2b – Process Abstraction in Unix

Process initialization

• Makes a copy of the parent PCB with new PID.
  • Child process has a copy of parent’s file descriptors table.
• When copying memory context, creates virtual address space of child process.
  • Copy-on-Write: On forking, new process’ virtual memory pages mapped to the same frame locations as parent
• Queued on the scheduler.

The master process init

Fork creates the process tree from the most common ancestor (PID = 1). Continues running until shutdown.

Process creation

Creates a new child process, a duplicate of the current executable image.

• Memory space is copied, not referenced.
• returns 0 if child, PID if parent.
• clone allows for partial copying.

New program execution

• execl, execlp, execle, execv, execvp, execvpe for different parameters
• argc: number of command line arguments
• argv char* string array. Each element in in argv[] is a character string. arg[0] is always the program name.

execl replaces currently executing process image with a new one.

int main() {
	execl( "/bin/ls", "ls", "-al", NULL);
} // executes "ls -al"

Parent/Child synchronization

wait(int *status) returns the PID of the terminated child process in the least significant byte of *status.

waitid() suspends until the child specified by the PID argument changes state.

• parent blocks until at least one child terminates
• remainder of child system resources is cleaned up (remove all zombie process)
• does not block if no child.

Scheduling Running –> Ready: When the OS decides you are taking up too much CPU time, you will be sent back to the ready queue

Process Termination

• returning result from main() implicitly calls exit(result) (open files are also flushed automatically)

Zombie Processes

• After a child process exits it immediately becomes a zombie as it still has a PCB in the process table.
• Needed for the parent to read its exit status.
• After parent calls wait to read the child process’ exit status, it is “reaped” and removed.
  • If parent dies before zombie reaped, zombie also becomes an orphan.

Orphan Processes

• parent process terminates before child.
• init process (which is the first process that boots in a Unix system) becomes the pseudo-parent of child.
  • wait() called by init periodically cleans up.