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769 lines
31 KiB
Plaintext
Debugging GNU Emacs
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Copyright (C) 1985, 2000, 2001, 2002, 2003, 2004,
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2005, 2006, 2007, 2008 Free Software Foundation, Inc.
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See the end of the file for license conditions.
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[People who debug Emacs on Windows using Microsoft debuggers
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should read the Windows-specific section near the end of this
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document.]
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** When you debug Emacs with GDB, you should start it in the directory
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where the executable was made. That directory has a .gdbinit file
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that defines various "user-defined" commands for debugging Emacs.
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(These commands are described below under "Examining Lisp object
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values" and "Debugging Emacs Redisplay problems".)
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** When you are trying to analyze failed assertions, it will be
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essential to compile Emacs either completely without optimizations or
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at least (when using GCC) with the -fno-crossjumping option. Failure
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to do so may make the compiler recycle the same abort call for all
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assertions in a given function, rendering the stack backtrace useless
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for identifying the specific failed assertion.
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** It is a good idea to run Emacs under GDB (or some other suitable
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debugger) *all the time*. Then, when Emacs crashes, you will be able
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to debug the live process, not just a core dump. (This is especially
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important on systems which don't support core files, and instead print
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just the registers and some stack addresses.)
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** If Emacs hangs, or seems to be stuck in some infinite loop, typing
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"kill -TSTP PID", where PID is the Emacs process ID, will cause GDB to
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kick in, provided that you run under GDB.
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** Getting control to the debugger
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`Fsignal' is a very useful place to put a breakpoint in.
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All Lisp errors go through there.
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It is useful, when debugging, to have a guaranteed way to return to
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the debugger at any time. When using X, this is easy: type C-z at the
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window where Emacs is running under GDB, and it will stop Emacs just
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as it would stop any ordinary program. When Emacs is running in a
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terminal, things are not so easy.
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The src/.gdbinit file in the Emacs distribution arranges for SIGINT
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(C-g in Emacs) to be passed to Emacs and not give control back to GDB.
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On modern POSIX systems, you can override that with this command:
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handle SIGINT stop nopass
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After this `handle' command, SIGINT will return control to GDB. If
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you want the C-g to cause a QUIT within Emacs as well, omit the
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`nopass'.
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A technique that can work when `handle SIGINT' does not is to store
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the code for some character into the variable stop_character. Thus,
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set stop_character = 29
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makes Control-] (decimal code 29) the stop character.
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Typing Control-] will cause immediate stop. You cannot
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use the set command until the inferior process has been started.
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Put a breakpoint early in `main', or suspend the Emacs,
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to get an opportunity to do the set command.
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When Emacs is running in a terminal, it is sometimes useful to use a separate
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terminal for the debug session. This can be done by starting Emacs as usual,
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then attaching to it from gdb with the `attach' command which is explained in
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the node "Attach" of the GDB manual.
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** Examining Lisp object values.
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When you have a live process to debug, and it has not encountered a
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fatal error, you can use the GDB command `pr'. First print the value
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in the ordinary way, with the `p' command. Then type `pr' with no
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arguments. This calls a subroutine which uses the Lisp printer.
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You can also use `pp value' to print the emacs value directly.
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To see the current value of a Lisp Variable, use `pv variable'.
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Note: It is not a good idea to try `pr', `pp', or `pv' if you know that Emacs
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is in deep trouble: its stack smashed (e.g., if it encountered SIGSEGV
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due to stack overflow), or crucial data structures, such as `obarray',
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corrupted, etc. In such cases, the Emacs subroutine called by `pr'
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might make more damage, like overwrite some data that is important for
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debugging the original problem.
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Also, on some systems it is impossible to use `pr' if you stopped
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Emacs while it was inside `select'. This is in fact what happens if
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you stop Emacs while it is waiting. In such a situation, don't try to
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use `pr'. Instead, use `s' to step out of the system call. Then
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Emacs will be between instructions and capable of handling `pr'.
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If you can't use `pr' command, for whatever reason, you can use the
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`xpr' command to print out the data type and value of the last data
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value, For example:
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p it->object
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xpr
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You may also analyze data values using lower-level commands. Use the
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`xtype' command to print out the data type of the last data value.
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Once you know the data type, use the command that corresponds to that
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type. Here are these commands:
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xint xptr xwindow xmarker xoverlay xmiscfree xintfwd xboolfwd xobjfwd
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xbufobjfwd xkbobjfwd xbuflocal xbuffer xsymbol xstring xvector xframe
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xwinconfig xcompiled xcons xcar xcdr xsubr xprocess xfloat xscrollbar
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Each one of them applies to a certain type or class of types.
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(Some of these types are not visible in Lisp, because they exist only
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internally.)
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Each x... command prints some information about the value, and
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produces a GDB value (subsequently available in $) through which you
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can get at the rest of the contents.
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In general, most of the rest of the contents will be additional Lisp
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objects which you can examine in turn with the x... commands.
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Even with a live process, these x... commands are useful for
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examining the fields in a buffer, window, process, frame or marker.
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Here's an example using concepts explained in the node "Value History"
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of the GDB manual to print values associated with the variable
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called frame. First, use these commands:
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cd src
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gdb emacs
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b set_frame_buffer_list
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r -q
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Then Emacs hits the breakpoint:
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(gdb) p frame
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$1 = 139854428
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(gdb) xpr
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Lisp_Vectorlike
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PVEC_FRAME
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$2 = (struct frame *) 0x8560258
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"emacs@localhost"
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(gdb) p *$
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$3 = {
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size = 1073742931,
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next = 0x85dfe58,
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name = 140615219,
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[...]
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}
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Now we can use `pr' to print the frame parameters:
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(gdb) pp $->param_alist
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((background-mode . light) (display-type . color) [...])
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The Emacs C code heavily uses macros defined in lisp.h. So suppose
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we want the address of the l-value expression near the bottom of
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`add_command_key' from keyboard.c:
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XVECTOR (this_command_keys)->contents[this_command_key_count++] = key;
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XVECTOR is a macro, so GDB only knows about it if Emacs has been compiled with
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preprocessor macro information. GCC provides this if you specify the options
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`-gdwarf-2' and `-g3'. In this case, GDB can evaluate expressions like
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"p XVECTOR (this_command_keys)".
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When this information isn't available, you can use the xvector command in GDB
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to get the same result. Here is how:
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(gdb) p this_command_keys
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$1 = 1078005760
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(gdb) xvector
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$2 = (struct Lisp_Vector *) 0x411000
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0
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(gdb) p $->contents[this_command_key_count]
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$3 = 1077872640
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(gdb) p &$
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$4 = (int *) 0x411008
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Here's a related example of macros and the GDB `define' command.
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There are many Lisp vectors such as `recent_keys', which contains the
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last 100 keystrokes. We can print this Lisp vector
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p recent_keys
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pr
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But this may be inconvenient, since `recent_keys' is much more verbose
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than `C-h l'. We might want to print only the last 10 elements of
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this vector. `recent_keys' is updated in keyboard.c by the command
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XVECTOR (recent_keys)->contents[recent_keys_index] = c;
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So we define a GDB command `xvector-elts', so the last 10 keystrokes
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are printed by
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xvector-elts recent_keys recent_keys_index 10
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where you can define xvector-elts as follows:
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define xvector-elts
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set $i = 0
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p $arg0
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xvector
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set $foo = $
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while $i < $arg2
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p $foo->contents[$arg1-($i++)]
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pr
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end
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document xvector-elts
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Prints a range of elements of a Lisp vector.
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xvector-elts v n i
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prints `i' elements of the vector `v' ending at the index `n'.
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end
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** Getting Lisp-level backtrace information within GDB
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The most convenient way is to use the `xbacktrace' command. This
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shows the names of the Lisp functions that are currently active.
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If that doesn't work (e.g., because the `backtrace_list' structure is
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corrupted), type "bt" at the GDB prompt, to produce the C-level
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backtrace, and look for stack frames that call Ffuncall. Select them
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one by one in GDB, by typing "up N", where N is the appropriate number
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of frames to go up, and in each frame that calls Ffuncall type this:
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p *args
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pr
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This will print the name of the Lisp function called by that level
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of function calling.
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By printing the remaining elements of args, you can see the argument
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values. Here's how to print the first argument:
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p args[1]
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pr
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If you do not have a live process, you can use xtype and the other
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x... commands such as xsymbol to get such information, albeit less
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conveniently. For example:
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p *args
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xtype
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and, assuming that "xtype" says that args[0] is a symbol:
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xsymbol
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** Debugging Emacs Redisplay problems
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The src/.gdbinit file defines many useful commands for dumping redisplay
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related data structures in a terse and user-friendly format:
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`ppt' prints value of PT, narrowing, and gap in current buffer.
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`pit' dumps the current display iterator `it'.
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`pwin' dumps the current window 'win'.
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`prow' dumps the current glyph_row `row'.
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`pg' dumps the current glyph `glyph'.
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`pgi' dumps the next glyph.
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`pgrow' dumps all glyphs in current glyph_row `row'.
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`pcursor' dumps current output_cursor.
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The above commands also exist in a version with an `x' suffix which
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takes an object of the relevant type as argument.
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** Following longjmp call.
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Recent versions of glibc (2.4+?) encrypt stored values for setjmp/longjmp which
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prevents GDB from being able to follow a longjmp call using `next'. To
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disable this protection you need to set the environment variable
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LD_POINTER_GUARD to 0.
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** Using GDB in Emacs
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Debugging with GDB in Emacs offers some advantages over the command line (See
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the GDB Graphical Interface node of the Emacs manual). There are also some
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features available just for debugging Emacs:
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1) The command gud-pp is available on the tool bar (the `pp' icon) and
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allows the user to print the s-expression of the variable at point,
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in the GUD buffer.
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2) Pressing `p' on a component of a watch expression that is a lisp object
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in the speedbar prints its s-expression in the GUD buffer.
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3) The STOP button on the tool bar is adjusted so that it sends SIGTSTP
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instead of the usual SIGINT.
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4) The command gud-pv has the global binding 'C-x C-a C-v' and prints the
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value of the lisp variable at point.
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** Debugging what happens while preloading and dumping Emacs
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Type `gdb temacs' and start it with `r -batch -l loadup dump'.
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If temacs actually succeeds when running under GDB in this way, do not
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try to run the dumped Emacs, because it was dumped with the GDB
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breakpoints in it.
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** Debugging `temacs'
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Debugging `temacs' is useful when you want to establish whether a
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problem happens in an undumped Emacs. To run `temacs' under a
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debugger, type "gdb temacs", then start it with `r -batch -l loadup'.
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** If you encounter X protocol errors
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The X server normally reports protocol errors asynchronously,
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so you find out about them long after the primitive which caused
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the error has returned.
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To get clear information about the cause of an error, try evaluating
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(x-synchronize t). That puts Emacs into synchronous mode, where each
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Xlib call checks for errors before it returns. This mode is much
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slower, but when you get an error, you will see exactly which call
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really caused the error.
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You can start Emacs in a synchronous mode by invoking it with the -xrm
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option, like this:
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emacs -xrm "emacs.synchronous: true"
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Setting a breakpoint in the function `x_error_quitter' and looking at
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the backtrace when Emacs stops inside that function will show what
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code causes the X protocol errors.
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Some bugs related to the X protocol disappear when Emacs runs in a
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synchronous mode. To track down those bugs, we suggest the following
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procedure:
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- Run Emacs under a debugger and put a breakpoint inside the
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primitive function which, when called from Lisp, triggers the X
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protocol errors. For example, if the errors happen when you
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delete a frame, put a breakpoint inside `Fdelete_frame'.
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- When the breakpoint breaks, step through the code, looking for
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calls to X functions (the ones whose names begin with "X" or
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"Xt" or "Xm").
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- Insert calls to `XSync' before and after each call to the X
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functions, like this:
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XSync (f->output_data.x->display_info->display, 0);
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where `f' is the pointer to the `struct frame' of the selected
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frame, normally available via XFRAME (selected_frame). (Most
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functions which call X already have some variable that holds the
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pointer to the frame, perhaps called `f' or `sf', so you shouldn't
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need to compute it.)
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If your debugger can call functions in the program being debugged,
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you should be able to issue the calls to `XSync' without recompiling
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Emacs. For example, with GDB, just type:
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call XSync (f->output_data.x->display_info->display, 0)
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before and immediately after the suspect X calls. If your
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debugger does not support this, you will need to add these pairs
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of calls in the source and rebuild Emacs.
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Either way, systematically step through the code and issue these
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calls until you find the first X function called by Emacs after
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which a call to `XSync' winds up in the function
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`x_error_quitter'. The first X function call for which this
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happens is the one that generated the X protocol error.
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- You should now look around this offending X call and try to figure
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out what is wrong with it.
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** If Emacs causes errors or memory leaks in your X server
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You can trace the traffic between Emacs and your X server with a tool
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like xmon, available at ftp://ftp.x.org/contrib/devel_tools/.
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Xmon can be used to see exactly what Emacs sends when X protocol errors
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happen. If Emacs causes the X server memory usage to increase you can
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use xmon to see what items Emacs creates in the server (windows,
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graphical contexts, pixmaps) and what items Emacs delete. If there
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are consistently more creations than deletions, the type of item
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and the activity you do when the items get created can give a hint where
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to start debugging.
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** If the symptom of the bug is that Emacs fails to respond
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Don't assume Emacs is `hung'--it may instead be in an infinite loop.
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To find out which, make the problem happen under GDB and stop Emacs
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once it is not responding. (If Emacs is using X Windows directly, you
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can stop Emacs by typing C-z at the GDB job.) Then try stepping with
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`step'. If Emacs is hung, the `step' command won't return. If it is
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looping, `step' will return.
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If this shows Emacs is hung in a system call, stop it again and
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examine the arguments of the call. If you report the bug, it is very
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important to state exactly where in the source the system call is, and
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what the arguments are.
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If Emacs is in an infinite loop, try to determine where the loop
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starts and ends. The easiest way to do this is to use the GDB command
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`finish'. Each time you use it, Emacs resumes execution until it
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exits one stack frame. Keep typing `finish' until it doesn't
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return--that means the infinite loop is in the stack frame which you
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just tried to finish.
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Stop Emacs again, and use `finish' repeatedly again until you get back
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to that frame. Then use `next' to step through that frame. By
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stepping, you will see where the loop starts and ends. Also, examine
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the data being used in the loop and try to determine why the loop does
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not exit when it should.
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** If certain operations in Emacs are slower than they used to be, here
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is some advice for how to find out why.
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Stop Emacs repeatedly during the slow operation, and make a backtrace
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each time. Compare the backtraces looking for a pattern--a specific
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function that shows up more often than you'd expect.
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If you don't see a pattern in the C backtraces, get some Lisp
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backtrace information by typing "xbacktrace" or by looking at Ffuncall
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frames (see above), and again look for a pattern.
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When using X, you can stop Emacs at any time by typing C-z at GDB.
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When not using X, you can do this with C-g. On non-Unix platforms,
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such as MS-DOS, you might need to press C-BREAK instead.
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** If GDB does not run and your debuggers can't load Emacs.
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On some systems, no debugger can load Emacs with a symbol table,
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perhaps because they all have fixed limits on the number of symbols
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and Emacs exceeds the limits. Here is a method that can be used
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in such an extremity. Do
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nm -n temacs > nmout
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strip temacs
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adb temacs
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0xd:i
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0xe:i
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14:i
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17:i
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:r -l loadup (or whatever)
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It is necessary to refer to the file `nmout' to convert
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numeric addresses into symbols and vice versa.
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It is useful to be running under a window system.
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Then, if Emacs becomes hopelessly wedged, you can create
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another window to do kill -9 in. kill -ILL is often
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useful too, since that may make Emacs dump core or return
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to adb.
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** Debugging incorrect screen updating.
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To debug Emacs problems that update the screen wrong, it is useful
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to have a record of what input you typed and what Emacs sent to the
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screen. To make these records, do
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(open-dribble-file "~/.dribble")
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(open-termscript "~/.termscript")
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The dribble file contains all characters read by Emacs from the
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terminal, and the termscript file contains all characters it sent to
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the terminal. The use of the directory `~/' prevents interference
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with any other user.
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If you have irreproducible display problems, put those two expressions
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in your ~/.emacs file. When the problem happens, exit the Emacs that
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you were running, kill it, and rename the two files. Then you can start
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another Emacs without clobbering those files, and use it to examine them.
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An easy way to see if too much text is being redrawn on a terminal is to
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evaluate `(setq inverse-video t)' before you try the operation you think
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will cause too much redrawing. This doesn't refresh the screen, so only
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newly drawn text is in inverse video.
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The Emacs display code includes special debugging code, but it is
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normally disabled. You can enable it by building Emacs with the
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pre-processing symbol GLYPH_DEBUG defined. Here's one easy way,
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suitable for Unix and GNU systems, to build such a debugging version:
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|
||
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
|
||
|
||
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"
|
||
export DEBUG_FILE=/usr/tmp/LESSTIF_TRACE
|
||
emacs &
|
||
|
||
causes LessTif to print traces from the three named source files to a
|
||
file in `/usr/tmp' (that file can get pretty large). The above should
|
||
be typed at the shell prompt before invoking Emacs, as shown by the
|
||
last line above.
|
||
|
||
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.
|
||
|
||
|
||
** 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.
|
||
|
||
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".
|
||
|
||
- 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 some malloc debugging packages
|
||
work a lot better with it than with 2.95 or later versions.
|
||
|
||
- Type "make" then "make -k install".
|
||
|
||
- If required, invoke the package-specific command to prepare
|
||
src/temacs for execution.
|
||
|
||
- cd ..; src/temacs
|
||
|
||
(Note that this runs `temacs' instead of the usual `emacs' executable.
|
||
This avoids problems with dumping Emacs mentioned above.)
|
||
|
||
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).
|
||
|
||
** 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'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.
|
||
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
|
||
'debug_print'.
|
||
|
||
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
|
||
threads.
|
||
|
||
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
|