gdb is my “tool of last resort”. When all other online diagnostics have failed, I know enough gdb to pull out a C level stack trace.
% gdb $SHELL $$ GNU gdb 6.3.50-20050815 (Apple version gdb-477) (Sun Apr 30 20:06:22 GMT 2006) /Users/dom/320: No such file or directory. Attaching to program: `/bin/zsh', process 320. Reading symbols for shared libraries ........ done 0x90006108 in syscall () (gdb) where #0 0x90006108 in syscall () #1 0x00054558 in signal_suspend () #2 0x0002c9a8 in waitforpid () #3 0x0002cb4c in waitjobs () #4 0x00012584 in execlist () #5 0x00011c24 in execlist () #6 0x00011874 in execode () #7 0x00026890 in loop () #8 0x0002945c in zsh_main () #9 0x00001d14 in start ()
It’s not terribly informative, but in the past, it’s given me just enough of a clue to start looking at the SSL libraries (for example).
Jamis Buck has gone one better—he’s pulled a ruby stacktrace from a running process. Which seems quite magical to me indeed. I also think that you can turn most of what he’s done into a gdb macro. I’ll have to have a look at some examples…
In the past, I’ve resorted installing a signal handler “just in case” to pull out this sort of information. All of my Perl apps have this in their startup files.
$SIG{ USR2 } = sub { Carp::cluck("Caught SIGUSR2 in $$") };
Which is all very well, if you know what you need in advance. Which is not usually the case.
Jamis++
Update: Ok, turning it into a gdb macro is dead easy. Save this lot into ~/ruby.gdb
# A quick hack to show the environment for a Ruby process.
define printenv
set $index = 0
while environ[$index]
x/1s environ[$index]
set $index = $index + 1
end
end
document printenv
Display the environment for the current process.
end
define rb_where
set $ary = (int)backtrace(-1)
set $count = *($ary+8)
set $index = 0
while $index < $count
x/1s *((int)rb_ary_entry($ary, $index)+12)
set $index = $index + 1
end
end
document rb_where
Show the ruby stacktrace.
end
To use it do source ~/ruby.gdb from the gdb session, and then you get two new commands: printenv and rb_where.
Oh yes, I do know how super-trivial all this would be if only I had DTrace. Roll on Leopard.
Jabbering Giraffe 