This is ../info/lispref.info, produced by makeinfo version 4.0b from lispref/lispref.texi. INFO-DIR-SECTION XEmacs Editor START-INFO-DIR-ENTRY * Lispref: (lispref). XEmacs Lisp Reference Manual. END-INFO-DIR-ENTRY 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 XEmacs Lisp Reference Manual (for 19.14 and 20.0) v3.1, March 1996 XEmacs Lisp Reference Manual (for 19.15 and 20.1, 20.2, 20.3) v3.2, April, May, November 1997 XEmacs Lisp Reference Manual (for 21.0) v3.3, April 1998 Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995 Free Software Foundation, Inc. Copyright (C) 1994, 1995 Sun Microsystems, Inc. Copyright (C) 1995, 1996 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: Disassembly, Next: Different Behavior, Prev: Compiled-Function 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'. (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 varref integer ; Get value of `integer' ; from the environment ; and push the value ; onto the stack. 1 constant 1 ; Push 1 onto stack. 2 eqlsign ; Pop top two values off stack, ; compare them, ; and push result onto stack. 3 goto-if-nil 1 ; Pop and test top of stack; ; if `nil', ; go to label 1 (which is also byte 7), ; else continue. 5 constant 1 ; Push 1 onto top of stack. 6 return ; Return the top element ; of the stack. 7:1 varref integer ; Push value of `integer' onto stack. 8 constant factorial ; Push `factorial' onto stack. 9 varref integer ; Push value of `integer' onto stack. 10 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' 12 mult ; Pop top two values off the stack, ; multiply them, ; pushing the result onto the stack. 13 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. 3:1 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 set `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-not-nil 1 ; Goto label 1 (byte 3) if `n' <= 0 ; (this exits the while loop). ; else pop top of stack ; and continue 11 varref t1 ; Push value of `t1' onto stack. 12 constant current-time-string ; Push ; `current-time-string' ; onto top of stack. 13 call 0 ; Call `current-time-string' again. 14 unbind 1 ; Unbind `t1' in local environment. 15 list2 ; Pop top two elements off stack, ; create a list of them, ; and push list onto stack. 16 return ; Return the top element of the stack. => nil  File: lispref.info, Node: Different Behavior, Prev: Disassembly, Up: Byte Compilation Different Behavior ================== The intent is that compiled byte-code and the corresponding code executed by the Lisp interpreter produce identical results. However, there are some circumstances where the results will differ. * Arithmetic operations may be rearranged for efficiency or compile-time evaluation. When floating point numbers are involved, this may produce different values or an overflow. * Some arithmetic operations may be optimized away. For example, the expression `(+ x)' may be optimized to simply `x'. If the value of `x' is a marker, then the value will be a marker instead of an integer. If the value of `x' is a cons cell, then the interpreter will issue an error, while the bytecode will not. If you're trying to use `(+ OBJECT 0)' to convert OBJECT to integer, consider using an explicit conversion function, which is clearer and guaranteed to work. Instead of `(+ MARKER 0)', use `(marker-position MARKER)'. Instead of `(+ CHAR 0)', use `(char-int CHAR)'. For maximal equivalence between interpreted and compiled code, the variables `byte-compile-delete-errors' and `byte-compile-optimize' can be set to `nil', but this is not recommended.  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 XEmacs 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 XEmacs 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 XEmacs 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::. - User Option: debug-on-signal This variable is similar to `debug-on-error' but breaks whenever an error is signalled, regardless of whether it would be handled. - User Option: debug-ignored-errors This variable specifies certain kinds of errors that should not enter the debugger. Its value is a list of error condition symbols and/or regular expressions. If the error has any of those condition symbols, or if the error message matches any of the regular expressions, then that error does not enter the debugger, regardless of the value of `debug-on-error'. The normal value of this variable lists several errors that happen often during editing but rarely result from bugs in Lisp programs. 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. *Please 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 &optional 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 &optional stream detailed 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'. The backtrace is normally printed to `standard-output', but this can be changed by specifying a value for STREAM. If DETAILED is non-`nil', the backtrace also shows places where currently active variable bindings, catches, condition-cases, and unwind-protects were made as well as function calls. 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 #) 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.  File: lispref.info, Node: Excess Open, Next: Excess Close, Up: Syntax Errors Excess Open Parentheses ----------------------- The first step is to find the defun that is unbalanced. If there is an excess open parenthesis, the way to do this is to insert a close parenthesis at the end of the file and type `C-M-b' (`backward-sexp'). This will move you to the beginning of the defun that is unbalanced. (Then type `C- C-_ C-u C-' to set the mark there, undo the insertion of the close parenthesis, and finally return to the mark.) The next step is to determine precisely what is wrong. There is no way to be sure of this except to study the program, but often the existing indentation is a clue to where the parentheses should have been. The easiest way to use this clue is to reindent with `C-M-q' and see what moves. Before you do this, make sure the defun has enough close parentheses. Otherwise, `C-M-q' will get an error, or will reindent all the rest of the file until the end. So move to the end of the defun and insert a close parenthesis there. Don't use `C-M-e' to move there, since that too will fail to work until the defun is balanced. Now you can go to the beginning of the defun and type `C-M-q'. Usually all the lines from a certain point to the end of the function will shift to the right. There is probably a missing close parenthesis, or a superfluous open parenthesis, near that point. (However, don't assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the `C-M-q' with `C-_', since the old indentation is probably appropriate to the intended parentheses. After you think you have fixed the problem, use `C-M-q' again. If the old indentation actually fit the intended nesting of parentheses, and you have put back those parentheses, `C-M-q' should not change anything.  File: lispref.info, Node: Excess Close, Prev: Excess Open, Up: Syntax Errors Excess Close Parentheses ------------------------ To deal with an excess close parenthesis, first insert an open parenthesis at the beginning of the file, back up over it, and type `C-M-f' to find the end of the unbalanced defun. (Then type `C- C-_ C-u C-' to set the mark there, undo the insertion of the open parenthesis, and finally return to the mark.) Then find the actual matching close parenthesis by typing `C-M-f' at the beginning of the defun. This will leave you somewhere short of the place where the defun ought to end. It is possible that you will find a spurious close parenthesis in that vicinity. If you don't see a problem at that point, the next thing to do is to type `C-M-q' at the beginning of the defun. A range of lines will probably shift left; if so, the missing open parenthesis or spurious close parenthesis is probably near the first of those lines. (However, don't assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the `C-M-q' with `C-_', since the old indentation is probably appropriate to the intended parentheses. After you think you have fixed the problem, use `C-M-q' again. If the old indentation actually fit the intended nesting of parentheses, and you have put back those parentheses, `C-M-q' should not change anything.  File: lispref.info, Node: Compilation Errors, Next: Edebug, Prev: Syntax Errors, Up: Debugging Debugging Problems in Compilation ================================= When an error happens during byte compilation, it is normally due to invalid syntax in the program you are compiling. The compiler prints a suitable error message in the `*Compile-Log*' buffer, and then stops. The message may state a function name in which the error was found, or it may not. Either way, here is how to find out where in the file the error occurred. What you should do is switch to the buffer ` *Compiler Input*'. (Note that the buffer name starts with a space, so it does not show up in `M-x list-buffers'.) This buffer contains the program being compiled, and point shows how far the byte compiler was able to read. If the error was due to invalid Lisp syntax, point shows exactly where the invalid syntax was _detected_. The cause of the error is not necessarily near by! Use the techniques in the previous section to find the error. If the error was detected while compiling a form that had been read successfully, then point is located at the end of the form. In this case, this technique can't localize the error precisely, but can still show you which function to check.  File: lispref.info, Node: Edebug, Prev: Compilation Errors, Up: Top Edebug ====== Edebug is a source-level debugger for XEmacs Lisp programs that provides the following features: * Step through evaluation, stopping before and after each expression. * Set conditional or unconditional breakpoints, install embedded breakpoints, or a global break event. * Trace slow or fast stopping briefly at each stop point, or each breakpoint. * Display expression results and evaluate expressions as if outside of Edebug. Interface with the custom printing package for printing circular structures. * Automatically reevaluate a list of expressions and display their results each time Edebug updates the display. * Output trace info on function enter and exit. * Errors stop before the source causing the error. * Display backtrace without Edebug calls. * Allow specification of argument evaluation for macros and defining forms. * Provide rudimentary coverage testing and display of frequency counts. The first three sections should tell you enough about Edebug to enable you to use it. * Menu: * Using Edebug:: Introduction to use of Edebug. * Instrumenting:: You must first instrument code. * Edebug Execution Modes:: Execution modes, stopping more or less often. * Jumping:: Commands to jump to a specified place. * Edebug Misc:: Miscellaneous commands. * Breakpoints:: Setting breakpoints to make the program stop. * Trapping Errors:: trapping errors with Edebug. * Edebug Views:: Views inside and outside of Edebug. * Edebug Eval:: Evaluating expressions within Edebug. * Eval List:: Automatic expression evaluation. * Reading in Edebug:: Customization of reading. * Printing in Edebug:: Customization of printing. * Tracing:: How to produce tracing output. * Coverage Testing:: How to test evaluation coverage. * The Outside Context:: Data that Edebug saves and restores. * Instrumenting Macro Calls:: Specifying how to handle macro calls. * Edebug Options:: Option variables for customizing Edebug.  File: lispref.info, Node: Using Edebug, Next: Instrumenting, Up: Edebug Using Edebug ------------ To debug an XEmacs Lisp program with Edebug, you must first "instrument" the Lisp code that you want to debug. If you want to just try it now, load `edebug.el', move point into a definition and do `C-u C-M-x' (`eval-defun' with a prefix argument). See *Note Instrumenting:: for alternative ways to instrument code. Once a function is instrumented, any call to the function activates Edebug. Activating Edebug may stop execution and let you step through the function, or it may update the display and continue execution while checking for debugging commands, depending on the selected Edebug execution mode. The initial execution mode is `step', by default, which does stop execution. *Note Edebug Execution Modes::. Within Edebug, you normally view an XEmacs buffer showing the source of the Lisp function you are debugging. This is referred to as the "source code buffer"--but note that it is not always the same buffer depending on which function is currently being executed. An arrow at the left margin indicates the line where the function is executing. Point initially shows where within the line the function is executing, but you can move point yourself. If you instrument the definition of `fac' (shown below) and then execute `(fac 3)', here is what you normally see. Point is at the open-parenthesis before `if'. (defun fac (n) =>-!-(if (< 0 n) (* n (fac (1- n))) 1)) The places within a function where Edebug can stop execution are called "stop points". These occur both before and after each subexpression that is a list, and also after each variable reference. Here we show with periods the stop points found in the function `fac': (defun fac (n) .(if .(< 0 n.). .(* n. .(fac (1- n.).).). 1).) While the source code buffer is selected, the special commands of Edebug are available in it, in addition to the commands of XEmacs Lisp mode. (The buffer is temporarily made read-only, however.) For example, you can type the Edebug command to execute until the next stop point. If you type once after entry to `fac', here is the display you will see: (defun fac (n) =>(if -!-(< 0 n) (* n (fac (1- n))) 1)) When Edebug stops execution after an expression, it displays the expression's value in the echo area. Other frequently used commands are `b' to set a breakpoint at a stop point, `g' to execute until a breakpoint is reached, and `q' to exit to the top-level command loop. Type `?' to display a list of all Edebug commands.  File: lispref.info, Node: Instrumenting, Next: Edebug Execution Modes, Prev: Using Edebug, Up: Edebug Instrumenting for Edebug ------------------------ In order to use Edebug to debug Lisp code, you must first "instrument" the code. Instrumenting a form inserts additional code into it which invokes Edebug at the proper places. Furthermore, if Edebug detects a syntax error while instrumenting, point is left at the erroneous code and an `invalid-read-syntax' error is signaled. Once you have loaded Edebug, the command `C-M-x' (`eval-defun') is redefined so that when invoked with a prefix argument on a definition, it instruments the definition before evaluating it. (The source code itself is not modified.) If the variable `edebug-all-defs' is non-`nil', that inverts the meaning of the prefix argument: then `C-M-x' instruments the definition _unless_ it has a prefix argument. The default value of `edebug-all-defs' is `nil'. The command `M-x edebug-all-defs' toggles the value of the variable `edebug-all-defs'. If `edebug-all-defs' is non-`nil', then the commands `eval-region', `eval-current-buffer', and `eval-buffer' also instrument any definitions they evaluate. Similarly, `edebug-all-forms' controls whether `eval-region' should instrument _any_ form, even non-defining forms. This doesn't apply to loading or evaluations in the minibuffer. The command `M-x edebug-all-forms' toggles this option. Another command, `M-x edebug-eval-top-level-form', is available to instrument any top-level form regardless of the value of `edebug-all-defs' or `edebug-all-forms'. Just before Edebug instruments any code, it calls any functions in the variable `edebug-setup-hook' and resets its value to `nil'. You could use this to load up Edebug specifications associated with a package you are using but only when you also use Edebug. For example, `my-specs.el' may be loaded automatically when you use `my-package' with Edebug by including the following code in `my-package.el'. (add-hook 'edebug-setup-hook (function (lambda () (require 'my-specs)))) While Edebug is active, the command `I' (`edebug-instrument-callee') instruments the definition of the function or macro called by the list form after point, if is not already instrumented. If the location of the definition is not known to Edebug, this command cannot be used. After loading Edebug, `eval-region' records the position of every definition it evaluates, even if not instrumenting it. Also see the command `i' (*Note Jumping::) which steps into the callee. Edebug knows how to instrument all the standard special forms, an interactive form with an expression argument, anonymous lambda expressions, and other defining forms. (Specifications for macros defined by `cl.el' (version 2.03) are provided in `cl-specs.el'.) Edebug cannot know what a user-defined macro will do with the arguments of a macro call so you must tell it. See *Note Instrumenting Macro Calls:: for the details. Note that a couple ways remain to evaluate expressions without instrumenting them. Loading a file via the `load' subroutine does not instrument expressions for Edebug. Evaluations in the minibuffer via `eval-expression' (`M-ESC') are not instrumented. To remove instrumentation from a definition, simply reevaluate it with one of the non-instrumenting commands, or reload the file. See *Note Edebug Eval:: for other evaluation functions available inside of Edebug.