@node Debugging, Read and Print, Advising Functions, Top
@chapter 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.
+ There are several ways to find and investigate problems in an Emacs
+Lisp program.
@itemize @bullet
@item
-If the problem occurs when you run the program, you can use a Lisp
-debugger to investigate what is happening during execution. In addition
-to the ordinary debugger, Emacs comes with a source-level debugger,
-Edebug. This chapter describes both of them.
+If a problem occurs when you run the program, you can use the built-in
+Emacs Lisp debugger to suspend the Lisp evaluator, and examine and/or
+alter its internal state.
@item
-If the problem is syntactic, so that Lisp cannot even read the program,
-you can use the Emacs facilities for editing Lisp to localize it.
+You can use Edebug, a source-level debugger for Emacs Lisp.
@item
-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.
+If a syntactic problem is preventing Lisp from even reading the
+program, you can locate it using Lisp editing commands.
+
+@item
+You can look at the error and warning messages produced by the byte
+compiler when it compiles the program. @xref{Compiler Errors}.
+
+@item
+You can use the Testcover package to perform coverage testing on the
+program.
+
+@item
+You can use the ERT package to write regression tests for the program.
+@xref{Top,the ERT manual,, ERT, ERT: Emacs Lisp Regression Testing}.
@end itemize
+ Other useful tools for debugging input and output problems are the
+dribble file (@pxref{Terminal Input}) and the @code{open-termscript}
+function (@pxref{Terminal Output}).
+
@menu
-* Debugger:: How the Emacs Lisp debugger is implemented.
+* Debugger:: A debugger for the Emacs Lisp evaluator.
* Edebug:: A source-level Emacs Lisp debugger.
* Syntax Errors:: How to find syntax errors.
* Test Coverage:: Ensuring you have tested all branches in your code.
-* Compilation Errors:: How to find errors that show up in byte compilation.
@end menu
- Another useful debugging tool is the dribble file. When a dribble
-file is open, Emacs copies all keyboard input characters to that file.
-Afterward, you can examine the file to find out what input was used.
-@xref{Terminal Input}.
-
- For debugging problems in terminal descriptions, the
-@code{open-termscript} function can be useful. @xref{Terminal Output}.
-
@node Debugger
@section The Lisp Debugger
@cindex debugger for Emacs Lisp
error.
However, entry to the debugger is not a normal consequence of an
-error. Many commands frequently cause Lisp errors when invoked
-inappropriately, and during ordinary editing it would be very
-inconvenient to enter the debugger each time this happens. So if you
-want errors to enter the debugger, set the variable
-@code{debug-on-error} to non-@code{nil}. (The command
-@code{toggle-debug-on-error} provides an easy way to do this.)
+error. Many commands signal Lisp errors when invoked inappropriately,
+and during ordinary editing it would be very inconvenient to enter the
+debugger each time this happens. So if you want errors to enter the
+debugger, set the variable @code{debug-on-error} to non-@code{nil}.
+(The command @code{toggle-debug-on-error} provides an easy way to do
+this.)
@defopt debug-on-error
This variable determines whether the debugger is called when an error
is signaled and not handled. If @code{debug-on-error} is @code{t},
all kinds of errors call the debugger, except those listed in
@code{debug-ignored-errors} (see below). If it is @code{nil}, none
-call the debugger. (Note that @code{eval-expression-debug-on-error}
-affects the setting of this variable in some cases; see below.)
+call the debugger.
+
+The value can also be a list of error conditions (@pxref{Signaling
+Errors}). Then the debugger is called only for error conditions in
+this list (except those also listed in @code{debug-ignored-errors}).
+For example, if you set @code{debug-on-error} to the list
+@code{(void-variable)}, the debugger is only called for errors about a
+variable that has no value.
-The value can also be a list of error conditions that should call the
-debugger. For example, if you set it to the list
-@code{(void-variable)}, then only errors about a variable that has no
-value invoke the debugger.
+Note that @code{eval-expression-debug-on-error} overrides this
+variable in some cases; see below.
When this variable is non-@code{nil}, Emacs does not create an error
handler around process filter functions and sentinels. Therefore,
@end defopt
@defopt 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
-@code{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. However,
-``rarely'' is not ``never''; if your program fails with an error that
-matches this list, you will need to change this list in order to debug
+This variable specifies errors which should not enter the debugger,
+regardless of the value of @code{debug-on-error}. 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.
+
+The normal value of this variable lists several errors that happen
+often during editing but rarely result from bugs in Lisp programs.
+However, ``rarely'' is not ``never''; if your program fails with an
+error that matches this list, you may try changing this list to debug
the error. The easiest way is usually to set
@code{debug-ignored-errors} to @code{nil}.
@end defopt
@defopt eval-expression-debug-on-error
-If this variable has a non-@code{nil} value, then
-@code{debug-on-error} is set to @code{t} when evaluating with the
-command @code{eval-expression}. If
-@code{eval-expression-debug-on-error} is @code{nil}, then the value of
-@code{debug-on-error} is not changed. @xref{Lisp Eval,, Evaluating
+If this variable has a non-@code{nil} value (the default), running the
+command @code{eval-expression} causes @code{debug-on-error} to be
+temporarily bound to to @code{t}. @xref{Lisp Eval,, Evaluating
Emacs-Lisp Expressions, emacs, The GNU Emacs Manual}.
-@end defopt
-@defopt debug-on-signal
-Normally, errors that are caught by @code{condition-case} never run the
-debugger, even if @code{debug-on-error} is non-@code{nil}. In other
-words, @code{condition-case} gets a chance to handle the error before
-the debugger gets a chance.
-
-If you set @code{debug-on-signal} to a non-@code{nil} value, then the
-debugger gets the first chance at every error; an error will invoke the
-debugger regardless of any @code{condition-case}, if it fits the
-criteria specified by the values of @code{debug-on-error} and
-@code{debug-ignored-errors}.
-
-@strong{Warning:} This variable is strong medicine! Various parts of
-Emacs handle errors in the normal course of affairs, and you may not
-even realize that errors happen there. If you set
-@code{debug-on-signal} to a non-@code{nil} value, those errors will
-enter the debugger.
-
-@strong{Warning:} @code{debug-on-signal} has no effect when
-@code{debug-on-error} is @code{nil}.
+If @code{eval-expression-debug-on-error} is @code{nil}, then the value
+of @code{debug-on-error} is not changed during @code{eval-expression}.
@end defopt
+@defvar debug-on-signal
+Normally, errors caught by @code{condition-case} never invoke the
+debugger. The @code{condition-case} gets a chance to handle the error
+before the debugger gets a chance.
+
+If you change @code{debug-on-signal} to a non-@code{nil} value, the
+debugger gets the first chance at every error, regardless of the
+presence of @code{condition-case}. (To invoke the debugger, the error
+must still fulfill the criteria specified by @code{debug-on-error} and
+@code{debug-ignored-errors}.)
+
+@strong{Warning:} Setting this variable to non-@code{nil} may have
+annoying effects. Various parts of Emacs catch errors in the normal
+course of affairs, and you may not even realize that errors happen
+there. If you need to debug code wrapped in @code{condition-case},
+consider using @code{condition-case-unless-debug} (@pxref{Handling
+Errors}).
+@end defvar
+
@defopt debug-on-event
If you set @code{debug-on-event} to a special event (@pxref{Special
Events}), Emacs will try to enter the debugger as soon as it receives
@cindex stopping an infinite loop
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 @kbd{C-g}, which causes a @dfn{quit}.
+problem is to stop the loop. On most operating systems, you can do
+this with @kbd{C-g}, which causes a @dfn{quit}. @xref{Quitting}.
Ordinary quitting gives no information about why the program was
looping. To get more information, you can set the variable
-@code{debug-on-quit} to non-@code{nil}. Quitting with @kbd{C-g} is not
-considered an error, and @code{debug-on-error} has no effect on the
-handling of @kbd{C-g}. Likewise, @code{debug-on-quit} has no effect on
-errors.
+@code{debug-on-quit} to non-@code{nil}. 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 may get enough information to solve the problem.
- 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.
+ Quitting with @kbd{C-g} is not considered an error, and
+@code{debug-on-error} has no effect on the handling of @kbd{C-g}.
+Likewise, @code{debug-on-quit} has no effect on errors.
@defopt debug-on-quit
-This variable determines whether the debugger is called when @code{quit}
-is signaled and not handled. If @code{debug-on-quit} is non-@code{nil},
-then the debugger is called whenever you quit (that is, type @kbd{C-g}).
-If @code{debug-on-quit} is @code{nil}, then the debugger is not called
-when you quit. @xref{Quitting}.
+This variable determines whether the debugger is called when
+@code{quit} is signaled and not handled. If @code{debug-on-quit} is
+non-@code{nil}, then the debugger is called whenever you quit (that
+is, type @kbd{C-g}). If @code{debug-on-quit} is @code{nil} (the
+default), then the debugger is not called when you quit.
@end defopt
@node Function Debugging
for examining the return value of a function.
If a function name is underlined, that means the debugger knows
-where its source code is located. You can click @kbd{Mouse-2} on that
-name, or move to it and type @key{RET}, to visit the source code.
+where its source code is located. You can click with the mouse on
+that name, or move to it and type @key{RET}, to visit the source code.
The debugger itself must be run byte-compiled, since it makes
assumptions about how many stack frames are used for the debugger
Edebug also has a coverage testing feature (@pxref{Coverage
Testing}). These features partly duplicate each other, and it would
be cleaner to combine them.
-
-@node Compilation Errors
-@section Debugging Problems in Compilation
-@cindex debugging byte compilation problems
-
- 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 @samp{*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 @w{@samp{ *Compiler Input*}}.
-(Note that the buffer name starts with a space, so it does not show
-up in @kbd{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 @emph{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.
HCoop / bpt / emacs.git / blobdiff