X-Git-Url: https://code.delx.au/gnu-emacs/blobdiff_plain/3102e429e643bf64f7d12cb97537cdf70c66f950..043dfe246086e70a991f4cee951ea14b1e417c26:/etc/DEBUG

diff --git a/etc/DEBUG b/etc/DEBUG
index f78656cb3b..b905bf48ea 100644
--- a/etc/DEBUG
+++ b/etc/DEBUG
@@ -1,28 +1,34 @@
 Debugging GNU Emacs
-Copyright (c) 1985, 2000, 2001 Free Software Foundation, Inc.
 
-   Permission is granted to anyone to make or distribute verbatim copies
-   of this document as received, in any medium, provided that the
-   copyright notice and permission notice are preserved,
-   and that the distributor grants the recipient permission
-   for further redistribution as permitted by this notice.
+Copyright (C) 1985, 2000, 2001, 2002, 2003, 2004,
+   2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc.
+See the end of the file for license conditions.
 
-   Permission is granted to distribute modified versions
-   of this document, or of portions of it,
-   under the above conditions, provided also that they
-   carry prominent notices stating who last changed them.
 
-[People who debug Emacs on Windows using native Windows debuggers
+[People who debug Emacs on Windows using Microsoft debuggers
 should read the Windows-specific section near the end of this
 document.]
 
-It is a good idea to run Emacs under GDB (or some other suitable
+** When you debug Emacs with GDB, you should start it in the directory
+where the executable was made.  That directory has a .gdbinit file
+that defines various "user-defined" commands for debugging Emacs.
+(These commands are described below under "Examining Lisp object
+values" and "Debugging Emacs Redisplay problems".)
+
+** When you are trying to analyze failed assertions, it will be
+essential to compile Emacs either completely without optimizations or
+at least (when using GCC) with the -fno-crossjumping option.  Failure
+to do so may make the compiler recycle the same abort call for all
+assertions in a given function, rendering the stack backtrace useless
+for identifying the specific failed assertion.
+
+** It is a good idea to run Emacs under GDB (or some other suitable
 debugger) *all the time*.  Then, when Emacs crashes, you will be able
 to debug the live process, not just a core dump.  (This is especially
 important on systems which don't support core files, and instead print
 just the registers and some stack addresses.)
 
-If Emacs hangs, or seems to be stuck in some infinite loop, typing
+** If Emacs hangs, or seems to be stuck in some infinite loop, typing
 "kill -TSTP PID", where PID is the Emacs process ID, will cause GDB to
 kick in, provided that you run under GDB.
 
@@ -32,7 +38,7 @@ kick in, provided that you run under GDB.
 All Lisp errors go through there.
 
 It is useful, when debugging, to have a guaranteed way to return to
-the debugger at any time.  When using X, this is easy: type C-c at the
+the debugger at any time.  When using X, this is easy: type C-z at the
 window where Emacs is running under GDB, and it will stop Emacs just
 as it would stop any ordinary program.  When Emacs is running in a
 terminal, things are not so easy.
@@ -41,7 +47,7 @@ The src/.gdbinit file in the Emacs distribution arranges for SIGINT
 (C-g in Emacs) to be passed to Emacs and not give control back to GDB.
 On modern POSIX systems, you can override that with this command:
 
-   handle int stop nopass
+   handle SIGINT stop nopass
 
 After this `handle' command, SIGINT will return control to GDB.  If
 you want the C-g to cause a QUIT within Emacs as well, omit the
@@ -58,6 +64,11 @@ use the set command until the inferior process has been started.
 Put a breakpoint early in `main', or suspend the Emacs,
 to get an opportunity to do the set command.
 
+When Emacs is running in a terminal, it is sometimes useful to use a separate
+terminal for the debug session.  This can be done by starting Emacs as usual,
+then attaching to it from gdb with the `attach' command which is explained in
+the node "Attach" of the GDB manual.
+
 ** Examining Lisp object values.
 
 When you have a live process to debug, and it has not encountered a
@@ -65,9 +76,13 @@ fatal error, you can use the GDB command `pr'.  First print the value
 in the ordinary way, with the `p' command.  Then type `pr' with no
 arguments.  This calls a subroutine which uses the Lisp printer.
 
-Note: It is not a good idea to try `pr' if you know that Emacs is in
-deep trouble: its stack smashed (e.g., if it encountered SIGSEGV due
-to stack overflow), or crucial data structures, such as `obarray',
+You can also use `pp value' to print the emacs value directly.
+
+To see the current value of a Lisp Variable, use `pv variable'.
+
+Note: It is not a good idea to try `pr', `pp', or `pv' if you know that Emacs
+is in deep trouble: its stack smashed (e.g., if it encountered SIGSEGV
+due to stack overflow), or crucial data structures, such as `obarray',
 corrupted, etc.  In such cases, the Emacs subroutine called by `pr'
 might make more damage, like overwrite some data that is important for
 debugging the original problem.
@@ -78,10 +93,17 @@ you stop Emacs while it is waiting.  In such a situation, don't try to
 use `pr'.  Instead, use `s' to step out of the system call.  Then
 Emacs will be between instructions and capable of handling `pr'.
 
-If you can't use `pr' command, for whatever reason, you can fall back
-on lower-level commands.  Use the `xtype' command to print out the
-data type of the last data value.  Once you know the data type, use
-the command that corresponds to that type.  Here are these commands:
+If you can't use `pr' command, for whatever reason, you can use the
+`xpr' command to print out the data type and value of the last data
+value, For example:
+
+    p it->object
+    xpr
+
+You may also analyze data values using lower-level commands.  Use the
+`xtype' command to print out the data type of the last data value.
+Once you know the data type, use the command that corresponds to that
+type.  Here are these commands:
 
     xint xptr xwindow xmarker xoverlay xmiscfree xintfwd xboolfwd xobjfwd
     xbufobjfwd xkbobjfwd xbuflocal xbuffer xsymbol xstring xvector xframe
@@ -101,41 +123,36 @@ objects which you can examine in turn with the x... commands.
 Even with a live process, these x...  commands are useful for
 examining the fields in a buffer, window, process, frame or marker.
 Here's an example using concepts explained in the node "Value History"
-of the GDB manual to print the variable frame from this line in
-xmenu.c:
-
-		  buf.frame_or_window = frame;
-
-First, use these commands:
+of the GDB manual to print values associated with the variable
+called frame.  First, use these commands:
 
     cd src
     gdb emacs
-    b xmenu.c:1296
-    r -q 
+    b set_frame_buffer_list
+    r -q
 
-Then type C-x 5 2 to create a new frame, and it hits the breakpoint:
+Then Emacs hits the breakpoint:
 
     (gdb) p frame
-    $1 = 1077872640
-    (gdb) xtype
+    $1 = 139854428
+    (gdb) xpr
     Lisp_Vectorlike
     PVEC_FRAME
-    (gdb) xframe
-    $2 = (struct frame *) 0x3f0800
+    $2 = (struct frame *) 0x8560258
+    "emacs@localhost"
     (gdb) p *$
     $3 = {
-      size = 536871989, 
-      next = 0x366240, 
-      name = 809661752, 
+      size = 1073742931,
+      next = 0x85dfe58,
+      name = 140615219,
       [...]
     }
-    (gdb) p $3->name
-    $4 = 809661752
 
-Now we can use `pr' to print the name of the frame:
+Now we can use `pr' to print the frame parameters:
+
+    (gdb) pp $->param_alist
+    ((background-mode . light) (display-type . color) [...])
 
-    (gdb) pr
-    "emacs@steenrod.math.nwu.edu"
 
 The Emacs C code heavily uses macros defined in lisp.h.  So suppose
 we want the address of the l-value expression near the bottom of
@@ -143,11 +160,13 @@ we want the address of the l-value expression near the bottom of
 
   XVECTOR (this_command_keys)->contents[this_command_key_count++] = key;
 
-XVECTOR is a macro, and therefore GDB does not know about it.
-GDB cannot evaluate "p XVECTOR (this_command_keys)".
+XVECTOR is a macro, so GDB only knows about it if Emacs has been compiled with
+preprocessor macro information.  GCC provides this if you specify the options
+`-gdwarf-2' and `-g3'.  In this case, GDB can evaluate expressions like
+"p XVECTOR (this_command_keys)".
 
-However, you can use the xvector command in GDB to get the same
-result.  Here is how:
+When this information isn't available, you can use the xvector command in GDB
+to get the same result.  Here is how:
 
     (gdb) p this_command_keys
     $1 = 1078005760
@@ -161,7 +180,7 @@ result.  Here is how:
 
 Here's a related example of macros and the GDB `define' command.
 There are many Lisp vectors such as `recent_keys', which contains the
-last 100 keystrokes.  We can print this Lisp vector
+last 300 keystrokes.  We can print this Lisp vector
 
 p recent_keys
 pr
@@ -173,7 +192,7 @@ this vector.  `recent_keys' is updated in keyboard.c by the command
   XVECTOR (recent_keys)->contents[recent_keys_index] = c;
 
 So we define a GDB command `xvector-elts', so the last 10 keystrokes
-are printed by 
+are printed by
 
     xvector-elts recent_keys recent_keys_index 10
 
@@ -185,7 +204,7 @@ where you can define xvector-elts as follows:
     xvector
     set $foo = $
     while $i < $arg2
-    p $foo->contents[$arg1-($i++)] 
+    p $foo->contents[$arg1-($i++)]
     pr
     end
     document xvector-elts
@@ -213,7 +232,7 @@ of function calling.
 
 By printing the remaining elements of args, you can see the argument
 values.  Here's how to print the first argument:
-  
+
    p args[1]
    pr
 
@@ -226,7 +245,50 @@ conveniently.  For example:
 
 and, assuming that "xtype" says that args[0] is a symbol:
 
-   xsymbol 
+   xsymbol
+
+** Debugging Emacs Redisplay problems
+
+The src/.gdbinit file defines many useful commands for dumping redisplay
+related data structures in a terse and user-friendly format:
+
+ `ppt' prints value of PT, narrowing, and gap in current buffer.
+ `pit' dumps the current display iterator `it'.
+ `pwin' dumps the current window 'win'.
+ `prow' dumps the current glyph_row `row'.
+ `pg' dumps the current glyph `glyph'.
+ `pgi' dumps the next glyph.
+ `pgrow' dumps all glyphs in current glyph_row `row'.
+ `pcursor' dumps current output_cursor.
+
+The above commands also exist in a version with an `x' suffix which
+takes an object of the relevant type as argument.
+
+** Following longjmp call.
+
+Recent versions of glibc (2.4+?) encrypt stored values for setjmp/longjmp which
+prevents GDB from being able to follow a longjmp call using `next'.  To
+disable this protection you need to set the environment variable
+LD_POINTER_GUARD to 0.
+
+** Using GDB in Emacs
+
+Debugging with GDB in Emacs offers some advantages over the command line (See
+the GDB Graphical Interface node of the Emacs manual).  There are also some
+features available just for debugging Emacs:
+
+1) The command gud-pp is available on the tool bar (the `pp' icon) and
+   allows the user to print the s-expression of the variable at point,
+   in the GUD buffer.
+
+2) Pressing `p' on a component of a watch expression that is a lisp object
+   in the speedbar prints its s-expression in the GUD buffer.
+
+3) The STOP button on the tool bar is adjusted so that it sends SIGTSTP
+   instead of the usual SIGINT.
+
+4) The command gud-pv has the global binding 'C-x C-a C-v' and prints the
+   value of the lisp variable at point.
 
 ** Debugging what happens while preloading and dumping Emacs
 
@@ -244,10 +306,80 @@ debugger, type "gdb temacs", then start it with `r -batch -l loadup'.
 
 ** If you encounter X protocol errors
 
-Try evaluating (x-synchronize t).  That puts Emacs into synchronous
-mode, where each Xlib call checks for errors before it returns.  This
-mode is much slower, but when you get an error, you will see exactly
-which call really caused the error.
+The X server normally reports protocol errors asynchronously,
+so you find out about them long after the primitive which caused
+the error has returned.
+
+To get clear information about the cause of an error, try evaluating
+(x-synchronize t).  That puts Emacs into synchronous mode, where each
+Xlib call checks for errors before it returns.  This mode is much
+slower, but when you get an error, you will see exactly which call
+really caused the error.
+
+You can start Emacs in a synchronous mode by invoking it with the -xrm
+option, like this:
+
+    emacs -xrm "emacs.synchronous: true"
+
+Setting a breakpoint in the function `x_error_quitter' and looking at
+the backtrace when Emacs stops inside that function will show what
+code causes the X protocol errors.
+
+Some bugs related to the X protocol disappear when Emacs runs in a
+synchronous mode.  To track down those bugs, we suggest the following
+procedure:
+
+  - Run Emacs under a debugger and put a breakpoint inside the
+    primitive function which, when called from Lisp, triggers the X
+    protocol errors.  For example, if the errors happen when you
+    delete a frame, put a breakpoint inside `Fdelete_frame'.
+
+  - When the breakpoint breaks, step through the code, looking for
+    calls to X functions (the ones whose names begin with "X" or
+    "Xt" or "Xm").
+
+  - Insert calls to `XSync' before and after each call to the X
+    functions, like this:
+
+       XSync (f->output_data.x->display_info->display, 0);
+
+    where `f' is the pointer to the `struct frame' of the selected
+    frame, normally available via XFRAME (selected_frame).  (Most
+    functions which call X already have some variable that holds the
+    pointer to the frame, perhaps called `f' or `sf', so you shouldn't
+    need to compute it.)
+
+    If your debugger can call functions in the program being debugged,
+    you should be able to issue the calls to `XSync' without recompiling
+    Emacs.  For example, with GDB, just type:
+
+       call XSync (f->output_data.x->display_info->display, 0)
+
+    before and immediately after the suspect X calls.  If your
+    debugger does not support this, you will need to add these pairs
+    of calls in the source and rebuild Emacs.
+
+    Either way, systematically step through the code and issue these
+    calls until you find the first X function called by Emacs after
+    which a call to `XSync' winds up in the function
+    `x_error_quitter'.  The first X function call for which this
+    happens is the one that generated the X protocol error.
+
+  - You should now look around this offending X call and try to figure
+    out what is wrong with it.
+
+** If Emacs causes errors or memory leaks in your X server
+
+You can trace the traffic between Emacs and your X server with a tool
+like xmon, available at ftp://ftp.x.org/contrib/devel_tools/.
+
+Xmon can be used to see exactly what Emacs sends when X protocol errors
+happen.  If Emacs causes the X server memory usage to increase you can
+use xmon to see what items Emacs creates in the server (windows,
+graphical contexts, pixmaps) and what items Emacs delete.  If there
+are consistently more creations than deletions, the type of item
+and the activity you do when the items get created can give a hint where
+to start debugging.
 
 ** If the symptom of the bug is that Emacs fails to respond
 
@@ -341,6 +473,41 @@ evaluate `(setq inverse-video t)' before you try the operation you think
 will cause too much redrawing.  This doesn't refresh the screen, so only
 newly drawn text is in inverse video.
 
+The Emacs display code includes special debugging code, but it is
+normally disabled.  You can enable it by building Emacs with the
+pre-processing symbol GLYPH_DEBUG defined.  Here's one easy way,
+suitable for Unix and GNU systems, to build such a debugging version:
+
+	 MYCPPFLAGS='-DGLYPH_DEBUG=1' make
+
+Building Emacs like that activates many assertions which scrutinize
+display code operation more than Emacs does normally.  (To see the
+code which tests these assertions, look for calls to the `xassert'
+macros.)  Any assertion that is reported to fail should be
+investigated.
+
+Building with GLYPH_DEBUG defined also defines several helper
+functions which can help debugging display code.  One such function is
+`dump_glyph_matrix'.  If you run Emacs under GDB, you can print the
+contents of any glyph matrix by just calling that function with the
+matrix as its argument.  For example, the following command will print
+the contents of the current matrix of the window whose pointer is in
+`w':
+
+  (gdb) p dump_glyph_matrix (w->current_matrix, 2)
+
+(The second argument 2 tells dump_glyph_matrix to print the glyphs in
+a long form.)  You can dump the selected window's current glyph matrix
+interactively with "M-x dump-glyph-matrix RET"; see the documentation
+of this function for more details.
+
+Several more functions for debugging display code are available in
+Emacs compiled with GLYPH_DEBUG defined; type "C-h f dump- TAB" and
+"C-h f trace- TAB" to see the full list.
+
+When you debug display problems running emacs under X, you can use
+the `ff' command to flush all pending display updates to the screen.
+
 
 ** Debugging LessTif
 
@@ -349,8 +516,8 @@ and keyboard events, or LessTif menus behave weirdly, it might be
 helpful to set the `DEBUGSOURCES' and `DEBUG_FILE' environment
 variables, so that one can see what LessTif was doing at this point.
 For instance
-  
-  export DEBUGSOURCES="RowColumn.c MenuShell.c MenuUtil.c"
+
+  export DEBUGSOURCES="RowColumn.c:MenuShell.c:MenuUtil.c"
   export DEBUG_FILE=/usr/tmp/LESSTIF_TRACE
   emacs &
 
@@ -365,17 +532,98 @@ appearing on another.  Then, when the bug happens, you can go back to
 the machine where you started GDB and use the debugger from there.
 
 
-** Running Emacs with Purify
+** Debugging problems which happen in GC
+
+The array `last_marked' (defined on alloc.c) can be used to display up
+to 500 last objects marked by the garbage collection process.
+Whenever the garbage collector marks a Lisp object, it records the
+pointer to that object in the `last_marked' array, which is maintained
+as a circular buffer.  The variable `last_marked_index' holds the
+index into the `last_marked' array one place beyond where the pointer
+to the very last marked object is stored.
 
-Some people who are willing to use non-free software use Purify.  We
-can't ethically ask you to you become a Purify user; but if you have
-it, and you test Emacs with it, we will not refuse to look at the
-results you find.
+The single most important goal in debugging GC problems is to find the
+Lisp data structure that got corrupted.  This is not easy since GC
+changes the tag bits and relocates strings which make it hard to look
+at Lisp objects with commands such as `pr'.  It is sometimes necessary
+to convert Lisp_Object variables into pointers to C struct's manually.
 
-Emacs compiled with Purify won't run without some hacking.  Here are
-some of the changes you might find necessary (SYSTEM-NAME and
-MACHINE-NAME are the names of your OS- and CPU-specific headers in the
-subdirectories of `src'):
+Use the `last_marked' array and the source to reconstruct the sequence
+that objects were marked.  In general, you need to correlate the
+values recorded in the `last_marked' array with the corresponding
+stack frames in the backtrace, beginning with the innermost frame.
+Some subroutines of `mark_object' are invoked recursively, others loop
+over portions of the data structure and mark them as they go.  By
+looking at the code of those routines and comparing the frames in the
+backtrace with the values in `last_marked', you will be able to find
+connections between the values in `last_marked'.  E.g., when GC finds
+a cons cell, it recursively marks its car and its cdr.  Similar things
+happen with properties of symbols, elements of vectors, etc.  Use
+these connections to reconstruct the data structure that was being
+marked, paying special attention to the strings and names of symbols
+that you encounter: these strings and symbol names can be used to grep
+the sources to find out what high-level symbols and global variables
+are involved in the crash.
+
+Once you discover the corrupted Lisp object or data structure, grep
+the sources for its uses and try to figure out what could cause the
+corruption.  If looking at the sources doesn't help, you could try
+setting a watchpoint on the corrupted data, and see what code modifies
+it in some invalid way.  (Obviously, this technique is only useful for
+data that is modified only very rarely.)
+
+It is also useful to look at the corrupted object or data structure in
+a fresh Emacs session and compare its contents with a session that you
+are debugging.
+
+** Debugging problems with non-ASCII characters
+
+If you experience problems which seem to be related to non-ASCII
+characters, such as \201 characters appearing in the buffer or in your
+files, set the variable byte-debug-flag to t.  This causes Emacs to do
+some extra checks, such as look for broken relations between byte and
+character positions in buffers and strings; the resulting diagnostics
+might pinpoint the cause of the problem.
+
+** Debugging the TTY (non-windowed) version
+
+The most convenient method of debugging the character-terminal display
+is to do that on a window system such as X.  Begin by starting an
+xterm window, then type these commands inside that window:
+
+  $ tty
+  $ echo $TERM
+
+Let's say these commands print "/dev/ttyp4" and "xterm", respectively.
+
+Now start Emacs (the normal, windowed-display session, i.e. without
+the `-nw' option), and invoke "M-x gdb RET emacs RET" from there.  Now
+type these commands at GDB's prompt:
+
+  (gdb) set args -nw -t /dev/ttyp4
+  (gdb) set environment TERM xterm
+  (gdb) run
+
+The debugged Emacs should now start in no-window mode with its display
+directed to the xterm window you opened above.
+
+Similar arrangement is possible on a character terminal by using the
+`screen' package.
+
+** Running Emacs built with malloc debugging packages
+
+If Emacs exhibits bugs that seem to be related to use of memory
+allocated off the heap, it might be useful to link Emacs with a
+special debugging library, such as Electric Fence (a.k.a. efence) or
+GNU Checker, which helps find such problems.
+
+Emacs compiled with such packages might not run without some hacking,
+because Emacs replaces the system's memory allocation functions with
+its own versions, and because the dumping process might be
+incompatible with the way these packages use to track allocated
+memory.  Here are some of the changes you might find necessary
+(SYSTEM-NAME and MACHINE-NAME are the names of your OS- and
+CPU-specific headers in the subdirectories of `src'):
 
   - In src/s/SYSTEM-NAME.h add "#define SYSTEM_MALLOC".
 
@@ -383,49 +631,42 @@ subdirectories of `src'):
     "#define CANNOT_UNEXEC".
 
   - Configure with a different --prefix= option.  If you use GCC,
-    version 2.7.2 is preferred, as Purify works a lot better with it
-    than with 2.95 or later versions.
+    version 2.7.2 is preferred, as some malloc debugging packages
+    work a lot better with it than with 2.95 or later versions.
 
-  - Type "make" then "make -k install".  You might need to run
-    "make -k install twice.
+  - Type "make" then "make -k install".
 
-  - cd src; purify -chain-length=40 gcc <link command line for temacs>
+  - If required, invoke the package-specific command to prepare
+    src/temacs for execution.
 
   - cd ..; src/temacs
 
-Note that Purify might print lots of false alarms for bitfields used
-by Emacs in some data structures.  If you want to get rid of the false
-alarms, you will have to hack the definitions of these data structures
-on the respective headers to remove the ":N" bitfield definitions
-(which will cause each such field to use a full int).
+(Note that this runs `temacs' instead of the usual `emacs' executable.
+This avoids problems with dumping Emacs mentioned above.)
 
-** Debugging problems which happen in GC
+Some malloc debugging libraries might print lots of false alarms for
+bitfields used by Emacs in some data structures.  If you want to get
+rid of the false alarms, you will have to hack the definitions of
+these data structures on the respective headers to remove the `:N'
+bitfield definitions (which will cause each such field to use a full
+int).
 
-The array `last_marked' (defined on alloc.c) can be used to display
-up to 500 last objects marked by the garbage collection process.  The
-variable `last_marked_index' holds the index into the `last_marked'
-array one place beyond where the very last marked object is stored.
+** How to recover buffer contents from an Emacs core dump file
 
-The single most important goal in debugging GC problems is to find the
-Lisp data structure that got corrupted.  This is not easy since GC
-changes the tag bits and relocates strings which make it hard to look
-at Lisp objects with commands such as `pr'.  It is sometimes necessary
-to convert Lisp_Object variables into pointers to C struct's manually.
-Use the `last_marked' array and the source to reconstruct the sequence
-that objects were marked.
-
-Once you discover the corrupted Lisp object or data structure, it is
-useful to look at it in a fresh session and compare its contents with
-a session that you are debugging.
+The file etc/emacs-buffer.gdb defines a set of GDB commands for
+recovering the contents of Emacs buffers from a core dump file.  You
+might also find those commands useful for displaying the list of
+buffers in human-readable format from within the debugger.
 
 ** Some suggestions for debugging on MS Windows:
 
    (written by Marc Fleischeuers, Geoff Voelker and Andrew Innes)
 
 To debug Emacs with Microsoft Visual C++, you either start emacs from
-the debugger or attach the debugger to a running emacs process.  To
-start emacs from the debugger, you can use the file bin/debug.bat.  The
-Microsoft Developer studio will start and under Project, Settings,
+the debugger or attach the debugger to a running emacs process.
+
+To start emacs from the debugger, you can use the file bin/debug.bat.
+The Microsoft Developer studio will start and under Project, Settings,
 Debug, General you can set the command-line arguments and Emacs's
 startup directory.  Set breakpoints (Edit, Breakpoints) at Fsignal and
 other functions that you want to examine.  Run the program (Build,
@@ -483,3 +724,45 @@ It is also possible to keep appropriately masked and typecast Lisp
 symbols in the Watch window, this is more convenient when steeping
 though the code.  For instance, on entering apply_lambda, you can
 watch (struct Lisp_Symbol *) (0xfffffff & args[0]).
+
+Optimizations often confuse the MS debugger.  For example, the
+debugger will sometimes report wrong line numbers, e.g., when it
+prints the backtrace for a crash.  It is usually best to look at the
+disassembly to determine exactly what code is being run--the
+disassembly will probably show several source lines followed by a
+block of assembler for those lines.  The actual point where Emacs
+crashes will be one of those source lines, but not necessarily the one
+that the debugger reports.
+
+Another problematic area with the MS debugger is with variables that
+are stored in registers: it will sometimes display wrong values for
+those variables.  Usually you will not be able to see any value for a
+register variable, but if it is only being stored in a register
+temporarily, you will see an old value for it.  Again, you need to
+look at the disassembly to determine which registers are being used,
+and look at those registers directly, to see the actual current values
+of these variables.
+
+
+This file is part of GNU Emacs.
+
+GNU Emacs is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+GNU Emacs is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
+
+
+Local variables:
+mode: outline
+paragraph-separate: "[ 	]*$"
+end:
+
+;;; arch-tag: fbf32980-e35d-481f-8e4c-a2eca2586e6b