The aim of this milestone is to design the RPC 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 provided in include/sos.h.
You should create a libsos.a
library that your applications
can link against.
At this stage you will not actually be able to implement most
of the system calls, however you should be able to partially
implement open/close, read/write
for the
console
device. This will allow you to run a simple
shell on your system, which will allow you to perform interactive
testing.
Other system calls should output system call not
implemented
.
When a program opens the file console
it should
access the console on the serial device. The console is a multiple
writer, single reader device, i.e., more than one process can
concurrently open the device for writing, but only one process can
open the device for reading.
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).
You may once again find the documentation on libserial handy.
You will need to create a new library and to change your top-level
SConstruct
to make it build both your libsos.a
and the provided sosh
application.
To add a new library you should create a new directory in the
libs/sos
directory with 2 subdirectories
libs/sos/include
and libs/sos/src
, and place your
source file into the src/
directory. You will also need to
create a SConscript
file to tell make system how to build your
new library. Its format should be self-evident from the other
libs/*/SConscript
files in other provided libraries. Finally
to include your new library and build the sosh application, you will need
to change your top-level SConstruct
file:
from: milestone = 0 to: milestone = 3
Once you have your libs/sos library set up, it should like like this:
libs/sos libs/sos/SConscript libs/sos/src/ libs/sos/include/ libs/sos/include/sos.h
You should copy the contents of tty_test/ttyout.c to libs/sos/src. You can consider this new file as the acorn from which the entire grand tree of -lsos will flourish.
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 descision is how to transfer data between the kernel and user processes. Some options you have are:
memcpy
data into it when you need to
transfer data.Whatever you do, remember the basic engineering rule: keep it simple, stupid!(KISS).
This milestone is larger than previous ones, and it is advisable that you split the workload between the group.
For this assessment you should be able to demonstrate
sosh
running, and SOS outputting system
call not implemented
for the relevant system calls.
sosh
makes use of the the SOS system calls in its
ls
and ps
commands. This should be
sufficient to demonstrate that your system calls work,
alternatively you can create your own test code.
As always you should be able to explain both the design and your implementation.