Wording improvements, from "Valentin I. Spitkovsky"

[pintos.git] / doc / debug.texi

@node Debugging Tools
@appendix Debugging Tools

Many tools lie at your disposal for debugging Pintos.  This appendix
introduces you to a few of them.

@menu
* printf::                      
* ASSERT::                      
* Function and Parameter Attributes::  
* Backtraces::                  
* GDB::                         
* Triple Faults::               
* Modifying Bochs::             
* Debugging Tips::              
@end menu

@node printf
@section @code{printf()}

Don't underestimate the value of @func{printf}.  The way
@func{printf} is implemented in Pintos, you can call it from
practically anywhere in the kernel, whether it's in a kernel thread or
an interrupt handler, almost regardless of what locks are held.

@func{printf} is useful for more than just examining data.
It can also help figure out when and where something goes wrong, even
when the kernel crashes or panics without a useful error message.  The
strategy is to sprinkle calls to @func{printf} with different strings
(e.g.@: @code{"<1>"}, @code{"<2>"}, @dots{}) throughout the pieces of
code you suspect are failing.  If you don't even see @code{<1>} printed,
then something bad happened before that point, if you see @code{<1>}
but not @code{<2>}, then something bad happened between those two
points, and so on.  Based on what you learn, you can then insert more
@func{printf} calls in the new, smaller region of code you suspect.
Eventually you can narrow the problem down to a single statement.
@xref{Triple Faults}, for a related technique.

@node ASSERT
@section @code{ASSERT}

Assertions are useful because they can catch problems early, before
they'd otherwise be noticed.  Ideally, each function should begin with a
set of assertions that check its arguments for validity.  (Initializers
for functions' local variables are evaluated before assertions are
checked, so be careful not to assume that an argument is valid in an
initializer.)  You can also sprinkle assertions throughout the body of
functions in places where you suspect things are likely to go wrong.
They are especially useful for checking loop invariants.

Pintos provides the @code{ASSERT} macro, defined in @file{<debug.h>},
for checking assertions.

@defmac ASSERT (expression)
Tests the value of @var{expression}.  If it evaluates to zero (false),
the kernel panics.  The panic message includes the expression that
failed, its file and line number, and a backtrace, which should help you
to find the problem.  @xref{Backtraces}, for more information.
@end defmac

@node Function and Parameter Attributes
@section Function and Parameter Attributes

These macros defined in @file{<debug.h>} tell the compiler special
attributes of a function or function parameter.  Their expansions are
GCC-specific.

@defmac UNUSED
Appended to a function parameter to tell the compiler that the
parameter might not be used within the function.  It suppresses the
warning that would otherwise appear.
@end defmac

@defmac NO_RETURN
Appended to a function prototype to tell the compiler that the
function never returns.  It allows the compiler to fine-tune its
warnings and its code generation.
@end defmac

@defmac NO_INLINE
Appended to a function prototype to tell the compiler to never emit
the function in-line.  Occasionally useful to improve the quality of
backtraces (see below).
@end defmac

@defmac PRINTF_FORMAT (@var{format}, @var{first})
Appended to a function prototype to tell the compiler that the function
takes a @func{printf}-like format string as the argument numbered
@var{format} (starting from 1) and that the corresponding value
arguments start at the argument numbered @var{first}.  This lets the
compiler tell you if you pass the wrong argument types.
@end defmac

@node Backtraces
@section Backtraces

When the kernel panics, it prints a ``backtrace,'' that is, a summary
of how your program got where it is, as a list of addresses inside the
functions that were running at the time of the panic.  You can also
insert a call to @func{debug_backtrace}, prototyped in
@file{<debug.h>}, to print a backtrace at any point in your code.

The addresses in a backtrace are listed as raw hexadecimal numbers,
which are difficult to interpret.  We provide a tool called
@command{backtrace} to translate these into function names and source
file line numbers.
Give it the name of your @file{kernel.o} as the first argument and the
hexadecimal numbers composing the backtrace (including the @samp{0x}
prefixes) as the remaining arguments.  It outputs the function name
and source file line numbers that correspond to each address.  

If the translated form of a backtrace is garbled, or doesn't make
sense (e.g.@: function A is listed above function B, but B doesn't
call A), then it's a good sign that you're corrupting a kernel
thread's stack, because the backtrace is extracted from the stack.
Alternatively, it could be that the @file{kernel.o} you passed to
@command{backtrace} is not the same kernel that produced
the backtrace.

Sometimes backtraces can be confusing without any corruption.
Compiler optimizations can cause surprising behavior.  When a function
has called another function as its final action (a @dfn{tail call}), the
calling function may not appear in a backtrace at all.  Similarly, when
function A calls another function B that never returns, the compiler may
optimize such that an unrelated function C appears in the backtrace
instead of A.  Function C is simply the function that happens to be in
memory just after A.  In the threads project, this is commonly seen in
backtraces for test failures; see @ref{The pass function fails, ,
@func{pass} Fails}, for more information.

@menu
* Backtrace Example::           
@end menu

@node Backtrace Example
@subsection Example

Here's an example.  Suppose that Pintos printed out this following call
stack, which is taken from an actual Pintos submission for the file
system project:

@example
Call stack: 0xc0106eff 0xc01102fb 0xc010dc22 0xc010cf67 0xc0102319
0xc010325a 0x804812c 0x8048a96 0x8048ac8.
@end example

You would then invoke the @command{backtrace} utility like shown below,
cutting and pasting the backtrace information into the command line.
This assumes that @file{kernel.o} is in the current directory.  You
would of course enter all of the following on a single shell command
line, even though that would overflow our margins here:

@example
backtrace kernel.o 0xc0106eff 0xc01102fb 0xc010dc22 0xc010cf67 
0xc0102319 0xc010325a 0x804812c 0x8048a96 0x8048ac8
@end example

The backtrace output would then look something like this:

@example
0xc0106eff: debug_panic (lib/debug.c:86)
0xc01102fb: file_seek (filesys/file.c:405)
0xc010dc22: seek (userprog/syscall.c:744)
0xc010cf67: syscall_handler (userprog/syscall.c:444)
0xc0102319: intr_handler (threads/interrupt.c:334)
0xc010325a: intr_entry (threads/intr-stubs.S:38)
0x0804812c: (unknown)
0x08048a96: (unknown)
0x08048ac8: (unknown)
@end example

(You will probably not see exactly the same addresses if you run the
command above on your own kernel binary, because the source code you
compiled and the compiler you used are probably different.)

The first line in the backtrace refers to @func{debug_panic}, the
function that implements kernel panics.  Because backtraces commonly
result from kernel panics, @func{debug_panic} will often be the first
function shown in a backtrace.

The second line shows @func{file_seek} as the function that panicked,
in this case as the result of an assertion failure.  In the source code
tree used for this example, line 405 of @file{filesys/file.c} is the
assertion

@example
ASSERT (file_ofs >= 0);
@end example

@noindent
(This line was also cited in the assertion failure message.)
Thus, @func{file_seek} panicked because it passed a negative file offset
argument.

The third line indicates that @func{seek} called @func{file_seek},
presumably without validating the offset argument.  In this submission,
@func{seek} implements the @code{seek} system call.

The fourth line shows that @func{syscall_handler}, the system call
handler, invoked @func{seek}.

The fifth and sixth lines are the interrupt handler entry path.

The remaining lines are for addresses below @code{PHYS_BASE}.  This
means that they refer to addresses in the user program, not in the
kernel.  If you know what user program was running when the kernel
panicked, you can re-run @command{backtrace} on the user program, like
so: (typing the command on a single line, of course):

@example
backtrace tests/filesys/extended/grow-too-big 0xc0106eff 0xc01102fb
0xc010dc22 0xc010cf67 0xc0102319 0xc010325a 0x804812c 0x8048a96
0x8048ac8
@end example

The results look like this:

@example
0xc0106eff: (unknown)
0xc01102fb: (unknown)
0xc010dc22: (unknown)
0xc010cf67: (unknown)
0xc0102319: (unknown)
0xc010325a: (unknown)
0x0804812c: test_main (...xtended/grow-too-big.c:20)
0x08048a96: main (tests/main.c:10)
0x08048ac8: _start (lib/user/entry.c:9)
@end example

You can even specify both the kernel and the user program names on
the command line, like so:

@example
backtrace kernel.o tests/filesys/extended/grow-too-big 0xc0106eff
0xc01102fb 0xc010dc22 0xc010cf67 0xc0102319 0xc010325a 0x804812c
0x8048a96 0x8048ac8
@end example

The result is a combined backtrace:

@example
In kernel.o:
0xc0106eff: debug_panic (lib/debug.c:86)
0xc01102fb: file_seek (filesys/file.c:405)
0xc010dc22: seek (userprog/syscall.c:744)
0xc010cf67: syscall_handler (userprog/syscall.c:444)
0xc0102319: intr_handler (threads/interrupt.c:334)
0xc010325a: intr_entry (threads/intr-stubs.S:38)
In tests/filesys/extended/grow-too-big:
0x0804812c: test_main (...xtended/grow-too-big.c:20)
0x08048a96: main (tests/main.c:10)
0x08048ac8: _start (lib/user/entry.c:9)
@end example

Here's an extra tip for anyone who read this far: @command{backtrace}
is smart enough to strip the @code{Call stack:} header and @samp{.}
trailer from the command line if you include them.  This can save you
a little bit of trouble in cutting and pasting.  Thus, the following
command prints the same output as the first one we used:

@example
backtrace kernel.o Call stack: 0xc0106eff 0xc01102fb 0xc010dc22
0xc010cf67 0xc0102319 0xc010325a 0x804812c 0x8048a96 0x8048ac8.
@end example

@node GDB
@section GDB

You can run Pintos under the supervision of the GDB debugger.
First, start Pintos with the @option{--gdb} option, e.g.@:
@command{pintos --gdb -- run mytest}.  Second, open a second terminal on
the same machine and
use @command{pintos-gdb} to invoke GDB on
@file{kernel.o}:@footnote{@command{pintos-gdb} is a wrapper around
@command{gdb} (80@var{x}86) or @command{i386-elf-gdb} (SPARC) that loads
the Pintos macros at startup.}
@example
pintos-gdb kernel.o
@end example
@noindent and issue the following GDB command:
@example
target remote localhost:1234
@end example

Now GDB is connected to the simulator over a local
network connection.  You can now issue any normal GDB
commands.  If you issue the @samp{c} command, the simulated BIOS will take
control, load Pintos, and then Pintos will run in the usual way.  You
can pause the process at any point with @key{Ctrl+C}.

@menu
* Using GDB::                   
* Example GDB Session::         
* Debugging User Programs::     
* GDB FAQ::                     
@end menu

@node Using GDB
@subsection Using GDB

You can read the GDB manual by typing @code{info gdb} at a
terminal command prompt.  Here's a few commonly useful GDB commands:

@deffn {GDB Command} c
Continues execution until @key{Ctrl+C} or the next breakpoint.
@end deffn

@deffn {GDB Command} break function
@deffnx {GDB Command} break file:line
@deffnx {GDB Command} break *address
Sets a breakpoint at @var{function}, at @var{line} within @var{file}, or
@var{address}.
(Use a @samp{0x} prefix to specify an address in hex.)

Use @code{break main} to make GDB stop when Pintos starts running.
@end deffn

@deffn {GDB Command} p expression
Evaluates the given @var{expression} and prints its value.
If the expression contains a function call, that function will actually
be executed.
@end deffn

@deffn {GDB Command} l *address
Lists a few lines of code around @var{address}.
(Use a @samp{0x} prefix to specify an address in hex.)
@end deffn

@deffn {GDB Command} bt
Prints a stack backtrace similar to that output by the
@command{backtrace} program described above.
@end deffn

@deffn {GDB Command} p/a address
Prints the name of the function or variable that occupies @var{address}.
(Use a @samp{0x} prefix to specify an address in hex.)
@end deffn

@deffn {GDB Command} diassemble function
Disassembles @var{function}.
@end deffn

We also provide a set of macros specialized for debugging Pintos,
written by Godmar Back @email{gback@@cs.vt.edu}.  You can type
@code{help user-defined} for basic help with the macros.  Here is an
overview of their functionality, based on Godmar's documentation:

@deffn {GDB Macro} debugpintos
Attach debugger to a waiting pintos process on the same machine.
Shorthand for @code{target remote localhost:1234}.
@end deffn

@deffn {GDB Macro} dumplist list type element
Prints the elements of @var{list}, which should be a @code{struct} list
that contains elements of the given @var{type} (without the word
@code{struct}) in which @var{element} is the @struct{list_elem} member
that links the elements.

Example: @code{dumplist all_list thread all_elem} prints all elements of
@struct{thread} that are linked in @code{struct list all_list} using the
@code{struct list_elem all_elem} which is part of @struct{thread}.
(This assumes that you have added @code{all_list} and @code{all_elem}
yourself.)
@end deffn

@deffn {GDB Macro} btthread thread
Shows the backtrace of @var{thread}, which is a pointer to the
@struct{thread} of the thread whose backtrace it should show.  For the
current thread, this is identical to the @code{bt} (backtrace) command.
It also works for any thread suspended in @func{schedule},
provided you know where its kernel stack page is located.
@end deffn

@deffn {GDB Macro} btthreadlist list element
Shows the backtraces of all threads in @var{list}, the @struct{list} in
which the threads are kept.  Specify @var{element} as the
@struct{list_elem} field used inside @struct{thread} to link the threads
together.

Example: @code{btthreadlist all_list all_elem} shows the backtraces of
all threads contained in @code{struct list all_list}, linked together by
@code{all_elem}.  This command is useful to determine where your threads
are stuck when a deadlock occurs.  Please see the example scenario below.
(This assumes that you have added @code{all_list} and @code{all_elem}
yourself.)
@end deffn

@deffn {GDB Macro} btpagefault
Print a backtrace of the current thread after a page fault exception.
Normally, when a page fault exception occurs, GDB will stop
with a message that might say:

@example
Program received signal 0, Signal 0.
0xc0102320 in intr0e_stub ()
@end example

In that case, the @code{bt} command might not give a useful
backtrace.  Use @code{btpagefault} instead.

You may also use @code{btpagefault} for page faults that occur in a user
process.  In this case, you may also wish to load the user program's
symbol table (@pxref{Debugging User Programs}).
@end deffn

@deffn {GDB Macro} hook-stop
GDB invokes this macro every time the simulation stops, which Bochs will
do for every processor exception, among other reasons.  If the
simulation stops due to a page fault, @code{hook-stop} will print a
message that says and explains further whether the page fault occurred
in the kernel or in user code.

If the exception occurred from user code, @code{hook-stop} will say:
@example
pintos-debug: a page fault exception occurred in user mode
pintos-debug: hit 'c' to continue, or 's' to step to intr_handler
@end example

In Project 2, a page fault in a user process leads to the termination of
the process.  You should expect those page faults to occur in the
robustness tests where we test that your kernel properly terminates
processes that try to access invalid addresses.  To debug those, set a
break point in @func{page_fault} in @file{exception.c}, which you will
need to modify accordingly.

In Project 3, a page fault in a user process no longer automatically
leads to the termination of a process.  Instead, it may require reading in
data for the page the process was trying to access, either
because it was swapped out or because this is the first time it's
accessed.  In either case, you will reach @func{page_fault} and need to
take the appropriate action there.

If the page fault did not occur in user mode while executing a user
process, then it occurred in kernel mode while executing kernel code.
In this case, @code{hook-stop} will print this message:
@example
pintos-debug: a page fault occurred in kernel mode
@end example
followed by the output of the @code{btpagefault} command.

Before Project 3, a page fault exception in kernel code is always a bug
in your kernel, because your kernel should never crash.  Starting with
Project 3, the situation will change if you use @func{get_user} and
@func{put_user} strategy to verify user memory accesses
(@pxref{Accessing User Memory}).

If you don't want GDB to stop for page faults, then issue the command
@code{handle SIGSEGV nostop}.  GDB will still print a message for
every page fault, but it will not come back to a command prompt.
@end deffn

@node Example GDB Session
@subsection Example GDB Session

This section narrates a sample GDB session, provided by Godmar Back.
This example illustrates how one might debug a Project 1 solution in
which occasionally a thread that calls @func{timer_sleep} is not woken
up.  With this bug, tests such as @code{mlfqs_load_1} get stuck.

This session was captured with a slightly older version of Bochs and the
GDB macros for Pintos, so it looks slightly different than it would now.
Program output is shown in normal type, user input in @strong{strong}
type.

First, I start Pintos:

@smallexample
$ @strong{pintos -v --gdb -- -q -mlfqs run mlfqs-load-1}
Writing command line to /tmp/gDAlqTB5Uf.dsk...
bochs -q
========================================================================
                       Bochs x86 Emulator 2.2.5
             Build from CVS snapshot on December 30, 2005
========================================================================
00000000000i[     ] reading configuration from bochsrc.txt
00000000000i[     ] Enabled gdbstub
00000000000i[     ] installing nogui module as the Bochs GUI
00000000000i[     ] using log file bochsout.txt
Waiting for gdb connection on localhost:1234
@end smallexample

@noindent Then, I open a second window on the same machine and start GDB:

@smallexample
$ @strong{pintos-gdb kernel.o}
GNU gdb Red Hat Linux (6.3.0.0-1.84rh)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-redhat-linux-gnu"...
Using host libthread_db library "/lib/libthread_db.so.1".
@end smallexample

@noindent Then, I tell GDB to attach to the waiting Pintos emulator:

@smallexample
(gdb) @strong{debugpintos}
Remote debugging using localhost:1234
0x0000fff0 in ?? ()
Reply contains invalid hex digit 78
@end smallexample

@noindent Now I tell Pintos to run by executing @code{c} (short for
@code{continue}) twice:

@smallexample
(gdb) @strong{c}
Continuing.
Reply contains invalid hex digit 78
(gdb) @strong{c}
Continuing.
@end smallexample

@noindent Now Pintos will continue and output:

@smallexample
Pintos booting with 4,096 kB RAM...
Kernel command line: -q -mlfqs run mlfqs-load-1
374 pages available in kernel pool.
373 pages available in user pool.
Calibrating timer...  102,400 loops/s.
Boot complete.
Executing 'mlfqs-load-1':
(mlfqs-load-1) begin
(mlfqs-load-1) spinning for up to 45 seconds, please wait...
(mlfqs-load-1) load average rose to 0.5 after 42 seconds
(mlfqs-load-1) sleeping for another 10 seconds, please wait...
@end smallexample

@noindent 
@dots{}until it gets stuck because of the bug I had introduced.  I hit
@key{Ctrl+C} in the debugger window:

@smallexample
Program received signal 0, Signal 0.
0xc010168c in next_thread_to_run () at ../../threads/thread.c:649
649       while (i <= PRI_MAX && list_empty (&ready_list[i]))
(gdb) 
@end smallexample

@noindent 
The thread that was running when I interrupted Pintos was the idle
thread.  If I run @code{backtrace}, it shows this backtrace:

@smallexample
(gdb) @strong{bt}
#0  0xc010168c in next_thread_to_run () at ../../threads/thread.c:649
#1  0xc0101778 in schedule () at ../../threads/thread.c:714
#2  0xc0100f8f in thread_block () at ../../threads/thread.c:324
#3  0xc0101419 in idle (aux=0x0) at ../../threads/thread.c:551
#4  0xc010145a in kernel_thread (function=0xc01013ff , aux=0x0)
    at ../../threads/thread.c:575
#5  0x00000000 in ?? ()
@end smallexample

@noindent 
Not terribly useful.  What I really like to know is what's up with the
other thread (or threads).  Since I keep all threads in a linked list
called @code{all_list}, linked together by a @struct{list_elem} member
named @code{all_elem}, I can use the @code{btthreadlist} macro from the
macro library I wrote.  @code{btthreadlist} iterates through the list of
threads and prints the backtrace for each thread:

@smallexample
(gdb) @strong{btthreadlist all_list all_elem}
pintos-debug: dumping backtrace of thread 'main' @@0xc002f000
#0  0xc0101820 in schedule () at ../../threads/thread.c:722
#1  0xc0100f8f in thread_block () at ../../threads/thread.c:324
#2  0xc0104755 in timer_sleep (ticks=1000) at ../../devices/timer.c:141
#3  0xc010bf7c in test_mlfqs_load_1 () at ../../tests/threads/mlfqs-load-1.c:49
#4  0xc010aabb in run_test (name=0xc0007d8c "mlfqs-load-1")
    at ../../tests/threads/tests.c:50
#5  0xc0100647 in run_task (argv=0xc0110d28) at ../../threads/init.c:281
#6  0xc0100721 in run_actions (argv=0xc0110d28) at ../../threads/init.c:331
#7  0xc01000c7 in main () at ../../threads/init.c:140

pintos-debug: dumping backtrace of thread 'idle' @@0xc0116000
#0  0xc010168c in next_thread_to_run () at ../../threads/thread.c:649
#1  0xc0101778 in schedule () at ../../threads/thread.c:714
#2  0xc0100f8f in thread_block () at ../../threads/thread.c:324
#3  0xc0101419 in idle (aux=0x0) at ../../threads/thread.c:551
#4  0xc010145a in kernel_thread (function=0xc01013ff , aux=0x0)
    at ../../threads/thread.c:575
#5  0x00000000 in ?? ()
@end smallexample

@noindent 
In this case, there are only two threads, the idle thread and the main
thread.  The kernel stack pages (to which the @struct{thread} points)
are at @t{0xc0116000} and @t{0xc002f000}, respectively.  The main thread
is stuck in @func{timer_sleep}, called from @code{test_mlfqs_load_1}.

Knowing where threads are stuck can be tremendously useful, for instance
when diagnosing deadlocks or unexplained hangs.

@node Debugging User Programs
@subsection Debugging User Programs

You can also use GDB to debug a user program running under
Pintos.  Start by issuing this GDB command to load the
program's symbol table:
@example
add-symbol-file @var{program}
@end example
@noindent
where @var{program} is the name of the program's executable (in the host
file system, not in the Pintos file system).  After this, you should be
able to debug the user program the same way you would the kernel, by
placing breakpoints, inspecting data, etc.  Your actions apply to every
user program running in Pintos, not just to the one you want to debug,
so be careful in interpreting the results.  Also, a name that appears in
both the kernel and the user program will actually refer to the kernel
name.  (The latter problem can be avoided by giving the user executable
name on the GDB command line, instead of @file{kernel.o}, and then using
@code{add-symbol-file} to load @file{kernel.o}.)

@node GDB FAQ
@subsection FAQ

@table @asis
@item GDB can't connect to Bochs.

If the @command{target remote} command fails, then make sure that both
GDB and @command{pintos} are running on the same machine by
running @command{hostname} in each terminal.  If the names printed
differ, then you need to open a new terminal for GDB on the
machine running @command{pintos}.

@item GDB doesn't recognize any of the macros.

If you start GDB with @command{pintos-gdb}, it should load the Pintos
macros automatically.  If you start GDB some other way, then you must
issue the command @code{source @var{pintosdir}/src/misc/gdb-macros},
where @var{pintosdir} is the root of your Pintos directory, before you
can use them.

@item Can I debug Pintos with DDD?

Yes, you can.  DDD invokes GDB as a subprocess, so you'll need to tell
it to invokes @command{pintos-gdb} instead:
@example
ddd --gdb --debugger pintos-gdb
@end example

@item Can I use GDB inside Emacs?

Yes, you can.  Emacs has special support for running GDB as a
subprocess.  Type @kbd{M-x gdb} and enter your @command{pintos-gdb}
command at the prompt.  The Emacs manual has information on how to use
its debugging features in a section titled ``Debuggers.''

@item GDB is doing something weird.

If you notice strange behavior while using GDB, there
are three possibilities: a bug in your
modified Pintos, a bug in Bochs's
interface to GDB or in GDB itself, or
a bug in the original Pintos code.  The first and second
are quite likely, and you should seriously consider both.  We hope
that the third is less likely, but it is also possible.
@end table

@node Triple Faults
@section Triple Faults

When a CPU exception handler, such as a page fault handler, cannot be
invoked because it is missing or defective, the CPU will try to invoke
the ``double fault'' handler.  If the double fault handler is itself
missing or defective, that's called a ``triple fault.''  A triple fault
causes an immediate CPU reset.

Thus, if you get yourself into a situation where the machine reboots in
a loop, that's probably a ``triple fault.''  In a triple fault
situation, you might not be able to use @func{printf} for debugging,
because the reboots might be happening even before everything needed for
@func{printf} is initialized.

There are at least two ways to debug triple faults.  First, you can run
Pintos in Bochs under GDB (@pxref{GDB}).  If Bochs has been built
properly for Pintos, a triple fault under GDB will cause it to print the
message ``Triple fault: stopping for gdb'' on the console and break into
the debugger.  (If Bochs is not running under GDB, a triple fault will
still cause it to reboot.)  You can then inspect where Pintos stopped,
which is where the triple fault occurred.

Another option is what I call ``debugging by infinite loop.''
Pick a place in the Pintos code, insert the infinite loop
@code{for (;;);} there, and recompile and run.  There are two likely
possibilities:

@itemize @bullet
@item
The machine hangs without rebooting.  If this happens, you know that
the infinite loop is running.  That means that whatever caused the
reboot must be @emph{after} the place you inserted the infinite loop.
Now move the infinite loop later in the code sequence.

@item
The machine reboots in a loop.  If this happens, you know that the
machine didn't make it to the infinite loop.  Thus, whatever caused the
reboot must be @emph{before} the place you inserted the infinite loop.
Now move the infinite loop earlier in the code sequence.
@end itemize

If you move around the infinite loop in a ``binary search'' fashion, you
can use this technique to pin down the exact spot that everything goes
wrong.  It should only take a few minutes at most.

@node Modifying Bochs
@section Modifying Bochs

An advanced debugging technique is to modify and recompile the
simulator.  This proves useful when the simulated hardware has more
information than it makes available to the OS.  For example, page
faults have a long list of potential causes, but the hardware does not
report to the OS exactly which one is the particular cause.
Furthermore, a bug in the kernel's handling of page faults can easily
lead to recursive faults, but a ``triple fault'' will cause the CPU to
reset itself, which is hardly conducive to debugging.

In a case like this, you might appreciate being able to make Bochs
print out more debug information, such as the exact type of fault that
occurred.  It's not very hard.  You start by retrieving the source
code for Bochs 2.2.6 from @uref{http://bochs.sourceforge.net} and
saving the file @file{bochs-2.2.6.tar.gz} into a directory.  
The script @file{pintos/src/misc/bochs-2.2.6-build.sh}
applies a number of patches contained in @file{pintos/src/misc}
to the Bochs tree, then builds Bochs and installs it in a directory
of your choice.
Run this script without arguments to learn usage instructions.
To use your @file{bochs} binary with @command{pintos},
put it in your @env{PATH}, and make sure that it is earlier than
@file{@value{localpintosbindir}/bochs}.

Of course, to get any good out of this you'll have to actually modify
Bochs.  Instructions for doing this are firmly out of the scope of
this document.  However, if you want to debug page faults as suggested
above, a good place to start adding @func{printf}s is
@func{BX_CPU_C::dtranslate_linear} in @file{cpu/paging.cc}.

@node Debugging Tips
@section Tips

The page allocator in @file{threads/palloc.c} and the block allocator in
@file{threads/malloc.c} clear all the bytes in memory to
@t{0xcc} at time of free.  Thus, if you see an attempt to
dereference a pointer like @t{0xcccccccc}, or some other reference to
@t{0xcc}, there's a good chance you're trying to reuse a page that's
already been freed.  Also, byte @t{0xcc} is the CPU opcode for ``invoke
interrupt 3,'' so if you see an error like @code{Interrupt 0x03 (#BP
Breakpoint Exception)}, then Pintos tried to execute code in a freed page or
block.

An assertion failure on the expression @code{sec_no < d->capacity}
indicates that Pintos tried to access a file through an inode that has
been closed and freed.  Freeing an inode clears its starting sector
number to @t{0xcccccccc}, which is not a valid sector number for disks
smaller than about 1.6 TB.