Process Creation

• There are four major system calls associated with creation of new processes
  • fork() - create a nearly identical copy of a process
  • exec - replace the memory (including code) of a process.
  • exit() - terminate a process
  • wait() - receive notification that a child process has terminated.
  • 
• fork

  •        #include <unistd.h>
           pid_t fork(void);
    	

  • Create a duplicate of the process except
    • Return value: 0 for child, pid of child for parent.
    • The Child has a unique pid.
    • The ppid of the child is the pid of the parent.
    • Some topics we have yet to discuss are different
      • Charged resources are at 0
      • Pending signals are cleared
      • Locks are not inherited.
      • Timers are not inherited
      • Some advanced I/O information is not shared.
  • So inherited attributes include
    • File descriptors
    • The entire virtual address space (all memory)
    • Open Directory streams.
  • Run forker
  • Change the sleep and run forker again.
• wait

  •        #include <sys/types.h>
           #include <sys/wait.h>
    
           pid_t wait(int *status);
    	

  • Suspend operation until a child process changes state.
  • This will be exit for a while for us.
  • It will not block if there are unhandled events for which wait would block.
  • The return value is the pid of the proces which exits.
  • The status parameter contains information, we will look at this later.
• Memory
  • For increased speed, linux (and most uinx versions) use a copy on write scheme for forked processes.
    • When a process is forked, the parent and child both share the same memory.
    • If one or the other writes before an exit, the memory is copied.
    • See myMemTest.C
  • At one point, before copy on write, vfork was implemented.
    • The author says to avoid use of vfork.