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This rule is a special case of SIG31-C. Do not access or modify 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 makes a special exception for errno in this case, saying the only thing that is allowed to go wrong is that errno takes on an indeterminate value. This makes it possible to call signal() from within a signal handler without risking completely unrestricted 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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