Attributes

• Table 4.1 attempts to summarize how process attributes are shared, copied, replaced, or separate for the four major ways to change a process.
• Instead of actually copying memory, a feature known as "copy-on-write" is frequently used in modern OSes like Linux.
• The mappings between virtual and physical memory are duplicated for the new process, but the new mappings are marked as read-only. When the process tries to write to these memory blocks, the exception handler allocates a new block of memory, copies the data to the new block, changes the mapping to point to the new block with write access, and then resumes the execution of the program.
• This feature reduces the overhead of forking a new process.
  

  Table 4.1: Process Attribute Inheritance

  Attribute	fork	thread	clone	execve
  Virtual Memory
  Code Segment	copy	shared	CLONE_VM	replaced
  Const Data Segment	don't care	shared	CLONE_VM	replaced
  Variable Data Segment	copy	shared	CLONE_VM	replaced
  stack	copy	separate	CLONE_VM	replaced
  mmap	copy	shared	CLONE_VM	replaced
  brk	copy	shared	CLONE_VM	replaced
  command line	copy	shared	CLONE_VM	replaced
  environment	copy	shared	CLONE_VM	replaced
  Files
  chroot, chdir, umask	copy	shared	CLONE_FS	copy
  File descriptor table	copy	shared	CLONE_FILES	copy
  file locks	separate	separate	CLONE_PID	same
  Signals
  Signal Handlers	copy	shared	CLONE_SIGHAND	reset
  Pending Signals	separate	separate	separate	reset
  Signal masks	separate	separate	separate	reset
  Process Id	different	different	CLONE_PID	same
  timeslice	separate	shared	CLONE_PID	same