Commit 437368fe authored by Eli Zaretskii's avatar Eli Zaretskii
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Expand and update, based on the instructions to

pretesters, nt/INSTALL, and private communications and experience.
parent e26f9ced
2001-01-27 Eli Zaretskii <>
* DEBUG: Expand and update, based on the instructions to
pretesters, nt/INSTALL, and private communications and experience.
2001-01-17 Dave Love <>
* termcap.src: New version from
Debugging GNU Emacs
Copyright (c) 1985, 2000 Free Software Foundation, Inc.
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
......@@ -12,7 +12,21 @@ Copyright (c) 1985, 2000 Free Software Foundation, Inc.
under the above conditions, provided also that they
carry prominent notices stating who last changed them.
** Some useful techniques
[People who debug Emacs on Windows using native Windows debuggers
should read the Windows-specific section near the end of this
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
"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.
All Lisp errors go through there.
......@@ -37,6 +51,9 @@ 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.
** Examining Lisp object values.
......@@ -45,9 +62,16 @@ 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.
If you can't use this command, either because the process can't run
a subroutine or because the data is invalid, you can fall back on
lower-level commands.
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',
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.
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
......@@ -65,9 +89,201 @@ Each x... command prints some information about the value, and
produces a GDB value (subsequently available in $) through which you
can get at the rest of the contents.
In general, most of the rest of the contents will be addition Lisp
In general, most of the rest of the contents will be additional Lisp
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
buf.frame_or_window = frame;
First, use these commands:
cd src
gdb emacs
b xmenu.c:1296
r -q
Then type C-x 5 2 to create a new frame, and it hits the breakpoint:
(gdb) p frame
$1 = 1077872640
(gdb) xtype
(gdb) xframe
$2 = (struct frame *) 0x3f0800
(gdb) p *$
$3 = {
size = 536871989,
next = 0x366240,
name = 809661752,
(gdb) p $3->name
$4 = 809661752
Now we can use `pr' to print the name of the frame:
(gdb) pr
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
`add_command_key' from keyboard.c:
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)".
However, you can use the xvector command in GDB to get the same
result. Here is how:
(gdb) p this_command_keys
$1 = 1078005760
(gdb) xvector
$2 = (struct Lisp_Vector *) 0x411000
(gdb) p $->contents[this_command_key_count]
$3 = 1077872640
(gdb) p &$
$4 = (int *) 0x411008
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
p recent_keys
But this may be inconvenient, since `recent_keys' is much more verbose
than `C-h l'. We might want to print only the last 10 elements of
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
xvector-elts recent_keys recent_keys_index 10
where you can define xvector-elts as follows:
define xvector-elts
set $i = 0
p $arg0
set $foo = $
while $i < $arg2
p $foo->contents[$arg1-($i++)]
document xvector-elts
Prints a range of elements of a Lisp vector.
xvector-elts v n i
prints `i' elements of the vector `v' ending at the index `n'.
** Getting Lisp-level backtrace information within GDB
The most convenient way is to use the `xbacktrace' command.
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
backtrace, and look for stack frames that call Ffuncall. Select them
one by one in GDB, by typing "up N", where N is the appropriate number
of frames to go up, and in each frame that calls Ffuncall type this:
p *args
This will print the name of the Lisp function called by that level
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]
If you do not have a live process, you can use xtype and the other
x... commands such as xsymbol to get such information, albeit less
conveniently. For example:
p *args
and, assuming that "xtype" says that args[0] is a symbol:
** Debugging what happens while preloading and dumping Emacs
Type `gdb temacs' and start it with `r -batch -l loadup dump'.
If temacs actually succeeds when running under GDB in this way, do not
try to run the dumped Emacs, because it was dumped with the GDB
breakpoints in it.
** Debugging `temacs'
Debugging `temacs' is useful when you want to establish whether a
problem happens in an undumped Emacs. To run `temacs' under a
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.
** 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.
To find out which, make the problem happen under GDB and stop Emacs
once it is not responding. (If Emacs is using X Windows directly, you
can stop Emacs by typing C-z at the GDB job.) Then try stepping with
`step'. If Emacs is hung, the `step' command won't return. If it is
looping, `step' will return.
If this shows Emacs is hung in a system call, stop it again and
examine the arguments of the call. If you report the bug, it is very
important to state exactly where in the source the system call is, and
what the arguments are.
If Emacs is in an infinite loop, try to determine where the loop
starts and ends. The easiest way to do this is to use the GDB command
`finish'. Each time you use it, Emacs resumes execution until it
exits one stack frame. Keep typing `finish' until it doesn't
return--that means the infinite loop is in the stack frame which you
just tried to finish.
Stop Emacs again, and use `finish' repeatedly again until you get back
to that frame. Then use `next' to step through that frame. By
stepping, you will see where the loop starts and ends. Also, examine
the data being used in the loop and try to determine why the loop does
not exit when it should.
** If certain operations in Emacs are slower than they used to be, here
is some advice for how to find out why.
Stop Emacs repeatedly during the slow operation, and make a backtrace
each time. Compare the backtraces looking for a pattern--a specific
function that shows up more often than you'd expect.
If you don't see a pattern in the C backtraces, get some Lisp
backtrace information by typing "xbacktrace" or by looking at Ffuncall
frames (see above), and again look for a pattern.
When using X, you can stop Emacs at any time by typing C-z at GDB.
When not using X, you can do this with C-g. On non-Unix platforms,
such as MS-DOS, you might need to press C-BREAK instead.
** If GDB does not run and your debuggers can't load Emacs.
On some systems, no debugger can load Emacs with a symbol table,
......@@ -117,3 +333,140 @@ An easy way to see if too much text is being redrawn on a terminal is to
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.
** Debugging LessTif
If you encounter bugs whereby Emacs built with LessTif grabs all mouse
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"
causes LessTif to print traces from the three named source files to a
file in `/usr/tmp' (that file can get pretty large).
Running GDB from another terminal could also help with such problems.
You can arrange for GDB to run on one machine, with the Emacs display
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
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'):
- In src/s/SYSTEM-NAME.h add "#define SYSTEM_MALLOC".
- In src/m/MACHINE-NAME.h add "#define CANNOT_DUMP" and
"#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.
- Type "make" then "make -k install". You might need to run
"make -k install twice.
- cd src; purify -chain-length=40 gcc <link command line for temacs>
- 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.
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.
** 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'
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.
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
find that execution is halted somewhere in user32.dll. Open the stack
trace window and go up the stack to w32_msg_pump. Now you can set
breakpoints in Emacs (Edit, Breakpoints). Continue the running Emacs
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
'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
the expression. Press 'Recalculate' and the output is sent to stderr,
and to the debugger via the OutputDebugString routine. The output
sent to stderr should be displayed in the console window that was
opened when the emacs.exe executable was started. The output sent to
the debugger should be displayed in the 'Debug' pane in the Output
window. If Emacs was started from the debugger, a console window was
opened at Emacs' startup; this console window also shows the output of
For example, start and run Emacs in the debugger until it is waiting
for user input. Then click on the `Break' button in the debugger to
halt execution. Emacs should halt in `ZwUserGetMessage' waiting for
an input event. Use the `Call Stack' window to select the procedure
`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'.
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
stack is not an Emacs procedure, then the debugger won't recognize
Emacs symbols. Instead, select a frame that is inside an Emacs
procedure and try using `debug_print' again.
If QuickWatch invokes debug_print but nothing happens, then check the
thread that is selected in the debugger. If the selected thread is
not the last thread to run (the "current" thread), then it cannot be
used to execute debug_print. Use the Debug menu to select the current
thread and try using debug_print again. Note that the debugger halts
execution (e.g., due to a breakpoint) in the context of the current
thread, so this should only be a problem if you've explicitly switched
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]).
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