Advanced Operating Systems
M4: System call interface
The aim of this milestone is to design
protocol for the system call interface. You should implement
both the client and system side of this interface. This
client-side system-call interface must conform to the interface
musl libc and sos
For example, the shell does not call
Not all syscalls have been implemented through musl libc as in
some cases the POSIX semantics are unnecessarily complex. You are
free to implement more of the functionality in any of the syscalls
At this stage you will not actually be able to implement most
of the SOS (server-side) system calls, however you should be able to partially
You should also change the implementation of
Other system calls defined in
When a program opens the file
Reading the console is a blocking operation. If a process reads from the console and there is no data available it should block until data becomes available.
Be careful not to implement the console device as a 'hack'. You should think about being able to support multiple serial ports and other stream devices in your design (although not necessarily implement them). This means designing a consistent interface for interacting with all devices. You may want to read up on how Linux treats devices.
You may once again find the documentation on libserial handy.
You will need to modify the libsos library to add implementations
of the interface defined in
You will also need to add sosh to the cpio archive that is linked
to SOS and instruct SOS to run this application. Both of these tasks
are achieved through the configuration menu. Run
At this stage of the project you will need to decide whether you want to have a simple single-threaded server, or to multi-thread it. A multi-threaded design could be advantageous to deal with the inherent concurrency your system will have (e.g. between paging, system calls, asynchronous I/O and clock interrupts), but it will require careful design of synchronisation in order to avoid race conditions and deadlocks. A single threaded model will require extra attention to ensure liveness.
Another design decision is how to transfer data between the kernel and user processes. Some options you have are:
Whatever you do, remember the basic engineering rule: keep it simple, stupid!(KISS).
This milestone is larger than it seems. The system call interface of an OS determines how user applications receive data they request. You will need to consider how you can move data in various quantities between your root server and clients. Scenarios to consider include:
Recall how OS/161 used an internal framework for safely transporting data between the OS and the application. The OS avoided simply reaching into application address spaces based on app-supplied pointers. You'll need to think about a framework (or at least a convention within your code) that can provide SOS with safe access to application memory. In later milestones, your approach will have to evolve to function in the presence in paging to disk, including paging application buffers.
Recall also that the OS can't rely on applications for correctness. Any system call you add should behave gracefully in the presence of a malicious application - i.e. use error checking on system call arguments, and returns errors when encoutering problems.
For this assessment you should be able to demonstrate
You should also show some sos application-level test code that
As always, you should be able to explain both the design and your implementation.
Note: In line with the comments in the advice section, streaming large blocks of data over multiple IPCs is a sure-fire way not to get full marks!
Last modified: 28 Aug 2015.