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Debugging GNU Emacs
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Copyright (C) 1985, 2000-2016 Free Software Foundation, Inc.
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See the end of the file for license conditions.
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** Preliminaries
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This section can be skipped if you are already familiar with building
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Emacs with debug info, configuring and starting GDB, and simple GDB
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debugging techniques.
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*** Configuring Emacs for debugging
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It is best to configure and build Emacs with special options that will
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make the debugging easier. Here's the configure-time options we
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recommend (they are in addition to any other options you might need,
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such as --prefix):
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CFLAGS='-O0 -g3' ./configure --enable-checking='yes,glyphs' --enable-check-lisp-object-type
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The CFLAGS value is important: debugging optimized code can be very
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hard. (If the problem only happens with optimized code, you may need
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to enable optimizations. If that happens, try using -Og first,
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instead of -O2, as the former will disable some optimizations that
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make debugging some code exceptionally hard.)
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Modern versions of GCC support more elaborate debug info that is
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available by just using the -g3 compiler switch. Try using -gdwarf-4
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in addition to -g3, and if that fails, try -gdwarf-3. This is
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especially important if you have to debug optimized code. More info
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about this is available below; search for "analyze failed assertions".
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The 2 --enable-* switches are optional. They don't have any effect on
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debugging with GDB, but will compile additional code that might catch
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the problem you are debugging much earlier, in the form of assertion
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violation. The --enable-checking option also enables additional
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functionality useful for debugging display problems; see more about
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this below under "Debugging Emacs redisplay problems".
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Emacs needs not be installed to be debugged, you can debug the binary
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created in the 'src' directory.
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*** Configuring GDB
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When you debug Emacs with GDB, you should start GDB in the directory
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where the Emacs executable was made (the 'src' directory in the Emacs
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source tree). That directory has a .gdbinit file that defines various
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"user-defined" commands for debugging Emacs. (These commands are
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described below under "Examining Lisp object values" and "Debugging
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Emacs Redisplay problems".)
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Starting the debugger from Emacs, via the "M-x gdb" command (described
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below), when the current buffer visits one of the Emacs C source files
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will automatically start GDB in the 'src' directory.
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Some GDB versions by default do not automatically load .gdbinit files
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in the directory where you invoke GDB. With those versions of GDB,
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you will see a warning when GDB starts, like this:
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warning: File ".../src/.gdbinit" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load".
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The simplest way to fix this is to add the following line to your
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~/.gdbinit file:
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add-auto-load-safe-path /path/to/emacs/src/.gdbinit
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There are other ways to overcome that difficulty, they are all
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described in the node "Auto-loading safe path" in the GDB user manual.
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If nothing else helps, type "source /path/to/.gdbinit RET" at the GDB
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prompt, to unconditionally load the GDB init file.
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*** Use the Emacs GDB UI front-end
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We recommend using the GUI front-end for GDB provided by Emacs. With
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it, you can start GDB by typing "M-x gdb RET". This will suggest the
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file name of the default binary to debug; if the suggested default is
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not the Emacs binary you want to debug, change the file name as
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needed. Alternatively, if you want to attach the debugger to an
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already running Emacs process, change the GDB command shown in the
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minibuffer to say this:
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gdb -i=mi -p PID
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where PID is the numerical process ID of the running Emacs process,
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displayed by system utilities such as 'top' or 'ps' on Posix hosts and
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Task Manager on MS-Windows.
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Once the debugger starts, open the additional windows provided by the
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GDB UI, by typing "M-x gdb-many-windows RET". (Alternatively, click
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Gud->GDB-MI->Display Other Windows" from the menu bar.) At this
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point, make your frame large enough (or full-screen) such that the
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windows you just opened have enough space to show the content without
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horizontal scrolling.
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You can later restore your window configuration with the companion
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command "M-x gdb-restore-windows RET", or by deselecting "Display
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Other Windows" from the menu bar.
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*** Setting initial breakpoints
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Before you let Emacs run, you should now set breakpoints in the code
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which you want to debug, so that Emacs stops there and lets GDB take
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control. If the code which you want to debug is executed under some
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rare conditions, or only when a certain Emacs command is manually
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invoked, then just set your breakpoint there, let Emacs run, and
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trigger the breakpoint by invoking that command or reproducing those
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rare conditions.
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If you are less lucky, and the code in question is run very
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frequently, you will have to find some way of avoiding triggering your
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breakpoint when the conditions for the buggy behavior did not yet
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happen. There's no single recipe for this, you will have to be
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creative and study the code to see what's appropriate. Some useful
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tricks for that:
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. Make your breakpoint conditional on certain buffer or string
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position. For example:
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(gdb) break foo.c:1234 if PT >= 9876
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. Set a break point in some rarely called function, then create the
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conditions for the bug, call that rare function, and when GDB gets
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control, set the breakpoint in the buggy code, knowing that it
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will now be called when the bug happens.
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. If the bug manifests itself as an error message, set a breakpoint
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in Fsignal, and when it breaks, look at the backtrace to see what
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triggers the error.
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Some additional techniques are described below under "Getting control
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to the debugger".
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You are now ready to start your debugging session.
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If you are starting a new Emacs session, type "run", followed by any
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command-line arguments (e.g., "-Q") into the *gud-emacs* buffer and
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press RET.
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If you attached the debugger to a running Emacs, type "continue" into
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the *gud-emacs* buffer and press RET.
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Many variables you will encounter while debugging are Lisp objects.
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These are displayed as integer values (or structures, if you used the
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"--enable-check-lisp-object-type" option at configure time) that are
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hard to interpret, especially if they represent long lists. You can
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use the 'pp' command to display them in their Lisp form. That command
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displays its output on the standard error stream, which you
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can redirect to a file using "M-x redirect-debugging-output".
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This means that if you attach GDB to a running Emacs that was invoked
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from a desktop icon, chances are you will not see the output at all,
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or it will wind up in an obscure place (check the documentation of
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your desktop environment).
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Additional information about displaying Lisp objects can be found
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under "Examining Lisp object values" below.
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The rest of this document describes specific useful techniques for
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debugging Emacs; we suggest reading it in its entirety the first time
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you are about to debug Emacs, then look up your specific issues
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whenever you need.
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Good luck!
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** When you are trying to analyze failed assertions or backtraces, it
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is essential to compile Emacs with flags suitable for debugging.
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With GCC 4.8 or later, you can invoke 'make' with CFLAGS="-Og -g3".
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With older GCC or non-GCC compilers, you can use CFLAGS="-O0 -g3".
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With GCC and higher optimization levels such as -O2, the
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-fno-omit-frame-pointer and -fno-crossjumping options are often
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essential. The latter prevents GCC from using the same abort call for
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all assertions in a given function, rendering the stack backtrace
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useless for identifying the specific failed assertion.
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Some versions of GCC support recent versions of the DWARF standard for
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debugging info, but default to older versions; for example, they could
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support -gdwarf-4 compiler option (for DWARF v4), but default to
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version 2 of the DWARF standard. For best results in debugging
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abilities, find out the highest version of DWARF your GCC can support,
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and use the corresponding -gdwarf-N switch instead of just -g (you
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will still need -g3, as in "-gdwarf-4 -g3").
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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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Setting a breakpoint in a strategic place, after loading Emacs into
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the debugger, but before running it, is the most efficient way of
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making sure control will be returned to the debugger when you need
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that.
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'Fsignal' is a very useful place to put a breakpoint in. All Lisp
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errors go through there. If you are only interested in errors that
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would fire the Lisp debugger, breaking at 'maybe_call_debugger' is
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useful.
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Another technique for get control to the debugger is to put a
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breakpoint in some rarely used function. One such convenient function
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is Fredraw_display, which you can invoke at will interactively with
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"M-x redraw-display RET".
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It is also useful to have a guaranteed way to return to the debugger
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at any arbitrary time. When using X, this is easy: type C-z at the
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window where you are interacting with GDB, and it will stop Emacs just
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as it would stop any ordinary program. (This doesn't work if GDB was
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attached to a running Emacs process; in that case, you will need to
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type C-z to the shell window from which Emacs was started, or use the
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"kill -TSTP" method described below.)
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When Emacs is displaying on a text terminal, things are not so easy,
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so we describe the various alternatives below (however, those of them
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that use signals only work on Posix systems).
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The src/.gdbinit file in the Emacs distribution arranges for SIGINT
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(C-g in Emacs on a text-mode frame) to be passed to Emacs and not give
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control back to GDB. On modern systems, you can override that with
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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 'nopass'.
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See the GDB manual for more details about signal handling and the
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'handle' command.
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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, so
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start Emacs with the 'start' command, to get an opportunity to do the
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above 'set' command.
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On a Posix host, you can also send a signal using the 'kill' command
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from a shell prompt, like this:
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kill -TSTP Emacs-PID
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where Emacs-PID is the process ID of Emacs being debugged. Other
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useful signals to send are SIGUSR1 and SIGUSR2; see "Error Debugging"
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in the ELisp manual for how to use those.
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When Emacs is displaying on a text terminal, it is useful to have a
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separate terminal for the debug session. This can be done by starting
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Emacs as usual, then attaching to it from gdb with the 'attach'
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command which is explained in the node "Attach" of the GDB manual.
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On MS-Windows, you can alternatively start Emacs from its own separate
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console by setting the new-console option before running Emacs under
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GDB:
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(gdb) set new-console 1
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(gdb) run
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If you do this, then typing C-c or C-BREAK into the console window
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through which you interact with GDB will stop Emacs and return control
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to the debugger, no matter if Emacs displays GUI or text-mode frames.
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This is the only reliable alternative on MS-Windows to get control to
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the debugger, besides setting breakpoints in advance.
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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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These commands send their output to stderr; if that is closed or
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redirected to some file you don't know, you won't see their output.
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This is particularly so for Emacs invoked on MS-Windows from the
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desktop shortcut. You can use the command 'redirect-debugging-output'
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to redirect stderr to a file.
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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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xchartable xsubchartable xboolvector xhashtable xlist xcoding
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xcharset xfontset xfont
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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 'pp' 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-N' (where N is 2 or higher) and '-g3'. In this case, GDB can
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evaluate expressions like "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 300 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:
|
||
|
||
xsymbol
|
||
|
||
** Debugging Emacs redisplay problems
|
||
|
||
If you configured Emacs with --enable-checking='glyphs', you can use redisplay
|
||
tracing facilities from a running Emacs session.
|
||
|
||
The command "M-x trace-redisplay RET" will produce a trace of what redisplay
|
||
does on the standard error stream. This is very useful for understanding the
|
||
code paths taken by the display engine under various conditions, especially if
|
||
some redisplay optimizations produce wrong results. (You know that redisplay
|
||
optimizations might be involved if "M-x redraw-display RET", or even just
|
||
typing "M-x", causes Emacs to correct the bad display.) Since the cursor
|
||
blinking feature triggers periodic redisplay cycles, we recommend disabling
|
||
'blink-cursor-mode' before invoking 'trace-redisplay', so that you have less
|
||
clutter in the trace. You can also have up to 30 last trace messages dumped to
|
||
standard error by invoking the 'dump-redisplay-history' command.
|
||
|
||
To find the code paths which were taken by the display engine, search xdisp.c
|
||
for the trace messages you see.
|
||
|
||
The command 'dump-glyph-matrix' is useful for producing on standard error
|
||
stream a full dump of the selected window's glyph matrix. See the function's
|
||
doc string for more details. If you are debugging redisplay issues in
|
||
text-mode frames, you may find the command 'dump-frame-glyph-matrix' useful.
|
||
|
||
Other commands useful for debugging redisplay are 'dump-glyph-row' and
|
||
'dump-tool-bar-row'.
|
||
|
||
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.)
|
||
|
||
The Emacs display code includes special debugging code, but it is normally
|
||
disabled. Configuring Emacs with --enable-checking='yes,glyphs' enables it.
|
||
|
||
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 'eassert'
|
||
macros.) Any assertion that is reported to fail should be investigated.
|
||
|
||
When you debug display problems running emacs under X, you can use
|
||
the 'ff' command to flush all pending display updates to the screen.
|
||
|
||
The src/.gdbinit file defines many useful commands for dumping redisplay
|
||
related data structures in a terse and user-friendly format:
|
||
|
||
'ppt' prints value of PT, narrowing, and gap in current buffer.
|
||
'pit' dumps the current display iterator 'it'.
|
||
'pwin' dumps the current window 'win'.
|
||
'prow' dumps the current glyph_row 'row'.
|
||
'pg' dumps the current glyph 'glyph'.
|
||
'pgi' dumps the next glyph.
|
||
'pgrow' dumps all glyphs in current glyph_row 'row'.
|
||
'pcursor' dumps current output_cursor.
|
||
|
||
The above commands also exist in a version with an 'x' suffix which takes an
|
||
object of the relevant type as argument. For example, 'pgrowx' dumps all
|
||
glyphs in its argument, which must be of type 'struct glyph_row'.
|
||
|
||
Since redisplay is performed by Emacs very frequently, you need to place your
|
||
breakpoints cleverly to avoid hitting them all the time, when the issue you are
|
||
debugging did not (yet) happen. Here are some useful techniques for that:
|
||
|
||
. Put a breakpoint at 'Fredraw_display' before running Emacs. Then do
|
||
whatever is required to reproduce the bad display, and invoke "M-x
|
||
redraw-display". The debugger will kick in, and you can set or enable
|
||
breakpoints in strategic places, knowing that the bad display will be
|
||
redrawn from scratch.
|
||
|
||
. For debugging incorrect cursor position, a good place to put a breakpoint is
|
||
in 'set_cursor_from_row'. The first time this function is called as part of
|
||
'redraw-display', Emacs is redrawing the minibuffer window, which is usually
|
||
not what you want; type "continue" to get to the call you want. In general,
|
||
always make sure 'set_cursor_from_row' is called for the right window and
|
||
buffer by examining the value of w->contents: it should be the buffer whose
|
||
display you are debugging.
|
||
|
||
. 'set_cursor_from_row' is also a good place to look at the contents of a
|
||
screen line (a.k.a. "glyph row"), by means of the 'pgrow' GDB command. Of
|
||
course, you need first to make sure the cursor is on the screen line which
|
||
you want to investigate. If you have set a breakpoint in 'Fredraw_display',
|
||
as advised above, move cursor to that line before invoking 'redraw-display'.
|
||
|
||
. If the problem happens only at some specific buffer position or for some
|
||
specific rarely-used character, you can make your breakpoints conditional on
|
||
those values. The display engine maintains the buffer and string position
|
||
it is processing in the it->current member; for example, the buffer
|
||
character position is in it->current.pos.charpos. Most redisplay functions
|
||
accept a pointer to a 'struct it' object as their argument, so you can make
|
||
conditional breakpoints in those functions, like this:
|
||
|
||
(gdb) break x_produce_glyphs if it->current.pos.charpos == 1234
|
||
|
||
For conditioning on the character being displayed, use it->c or
|
||
it->char_to_display.
|
||
|
||
. You can also make the breakpoints conditional on what object is being used
|
||
for producing glyphs for display. The it->method member has the value
|
||
GET_FROM_BUFFER for displaying buffer contents, GET_FROM_STRING for
|
||
displaying a Lisp string (e.g., a 'display' property or an overlay string),
|
||
GET_FROM_IMAGE for displaying an image, etc. See 'enum it_method' in
|
||
dispextern.h for the full list of values.
|
||
|
||
** Following longjmp call.
|
||
|
||
Recent versions of glibc (2.4+?) encrypt stored values for setjmp/longjmp which
|
||
prevents GDB from being able to follow a longjmp call using 'next'. To
|
||
disable this protection you need to set the environment variable
|
||
LD_POINTER_GUARD to 0.
|
||
|
||
** Using GDB in Emacs
|
||
|
||
Debugging with GDB in Emacs offers some advantages over the command line (See
|
||
the GDB Graphical Interface node of the Emacs manual). There are also some
|
||
features available just for debugging Emacs:
|
||
|
||
1) The command gud-print is available on the tool bar (the 'p' 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 and the Signals->STOP menu-bar menu
|
||
item are adjusted so that they send 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
|
||
|
||
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 need to debug what happens during dumping, start it with 'r -batch -l
|
||
loadup dump' instead. For debugging the bootstrap dumping, use "loadup
|
||
bootstrap" instead of "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.
|
||
|
||
** If you encounter X protocol errors
|
||
|
||
The X server normally reports protocol errors asynchronously,
|
||
so you find out about them long after the primitive which caused
|
||
the error has returned.
|
||
|
||
To get clear information about the cause of an error, try evaluating
|
||
(x-synchronize t). That puts Emacs into synchronous mode, where each
|
||
Xlib call checks for errors before it returns. This mode is much
|
||
slower, but when you get an error, you will see exactly which call
|
||
really caused the error.
|
||
|
||
You can start Emacs in a synchronous mode by invoking it with the -xrm
|
||
option, like this:
|
||
|
||
emacs -xrm "emacs.synchronous: true"
|
||
|
||
Setting a breakpoint in the function 'x_error_quitter' and looking at
|
||
the backtrace when Emacs stops inside that function will show what
|
||
code causes the X protocol errors.
|
||
|
||
Some bugs related to the X protocol disappear when Emacs runs in a
|
||
synchronous mode. To track down those bugs, we suggest the following
|
||
procedure:
|
||
|
||
- Run Emacs under a debugger and put a breakpoint inside the
|
||
primitive function which, when called from Lisp, triggers the X
|
||
protocol errors. For example, if the errors happen when you
|
||
delete a frame, put a breakpoint inside 'Fdelete_frame'.
|
||
|
||
- When the breakpoint breaks, step through the code, looking for
|
||
calls to X functions (the ones whose names begin with "X" or
|
||
"Xt" or "Xm").
|
||
|
||
- Insert calls to 'XSync' before and after each call to the X
|
||
functions, like this:
|
||
|
||
XSync (f->output_data.x->display_info->display, 0);
|
||
|
||
where 'f' is the pointer to the 'struct frame' of the selected
|
||
frame, normally available via XFRAME (selected_frame). (Most
|
||
functions which call X already have some variable that holds the
|
||
pointer to the frame, perhaps called 'f' or 'sf', so you shouldn't
|
||
need to compute it.)
|
||
|
||
If your debugger can call functions in the program being debugged,
|
||
you should be able to issue the calls to 'XSync' without recompiling
|
||
Emacs. For example, with GDB, just type:
|
||
|
||
call XSync (f->output_data.x->display_info->display, 0)
|
||
|
||
before and immediately after the suspect X calls. If your
|
||
debugger does not support this, you will need to add these pairs
|
||
of calls in the source and rebuild Emacs.
|
||
|
||
Either way, systematically step through the code and issue these
|
||
calls until you find the first X function called by Emacs after
|
||
which a call to 'XSync' winds up in the function
|
||
'x_error_quitter'. The first X function call for which this
|
||
happens is the one that generated the X protocol error.
|
||
|
||
- You should now look around this offending X call and try to figure
|
||
out what is wrong with it.
|
||
|
||
** If Emacs causes errors or memory leaks in your X server
|
||
|
||
You can trace the traffic between Emacs and your X server with a tool
|
||
like xmon, available at ftp://ftp.x.org/contrib/devel_tools/.
|
||
|
||
Xmon can be used to see exactly what Emacs sends when X protocol errors
|
||
happen. If Emacs causes the X server memory usage to increase you can
|
||
use xmon to see what items Emacs creates in the server (windows,
|
||
graphical contexts, pixmaps) and what items Emacs delete. If there
|
||
are consistently more creations than deletions, the type of item
|
||
and the activity you do when the items get created can give a hint where
|
||
to start debugging.
|
||
|
||
** If the symptom of the bug is that Emacs fails to respond
|
||
|
||
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. On MS-Windows, run Emacs
|
||
as usual, and then attach GDB to it -- that will usually interrupt
|
||
whatever Emacs is doing and let you perform the steps described
|
||
below.)
|
||
|
||
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.
|
||
|
||
On GNU and Unix systems, you can also trying sending Emacs SIGUSR2,
|
||
which, if 'debug-on-event' has its default value, will cause Emacs to
|
||
attempt to break it out of its current loop and into the Lisp
|
||
debugger. (See the node "Debugging" in the ELisp manual for the
|
||
details about the Lisp debugger.) This feature is useful when a
|
||
C-level debugger is not conveniently available.
|
||
|
||
** 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,
|
||
perhaps because they all have fixed limits on the number of symbols
|
||
and Emacs exceeds the limits. Here is a method that can be used
|
||
in such an extremity. Do
|
||
|
||
nm -n temacs > nmout
|
||
strip temacs
|
||
adb temacs
|
||
0xd:i
|
||
0xe:i
|
||
14:i
|
||
17:i
|
||
:r -l loadup (or whatever)
|
||
|
||
It is necessary to refer to the file 'nmout' to convert
|
||
numeric addresses into symbols and vice versa.
|
||
|
||
It is useful to be running under a window system.
|
||
Then, if Emacs becomes hopelessly wedged, you can create another
|
||
window to do kill -9 in. kill -ILL is often useful too, since that
|
||
may make Emacs dump core or return to adb.
|
||
|
||
** Debugging incorrect screen updating on a text terminal.
|
||
|
||
To debug Emacs problems that update the screen wrong, it is useful
|
||
to have a record of what input you typed and what Emacs sent to the
|
||
screen. To make these records, do
|
||
|
||
(open-dribble-file "~/.dribble")
|
||
(open-termscript "~/.termscript")
|
||
|
||
The dribble file contains all characters read by Emacs from the
|
||
terminal, and the termscript file contains all characters it sent to
|
||
the terminal. The use of the directory '~/' prevents interference
|
||
with any other user.
|
||
|
||
If you have irreproducible display problems, put those two expressions
|
||
in your ~/.emacs file. When the problem happens, exit the Emacs that
|
||
you were running, kill it, and rename the two files. Then you can start
|
||
another Emacs without clobbering those files, and use it to examine them.
|
||
|
||
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"
|
||
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.
|
||
|
||
On MS-Windows, you can start Emacs in its own separate console by
|
||
setting the new-console option before running Emacs under GDB:
|
||
|
||
(gdb) set new-console 1
|
||
(gdb) run
|
||
|
||
** 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:
|
||
|
||
- Edit configure, to set system_malloc and CANNOT_DUMP to "yes".
|
||
|
||
- 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.
|
||
|
||
|
||
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:
|