
provided for built-in functions and for low-level functions and macros
defined in @file{subr.el}. You can assign these properties for the
functions you write.
-
@end table
-
-
-@c Emacs versions prior to 19 did not have inline functions.
+@end ignore
@node Related Topics
@section Other Topics Related to Functions
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