Signals in disastrOS - SO Project

Summary

The goal of this project is to implement a simple signals system within disastrOS, a program provided by our professors.
Given the PCB of a generic process, a system call defined by us will be invoked at a specific time and will set a signal number for the aforementioned process.

How?

An array of contexts has been added to the process PCB, in which each cell will contain the context associated with the same number as the received signal, along with a common stack that all the signal contexts will share.
We used two bitmaps to handle the signals, one for the received signals and one for the signal being served.
We have defined a new syscall, called internal_signal, which will run at fixed intervals (defined in signal.h) and the function works as follows:

1. it analyzes which signal must be sent in that instant of time
2. we scan the ready_list to check if it's not empty
   • if not empty
     • we find the PCB of the last process in the ready_list
     • we check if that process is init, in which case the function will return
     • we check if the last signal received is the same as the one we are trying to send or the same signal is served, in which case the function will print a message
     • otherwise, we create a new context which will be used by the signal_handler
     • if the signals_mask is not set, we set the bit representing that signal we just sent
   • otherwise, we go to 3
3. we scan the waiting_list to check if it's not empty
   • if not empty
     • we find the PCB of the last process in the waiting_list
     • we check if that process is init, in which case the function will return
     • we check if the last signal received is the same as the one we are trying to send or the same signal is served, in which case the function will print a message
     • otherwise, we create a new context which will be used by the signal_handler
     • if the signals_mask is not set, we set the bit representing that signal we just sent
4. if 2 and 3 fail, the program will crash because at least one process must be active

Before each process starts running, we check if any of the signals are active by checking the signals value in the PCB: if the value is not zero, we jump to the signal main context, otherwise the process proceeds in its execution.
Assuming that we are in the signal main context, we begin to scan the mask of the signals received until we find a cell with its bit set, then we move on to the context inside the cell of the array of contexts with the same index if the mask of the signal served is not set.

Signals defined

We have defined two signals:

1. MovUp: print an informative message and set a variable on the PCB to "true", which will be used the next time the process performs a wait
2. Kill: print an informative message meaning the process would be killed

How to execute

Type in shell the following:

$ make
$ ./disastrOS_test

Source files that have been changed:

• disastrOS_constants.h
• disastrOS_pcb.c
• disastrOS_pcb.h
• disastrOS_sleep.c
• disastrOS_spawn.c
• disastrOS_syscalls.h
• disastrOS.c
• disastrOS.h

Added files:

• disastrOS_sendSignal.c
• sigKill.c
• sigMovUp.c
• signalMakeContext.c
• signals.h