22.7. Implementation and Portability of signal()

In this section, we show how to implement signal() using sigaction(). The implementation is straightforward, but needs to account for the fact that, historically and across different UNIX implementations, signal() has had different semantics. In particular, early implementations of signals were unreliable, meaning that:

• On entry to a signal handler, the disposition of the signal was reset to its default. (This corresponds to the SA_RESETHAND flag described in Section 20.13.) In order to have the signal handler invoked again for a subsequent delivery of the same signal, the programmer needed to make a call to signal() from within the handler to explicitly reestablish the handler. The problem in this scenario is that there is a small window of time between entering the signal handler and reestablishment of the handler, during which, if the signal arrives a second time, it would be processed according to its default disposition.

• Delivery of further occurrences of a signal was not blocked during execution of a signal handler. (This corresponds to the SA_NODEFER flag described in Section 20.13.) This meant that if the signal was delivered again while the handler was still executing, then the handler would be recursively invoked. Given a sufficiently rapid stream of signals, the resulting recursive invocations of the handler could overflow the stack.