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sigchain API
============
 
Code often wants to set a signal handler to clean up temporary files or
other work-in-progress when we die unexpectedly. For multiple pieces of
code to do this without conflicting, each piece of code must remember
the old value of the handler and restore it either when:
 
  1. The work-in-progress is finished, and the handler is no longer
     necessary. The handler should revert to the original behavior
     (either another handler, SIG_DFL, or SIG_IGN).
 
  2. The signal is received. We should then do our cleanup, then chain
     to the next handler (or die if it is SIG_DFL).
 
Sigchain is a tiny library for keeping a stack of handlers. Your handler
and installation code should look something like:
 
------------------------------------------
  void clean_foo_on_signal(int sig)
  {
	  clean_foo();
	  sigchain_pop(sig);
	  raise(sig);
  }
 
  void other_func()
  {
	  sigchain_push_common(clean_foo_on_signal);
	  mess_up_foo();
	  clean_foo();
  }
------------------------------------------
 
Handlers are given the typdef of sigchain_fun. This is the same type
that is given to signal() or sigaction(). It is perfectly reasonable to
push SIG_DFL or SIG_IGN onto the stack.
 
You can sigchain_push and sigchain_pop individual signals. For
convenience, sigchain_push_common will push the handler onto the stack
for many common signals.