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According to [ISO/IEC 9899:2011], the behavior a program is undefined 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.

(See undefined behavior 133 of Annex J.)

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 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 can take on an indeterminate value. This special exception 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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The call to perror() from handler() also violates SIG30-C. Call only asynchronous-safe functions within signal handlers.

Compliant Solution

The compliant solution does not reference errno and does not return from the signal handler if the signal() call fails.

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POSIX is less restrictive than C about what applications can do in signal handlers. It has a long list of asynchronous-safe functions that can be called. (See SIG30-C. Call only asynchronous-safe functions within signal handlers.) Many of these functions set errno on error, which can lead to a signal handler being executed between a call to a failed function and the subsequent inspection of errno. Consequently, the value inspected is not the one set by that function but the one set by a function call in the signal handler. POSIX applications can avoid this problem by ensuring that signal handlers containing code that might alter errno always save the value of errno on entry and restore it before returning.

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