Start of Lecture on January 17, Chapter 3: Processes

Transcription

1 Start of Lecture on January 17,

2 Reminders No reminders What are your questions or comments? 2

3 Example: Creating with fork() Run fork1.c and fork2.c parent resumes here if ( fork() ) { } else { exec( ) Run dumbshell.c Each call to fork() adds a new process control block to the process table (which is a list of PCBs) if ( fork() ) { } else { exec( ) } before fork() child resumes here if ( fork() ) { } else { exec( ) } } after fork() child resumes here after exec() 3

4 Review of fork() with a simple example #include <stdio.h> #include <sys/types.h> #include <unistd.h> int main(void) { printf("hello \n"); fork(); printf("bye\n"); return 0; } How many people think Hello is printed out once? More than once? How many people think bye is printed out once? More than once? What happens if exec() is not called? 4

5 Terminating processes: exit() and wait() Process executes last statement and then asks the operating system to delete it using the exit() system call Returns status data from child to parent (via wait() system call) The resources of the process are deallocated by the OS Parent may terminate execution of children processes using abort() system call, for reasons such as: Child has exceeded allocated resources Task assigned to child no longer required Parent is exiting and OS does not allow child to continue if parent terminates 5

6 Example: terminating child processes Run zombie.c If child process terminates before the parent has called wait(&status), it hangs around as a zombie If parent terminates, with children still not terminated, and does not call wait(&status), then children are orphans. Why do processes hang around as zombies before wait called by the parent process? What do you think happens with orphaned processes? 6

7 Unusual termination Zombies are not really a problem as they usually exist briefly, but could indicate a bug what might be a negative effect of too many zombies being created? If a process terminates, its children must also be terminated: called cascading termination 7

8 Summary of UNIX system initialization and bootstrap process 0 swapper shutdown wait until init exits system shutdown fork process 1 init wait until all children exit fork. exit exec exec getty login SHELL fork user commands as many as available terminals exit user environment 8

9 Video Break: as brought to you by another stupendous classmate 9

10 Interprocess Communication (IPC) 10

11 Why do we need inter-process communication? So that processes can cooperate! Information sharing: several users may require concurrent access to piece of information (e.g. file) Computational speedup: can break task into subtasks which execute in parallel Modularity: construct system in modular fashion, to promote writing, understanding and testing the system Privilege separation: separate processes can decide what to communicate with minimal risk 11

12 Examples of usefulness of IPC Chrome browser uses multiprocess architecture; these processes need to cooperate Three types of processes: browser process manage interface, disk/network I/O Renderer contain logic to render web page plug-in process e.g. Flash or Quicktime This architecture is modular (good code design) with safer separation particularly from crashes and security exploits in individual tabs What are some other examples of real-world scenarios where processes need to communicate? 12

13 Two basic models for IPC shared memory (communicated) data are directly available to each process in their address space e.g UNIX pipes (a special type of file): ps -el grep myprogram message passing data are explicitly exchanged e.g. UNIX sockets signals are a rudimentary form of IPC; atypical because their is no operation to receive a signal, but can declare a function to handle it 13

14 Shared memory versus Message Passing shared memory process A write A A read A (e.g., UNIX pipes) process B A message passing send A receive A (e.g., UNIX sockets) process A process B 14