Processes: Fork

• The general pattern to starting a new process is

• fork()
  
  if new process
     exec new code
     exit
  else
     wait for new process to exit
     exit
  

• A Word of Caution
  • You can kill a system with the thing we are about to do.
  • Be careful when writing code.
  • Do not put a call to fork in a loop yet.
  • Run your code on one of the lab machines (NOT cslab100 and cslab101)
  • Let Austin and I know if you do something bad (fork bomb) so we can fix it.
• This is spread across several chapters.
• We will start with chapter 24. You should read it.
• Fork
  • needs unistd.h
  • pid_t fork(void);
  • return
    • -1 on error
    • 0 to child on success
    • child_pid to parent on success.
  • The child and parent return at the same place except for different return values.
  • The child has a new process id.
  • The child gets (eventually) a new copy of all data and files.
  • There are slight changes in the new process
    • We will discuss these as they come up.
  • But for the most part they are the same process.
• it is unknown which process (parent or child) will execute first.
• Notice in this program both processes
  • Get copies of the variables, but they are not shared.
  • share output streams (cout and the open file)
    • Far more on this in the future.
  • On linux systems, the parent seems to go first.
• I/O is mixed, so first come first served
  • I resist input
  • I label output.
• Some things are problematic

  • srand(time(nullptr))
    	  

• Memory
  • In theory, all process memory is copied on fork.
  • This is not quite true.
    • The linux kernel does a copy on write
    • The processes share memory until one of them tries to write.
    • At this point the memory is copied and each get a unique copy of the memory.
    • For a detailed description see the chapter.
• Some Examples:
  • Look at fork.C
  • Look at zombie.C
  • Look at orphan.C
• There is a vfork but
  • It should only be followed by exec
• So what happens here

  • for(i=0;i<5;i++) {
       pid = fork();
    }
    	 

  • See forkBomb.C