Process Creation

COMP124 Lectures 2021-03-06

On most systems, processes are created (spawned) by other processes. The original process is the parent, and the new process is the child.

Hence we end up with a kind of ancestral tree of processes.

Unix

• The exec() system call allows one program to execute another:
  • They stay in same process.
  • New program overwrites caller
• The fork() call spawns a new process:
  • Child is identical to parent.
• The wait() call suspends parent until child terminates.

Execution Scenarios

1. P1 execs P2:

    graph LR
    P1 --> P2 
    P2--> 1[ ]
   

2. P1 forks, then child execs P2:

    graph LR
    P1 --> P2 
    P1 --> 1[ ]
    P2 --> 2[ ]
   

3. The same as before by now P1 issues a wait() call.

    graph LR
    P1 --> P2 
    P1 -.-> 1[ ]
    P2 --> 1[ ]
    1 --> 2[ ]
   

Unix Processes

1. At startup, kernel creates process 0:
   • Kernel-level process, so can access kernel code and data structures directly.
   • Eventually becomes swapper, responsible for moving processes to/from disk.
   • Before this, it spawns other kernel-level processes.
2. Process 1 is init (now systemd on Linux):
   • Parent of all other processes.
   • Spawns daemon processes.
   • Spawns process for user login.
3. Unix processes:
   • When user logs on, the login process checks name and password against stored details, then execs shell.
   • User can alter their preferred choice of shell.
4. Shell prompts for commands, then forks and execs to carry out those commands.
5. At logout, shell process dies:
   • init/systemd is informed, and creates new login process.

Zombies and Orphans

Parent processes usually wait for their children to die. If the death of a child is not acknowledged by the parent (via wait call), the child becomes a zombie:

• No resources, but still present in process table.

If parent dies before its children, the children become orphans

• Get “adopted” by init process.

Daemons

• Not associated with any user or terminal.
• Run permanently in background.
• Perform tasks requested of them by other processes.
  • Act as server process.
• Carry out the background tasks of the OS:
  • Subsystem managers are background processes.
  • They need their own time on the CPU.