142 lines
6.2 KiB
Text
142 lines
6.2 KiB
Text
# Here's the script I'll use to demonstrate - it just loops forever:
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$ cat test.rb
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#!/usr/bin/env ruby
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loop do
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sleep 1
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end
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# Now, I'll start the script in the background, and redirect stdout and stderr
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# to /dev/null:
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$ ruby ./test.rb >/dev/null 2>/dev/null &
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[1] 1343
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# Next, I'll grab the PID of the script (1343):
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$ ps aux | grep test.rb
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vagrant 1343 0.0 0.4 3884 1652 pts/0 S 14:42 0:00 ruby ./test.rb
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vagrant 1345 0.0 0.2 4624 852 pts/0 S+ 14:42 0:00 grep --color=auto test.rb
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# Now I start gdb. Note that I'm using sudo here. This may or may not be
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# necessary in your setup. I'd try without sudo first, and fall back to adding
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# it if the next step fails:
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$ sudo gdb
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GNU gdb (Ubuntu/Linaro 7.4-2012.04-0ubuntu2.1) 7.4-2012.04
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Copyright (C) 2012 Free Software Foundation, Inc.
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License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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This is free software: you are free to change and redistribute it.
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There is NO WARRANTY, to the extent permitted by law. Type "show copying"
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and "show warranty" for details.
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This GDB was configured as "i686-linux-gnu".
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For bug reporting instructions, please see:
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<http://bugs.launchpad.net/gdb-linaro/>.
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# OK, now I'm in gdb, and I want to instruct it to attach to our Ruby process.
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# I can do that using the 'attach' command, which takes a PID (the one we
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# gathered above):
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(gdb) attach 1343
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Attaching to process 1343
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Reading symbols from /opt/vagrant_ruby/bin/ruby...done.
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Reading symbols from /lib/i386-linux-gnu/librt.so.1...(no debugging symbols found)...done.
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Loaded symbols for /lib/i386-linux-gnu/librt.so.1
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Reading symbols from /lib/i386-linux-gnu/libdl.so.2...(no debugging symbols found)...done.
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Loaded symbols for /lib/i386-linux-gnu/libdl.so.2
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Reading symbols from /lib/i386-linux-gnu/libcrypt.so.1...(no debugging symbols found)...done.
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Loaded symbols for /lib/i386-linux-gnu/libcrypt.so.1
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Reading symbols from /lib/i386-linux-gnu/libm.so.6...(no debugging symbols found)...done.
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Loaded symbols for /lib/i386-linux-gnu/libm.so.6
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Reading symbols from /lib/i386-linux-gnu/libc.so.6...(no debugging symbols found)...done.
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Loaded symbols for /lib/i386-linux-gnu/libc.so.6
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Reading symbols from /lib/i386-linux-gnu/libpthread.so.0...(no debugging symbols found)...done.
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[Thread debugging using libthread_db enabled]
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Using host libthread_db library "/lib/i386-linux-gnu/libthread_db.so.1".
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Loaded symbols for /lib/i386-linux-gnu/libpthread.so.0
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Reading symbols from /lib/ld-linux.so.2...(no debugging symbols found)...done.
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Loaded symbols for /lib/ld-linux.so.2
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0xb770c424 in __kernel_vsyscall ()
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# Great, now gdb is attached to the target process. If the step above fails, try
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# going back and running gdb under sudo. The next thing I want to do is gather
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# C-level backtraces from all threads in the process. The following command
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# stands for 'thread apply all backtrace':
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(gdb) t a a bt
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Thread 1 (Thread 0xb74d76c0 (LWP 1343)):
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#0 0xb770c424 in __kernel_vsyscall ()
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#1 0xb75d7abd in select () from /lib/i386-linux-gnu/libc.so.6
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#2 0x08069c56 in rb_thread_wait_for (time=...) at eval.c:11376
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#3 0x080a20fd in rb_f_sleep (argc=1, argv=0xbf85f490) at process.c:1633
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#4 0x0805e0e2 in call_cfunc (argv=0xbf85f490, argc=1, len=-1, recv=3075299660, func=0x80a20b0 <rb_f_sleep>)
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at eval.c:5778
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#5 rb_call0 (klass=3075304600, recv=3075299660, id=9393, oid=9393, argc=1, argv=0xbf85f490, body=0xb74c85a8, flags=2)
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at eval.c:5928
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#6 0x0805e35d in rb_call (klass=3075304600, recv=3075299660, mid=9393, argc=1, argv=0xbf85f490, scope=1,
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self=<optimized out>) at eval.c:6176
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#7 0x080651ec in rb_eval (self=3075299660, n=0xb74c4e1c) at eval.c:3521
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#8 0x0805c31c in rb_yield_0 (val=6, self=3075299660, klass=<optimized out>, flags=0, avalue=0) at eval.c:5095
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#9 0x0806a1e5 in loop_i () at eval.c:5227
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#10 0x08058dbd in rb_rescue2 (b_proc=0x806a1c0 <loop_i>, data1=0, r_proc=0, data2=0) at eval.c:5491
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#11 0x08058f28 in rb_f_loop () at eval.c:5252
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#12 0x0805e0c1 in call_cfunc (argv=0x0, argc=0, len=0, recv=3075299660, func=0x8058ef0 <rb_f_loop>) at eval.c:5781
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#13 rb_call0 (klass=3075304600, recv=3075299660, id=4121, oid=4121, argc=0, argv=0x0, body=0xb74d4dbc, flags=2)
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at eval.c:5928
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#14 0x0805e35d in rb_call (klass=3075304600, recv=3075299660, mid=4121, argc=0, argv=0x0, scope=1, self=<optimized out>)
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at eval.c:6176
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#15 0x080651ec in rb_eval (self=3075299660, n=0xb74c4dcc) at eval.c:3521
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#16 0x080662c6 in rb_eval (self=3075299660, n=0xb74c4de0) at eval.c:3236
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#17 0x08068ee4 in ruby_exec_internal () at eval.c:1654
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#18 0x08068f24 in ruby_exec () at eval.c:1674
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#19 0x0806b2cd in ruby_run () at eval.c:1684
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#20 0x08053771 in main (argc=2, argv=0xbf860204, envp=0xbf860210) at main.c:48
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# C backtraces are sometimes sufficient, but often Ruby backtraces are necessary
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# for debugging as well. Ruby has a built-in function called rb_backtrace() that
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# we can use to dump out a Ruby backtrace, but it prints to stdout or stderr
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# (depending on your Ruby version), which might have been redirected to a file
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# or to /dev/null (as in our example) when the process started up.
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#
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# To get aroundt this, we'll do a little trick and redirect the target process's
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# stdout and stderr to the current TTY, so that any output from the process
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# will appear directly on our screen.
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# First, let's close the existing file descriptors for stdout and stderr
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# (FD 1 and 2, respectively):
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(gdb) call (void) close(1)
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(gdb) call (void) close(2)
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# Next, we need to figure out the device name for the current TTY:
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(gdb) shell tty
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/dev/pts/0
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# OK, now we can pass the device name obtained above to open() and attach
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# file descriptors 1 and 2 back to the current TTY with these calls:
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(gdb) call (int) open("/dev/pts/0", 2, 0)
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$1 = 1
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(gdb) call (int) open("/dev/pts/0", 2, 0)
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$2 = 2
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# Finally, we call rb_backtrace() in order to dump the Ruby backtrace:
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(gdb) call (void) rb_backtrace()
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from ./test.rb:4:in `sleep'
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from ./test.rb:4
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from ./test.rb:3:in `loop'
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from ./test.rb:3
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# And here's how we get out of gdb. Once you've quit, you'll probably want to
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# clean up the stuck process by killing it.
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(gdb) quit
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A debugging session is active.
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Inferior 1 [process 1343] will be detached.
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Quit anyway? (y or n) y
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Detaching from program: /opt/vagrant_ruby/bin/ruby, process 1343
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$
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