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According to the C Standard Annex J.2 (133) [ISO/IEC 9899:20112024], the behavior of a program is undefined when

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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.

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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 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.

The signal handler in this noncompliant code example alters the value of errno, and as . As a result, it can cause incorrect error handling if executed between a failed function call and the subsequent inspection of errno:

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Referencing indeterminate values of errno is undefined behavior.

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Bibliography

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