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According to \[[ISO/IEC 9899-1999| AA. C References#ISO/IEC 9899-1999]\] (see [undefined behavior 126| CC. Undefined Behavior#ub_126] of Annex J), the behavior a program is [undefined| BB. Definitions#undefined behavior] when

the value of errno is referred to after a signal occurred other than as the result of calling the abort or raise function and the corresponding signal handler obtained a SIG_ERR return from a call to the signal function.

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 call signal() on the signal currently being handled, and if signal() fails, the value of errno is indeterminate.

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 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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