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System calls for Synchronization

Wait system call

Arguments

Process Identifier of the process for which the current process has to wait.

Return Values
0 Success
-1 Given process identifier is invalid or it is the pid of the same process invoking wait

Description

The current process is blocked till the process with PID given as argument executes a Signal system call or exits. The system call will fail if a process attempts to wait for itself. The only data structure updated is Process Table. The mode flag in the Process Table has to be set to Kernel mode when the process enters the system call and reset before exiting from the system call.

Algorithm


Set the MODE_FLAG in the process table entry to 13.

Switch to the Kernel Stack.     /* See kernel stack management during system calls */
Save the value of SP to the USER SP field in the Process Table entry of the process.
Set the value of SP to the beginning of User Area Page.

If process is intending to wait for itself or for a non-existent process, return -1.    /* Check the status from Process table.  */ 

Change the status from (RUNNING,_ ) to (WAIT_PROCESS, Argument_PID ) in the Process Table.

Invoke the Scheduler by calling the switch_context() function in the Scheduler Module.

/* The following code excutes only when scheduled again after the occurance of a signal/exit of the process waiting for. */

Restore SP to the USER SP stored in the process table.

Reset the MODE_FLAG in the process table entry to 0.

Return 0.  /* Success */

Note:  At each point of return from the system call, remember to reset the MODE FLAG and switch back to the user stack.

Signal system call

Arguments

None

Return Value:
0 Success

Description

All processes waiting for the signalling process are resumed. The system call does not fail. The only data structure updated is Process Table.The mode flag in the Process Table has to be set to Kernel mode when the process enters the system call and reset before exiting from the system call.

Algorithm


Set the MODE_FLAG in the process table entry to 14.

Wake up all processes waiting for the current process.
    Traverse through the Process Table
        If the process is in state (WAIT_PROCESS, Pid) where Pid matches with the PID of the current process.
            Change the status to (READY, _ ).

Reset the MODE_FLAG in the process table entry to 0.

Return 0.   /* Success */
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