Debugging GNU Emacs
-Copyright (c) 1985, 2000, 2001 Free Software Foundation, Inc.
+Copyright (C) 1985, 2000, 2001, 2002, 2003, 2004,
+ 2005, 2006 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
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.
+
+** 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.
** Getting control to the debugger
-`Fsignal' is a very useful place to stop in.
+`Fsignal' is a very useful place to put a breakpoint in.
All Lisp errors go through there.
-It is useful, when debugging, to have a guaranteed way
-to return to the debugger at any time. If you are using
-interrupt-driven input, which is the default, then Emacs is using
-RAW mode and the only way you can do it is to store
-the code for some character into the variable stop_character:
+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-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.
+
+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 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
+`nopass'.
+
+A technique that can work when `handle SIGINT' does not is to store
+the code for some character into the variable stop_character. Thus,
set stop_character = 29
Put a breakpoint early in `main', or suspend the Emacs,
to get an opportunity to do the set command.
-If you are using cbreak input (see the Lisp function set-input-mode),
-then typing Control-g will cause a SIGINT, which will return control
-to GDB immediately if you type this command first:
-
- handle 2 stop
-
-By default the src/.gdbinit file in the Emacs distribution arranges
-for SIGINT to be passed to Emacs. You can type C-z, which generates
-SIGTSTP, to cause GDB to regain control.
+When Emacs is running in a terminal, it is 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.
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.
+
+Note: It is not a good idea to try `pr' or `pp' 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.
-If you can't use `pr' command, either because the process can't run a
-subroutine or because the data is invalid or Emacs already got a fatal
-signal, you can fall back on lower-level commands.
+Also, on some systems it is impossible to use `pr' if you stopped
+Emacs while it was inside `select'. This is in fact what happens if
+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'.
-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 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:
xint xptr xwindow xmarker xoverlay xmiscfree xintfwd xboolfwd xobjfwd
xbufobjfwd xkbobjfwd xbuflocal xbuffer xsymbol xstring xvector xframe
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
+ $1 = 139854428
(gdb) xtype
Lisp_Vectorlike
PVEC_FRAME
(gdb) xframe
- $2 = (struct frame *) 0x3f0800
+ $2 = (struct frame *) 0x8560258
(gdb) p *$
$3 = {
- size = 536871989,
- next = 0x366240,
- name = 809661752,
+ size = 1073742931,
+ next = 0x85dfe58,
+ name = 140615219,
[...]
}
(gdb) p $3->name
- $4 = 809661752
+ $4 = 140615219
Now we can use `pr' to print the name of the frame:
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
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
xvector
set $foo = $
while $i < $arg2
- p $foo->contents[$arg1-($i++)]
+ p $foo->contents[$arg1-($i++)]
pr
end
document xvector-elts
** Getting Lisp-level backtrace information within GDB
-The most convenient way is to use the `xbacktrace' command.
+The most convenient way is to use the `xbacktrace' command. This
+shows the names of the Lisp functions that are currently active.
If that doesn't work (e.g., because the `backtrace_list' structure is
corrupted), type "bt" at the GDB prompt, to produce the C-level
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
and, assuming that "xtype" says that args[0] is a symbol:
- xsymbol
+ xsymbol
+
+** 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 isavailable 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
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
Don't assume Emacs is `hung'--it may instead be in an infinite loop.
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
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 &
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.
+
+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".
"#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).
-
-
-** 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. The
-variable `last_marked_index' holds the index into the `last_marked'
-array one place beyond where the very last marked object is stored.
+(Note that this runs `temacs' instead of the usual `emacs' executable.
+This avoids problems with dumping Emacs mentioned above.)
-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.
+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).
-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.
+** How to recover buffer contents from an Emacs core dump file
+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,
-Debug, General you can set the command-line arguments and emacs'
+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,
+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,
Start debug). Emacs will start and the debugger will take control as
soon as a breakpoint is hit.
-You can also attach the debugger to an already running emacs process.
+You can also attach the debugger to an already running Emacs process.
To do this, start up the Microsoft Developer studio and select Build,
Start debug, Attach to process. Choose the Emacs process from the
list. Send a break to the running process (Debug, Break) and you will
process (Debug, Step out) and control will return to Emacs, until a
breakpoint is hit.
-To examine the contents of a lisp variable, you can use the function
+To examine the contents of a Lisp variable, you can use the function
'debug_print'. Right-click on a variable, select QuickWatch (it has
an eyeglass symbol on its button in the toolbar), and in the text
field at the top of the window, place 'debug_print(' and ')' around
`w32_msp_pump' up the call stack (see below for why you have to do
this). Open the QuickWatch window and enter
"debug_print(Vexec_path)". Evaluating this expression will then print
-out the contents of the lisp variable `exec-path'.
+out the contents of the Lisp variable `exec-path'.
If QuickWatch reports that the symbol is unknown, then check the call
stack in the `Call Stack' window. If the selected frame in the call
thread, so this should only be a problem if you've explicitly switched
threads.
-It is also possible to keep appropriately masked and typecast lisp
+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 neccesarily 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.
+
+;;; arch-tag: fbf32980-e35d-481f-8e4c-a2eca2586e6b