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A signal handler is allowed to call signal(), and if that fails, signal() returns SIG_ERR and sets errno to a positive value. However, if the event that caused a signal was external (not the result of the program calling abort() or raise()), the only functions the signal handler may call are _Exit() or abort(), or it may call signal() on the signal currently being handled, and if signal() fails, the value of errno is indeterminate.
This rule is an instance of ERR33-C. Detect and handle standard library errors.
This rule is also a special case of SIG31-C. Do not access shared objects in signal handlers. The object designated by errno is of static storage duration and is not a volatile sig_atomic_t. As a result, performing any action that would require errno to be set would normally cause undefined behavior. The C Standard in subclause 7.14.1.1 paragraph 5 makes a special exception for errno in this case, saying the only thing that is allowed to go wrong is that errno can take on an indeterminate value. This special exception makes it possible to call signal() from within a signal handler without risking undefined behavior, but the handler, and any code executed after the handler returns, must not depend on the value of errno being meaningful.
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