Debugging Tools

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


Don't underestimate the value of printf(). The way 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. 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 printf() with different strings (e.g. "<1>", "<2>", . . . ) throughout the pieces of code you suspect are failing. If you don't even see <1> printed, then something bad happened before that point, if you see <1> but not <2>, then something bad happened between those two points, and so on. Based on what you learn, you can then insert more printf() calls in the new, smaller region of code you suspect. Eventually you can narrow the problem down to a single statement. See Triple Faults, for a related technique.


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 ASSERT macro, defined in <debug.h>, for checking assertions.

#define ASSERT(expression) { /* Omit details */ }

Tests the value of 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. See Backtraces for more information.

Function and Parameter Attributes

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

#define UNUSED { /* Omit details */ }

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.

#define NO_RETURN { /* Omit details */ }

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.

#define NO_INLINE { /* Omit details */ }

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).

#define PRINTF_FORMAT(format, first) { /* Omit details */ }

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


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 debug_backtrace(), prototyped in <debug.h>, to print a backtrace at any point in your code. debug_backtrace_all(), also declared in <debug.h>, prints backtraces of all threads.

The addresses in a backtrace are listed as raw hexadecimal numbers, which are difficult to interpret. We provide a tool called backtrace to translate these into function names and source file line numbers. Give it the name of your kernel.o as the first argument and the hexadecimal numbers composing the backtrace (including the 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 kernel.o you passed to 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 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.


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:

Call stack: 0x800426eff 0x8004202fb 0x80042dc22 0x80042cf67 0x800422319
0x80042325a 0x40012c 0x400a96 0x400ac8.

You would then invoke the backtrace utility like shown below, cutting and pasting the backtrace information into the command line. This assumes that 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:

backtrace 0x800426eff 0x8004202fb 0x80042dc22 0x80042cf67 0x800422319
0x80042325a 0x400012c 0x400a96 0x4000ac8

The backtrace output would then look something like this:

0x0000000800426eff: debug_panic (lib/debug.c:86)
0x00000008004202fb: file_seek (filesys/file.c:405)
0x000000080042dc22: seek (userprog/syscall.c:744)
0x000000080042cf67: syscall_handler (userprog/syscall.c:444)
0x0000000800422319: intr_handler (threads/interrupt.c:334)
0x000000080042325a: intr_entry (threads/intr-stubs.S:38)
0x000000000040012c: (unknown)
0x0000000000400a96: (unknown)
0x0000000000400ac8: (unknown)

(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 debug_panic(), the function that implements kernel panics. Because backtraces commonly result from kernel panics, debug_panic() will often be the first function shown in a backtrace. The second line shows 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 filesys/file.c is the assertion

ASSERT (file_ofs >= 0);

(This line was also cited in the assertion failure message.) Thus, file_seek() panicked because it passed a negative file offset argument. The third line indicates that seek() called file_seek(), presumably without validating the offset argument. In this submission, seek() implements the seek system call. The fourth line shows that syscall_handler(), the system call handler, invoked seek(). The fifth and sixth lines are the interrupt handler entry path. The remaining lines are for addresses below KERN_BASE. This means that they refer to addresses in the user program, not in the kernel.


You can run Pintos under the supervision of the GDB debugger. First, start Pintos with the --gdb option, e.g. pintos --gdb -- run mytest. Second, open a second terminal on the same machine and use gdb to invoke GDB on 'kernel.o'

gdb kernel.o

and issue the following GDB command:

target remote localhost:1234

Now GDB is connected to the simulator over a local network connection. You can now issue any normal GDB commands. If you issue the '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 Ctrl+C.

Using GDB

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

  • c [GDB Command]

Continues execution until hCtrl+Ci or the next breakpoint.

  • break function [GDB Command]

  • break file:line [GDB Command]

  • break address [GDB Command]

Sets a breakpoint at function, at line within file, or address. (Use a '0x' prefix to specify an address in hex.) Use break main to make GDB stop when Pintos starts running.

  • p expression [GDB Command]

Evaluates the given expression and prints its value. If the expression contains a function call, that function will actually be executed.

  • l address [GDB Command]

Lists a few lines of code around address. (Use a '0x' prefix to specify an address in hex.)

  • bt [GDB Command]

Prints a stack backtrace similar to that output by the backtrace program described above.

  • p/a address [GDB Command]

Prints the name of the function or variable that occupies address. (Use a '0x' prefix to specify an address in hex.)

  • diassemble function [GDB Command]

Disassembles function.

Here's an extra tip for anyone who read this: install gdb script such as pwndbg. This makes your life easier!


  • Can I use GDB inside Emacs?

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

  • 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.

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 printf() for debugging, because the reboots might be happening even before everything needed for printf() is initialized.

There exists way to debug triple faults.

Pick a place in the Pintos code, insert the infinite loop for (;;); there, and recompile and run. There are two likely possibilities:

  • 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 after the place you inserted the infinite loop. Now move the infinite loop later in the code sequence.

  • 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 before the place you inserted the infinite loop. Now move the infinite loop earlier in the code sequence.

    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.


The page allocator in threads/palloc.c and the block allocator in threads/malloc.c clear all the bytes in memory to '0xcc' at time of free. Thus, if you see an attempt to dereference a pointer like 0xcccccccc, or some other reference to 0xcc, there's a good chance you're trying to reuse a page that's already been freed. Also, byte 0xcc is the CPU opcode for "invoke interrupt 3," so if you see an error like Interrupt 0x03 (#BP Breakpoint Exception), then Pintos tried to execute code in a freed page or block. An assertion failure on the expression 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 0xcccccccc, which is not a valid sector number for disks smaller than about 1.6 TB.

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