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FAQ

Questions

  1. How much code will I need to write?
  2. The kernel always panics when I run a custom test case.
  3. The kernel always panics with assertion is_thread(t) failed.
  4. All my user programs die with page faults.
  5. All my user programs die upon making a syscall.
  6. How can I disassemble user programs?
  7. Why do many C include files not work in Pintos programs? Can I use libfoo in my Pintos programs?
  8. How do I compile new user programs?
  9. Can I run user programs under a debugger?
  10. What’s the difference between tid_t and pid_t?

How much code will I need to write?

Here’s a summary of our reference solution. The reference solution represents just one possible solution. Many other solutions are also possible and many of those differ greatly from the reference solution. Some excellent solutions may not modify all the files modified by the reference solution, and some may modify files not modified by the reference solution.

lib/float.h | 6
threads/init.c | 7
threads/interrupt.h | 1
threads/intr-stubs.S | 6
threads/start.S | 5
threads/switch.S | 4
threads/switch.h | 5
threads/thread.c | 4
threads/thread.h | 1
userprog/exception.c | 7
userprog/process.c | 268 +++++++++++++++++++++++++++++-----
userprog/process.h | 37 ++++
userprog/syscall.c | 392 +++++++++++++++++++++++++++++++++++++++++++++++++--
userprog/syscall.h | 20 ++
14 files changed, 697 insertions(+), 66 deletions(-)

The kernel always panics when I run a custom test case.

Is your file name too long? The file system limits file names to 14 characters. Is the file system full? Does the file system already contain 16 files? The base Pintos file system has a 16-file limit.

The kernel always panics with assertion is_thread(t) failed.

This happens when you overflow your kernel stack. If you’re allocating large structures or buffers on the stack, try moving them to static memory or the heap instead. It’s also possible that you’ve made your struct thread too large. See the comment underneath the kernel stack diagram in threads/thread.h about the importance of keeping your struct thread small.

All my user programs die with page faults.

This will happen if you haven’t implemented argument passing (or haven’t done so correctly). The basic C library for user programs tries to read argc and argv off the stack. If the stack isn’t properly set up, this causes a page fault.

All my user programs die upon making a syscall.

You’ll have to implement syscall before you see anything else. Every reasonable program tries to make at least one syscall (exit) and most programs make more than that. Notably, printf invokes the write syscall. The default syscall handler just handles exit(). Until you have implemented syscalls sufficiently, you can use hex_dump to check your argument passing implementation (see Startup).

How can I disassemble user programs?

The objdump (80x86) or i386-elf-objdump (SPARC) utility can disassemble entire user programs or object files. Invoke it as objdump -d <file>. You can use GDB’s codedisassemble command to disassemble individual functions.

Why do many C include files not work in Pintos programs? Can I use libfoo in my Pintos programs?

The C library we provide is very limited. It does not include many of the features that are expected of a real operating system’s C library. The C library must be built specifically for the operating system (and architecture), since it must make syscalls for I/O and memory allocation. Not all functions do, of course, but usually the library is compiled as a unit.

If the library makes syscalls (e.g, parts of the C standard library), then they almost certainly will not work with Pintos. Pintos does not support as rich a syscall interfaces as real operating systems (e.g., Linux, FreeBSD), and furthermore, uses a different interrupt number (0x30) for syscalls than is used in Linux (0x80).

The chances are good that the library you want uses parts of the C library that Pintos doesn’t implement. It will probably take at least some porting effort to make it work under Pintos. Notably, the Pintos user program C library does not have a malloc implementation.

How do I compile new user programs?

Modify examples/Makefile, then run make.

Can I run user programs under a debugger?

Yes, with some limitations. See Adding new test to Pintos.

What’s the difference between tid_t and pid_t?

A tid_t identifies a kernel thread, which may have a user process running in it (if created with process_execute) or not (if created with thread_create). It is a data type used only in the kernel. A pid_t identifies a user process. It is used by user processes and the kernel in the exec and wait syscalls. You can choose whatever suitable types you like for tid_t and pid_t. By default, they’re both int. You can make them a one-to-one mapping, so that the same values in both identify the same process, or you can use a more complex mapping. It’s up to you.


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