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This is Info file ../../info/lispref.info, produced by Makeinfo-1.63 from the input file lispref.texi. Edition History: GNU Emacs Lisp Reference Manual Second Edition (v2.01), May 1993 GNU Emacs Lisp Reference Manual Further Revised (v2.02), August 1993 Lucid Emacs Lisp Reference Manual (for 19.10) First Edition, March 1994 XEmacs Lisp Programmer's Manual (for 19.12) Second Edition, April 1995 GNU Emacs Lisp Reference Manual v2.4, June 1995 XEmacs Lisp Programmer's Manual (for 19.13) Third Edition, July 1995 Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995 Free Software Foundation, Inc. Copyright (C) 1994, 1995 Sun Microsystems, Inc. Copyright (C) 1995 Amdahl Corporation. Copyright (C) 1995 Ben Wing. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled "GNU General Public License" is included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the section entitled "GNU General Public License" may be included in a translation approved by the Free Software Foundation instead of in the original English. File: lispref.info, Node: Compilation Functions, Next: Docs and Compilation, Prev: Speed of Byte-Code, Up: Byte Compilation The Compilation Functions ========================= You can byte-compile an individual function or macro definition with the `byte-compile' function. You can compile a whole file with `byte-compile-file', or several files with `byte-recompile-directory' or `batch-byte-compile'. When you run the byte compiler, you may get warnings in a buffer called `*Compile-Log*'. These report things in your program that suggest a problem but are not necessarily erroneous. Be careful when byte-compiling code that uses macros. Macro calls are expanded when they are compiled, so the macros must already be defined for proper compilation. For more details, see *Note Compiling Macros::. Normally, compiling a file does not evaluate the file's contents or load the file. But it does execute any `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 (*note Named Features::.). To avoid loading the macro definition files when someone *runs* the compiled program, write `eval-when-compile' around the `require' calls (*note Eval During Compile::.). - Function: byte-compile SYMBOL This function byte-compiles the function definition of SYMBOL, replacing the previous definition with the compiled one. The function definition of SYMBOL must be the actual code for the function; i.e., the compiler does not follow indirection to another symbol. `byte-compile' returns the new, compiled definition of SYMBOL. If SYMBOL's definition is a byte-code function object, `byte-compile' does nothing and returns `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." (defun factorial (integer) "Compute factorial of INTEGER." (if (= 1 integer) 1 (* integer (factorial (1- integer))))) => factorial (byte-compile 'factorial) => #<byte-code (integer) "┴U½é┴ç┬S!_ç" [integer 1 factorial] 3 "Compute factorial of INTEGER."> 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. - 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. If ARG is non-`nil', the result is inserted in the current buffer after the form; otherwise, it is printed in the minibuffer. - Command: byte-compile-file FILENAME &optional LOAD This function compiles a file of Lisp code named FILENAME into a file of byte-code. The output file's name is made by appending `c' to the end of FILENAME. If `load' is non-`nil', the file is loaded after having been compiled. 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 `t'. When called interactively, it prompts for the file name. % ls -l push* -rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el (byte-compile-file "~/emacs/push.el") => t % 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 - Command: byte-recompile-directory DIRECTORY &optional FLAG This function recompiles every `.el' file in DIRECTORY that needs recompilation. A file needs recompilation if a `.elc' file exists but is older than the `.el' file. When a `.el' file has no corresponding `.elc' file, then FLAG says what to do. If it is `nil', these files are ignored. If it is non-`nil', the user is asked whether to compile each such file. The returned value of this command is unpredictable. - Function: batch-byte-compile This function runs `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. (The file that gets the error will not, of course, produce any compiled code.) % emacs -batch -f batch-byte-compile *.el - Function: batch-byte-recompile-directory This function is similar to `batch-byte-compile' but runs the command `byte-recompile-directory' on the files remaining on the command line. - Variable: byte-recompile-directory-ignore-errors-p If non-`nil', this specifies that `byte-recompile-directory' will continue compiling even when an error occurs in a file. This is normally `nil', but is bound to `t' by `batch-byte-recompile-directory'. - Function: byte-code CODE-STRING DATA-VECTOR MAX-STACK This function actually interprets byte-code. A byte-compiled function is actually defined with a body that calls `byte-code'. Don't call this function yourself. Only the byte compiler knows how to generate valid calls to this function. In newer Emacs versions (19 and up), byte-code is usually executed as part of a byte-code function object, and only rarely due to an explicit call to `byte-code'. File: lispref.info, Node: Docs and Compilation, Next: Dynamic Loading, Prev: Compilation Functions, Up: Byte Compilation Documentation Strings and Compilation ===================================== 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: * 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. * If you alter the compiled file (such as by compiling a new version), then further access to documentation strings in this file will give nonsense results. 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. Byte-compiled files made with Emacs 19.29 will not load into older versions because the older versions don't support this feature. You can turn off this feature by setting `byte-compile-dynamic-docstrings' to `nil'. Once this is done, you can compile files that will load into older Emacs versions. You can do this globally, or for one source file by specifying a file-local binding for the variable. Here's one way to do that: -*-byte-compile-dynamic-docstrings: nil;-*- - Variable: byte-compile-dynamic-docstrings If this is non-`nil', the byte compiler generates compiled files that are set up for dynamic loading of documentation strings. The dynamic documentation string feature writes compiled files that use a special Lisp reader construct, `#@COUNT'. This construct skips the next COUNT characters. It also uses the `#$' construct, which stands for "the name of this file, as a string." It is best not to use these constructs in Lisp source files. File: lispref.info, Node: Dynamic Loading, Next: Eval During Compile, Prev: Docs and Compilation, Up: Byte Compilation Dynamic Loading of Individual Functions ======================================= When you compile a file, you can optionally enable the "dynamic function loading" feature (also known as "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 commands, provided that using one of them does not imply you will soon (or ever) 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: * If you delete or move the compiled file after loading it, Emacs can no longer load the remaining function definitions not already loaded. * If you alter the compiled file (such as by compiling a new version), then trying to load any function not already loaded will get nonsense results. If you compile a new version of the file, the best thing to do is immediately load the new compiled file. That will prevent any future problems. The byte compiler uses the dynamic function loading feature if the variable `byte-compile-dynamic' is non-`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, like this: -*-byte-compile-dynamic: t;-*- - Variable: byte-compile-dynamic If this is non-`nil', the byte compiler generates compiled files that are set up for dynamic function loading. - Function: fetch-bytecode FUNCTION This immediately finishes loading the definition of FUNCTION from its byte-compiled file, if it is not fully loaded already. The argument FUNCTION may be a byte-code function object or a function name. File: lispref.info, Node: Eval During Compile, Next: Byte-Code Objects, Prev: Dynamic Loading, Up: Byte Compilation Evaluation During Compilation ============================= These features permit you to write code to be evaluated during compilation of a program. - Special Form: eval-and-compile BODY This form marks 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 BODY in a separate file and referring to that file with `require'. Using `require' is preferable if there is a substantial amount of code to be executed in this way. - Special Form: eval-when-compile BODY This form marks BODY to be evaluated at compile time and not when the compiled program is loaded. The result of evaluation by the compiler becomes a constant which appears in the compiled program. When the program is interpreted, not compiled at all, BODY is evaluated normally. At top level, this is analogous to the Common Lisp idiom `(eval-when (compile eval) ...)'. Elsewhere, the Common Lisp `#.' reader macro (but not when interpreting) is closer to what `eval-when-compile' does. File: lispref.info, Node: Byte-Code Objects, Next: Disassembly, Prev: Eval During Compile, Up: Byte Compilation Byte-Code Function Objects ========================== Byte-compiled functions have a special data type: they are "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 begins with `#<byte-code' and ends with `>'. In Emacs version 18, there was no byte-code function object data type; compiled functions used the function `byte-code' to run the byte code. A byte-code function object must have at least four elements; there is no maximum number, but only the first six elements are actually used. They are: ARGLIST The list of argument symbols. BYTE-CODE The string containing the byte-code instructions. CONSTANTS The vector of Lisp objects referenced by the byte code. These include symbols used as function names and variable names. STACKSIZE The maximum stack size this function needs. DOCSTRING The documentation string (if any); otherwise, `nil'. The value may be a number or a list, in case the documentation string is stored in a file. Use the function `documentation' to get the real documentation string (*note Accessing Documentation::.). INTERACTIVE The interactive spec (if any). This can be a string or a Lisp expression. It is `nil' for a function that isn't interactive. Here's an example of a byte-code function object, in printed representation. It is the definition of the command `backward-sexp'. #[(&optional arg) "^H\204^F^@\301^P\302^H[!\207" [arg 1 forward-sexp] 2 254435 "p"] The primitive way to create a byte-code object is with `make-byte-code': - Function: make-byte-code &rest ELEMENTS This function constructs and returns a byte-code function object with ELEMENTS as its elements. 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 `aref'; you can also use `vconcat' to create a vector with the same elements. File: lispref.info, Node: Disassembly, Prev: Byte-Code Objects, Up: Byte Compilation 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. - Command: disassemble OBJECT &optional STREAM This function prints the disassembled code for OBJECT. If STREAM is supplied, then output goes there. Otherwise, the disassembled code is printed to the stream `standard-output'. The argument OBJECT can be a function name or a lambda expression. As a special exception, if this function is used interactively, it outputs to a buffer named `*Disassemble*'. Here are two examples of using the `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 `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. (defun factorial (integer) "Compute factorial of an integer." (if (= 1 integer) 1 (* integer (factorial (1- integer))))) => factorial (factorial 4) => 24 (disassemble 'factorial) -| byte-code for factorial: doc: Compute factorial of an integer. args: (integer) 0 constant 1 ; Push 1 onto stack. 1 varref integer ; Get value of `integer' ; from the environment ; and push the value ; onto the stack. 2 eqlsign ; Pop top two values off stack, ; compare them, ; and push result onto stack. 3 goto-if-nil 10 ; Pop and test top of stack; ; if `nil', go to 10, ; else continue. 6 constant 1 ; Push 1 onto top of stack. 7 goto 17 ; Go to 17 (in this case, 1 will be ; returned by the function). 10 constant * ; Push symbol `*' onto stack. 11 varref integer ; Push value of `integer' onto stack. 12 constant factorial ; Push `factorial' onto stack. 13 varref integer ; Push value of `integer' onto stack. 14 sub1 ; Pop `integer', decrement value, ; push new value onto stack. ; Stack now contains: ; - decremented value of `integer' ; - `factorial' ; - value of `integer' ; - `*' 15 call 1 ; Call function `factorial' using ; the first (i.e., the top) element ; of the stack as the argument; ; push returned value onto stack. ; Stack now contains: ; - result of recursive ; call to `factorial' ; - value of `integer' ; - `*' 16 call 2 ; Using the first two ; (i.e., the top two) ; elements of the stack ; as arguments, ; call the function `*', ; pushing the result onto the stack. 17 return ; Return the top element ; of the stack. => nil The `silly-loop' function is somewhat more complex: (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)))) => silly-loop (disassemble 'silly-loop) -| byte-code for silly-loop: doc: Return time before and after N iterations of a loop. args: (n) 0 constant current-time-string ; Push ; `current-time-string' ; onto top of stack. 1 call 0 ; Call `current-time-string' ; with no argument, ; pushing result onto stack. 2 varbind t1 ; Pop stack and bind `t1' ; to popped value. 3 varref n ; Get value of `n' from ; the environment and push ; the value onto the stack. 4 sub1 ; Subtract 1 from top of stack. 5 dup ; Duplicate the top of the stack; ; i.e., copy the top of ; the stack and push the ; copy onto the stack. 6 varset n ; Pop the top of the stack, ; and bind `n' to the value. ; In effect, the sequence `dup varset' ; copies the top of the stack ; into the value of `n' ; without popping it. 7 constant 0 ; Push 0 onto stack. 8 gtr ; Pop top two values off stack, ; test if N is greater than 0 ; and push result onto stack. 9 goto-if-nil-else-pop 17 ; Goto 17 if `n' <= 0 ; (this exits the while loop). ; else pop top of stack ; and continue 12 constant nil ; Push `nil' onto stack ; (this is the body of the loop). 13 discard ; Discard result of the body ; of the loop (a while loop ; is always evaluated for ; its side effects). 14 goto 3 ; Jump back to beginning ; of while loop. 17 discard ; Discard result of while loop ; by popping top of stack. ; This result is the value `nil' that ; was not popped by the goto at 9. 18 varref t1 ; Push value of `t1' onto stack. 19 constant current-time-string ; Push ; `current-time-string' ; onto top of stack. 20 call 0 ; Call `current-time-string' again. 21 list2 ; Pop top two elements off stack, ; create a list of them, ; and push list onto stack. 22 unbind 1 ; Unbind `t1' in local environment. 23 return ; Return value of the top of stack. => nil File: lispref.info, Node: Debugging, Next: Read and Print, Prev: Byte Compilation, Up: Top Debugging Lisp Programs *********************** There are three ways to investigate a problem in an Emacs Lisp program, depending on what you are doing with the program when the problem appears. * If the problem occurs when you run the program, you can use a Lisp debugger (either the default debugger or Edebug) to investigate what is happening during execution. * If the problem is syntactic, so that Lisp cannot even read the program, you can use the XEmacs facilities for editing Lisp to localize it. * If the problem occurs when trying to compile the program with the byte compiler, you need to know how to examine the compiler's input buffer. * Menu: * Debugger:: How the Emacs Lisp debugger is implemented. * Syntax Errors:: How to find syntax errors. * Compilation Errors:: How to find errors that show up in byte compilation. * Edebug:: A source-level Emacs Lisp debugger. Another useful debugging tool is the dribble file. When a dribble file is open, XEmacs copies all keyboard input characters to that file. Afterward, you can examine the file to find out what input was used. *Note Terminal Input::. For debugging problems in terminal descriptions, the `open-termscript' function can be useful. *Note Terminal Output::. File: lispref.info, Node: Debugger, Next: Syntax Errors, Up: Debugging The Lisp Debugger ================= The "Lisp debugger" provides the ability to suspend evaluation of a form. While evaluation is suspended (a state that is commonly known as a "break"), you may examine the run time stack, examine the values of local or global variables, or change those values. Since a break is a recursive edit, all the usual editing facilities of XEmacs are available; you can even run programs that will enter the debugger recursively. *Note Recursive Editing::. * Menu: * Error Debugging:: Entering the debugger when an error happens. * Infinite Loops:: Stopping and debugging a program that doesn't exit. * Function Debugging:: Entering it when a certain function is called. * Explicit Debug:: Entering it at a certain point in the program. * Using Debugger:: What the debugger does; what you see while in it. * Debugger Commands:: Commands used while in the debugger. * Invoking the Debugger:: How to call the function `debug'. * Internals of Debugger:: Subroutines of the debugger, and global variables. File: lispref.info, Node: Error Debugging, Next: Infinite Loops, Up: Debugger Entering the Debugger on an Error --------------------------------- The most important time to enter the debugger is when a Lisp error happens. This allows you to investigate the immediate causes of the error. However, entry to the debugger is not a normal consequence of an error. Many commands frequently get Lisp errors when invoked in inappropriate contexts (such as `C-f' at the end of the buffer) and during ordinary editing it would be very unpleasant to enter the debugger each time this happens. If you want errors to enter the debugger, set the variable `debug-on-error' to non-`nil'. - User Option: debug-on-error This variable determines whether the debugger is called when an error is signaled and not handled. If `debug-on-error' is `t', all errors call the debugger. If it is `nil', none call the debugger. The value can also be a list of error conditions that should call the debugger. For example, if you set it to the list `(void-variable)', then only errors about a variable that has no value invoke the debugger. When this variable is non-`nil', Emacs does not catch errors that happen in process filter functions and sentinels. Therefore, these errors also can invoke the debugger. *Note Processes::. To debug an error that happens during loading of the `.emacs' file, use the option `-debug-init', which binds `debug-on-error' to `t' while `.emacs' is loaded and inhibits use of `condition-case' to catch init file errors. If your `.emacs' file sets `debug-on-error', the effect may not last past the end of loading `.emacs'. (This is an undesirable byproduct of the code that implements the `-debug-init' command line option.) The best way to make `.emacs' set `debug-on-error' permanently is with `after-init-hook', like this: (add-hook 'after-init-hook '(lambda () (setq debug-on-error t))) File: lispref.info, Node: Infinite Loops, Next: Function Debugging, Prev: Error Debugging, Up: Debugger Debugging Infinite Loops ------------------------ When a program loops infinitely and fails to return, your first problem is to stop the loop. On most operating systems, you can do this with `C-g', which causes quit. Ordinary quitting gives no information about why the program was looping. To get more information, you can set the variable `debug-on-quit' to non-`nil'. Quitting with `C-g' is not considered an error, and `debug-on-error' has no effect on the handling of `C-g'. Likewise, `debug-on-quit' has no effect on errors. Once you have the debugger running in the middle of the infinite loop, you can proceed from the debugger using the stepping commands. If you step through the entire loop, you will probably get enough information to solve the problem. - User Option: debug-on-quit This variable determines whether the debugger is called when `quit' is signaled and not handled. If `debug-on-quit' is non-`nil', then the debugger is called whenever you quit (that is, type `C-g'). If `debug-on-quit' is `nil', then the debugger is not called when you quit. *Note Quitting::. File: lispref.info, Node: Function Debugging, Next: Explicit Debug, Prev: Infinite Loops, Up: Debugger Entering the Debugger on a Function Call ---------------------------------------- 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. - Command: debug-on-entry FUNCTION-NAME This function requests FUNCTION-NAME to invoke the debugger each time it is called. It works by inserting the form `(debug 'debug)' into the function definition as the first form. Any function 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't debug primitive functions (i.e., those written in C) this way. When `debug-on-entry' is called interactively, it prompts for FUNCTION-NAME in the minibuffer. If the function is already set up to invoke the debugger on entry, `debug-on-entry' does nothing. *Note:* if you redefine a function after using `debug-on-entry' on it, the code to enter the debugger is lost. `debug-on-entry' returns FUNCTION-NAME. (defun fact (n) (if (zerop n) 1 (* n (fact (1- n))))) => fact (debug-on-entry 'fact) => fact (fact 3) ------ Buffer: *Backtrace* ------ Entering: * fact(3) eval-region(4870 4878 t) byte-code("...") eval-last-sexp(nil) (let ...) eval-insert-last-sexp(nil) * call-interactively(eval-insert-last-sexp) ------ Buffer: *Backtrace* ------ (symbol-function 'fact) => (lambda (n) (debug (quote debug)) (if (zerop n) 1 (* n (fact (1- n))))) - Command: cancel-debug-on-entry FUNCTION-NAME This function undoes the effect of `debug-on-entry' on FUNCTION-NAME. When called interactively, it prompts for FUNCTION-NAME in the minibuffer. If FUNCTION-NAME is `nil' or the empty string, it cancels debugging for all functions. If `cancel-debug-on-entry' is called more than once on the same function, the second call does nothing. `cancel-debug-on-entry' returns FUNCTION-NAME. File: lispref.info, Node: Explicit Debug, Next: Using Debugger, Prev: Function Debugging, Up: Debugger Explicit Entry to the Debugger ------------------------------ You can cause the debugger to be called at a certain point in your program by writing the expression `(debug)' at that point. To do this, visit the source file, insert the text `(debug)' at the proper place, and type `C-M-x'. Be sure to undo this insertion before you save the file! The place where you insert `(debug)' must be a place where an additional form can be evaluated and its value ignored. (If the value of `(debug)' isn't ignored, it will alter the execution of the program!) The most common suitable places are inside a `progn' or an implicit `progn' (*note Sequencing::.). File: lispref.info, Node: Using Debugger, Next: Debugger Commands, Prev: Explicit Debug, Up: Debugger Using the Debugger ------------------ When the debugger is entered, it displays the previously selected buffer in one window and a buffer named `*Backtrace*' in another window. The backtrace buffer contains one line for each level of Lisp function execution currently going on. At the beginning of this buffer is a message describing the reason that the debugger was invoked (such as the error message and associated data, if it was invoked due to an error). The backtrace buffer is read-only and uses a special major mode, Debugger mode, in which letters are defined as debugger commands. The usual XEmacs editing commands are available; thus, you can switch windows to examine the buffer that was being edited at the time of the error, switch buffers, visit files, or do any other sort of editing. However, the debugger is a recursive editing level (*note Recursive Editing::.) and it is wise to go back to the backtrace buffer and exit the debugger (with the `q' command) when you are finished with it. Exiting the debugger gets out of the recursive edit and kills the backtrace buffer. The backtrace buffer shows you the functions that are executing and their argument values. It also allows you to specify a stack frame by moving point to the line describing that frame. (A stack frame is the place where the Lisp interpreter records information about a particular invocation of a function.) The frame whose line point is on is considered the "current frame". Some of the debugger commands operate on the current frame. The debugger itself must be run byte-compiled, since it makes assumptions about how many stack frames are used for the debugger itself. These assumptions are false if the debugger is running interpreted. File: lispref.info, Node: Debugger Commands, Next: Invoking the Debugger, Prev: Using Debugger, Up: Debugger Debugger Commands ----------------- Inside the debugger (in Debugger mode), these special commands are available in addition to the usual cursor motion commands. (Keep in mind that all the usual facilities of XEmacs, such as switching windows or buffers, are still available.) The most important use of debugger commands is for stepping through code, so that you can see how control flows. The debugger can step through the control structures of an interpreted function, but cannot do so in a byte-compiled function. If you would like to step through a byte-compiled function, replace it with an interpreted definition of the same function. (To do this, visit the source file for the function and type `C-M-x' on its definition.) Here is a list of Debugger mode commands: `c' Exit the debugger and continue execution. This resumes execution of the program as if the debugger had never been entered (aside from the effect of any variables or data structures you may have changed while inside the debugger). Continuing when an error or quit was signalled will cause the normal action of the signalling to take place. If you do not want this to happen, but instead want the program execution to continue as if the call to `signal' did not occur, use the `r' command. `d' Continue execution, but enter the debugger the next time any Lisp function is called. This allows you to step through the subexpressions of an expression, seeing what values the subexpressions compute, and what else they do. The stack frame made for the function call which enters the debugger in this way will be flagged automatically so that the debugger will be called again when the frame is exited. You can use the `u' command to cancel this flag. `b' Flag the current frame so that the debugger will be entered when the frame is exited. Frames flagged in this way are marked with stars in the backtrace buffer. `u' Don't enter the debugger when the current frame is exited. This cancels a `b' command on that frame. `e' Read a Lisp expression in the minibuffer, evaluate it, and print the value in the echo area. The debugger alters certain important variables, and the current buffer, as part of its operation; `e' temporarily restores their outside-the-debugger values so you can examine them. This makes the debugger more transparent. By contrast, `M-:' does nothing special in the debugger; it shows you the variable values within the debugger. `q' Terminate the program being debugged; return to top-level XEmacs command execution. If the debugger was entered due to a `C-g' but you really want to quit, and not debug, use the `q' command. `r' Return a value from the debugger. The value is computed by reading an expression with the minibuffer and evaluating it. The `r' command is useful when the debugger was invoked due to exit from a Lisp call frame (as requested with `b'); then the value specified in the `r' command is used as the value of that frame. It is also useful if you call `debug' and use its return value. If the debugger was entered at the beginning of a function call, `r' has the same effect as `c', and the specified return value does not matter. If the debugger was entered through a call to `signal' (i.e. as a result of an error or quit), then returning a value will cause the call to `signal' itself to return, rather than throwing to top-level or invoking a handler, as is normal. This allows you to correct an error (e.g. the type of an argument was wrong) or continue from a `debug-on-quit' as if it never happened. Note that some errors (e.g. any error signalled using the `error' function, and many errors signalled from a primitive function) are not continuable. If you return a value from them and continue execution, then the error will immediately be signalled again. Other errors (e.g. wrong-type-argument errors) will be continually resignalled until the problem is corrected. File: lispref.info, Node: Invoking the Debugger, Next: Internals of Debugger, Prev: Debugger Commands, Up: Debugger Invoking the Debugger --------------------- Here we describe fully the function used to invoke the debugger. - Function: debug &rest DEBUGGER-ARGS This function enters the debugger. It switches buffers to a buffer named `*Backtrace*' (or `*Backtrace*<2>' if it is the second recursive entry to the debugger, etc.), and fills it with information about the stack of Lisp function calls. It then enters a recursive edit, showing the backtrace buffer in Debugger mode. The Debugger mode `c' and `r' commands exit the recursive edit; then `debug' switches back to the previous buffer and returns to whatever called `debug'. This is the only way the function `debug' can return to its caller. If the first of the DEBUGGER-ARGS passed to `debug' is `nil' (or if it is not one of the special values in the table below), then `debug' displays the rest of its arguments at the top of the `*Backtrace*' buffer. This mechanism is used to display a message to the user. However, if the first argument passed to `debug' is one of the following special values, then it has special significance. Normally, these values are passed to `debug' only by the internals of XEmacs and the debugger, and not by programmers calling `debug'. The special values are: `lambda' A first argument of `lambda' means `debug' was called because of entry to a function when `debug-on-next-call' was non-`nil'. The debugger displays `Entering:' as a line of text at the top of the buffer. `debug' `debug' as first argument indicates a call to `debug' because of entry to a function that was set to debug on entry. The debugger displays `Entering:', just as in the `lambda' case. It also marks the stack frame for that function so that it will invoke the debugger when exited. `t' When the first argument is `t', this indicates a call to `debug' due to evaluation of a list form when `debug-on-next-call' is non-`nil'. The debugger displays the following as the top line in the buffer: Beginning evaluation of function call form: `exit' When the first argument is `exit', it indicates the exit of a stack frame previously marked to invoke the debugger on exit. The second argument given to `debug' in this case is the value being returned from the frame. The debugger displays `Return value:' on the top line of the buffer, followed by the value being returned. `error' When the first argument is `error', the debugger indicates that it is being entered because an error or `quit' was signaled and not handled, by displaying `Signaling:' followed by the error signaled and any arguments to `signal'. For example, (let ((debug-on-error t)) (/ 1 0)) ------ Buffer: *Backtrace* ------ Signaling: (arith-error) /(1 0) ... ------ Buffer: *Backtrace* ------ If an error was signaled, presumably the variable `debug-on-error' is non-`nil'. If `quit' was signaled, then presumably the variable `debug-on-quit' is non-`nil'. `nil' Use `nil' as the first of the DEBUGGER-ARGS when you want to enter the debugger explicitly. The rest of the DEBUGGER-ARGS are printed on the top line of the buffer. You can use this feature to display messages--for example, to remind yourself of the conditions under which `debug' is called. File: lispref.info, Node: Internals of Debugger, Prev: Invoking the Debugger, Up: Debugger Internals of the Debugger ------------------------- This section describes functions and variables used internally by the debugger. - Variable: debugger The value of this variable is the function to call to invoke the debugger. Its value must be a function of any number of arguments (or, more typically, the name of a function). Presumably this function will enter some kind of debugger. The default value of the variable is `debug'. The first argument that Lisp hands to the function indicates why it was called. The convention for arguments is detailed in the description of `debug'. - Command: backtrace This function prints a trace of Lisp function calls currently active. This is the function used by `debug' to fill up the `*Backtrace*' buffer. It is written in C, since it must have access to the stack to determine which function calls are active. The return value is always `nil'. In the following example, a Lisp expression calls `backtrace' explicitly. This prints the backtrace to the stream `standard-output': in this case, to the buffer `backtrace-output'. Each line of the backtrace represents one function call. The line shows the values of the function's arguments if they are all known. If they are still being computed, the line says so. The arguments of special forms are elided. (with-output-to-temp-buffer "backtrace-output" (let ((var 1)) (save-excursion (setq var (eval '(progn (1+ var) (list 'testing (backtrace)))))))) => nil ----------- Buffer: backtrace-output ------------ backtrace() (list ...computing arguments...) (progn ...) eval((progn (1+ var) (list (quote testing) (backtrace)))) (setq ...) (save-excursion ...) (let ...) (with-output-to-temp-buffer ...) eval-region(1973 2142 #<buffer *scratch*>) byte-code("... for eval-print-last-sexp ...") eval-print-last-sexp(nil) * call-interactively(eval-print-last-sexp) ----------- Buffer: backtrace-output ------------ The character `*' indicates a frame whose debug-on-exit flag is set. - Variable: debug-on-next-call If this variable is non-`nil', it says to call the debugger before the next `eval', `apply' or `funcall'. Entering the debugger sets `debug-on-next-call' to `nil'. The `d' command in the debugger works by setting this variable. - Function: backtrace-debug LEVEL FLAG This function sets the debug-on-exit flag of the stack frame LEVEL levels down the stack, giving it the value FLAG. If FLAG is non-`nil', this will cause the debugger to be entered when that frame later exits. Even a nonlocal exit through that frame will enter the debugger. This function is used only by the debugger. - Variable: command-debug-status This variable records the debugging status of the current interactive command. Each time a command is called interactively, this variable is bound to `nil'. The debugger can set this variable to leave information for future debugger invocations during the same command. The advantage, for the debugger, of using this variable rather than another global variable is that the data will never carry over to a subsequent command invocation. - Function: backtrace-frame FRAME-NUMBER The function `backtrace-frame' is intended for use in Lisp debuggers. It returns information about what computation is happening in the stack frame FRAME-NUMBER levels down. If that frame has not evaluated the arguments yet (or is a special form), the value is `(nil FUNCTION ARG-FORMS...)'. If that frame has evaluated its arguments and called its function already, the value is `(t FUNCTION ARG-VALUES...)'. In the return value, FUNCTION is whatever was supplied as the CAR of the evaluated list, or a `lambda' expression in the case of a macro call. If the function has a `&rest' argument, that is represented as the tail of the list ARG-VALUES. If FRAME-NUMBER is out of range, `backtrace-frame' returns `nil'. File: lispref.info, Node: Syntax Errors, Next: Compilation Errors, Prev: Debugger, Up: Debugging Debugging Invalid Lisp Syntax ============================= The Lisp reader reports invalid syntax, but cannot say where the real problem is. For example, the error "End of file during parsing" in evaluating an expression indicates an excess of open parentheses (or square brackets). The reader detects this imbalance at the end of the file, but it cannot figure out where the close parenthesis should have been. Likewise, "Invalid read syntax: ")"" indicates an excess close parenthesis or missing open parenthesis, but does not say where the missing parenthesis belongs. How, then, to find what to change? If the problem is not simply an imbalance of parentheses, a useful technique is to try `C-M-e' at the beginning of each defun, and see if it goes to the place where that defun appears to end. If it does not, there is a problem in that defun. However, unmatched parentheses are the most common syntax errors in Lisp, and we can give further advice for those cases. * Menu: * Excess Open:: How to find a spurious open paren or missing close. * Excess Close:: How to find a spurious close paren or missing open.