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Process Model

We can design program that run other program. We should first know:

Program: The executable instructions of a program, it either can be:

  • Source Code
  • Compiled machine code

Process: Running instance of a program

  • Machine code of the program
  • Infor about current state, presented by
    • Code
    • Global
    • Heap
    • Stack
    • Notice: Memory is not shared between processes.
  • operating system decides process order

Process hold by a Process Control Block

  • PID (Process ID): the id of current process
  • PC (Program Counter): the next instruction to be executed
  • SP (Stack Pointer): identifies the top of stack
  • actions

The number processor labeled as CPUs control how many processes can be executing an instruction at the same time.

Processes in different states:

  • Running State: the running processes
  • Ready State: the processes wait CPUs available
  • Blocked State the processes wait an event to occur

Use System Call create Process​

fork: create a child process (duplicate a process)

  • same code with parent process; same PC, SP
  • variable value may difference with using parent/child process (like PID)
  • inherit most variable values, even loops
  • different PID
  • different return value (child process return PID or -1 when fails)
    • return 0 in child process (parent fork child, child fork)

execl: the same process to execute different code, will not return back to original code if not fails

Process Ordering​

If a parent process terminates before children processes terminate, then the children's parent will be PID 1 process (init process), that is 'children'(orphan) become 'parent'

If the children process terminate but without parent call, then the termination status will be collected. As the parent process terminates, those process will be deleted.

Conclusion with example​

when we run program on shell

  1. shell is a process, t use fork execute the program we ask as a new process
  2. this child process call execl to load different program
  3. shell call wait and blocks 'till the child process finishes executing
  4. process terminated, shell ready for next command

Some system function​

  • int getpid(): get process id of current process
  • int getppid(): get parent process id of current process
  • usleep: suspend execution for microsecond intervals
  • int wait(): wait for process termination (only for child process)
    • WIFEXITED(int status): true if child process ended normally
      • WEXITSTATUS(int status): get the exit code of child process
    • WIFSIGNALED(int status): True when a signal caused to exit
      • WTERMSIG(int status): Use to determine which signal raised by the child process to terminate
    • WIFSTOPPED(int status): True when child process stopped
      • WSTOPSIG(int status): Determine which signal stopped child process
  • int waitpid(int pid, int *stat, int options):
    • options = WNOHANG return -1 immediately instead waiting if no child process ready to be noticed and set ERRNO to ECHILD (means no child process), else 0exists some zombie child process
  • abort: cause abnormal process termination
  • pipe: create pipe
  • signal: ANSI C signal handling
  • int execl(const char*path, const char*arg0,...): execute a file
  • there are family of execl function, we can determinate what we want to use by the letter:
    • l: list
    • v: vector
    • p: path (no p means empty path or path already given)
    • e: environment
  • int execvp(const char *file, char * argv[]):