/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: Communicating Processes


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 SendingProcess:

  • has to initialize the shared memory objects Use the predefined function ”initSharedMemoriesSendingProcess()”.
    • The request object must be readable and writeable!
    • The response object however only reads and thus needs to be read-only!
    • The locks object is readable and writeable
  • has to take care of mapping those objects to its virtual memory. Use the predefined function ”initMmapingsSendingProcess()”.
    Be careful about the flags! The mappings have to fit the shared memory objects.
  • has initialized the locks. Use the predefined function ”initLocks()”.
  • has to launch the AnsweringProcess. Use the predefined ”initProcess()’‘.
    • 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 SendingProcess asks for the input, publishes it on the shared memory, and makes it available for the other process. For initializing the shared memory CAREFULLY take a look at the defines! The name of those objects is predefined and can also be found in the list of defines.

The AnsweringProcess:

  • has to open the shared memory objects Use the predefined function ”initSharedMemoriesAnsweringProcess()”.
    • The response object must be readable and writeable!
    • The request object however only reads and thus needs to be read-only!
    • The locks object is readable and writeable
  • has to take care of mapping those objects to its virtual memory. Use the predefined function ”initMmapingsAnsweringProcess()”.
    Be careful about the flags! The mappings have to fit the shared memory objects.
  • has to dispatch the messages got by the request object and return the correct answers. Use the predefined function ”dispatcher()”.
  • has to be synchronized properly as well

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

Utils.c:

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

Utils.h:

  • The only change allowed in this file is adding semaphores to the semaphore struct. However, renaming, removing, or replacing semaphores will lead to problems. 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 easier.

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 to 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 for accessing data easy 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 and mutex 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.
  • I would recommend completing all tasks in the SendingProcess.c and in the AnsweringProcess.c functions first!
  • SendingProcess.h and AnsweringProcess.h don’t require any changes
  • You are allowed to add code everywhere in the program and extends structs as you like. But don’t remove or move around existing code ever (this would lead to point deductions).

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 bs-helpline@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.

Alina Steinberger <alina.steinberger@student.tugraz.at>