Q2.1.15: How to debug a SXEmacs problem with a debugger

If SXEmacs does crash on you, one of the most productive things you can do to help get the bug fixed is to poke around a bit with the debugger. Here are some hints:

• First of all, if the crash is at all reproducible, consider very strongly recompiling your SXEmacs with debugging symbols and with no optimisation (e.g. with GCC use the compiler flags ‘-g -O0’ – that’s an "oh" followed by a zero), and with the configure options ‘--debug=yes’ and ‘--error-checking=all’. This will make your SXEmacs run somewhat slower, but you are a lot more likely to catch the problem earlier (closer to its source). It makes it a lot easier to determine what’s going on with a debugger.
• If it’s not a true crash (i.e., SXEmacs is hung, or a zombie process), or it’s inconvenient to run SXEmacs again because SXEmacs is already running or is running in batch mode as part of a bunch of scripts, you may be able to attach to the existing process with your debugger. Most debuggers let you do this by substituting the process ID for the core file when you invoke the debugger from the command line, or by using the attach command or something similar.
• If you’re able to run SXEmacs under a debugger and reproduce the crash, here are some things you can do:
• If SXEmacs is hitting an assertion failure, put a breakpoint on assert_failed().
• If SXEmacs is hitting some weird Lisp error that’s causing it to crash (e.g. during startup), put a breakpoint on signal_1()—this is declared static in eval.c.
• If SXEmacs is outputting lots of X errors, put a breakpoint on x_error_handler(); that will tell you which call is causing them.
• Internally, you will probably see lots of variables that hold objects of type Lisp_Object. These are references to Lisp objects. Printing them out with the debugger probably won’t be too useful—you’ll likely just see a number. To decode them, do this:

  call dp (OBJECT)
  

  where OBJECT is whatever you want to decode (it can be a variable, a function call, etc.). This uses the Lisp printing routines to out a readable representation on the TTY from which the sxemacs process was invoked.

• If you want to get a Lisp backtrace showing the Lisp call stack, do this:

  call db ()
  

• Using dp and db has two disadvantages - they can only be used with a running (including hung or zombie) sxemacs process, and they do not display the internal C structure of a Lisp Object. Even if all you’ve got is a core dump, all is not lost.

  If you’re using GDB, there are some macros in the file src/.gdbinit in the SXEmacs source distribution that should make it easier for you to decode Lisp objects. This file is automatically read by gdb if gdb is run in the directory where sxemacs was built, and contains these useful macros to inspect the state of sxemacs:

  pobj

  Usage: pobj lisp_object
  Print the internal C representation of a lisp object.

  xtype

  Usage: xtype lisp_object
  Print the Lisp type of a lisp object.

  lbt

  Usage: lbt
  Print the current Lisp stack trace. Requires a running sxemacs process. (It works by calling the db routine described above.)

  ldp

  Usage: ldp lisp_object
  Print a Lisp Object value using the Lisp printer. Requires a running sxemacs process. (It works by calling the dp routine described above.)

  run-temacs

  Usage: run-temacs
  Run temacs interactively, like sxemacs. Use this with debugging tools (like purify) that cannot deal with dumping, or when temacs builds successfully, but sxemacs does not.

  dump-temacs

  Usage: dump-temacs
  Run the dumping part of the build procedure. Use when debugging temacs, not sxemacs! Use this when temacs builds successfully, but sxemacs does not.

  check-sxemacs

  Usage: check-sxemacs
  Run the test suite. Equivalent to ’make check’.

  check-temacs

  Usage: check-temacs
  Run the test suite on temacs. Equivalent to ’make check-temacs’. Use this with debugging tools (like purify) that cannot deal with dumping, or when temacs builds successfully, but sxemacs does not.

  If you are using Sun’s dbx debugger, there is an equivalent file src/.dbxrc, which defines the same commands for dbx.

• If you’re using a debugger to get a C stack backtrace and you’re seeing stack traces with some of the innermost frames mangled, it may be due to dynamic linking. (This happens especially under Linux.) Consider reconfiguring with ‘--dynamic=no’. Also, sometimes (again under Linux), stack backtraces of core dumps will have the frame where the fatal signal occurred mangled; if you can obtain a stack trace while running the SXEmacs process under a debugger, the stack trace should be clean.
• If you’re using a debugger to get a C stack backtrace and you’re getting a completely mangled and bogus stack trace, it’s probably due to one of the following:
  1. Your executable has been stripped. Bad news. Tell your sysadmin not to do this—it doesn’t accomplish anything except to save a bit of disk space, and makes debugging much much harder.
  2. Your stack is getting trashed. Debugging this is hard; you have to do a binary-search type of narrowing down where the crash occurs, until you figure out exactly which line is causing the problem. Of course, this only works if the bug is highly reproducible. Also, in many cases if you run SXEmacs from the debugger, the debugger can protect the stack somewhat. However, if the stack is being smashed, it is typically the case that there is a wild pointer somewhere in the program, often quite far from where the crash occurs.
  3. If your stack trace has exactly one frame in it, with address 0x0, this could simply mean that SXEmacs attempted to execute code at that address, e.g. through jumping to a null function pointer. Unfortunately, under those circumstances, GDB under Linux doesn’t know how to get a stack trace. Yes, this is the fourth Linux-related problem I’ve mentioned. I have no idea why GDB under Linux is so bogus. Complain to the GDB authors, or to comp.os.linux.development.system. Again, you’ll have to use the narrowing-down process described above.
  4. You will get a Lisp backtrace output when SXEmacs crashes, so you’ll have something useful.