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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.