If a call to the signal() function occurs other than as the result of calling the abort() or raise() function, and the XXX results in a SIG_ERR return, the value of errno is indeterminate.
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.
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