/Teaching/System Level Programming/Assignments/A4


Pull from upstream before solving this task.


Task: Interprocess Communication (IPC)

This exercise should teach you what interprocess communication is for and how you can realize it.

Main Idea

Everybody of us has used interprocess communication already. Mostly unintentionally at this point in your studies. This is why we wanted to take this specific topic into this semester’s course.

To understand the concept of IPC, some major concepts must be learned and understood beforehand.

  • Virtual Memory
  • Process vs. Thread
  • Shared Resources
  • Locking

Some of those terms are already familiar to you, others not. I will not entirely go into details for this assignment, but it’s always useful looking up information based on those keywords.

Implementation details: TicTacToe


You MUST NOT change predefined function signatures, sequences of checks or similar. Exploits will automatically result in deductions! Do not remove or add any usleep or assert statements!

The idea of this assignment is, that the Player sends commands to the Computer and the Computer responds accordingly. The game that is played is TicTacToe.

You start the Player-process, which has to then start the Computer-process. These two processes shall communicate using shared memory. The Player process reads commands from STDIN (terminal input) and sends them to the computer (there are two exceptions, described in Command Handling). The computer replies with the appropriate answer.

Player.c:

  • ”initSharedMemoriesPlayer()”
    • initialize the shared memory objects.
    • Make sure to only set the permissions the process needs!
  • ”initMmapingsPlayer()”
    • map those objects to virtual memory.
    • The permissions need to match the ones you used for the SHM objects!
  • ”initLocks()”
    • initialize any locks you might need.
  • ”initProcess()”
    • launch the Computer.
    • Ensure you launch a new process.
    • Ensure you execute the right executable.
  • has to be synchronized properly
  • has to be cleaned up properly (also stop and clean up everything after getCommand() returns 0)

The Player asks for the input, publishes it on the shared memory, and makes it available for the Computer-process. For initializing the shared memory, CAREFULLY take a look at the defines! The names of those objects are predefined and can be found in the list of defines in util.h:.

DO NOT remove or relocate code for checking your approach. Otherwise, we will deduce points.

Computer.c:

  • ”initSharedMemoriesComputer()”
    • open the shared memory objects.
    • Make sure to only set the permissions the process needs!
  • ”initMmapingsComputer()”
    • map those objects to virtual memory.
    • The permissions need to match the ones you used for the SHM objects!
  • ”cmdHandler()”
    • handle the commands received in the request object and return the appropriate answer in the response object.
  • has to be synchronized properly as well
  • has to be cleaned up properly

Command handling:

  • playerstart/computerstart: These commands should be handled by the Player-process, before sending them to the Computer. playerstart sets the player_start variable in the shmgamestate struct to 1, computerstart sets it to 0. The computer should treat this as a command it does not understand. (Take a look at the predefined replies!)
  • start: set the live variable in the shmgamestate struct to 1. The computer makes the first move if player_start is 0. If the game has already started, respond accordingly.
  • 1-9: Places the players symbol at one of the 9 positions (left to right, row-wise). The live array in shmgamestate has to be updated (‘ ‘ = empty field, ‘X’ = player one, ‘O’ = player two). If a move is not possible, the Computer should reply appropriately and not make a move itself. If the move is valid and the player has won or the game is a tie,  set live to 0 again and respond with the correct message. If the move is valid and the game is not over, the computer makes a move. If the computer wins, again, end the game and reply with the correct message. If the game has not yet started, respond accordingly.
  • exit: the Computer responds with “Shutting down!” and both processes are terminated.
  • any other input should be answered with “I did not understand that!”.

DO NOT remove or relocate code for checking your approach. Otherwise, we will deduce points.

util.c:

  • You can add checks in the functions provided, but when tested, all changes made will be ignored.

util.h:

  • The only change allowed in this file is adding synchronization primitives to the shmlocks struct. All other changes made in this file will make your program not testable. Do not change anything but carefully look at the extern variables, defines, and structs.

IPC in a (way too oversimplified) nutshell

Those simplified explanations should not replace your attendance and attention in the lecture nor serve you the detailed solution to this assignment. It should help you to understand the central concept briefly to make research more accessible.

Virtual Memory Virtual memory is part of the concept of modern operating systems. As you may suspect, there is a difference between physical and virtual memory. The physical memory provided by, e.g. (SO-)DIMMs has to be managed by the operating system that uses virtual addresses for locating data. So there has to be a sort of translation between the physical and virtual memory addresses. If you are interested in this topic, you can look up this and related articles [1].

Process vs. Threads As many of you already know from previous assignments, processes are treated differently in contrast to threads – well, kind of. As you discovered, a thread can operate on the whole memory the program has mapped. This means, e.g., the heap-allocated by one thread can be used with any other thread the program launches. It is possible since they share the same virtual address space, as they are all used in one program launched before. So we can deduce that each process has its own virtual memory space. Those virtual memory spaces are strictly separated from each other.
Thus, sharing resources ”between” two processes are realized differently from sharing resources ”in” a process. So we have to share memory between two processes. This memory is surprisingly called shared memory, and it is a central part of IPC [2].

Shared resources Shared resources are “saved” in files, as the file system (FS) is accessible in both processes – well, kind of again. But besides some exceptions, all files saved on the FS can be accessed depending on the rights a user has. But some of you may wonder, how to access this data? Do we have to use ”read()” and ”write()” for sharing information? Well, this is possible with one big limitation. This limitation is that we cannot use virtual addresses to access data easily and fast. Well, there is a solution called ”mmap()”, which takes a file descriptor (fd) that maps the file (full size) to our virtual memory space of the program. That’s it, well, kind of. How this mapping works in the kernel will be taught in the following course called operating systems.

Locking When we use IPC, mostly, some locking mechanisms have to be used. In our case, semaphores or mutexes and condition variables are the way to go [3].


Holding a lock while going to sleep will lead to point deductions.

Some further hints

What to do before you start?

  • Pull from upstream!
  • Try to understand the program structure.
  • Look at the Manpage and what those parameters of the needed functions are for and how they are used.
  • Only begin, if you understand the basic concept of processes, virtual memory, shared resources as well as mapping them. Bruteforcing will lead to a severe amount of wasted time.
  • Player.h and Computer.h MUST NOT be changed,
  • You are only allowed to add code inbetween the TOTOs and in the utils.c to create your own tests. Don’t remove or move around existing code ever (this would lead to point deductions).
  • Do not push binary files or any other junk files.
  • Carefully read the TODOs. Some contain crucial information!

Submission

Modify the files in your git repository. You can find this file in directory A4. Tag the submission with A4 and push it to the server.

Assignment Tutor

If you have any questions regarding this assignment, try Discord first, and slp@iaik.tugraz.at second. If you have a more direct question regarding your specific solution, you can also ask the tutor who organizes this assignment.

Aaron Giner, aaron.giner@student.tugraz.at