Operating System Project
Budget: $2 – $8 USD
Use the QEMU and Docker to run the operating system: Open two terminal windows. In one, enter make qemu-gdb (or make qemu-nox-gdb). This starts up QEMU, but QEMU stops just before the processor executes the first instruction and waits for a debugging connection from GDB. In the second terminal, from the same directory you ran make, run gdb.
xv6 repository: https://github.com/mit-pdos/xv6-public
for the docker image use: grantbot/xv6
Extend the current xv6 process implementation to maintain an exit status. To get this done, add a field to the process structure (see proc.h) in order to save an exit status of the terminated process. We will need this for implementing wait. Next, you have to change all system calls affected by this change (e.g., exit, wait etc.).
a) Change the exit system call signature to void exit(int status). The exit system call must act as previously defined (i.e., terminate the current process) but it must also store the exit status of the terminated process in the corresponding structure. In order to make the changes in exit system call you must update the following files: user.h, defs.h, sysproc.c, proc.c and all the user space programs that uses exit system call. Note, from now on, all user space programs must supply an exit status when terminated.
Hassle: one hassle that this change (and the one in part b below) introduces is that all existing places that used exit(), including ones that are in test programs have now to be changed to use the new prototype. You can either do that yourself (e.g., use grep to find all locations of this call and change them), or create a new exit call to match the new prototype.
Goals of this part of the assignment: Get familiar with system call arguments and how arguments are passed given the presence of two stacks (user mode and kernel mode). Understand the backward compatibility hassles that come from modifying the system call prototype. Carry out a gentle modification to an existing system call and to the Process Control Block (PCB), which will be needed by the next part of the Lab.
b) Update the wait system call signature to int wait(int *status). The wait system call must prevent the current process from execution until any of its child processes is terminated (if any exists) and return the terminated child exit status through the status argument. The system call must return the process id of the child that was terminated or -1 if no child exists (or unexpected error occurred). Note that the wait system call can receive NULL as an argument. In this case the child’s exit status must be discarded.
Goal of this part of the assignment: Continue to get familiar with system call arguments, in this case with how to return a value.
c) Add a waitpid system call: int waitpid(int pid, int *status, int options). This system call must act like wait system call with the following additional properties: The system call must wait for a process (not necessary a child process) with a pid that equals to one provided by the pid argument. The return value must be the process id of the process that was terminated or -1 if this process does not exist or if an unexpected error occurred. We are required only to implement a blocking waitpid where the kernel prevents the current process from execution until a process with the given pid terminates.
d) Write an example program to illustrate that your waitpid works. You have to modify the makefile to add your example program so that it can be executed from inside the shell once xv6 boots.
5. WHAT TO SUBMIT
You need to submit a report that includes the folowing:
All code changes you made on xv6 source code (hint: diff -r original_xv6 your_xv6)
Detailed explanation about these changes
Screenshots about how you run the related program(s) and results
xv6 repository: https://github.com/mit-pdos/xv6-public
for the docker image use: grantbot/xv6
Extend the current xv6 process implementation to maintain an exit status. To get this done, add a field to the process structure (see proc.h) in order to save an exit status of the terminated process. We will need this for implementing wait. Next, you have to change all system calls affected by this change (e.g., exit, wait etc.).
a) Change the exit system call signature to void exit(int status). The exit system call must act as previously defined (i.e., terminate the current process) but it must also store the exit status of the terminated process in the corresponding structure. In order to make the changes in exit system call you must update the following files: user.h, defs.h, sysproc.c, proc.c and all the user space programs that uses exit system call. Note, from now on, all user space programs must supply an exit status when terminated.
Hassle: one hassle that this change (and the one in part b below) introduces is that all existing places that used exit(), including ones that are in test programs have now to be changed to use the new prototype. You can either do that yourself (e.g., use grep to find all locations of this call and change them), or create a new exit call to match the new prototype.
Goals of this part of the assignment: Get familiar with system call arguments and how arguments are passed given the presence of two stacks (user mode and kernel mode). Understand the backward compatibility hassles that come from modifying the system call prototype. Carry out a gentle modification to an existing system call and to the Process Control Block (PCB), which will be needed by the next part of the Lab.
b) Update the wait system call signature to int wait(int *status). The wait system call must prevent the current process from execution until any of its child processes is terminated (if any exists) and return the terminated child exit status through the status argument. The system call must return the process id of the child that was terminated or -1 if no child exists (or unexpected error occurred). Note that the wait system call can receive NULL as an argument. In this case the child’s exit status must be discarded.
Goal of this part of the assignment: Continue to get familiar with system call arguments, in this case with how to return a value.
c) Add a waitpid system call: int waitpid(int pid, int *status, int options). This system call must act like wait system call with the following additional properties: The system call must wait for a process (not necessary a child process) with a pid that equals to one provided by the pid argument. The return value must be the process id of the process that was terminated or -1 if this process does not exist or if an unexpected error occurred. We are required only to implement a blocking waitpid where the kernel prevents the current process from execution until a process with the given pid terminates.
d) Write an example program to illustrate that your waitpid works. You have to modify the makefile to add your example program so that it can be executed from inside the shell once xv6 boots.
5. WHAT TO SUBMIT
You need to submit a report that includes the folowing:
All code changes you made on xv6 source code (hint: diff -r original_xv6 your_xv6)
Detailed explanation about these changes
Screenshots about how you run the related program(s) and results