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The C Standard, 7.14.1.1, paragraph 5 [ISO/IEC 9899:20112024], states that if the signal occurs other than as the result of calling the abort() or raise() function, the behavior is undefined if if

If the signal occurs other than as the result of calling the abort or raise function, the behavior is undefined if the signal handler refers to any object with static or thread storage duration that is not a lock-free atomic object and that is not declared with the constexpr storage-class specifier other than by assigning a value to an object declared as volatile sig_atomic_t, or ... the signal handler calls any function in the standard library other than the 

—  the abort function,

the 

—  the _Exit function,

the quick_exit function, or the 

—  the quick_exit function,

—  the functions in <stdatomic.h> (except where explicitly stated otherwise) when the atomic arguments are lock-free,

—  the atomic_is_lock_free function with any atomic argument, or

—  the signal function with the first argument equal to the signal number corresponding to the signal that caused the invocation of the handler. Furthermore, if such a call to the signal function results in a SIG_ERR return, the object designated by errno has an indeterminate representation.294)

Implementations may define a list of additional asynchronous-safe functions. These functions can also be called within a signal handler. This restriction applies to library functions as well as application-defined functions.

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