@c -*-texinfo-*- @c This is part of the GNU Emacs Lisp Reference Manual. @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 2001, 2002, 2003, 2004, @c 2005, 2006, 2007, 2008 Free Software Foundation, Inc. @c See the file elisp.texi for copying conditions. @setfilename ../info/compile @node Byte Compilation, Advising Functions, Loading, Top @chapter Byte Compilation @cindex byte compilation @cindex byte-code @cindex compilation (Emacs Lisp) Emacs Lisp has a @dfn{compiler} that translates functions written in Lisp into a special representation called @dfn{byte-code} that can be executed more efficiently. The compiler replaces Lisp function definitions with byte-code. When a byte-code function is called, its definition is evaluated by the @dfn{byte-code interpreter}. Because the byte-compiled code is evaluated by the byte-code interpreter, instead of being executed directly by the machine's hardware (as true compiled code is), byte-code is completely transportable from machine to machine without recompilation. It is not, however, as fast as true compiled code. Compiling a Lisp file with the Emacs byte compiler always reads the file as multibyte text, even if Emacs was started with @samp{--unibyte}, unless the file specifies otherwise. This is so that compilation gives results compatible with running the same file without compilation. @xref{Loading Non-ASCII}. In general, any version of Emacs can run byte-compiled code produced by recent earlier versions of Emacs, but the reverse is not true. @vindex no-byte-compile If you do not want a Lisp file to be compiled, ever, put a file-local variable binding for @code{no-byte-compile} into it, like this: @example ;; -*-no-byte-compile: t; -*- @end example @xref{Compilation Errors}, for how to investigate errors occurring in byte compilation. @menu * Speed of Byte-Code:: An example of speedup from byte compilation. * Compilation Functions:: Byte compilation functions. * Docs and Compilation:: Dynamic loading of documentation strings. * Dynamic Loading:: Dynamic loading of individual functions. * Eval During Compile:: Code to be evaluated when you compile. * Compiler Errors:: Handling compiler error messages. * Byte-Code Objects:: The data type used for byte-compiled functions. * Disassembly:: Disassembling byte-code; how to read byte-code. @end menu @node Speed of Byte-Code @section Performance of Byte-Compiled Code A byte-compiled function is not as efficient as a primitive function written in C, but runs much faster than the version written in Lisp. Here is an example: @example @group (defun silly-loop (n) "Return time before and after N iterations of a loop." (let ((t1 (current-time-string))) (while (> (setq n (1- n)) 0)) (list t1 (current-time-string)))) @result{} silly-loop @end group @group (silly-loop 100000) @result{} ("Fri Mar 18 17:25:57 1994" "Fri Mar 18 17:26:28 1994") ; @r{31 seconds} @end group @group (byte-compile 'silly-loop) @result{} @r{[Compiled code not shown]} @end group @group (silly-loop 100000) @result{} ("Fri Mar 18 17:26:52 1994" "Fri Mar 18 17:26:58 1994") ; @r{6 seconds} @end group @end example In this example, the interpreted code required 31 seconds to run, whereas the byte-compiled code required 6 seconds. These results are representative, but actual results will vary greatly. @node Compilation Functions @comment node-name, next, previous, up @section The Compilation Functions @cindex compilation functions You can byte-compile an individual function or macro definition with the @code{byte-compile} function. You can compile a whole file with @code{byte-compile-file}, or several files with @code{byte-recompile-directory} or @code{batch-byte-compile}. The byte compiler produces error messages and warnings about each file in a buffer called @samp{*Compile-Log*}. These report things in your program that suggest a problem but are not necessarily erroneous. @cindex macro compilation Be careful when writing macro calls in files that you may someday byte-compile. Macro calls are expanded when they are compiled, so the macros must already be defined for proper compilation. For more details, see @ref{Compiling Macros}. If a program does not work the same way when compiled as it does when interpreted, erroneous macro definitions are one likely cause (@pxref{Problems with Macros}). Inline (@code{defsubst}) functions are less troublesome; if you compile a call to such a function before its definition is known, the call will still work right, it will just run slower. Normally, compiling a file does not evaluate the file's contents or load the file. But it does execute any @code{require} calls at top level in the file. One way to ensure that necessary macro definitions are available during compilation is to require the file that defines them (@pxref{Named Features}). To avoid loading the macro definition files when someone @emph{runs} the compiled program, write @code{eval-when-compile} around the @code{require} calls (@pxref{Eval During Compile}). @defun byte-compile symbol This function byte-compiles the function definition of @var{symbol}, replacing the previous definition with the compiled one. The function definition of @var{symbol} must be the actual code for the function; i.e., the compiler does not follow indirection to another symbol. @code{byte-compile} returns the new, compiled definition of @var{symbol}. If @var{symbol}'s definition is a byte-code function object, @code{byte-compile} does nothing and returns @code{nil}. Lisp records only one function definition for any symbol, and if that is already compiled, non-compiled code is not available anywhere. So there is no way to ``compile the same definition again.'' @example @group (defun factorial (integer) "Compute factorial of INTEGER." (if (= 1 integer) 1 (* integer (factorial (1- integer))))) @result{} factorial @end group @group (byte-compile 'factorial) @result{} #[(integer) "^H\301U\203^H^@@\301\207\302^H\303^HS!\"\207" [integer 1 * factorial] 4 "Compute factorial of INTEGER."] @end group @end example @noindent The result is a byte-code function object. The string it contains is the actual byte-code; each character in it is an instruction or an operand of an instruction. The vector contains all the constants, variable names and function names used by the function, except for certain primitives that are coded as special instructions. If the argument to @code{byte-compile} is a @code{lambda} expression, it returns the corresponding compiled code, but does not store it anywhere. @end defun @deffn Command compile-defun &optional arg This command reads the defun containing point, compiles it, and evaluates the result. If you use this on a defun that is actually a function definition, the effect is to install a compiled version of that function. @code{compile-defun} normally displays the result of evaluation in the echo area, but if @var{arg} is non-@code{nil}, it inserts the result in the current buffer after the form it compiled. @end deffn @deffn Command byte-compile-file filename &optional load This function compiles a file of Lisp code named @var{filename} into a file of byte-code. The output file's name is made by changing the @samp{.el} suffix into @samp{.elc}; if @var{filename} does not end in @samp{.el}, it adds @samp{.elc} to the end of @var{filename}. Compilation works by reading the input file one form at a time. If it is a definition of a function or macro, the compiled function or macro definition is written out. Other forms are batched together, then each batch is compiled, and written so that its compiled code will be executed when the file is read. All comments are discarded when the input file is read. This command returns @code{t} if there were no errors and @code{nil} otherwise. When called interactively, it prompts for the file name. If @var{load} is non-@code{nil}, this command loads the compiled file after compiling it. Interactively, @var{load} is the prefix argument. @example @group % ls -l push* -rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el @end group @group (byte-compile-file "~/emacs/push.el") @result{} t @end group @group % ls -l push* -rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el -rw-rw-rw- 1 lewis 638 Oct 8 20:25 push.elc @end group @end example @end deffn @deffn Command byte-recompile-directory directory &optional flag force @cindex library compilation This command recompiles every @samp{.el} file in @var{directory} (or its subdirectories) that needs recompilation. A file needs recompilation if a @samp{.elc} file exists but is older than the @samp{.el} file. When a @samp{.el} file has no corresponding @samp{.elc} file, @var{flag} says what to do. If it is @code{nil}, this command ignores these files. If @var{flag} is 0, it compiles them. If it is neither @code{nil} nor 0, it asks the user whether to compile each such file, and asks about each subdirectory as well. Interactively, @code{byte-recompile-directory} prompts for @var{directory} and @var{flag} is the prefix argument. If @var{force} is non-@code{nil}, this command recompiles every @samp{.el} file that has a @samp{.elc} file. The returned value is unpredictable. @end deffn @defun batch-byte-compile &optional noforce This function runs @code{byte-compile-file} on files specified on the command line. This function must be used only in a batch execution of Emacs, as it kills Emacs on completion. An error in one file does not prevent processing of subsequent files, but no output file will be generated for it, and the Emacs process will terminate with a nonzero status code. If @var{noforce} is non-@code{nil}, this function does not recompile files that have an up-to-date @samp{.elc} file. @example % emacs -batch -f batch-byte-compile *.el @end example @end defun @defun byte-code code-string data-vector max-stack @cindex byte-code interpreter This function actually interprets byte-code. A byte-compiled function is actually defined with a body that calls @code{byte-code}. Don't call this function yourself---only the byte compiler knows how to generate valid calls to this function. In Emacs version 18, byte-code was always executed by way of a call to the function @code{byte-code}. Nowadays, byte-code is usually executed as part of a byte-code function object, and only rarely through an explicit call to @code{byte-code}. @end defun @node Docs and Compilation @section Documentation Strings and Compilation @cindex dynamic loading of documentation Functions and variables loaded from a byte-compiled file access their documentation strings dynamically from the file whenever needed. This saves space within Emacs, and makes loading faster because the documentation strings themselves need not be processed while loading the file. Actual access to the documentation strings becomes slower as a result, but this normally is not enough to bother users. Dynamic access to documentation strings does have drawbacks: @itemize @bullet @item If you delete or move the compiled file after loading it, Emacs can no longer access the documentation strings for the functions and variables in the file. @item If you alter the compiled file (such as by compiling a new version), then further access to documentation strings in this file will probably give nonsense results. @end itemize If your site installs Emacs following the usual procedures, these problems will never normally occur. Installing a new version uses a new directory with a different name; as long as the old version remains installed, its files will remain unmodified in the places where they are expected to be. However, if you have built Emacs yourself and use it from the directory where you built it, you will experience this problem occasionally if you edit and recompile Lisp files. When it happens, you can cure the problem by reloading the file after recompiling it. You can turn off this feature at compile time by setting @code{byte-compile-dynamic-docstrings} to @code{nil}; this is useful mainly if you expect to change the file, and you want Emacs processes that have already loaded it to keep working when the file changes. You can do this globally, or for one source file by specifying a file-local binding for the variable. One way to do that is by adding this string to the file's first line: @example -*-byte-compile-dynamic-docstrings: nil;-*- @end example @defvar byte-compile-dynamic-docstrings If this is non-@code{nil}, the byte compiler generates compiled files that are set up for dynamic loading of documentation strings. @end defvar @cindex @samp{#@@@var{count}} @cindex @samp{#$} The dynamic documentation string feature writes compiled files that use a special Lisp reader construct, @samp{#@@@var{count}}. This construct skips the next @var{count} characters. It also uses the @samp{#$} construct, which stands for ``the name of this file, as a string.'' It is usually best not to use these constructs in Lisp source files, since they are not designed to be clear to humans reading the file. @node Dynamic Loading @section Dynamic Loading of Individual Functions @cindex dynamic loading of functions @cindex lazy loading When you compile a file, you can optionally enable the @dfn{dynamic function loading} feature (also known as @dfn{lazy loading}). With dynamic function loading, loading the file doesn't fully read the function definitions in the file. Instead, each function definition contains a place-holder which refers to the file. The first time each function is called, it reads the full definition from the file, to replace the place-holder. The advantage of dynamic function loading is that loading the file becomes much faster. This is a good thing for a file which contains many separate user-callable functions, if using one of them does not imply you will probably also use the rest. A specialized mode which provides many keyboard commands often has that usage pattern: a user may invoke the mode, but use only a few of the commands it provides. The dynamic loading feature has certain disadvantages: @itemize @bullet @item If you delete or move the compiled file after loading it, Emacs can no longer load the remaining function definitions not already loaded. @item If you alter the compiled file (such as by compiling a new version), then trying to load any function not already loaded will usually yield nonsense results. @end itemize These problems will never happen in normal circumstances with installed Emacs files. But they are quite likely to happen with Lisp files that you are changing. The easiest way to prevent these problems is to reload the new compiled file immediately after each recompilation. The byte compiler uses the dynamic function loading feature if the variable @code{byte-compile-dynamic} is non-@code{nil} at compilation time. Do not set this variable globally, since dynamic loading is desirable only for certain files. Instead, enable the feature for specific source files with file-local variable bindings. For example, you could do it by writing this text in the source file's first line: @example -*-byte-compile-dynamic: t;-*- @end example @defvar byte-compile-dynamic If this is non-@code{nil}, the byte compiler generates compiled files that are set up for dynamic function loading. @end defvar @defun fetch-bytecode function If @var{function} is a byte-code function object, this immediately finishes loading the byte code of @var{function} from its byte-compiled file, if it is not fully loaded already. Otherwise, it does nothing. It always returns @var{function}. @end defun @node Eval During Compile @section Evaluation During Compilation These features permit you to write code to be evaluated during compilation of a program. @defspec eval-and-compile body@dots{} This form marks @var{body} to be evaluated both when you compile the containing code and when you run it (whether compiled or not). You can get a similar result by putting @var{body} in a separate file and referring to that file with @code{require}. That method is preferable when @var{body} is large. Effectively @code{require} is automatically @code{eval-and-compile}, the package is loaded both when compiling and executing. @code{autoload} is also effectively @code{eval-and-compile} too. It's recognized when compiling, so uses of such a function don't produce ``not known to be defined'' warnings. Most uses of @code{eval-and-compile} are fairly sophisticated. If a macro has a helper function to build its result, and that macro is used both locally and outside the package, then @code{eval-and-compile} should be used to get the helper both when compiling and then later when running. If functions are defined programmatically (with @code{fset} say), then @code{eval-and-compile} can be used to have that done at compile-time as well as run-time, so calls to those functions are checked (and warnings about ``not known to be defined'' suppressed). @end defspec @defspec eval-when-compile body@dots{} This form marks @var{body} to be evaluated at compile time but not when the compiled program is loaded. The result of evaluation by the compiler becomes a constant which appears in the compiled program. If you load the source file, rather than compiling it, @var{body} is evaluated normally. @cindex compile-time constant If you have a constant that needs some calculation to produce, @code{eval-when-compile} can do that at compile-time. For example, @lisp (defvar my-regexp (eval-when-compile (regexp-opt '("aaa" "aba" "abb")))) @end lisp @cindex macros, at compile time If you're using another package, but only need macros from it (the byte compiler will expand those), then @code{eval-when-compile} can be used to load it for compiling, but not executing. For example, @lisp (eval-when-compile (require 'my-macro-package)) ;; only macros needed from this @end lisp The same sort of thing goes for macros and @code{defsubst} functions defined locally and only for use within the file. They are needed for compiling the file, but in most cases they are not needed for execution of the compiled file. For example, @lisp (eval-when-compile (unless (fboundp 'some-new-thing) (defmacro 'some-new-thing () (compatibility code)))) @end lisp @noindent This is often good for code that's only a fallback for compatibility with other versions of Emacs. @strong{Common Lisp Note:} At top level, @code{eval-when-compile} is analogous to the Common Lisp idiom @code{(eval-when (compile eval) @dots{})}. Elsewhere, the Common Lisp @samp{#.} reader macro (but not when interpreting) is closer to what @code{eval-when-compile} does. @end defspec @node Compiler Errors @section Compiler Errors @cindex compiler errors Byte compilation outputs all errors and warnings into the buffer @samp{*Compile-Log*}. The messages include file names and line numbers that identify the location of the problem. The usual Emacs commands for operating on compiler diagnostics work properly on these messages. However, the warnings about functions that were used but not defined are always ``located'' at the end of the file, so these commands won't find the places they are really used. To do that, you must search for the function names. You can suppress the compiler warning for calling an undefined function @var{func} by conditionalizing the function call on an @code{fboundp} test, like this: @example (if (fboundp '@var{func}) ...(@var{func} ...)...) @end example @noindent The call to @var{func} must be in the @var{then-form} of the @code{if}, and @var{func} must appear quoted in the call to @code{fboundp}. (This feature operates for @code{cond} as well.) Likewise, you can suppress a compiler warning for an unbound variable @var{variable} by conditionalizing its use on a @code{boundp} test, like this: @example (if (boundp '@var{variable}) ...@var{variable}...) @end example @noindent The reference to @var{variable} must be in the @var{then-form} of the @code{if}, and @var{variable} must appear quoted in the call to @code{boundp}. You can suppress any compiler warnings using the construct @code{with-no-warnings}: @c This is implemented with a defun, but conceptually it is @c a special form. @defspec with-no-warnings body@dots{} In execution, this is equivalent to @code{(progn @var{body}...)}, but the compiler does not issue warnings for anything that occurs inside @var{body}. We recommend that you use this construct around the smallest possible piece of code. @end defspec @node Byte-Code Objects @section Byte-Code Function Objects @cindex compiled function @cindex byte-code function Byte-compiled functions have a special data type: they are @dfn{byte-code function objects}. Internally, a byte-code function object is much like a vector; however, the evaluator handles this data type specially when it appears as a function to be called. The printed representation for a byte-code function object is like that for a vector, with an additional @samp{#} before the opening @samp{[}. A byte-code function object must have at least four elements; there is no maximum number, but only the first six elements have any normal use. They are: @table @var @item arglist The list of argument symbols. @item byte-code The string containing the byte-code instructions. @item constants The vector of Lisp objects referenced by the byte code. These include symbols used as function names and variable names. @item stacksize The maximum stack size this function needs. @item docstring The documentation string (if any); otherwise, @code{nil}. The value may be a number or a list, in case the documentation string is stored in a file. Use the function @code{documentation} to get the real documentation string (@pxref{Accessing Documentation}). @item interactive The interactive spec (if any). This can be a string or a Lisp expression. It is @code{nil} for a function that isn't interactive. @end table Here's an example of a byte-code function object, in printed representation. It is the definition of the command @code{backward-sexp}. @example #[(&optional arg) "^H\204^F^@@\301^P\302^H[!\207" [arg 1 forward-sexp] 2 254435 "p"] @end example The primitive way to create a byte-code object is with @code{make-byte-code}: @defun make-byte-code &rest elements This function constructs and returns a byte-code function object with @var{elements} as its elements. @end defun You should not try to come up with the elements for a byte-code function yourself, because if they are inconsistent, Emacs may crash when you call the function. Always leave it to the byte compiler to create these objects; it makes the elements consistent (we hope). You can access the elements of a byte-code object using @code{aref}; you can also use @code{vconcat} to create a vector with the same elements. @node Disassembly @section Disassembled Byte-Code @cindex disassembled byte-code People do not write byte-code; that job is left to the byte compiler. But we provide a disassembler to satisfy a cat-like curiosity. The disassembler converts the byte-compiled code into humanly readable form. The byte-code interpreter is implemented as a simple stack machine. It pushes values onto a stack of its own, then pops them off to use them in calculations whose results are themselves pushed back on the stack. When a byte-code function returns, it pops a value off the stack and returns it as the value of the function. In addition to the stack, byte-code functions can use, bind, and set ordinary Lisp variables, by transferring values between variables and the stack. @deffn Command disassemble object &optional buffer-or-name This command displays the disassembled code for @var{object}. In interactive use, or if @var{buffer-or-name} is @code{nil} or omitted, the output goes in a buffer named @samp{*Disassemble*}. If @var{buffer-or-name} is non-@code{nil}, it must be a buffer or the name of an existing buffer. Then the output goes there, at point, and point is left before the output. The argument @var{object} can be a function name, a lambda expression or a byte-code object. If it is a lambda expression, @code{disassemble} compiles it and disassembles the resulting compiled code. @end deffn Here are two examples of using the @code{disassemble} function. We have added explanatory comments to help you relate the byte-code to the Lisp source; these do not appear in the output of @code{disassemble}. These examples show unoptimized byte-code. Nowadays byte-code is usually optimized, but we did not want to rewrite these examples, since they still serve their purpose. @example @group (defun factorial (integer) "Compute factorial of an integer." (if (= 1 integer) 1 (* integer (factorial (1- integer))))) @result{} factorial @end group @group (factorial 4) @result{} 24 @end group @group (disassemble 'factorial) @print{} byte-code for factorial: doc: Compute factorial of an integer. args: (integer) @end group @group 0 constant 1 ; @r{Push 1 onto stack.} 1 varref integer ; @r{Get value of @code{integer}} ; @r{from the environment} ; @r{and push the value} ; @r{onto the stack.} @end group @group 2 eqlsign ; @r{Pop top two values off stack,} ; @r{compare them,} ; @r{and push result onto stack.} @end group @group 3 goto-if-nil 10 ; @r{Pop and test top of stack;} ; @r{if @code{nil}, go to 10,} ; @r{else continue.} @end group @group 6 constant 1 ; @r{Push 1 onto top of stack.} 7 goto 17 ; @r{Go to 17 (in this case, 1 will be} ; @r{returned by the function).} @end group @group 10 constant * ; @r{Push symbol @code{*} onto stack.} 11 varref integer ; @r{Push value of @code{integer} onto stack.} @end group @group 12 constant factorial ; @r{Push @code{factorial} onto stack.} 13 varref integer ; @r{Push value of @code{integer} onto stack.} 14 sub1 ; @r{Pop @code{integer}, decrement value,} ; @r{push new value onto stack.} @end group @group ; @r{Stack now contains:} ; @minus{} @r{decremented value of @code{integer}} ; @minus{} @r{@code{factorial}} ; @minus{} @r{value of @code{integer}} ; @minus{} @r{@code{*}} @end group @group 15 call 1 ; @r{Call function @code{factorial} using} ; @r{the first (i.e., the top) element} ; @r{of the stack as the argument;} ; @r{push returned value onto stack.} @end group @group ; @r{Stack now contains:} ; @minus{} @r{result of recursive} ; @r{call to @code{factorial}} ; @minus{} @r{value of @code{integer}} ; @minus{} @r{@code{*}} @end group @group 16 call 2 ; @r{Using the first two} ; @r{(i.e., the top two)} ; @r{elements of the stack} ; @r{as arguments,} ; @r{call the function @code{*},} ; @r{pushing the result onto the stack.} @end group @group 17 return ; @r{Return the top element} ; @r{of the stack.} @result{} nil @end group @end example The @code{silly-loop} function is somewhat more complex: @example @group (defun silly-loop (n) "Return time before and after N iterations of a loop." (let ((t1 (current-time-string))) (while (> (setq n (1- n)) 0)) (list t1 (current-time-string)))) @result{} silly-loop @end group @group (disassemble 'silly-loop) @print{} byte-code for silly-loop: doc: Return time before and after N iterations of a loop. args: (n) 0 constant current-time-string ; @r{Push} ; @r{@code{current-time-string}} ; @r{onto top of stack.} @end group @group 1 call 0 ; @r{Call @code{current-time-string}} ; @r{ with no argument,} ; @r{ pushing result onto stack.} @end group @group 2 varbind t1 ; @r{Pop stack and bind @code{t1}} ; @r{to popped value.} @end group @group 3 varref n ; @r{Get value of @code{n} from} ; @r{the environment and push} ; @r{the value onto the stack.} @end group @group 4 sub1 ; @r{Subtract 1 from top of stack.} @end group @group 5 dup ; @r{Duplicate the top of the stack;} ; @r{i.e., copy the top of} ; @r{the stack and push the} ; @r{copy onto the stack.} @end group @group 6 varset n ; @r{Pop the top of the stack,} ; @r{and bind @code{n} to the value.} ; @r{In effect, the sequence @code{dup varset}} ; @r{copies the top of the stack} ; @r{into the value of @code{n}} ; @r{without popping it.} @end group @group 7 constant 0 ; @r{Push 0 onto stack.} @end group @group 8 gtr ; @r{Pop top two values off stack,} ; @r{test if @var{n} is greater than 0} ; @r{and push result onto stack.} @end group @group 9 goto-if-nil-else-pop 17 ; @r{Goto 17 if @code{n} <= 0} ; @r{(this exits the while loop).} ; @r{else pop top of stack} ; @r{and continue} @end group @group 12 constant nil ; @r{Push @code{nil} onto stack} ; @r{(this is the body of the loop).} @end group @group 13 discard ; @r{Discard result of the body} ; @r{of the loop (a while loop} ; @r{is always evaluated for} ; @r{its side effects).} @end group @group 14 goto 3 ; @r{Jump back to beginning} ; @r{of while loop.} @end group @group 17 discard ; @r{Discard result of while loop} ; @r{by popping top of stack.} ; @r{This result is the value @code{nil} that} ; @r{was not popped by the goto at 9.} @end group @group 18 varref t1 ; @r{Push value of @code{t1} onto stack.} @end group @group 19 constant current-time-string ; @r{Push} ; @r{@code{current-time-string}} ; @r{onto top of stack.} @end group @group 20 call 0 ; @r{Call @code{current-time-string} again.} @end group @group 21 list2 ; @r{Pop top two elements off stack,} ; @r{create a list of them,} ; @r{and push list onto stack.} @end group @group 22 unbind 1 ; @r{Unbind @code{t1} in local environment.} 23 return ; @r{Return value of the top of stack.} @result{} nil @end group @end example @ignore arch-tag: f78e3050-2f0a-4dee-be27-d9979a0a2289 @end ignore