The Great Symbian

• When the OS or a user process decides to create a new process, it can proceed as follows:
  - Assign a new process identifier and add its entry to the primary process table.
  - Allocate space for the process (program+data) and user stack. The amount of space required can set to default values depending on the process type. If a user process spawns a new process, the parent process can pass these values to the OS.
  - Create process control block.
  - Set appropriate linkage to a queue (ready) is set.
  - Create other necessary data structures (e.g. to store accounting information).
• Parent process creates children processes, which, in turn create other processes, forming a tree of processes.
• Resource sharing possibilities
  - Parent and children share all resources.
  - Children share subset of parent’s resources.
  - Parent and child share no resources.
• Execution possibilities
  - Parent and children execute concurrently.
  - Parent waits until children terminate.
  
• Address space possibilities
  - Child duplicate of parent.
  - Child has a program loaded into it.

B. PROCESS TERMINATION

• A process terminates when it executes last statement and asks the operating system to delete it by using exit system call. At that time, the child process
  - Output data from child to parent (via wait).
  - Process’ resources are deallocated by operating system.
• Parent may terminate execution of children processes via appropriate system called (e.g. abort). A parent may terminate the execution of one of its children for the following reasons:
  - Child has exceeded allocated resources.
  - Task assigned to child is no longer required.
  - Parent is exiting.
• Operating system does not allow child to continue if its parent terminates.
  - Cascading termination.