The siglongjmp function restores the environment saved by the
    most recent call to sigsetjmp in the same process with the
    corresponding sigjmp_buf argument.

    All accessible objects have values when siglongjmp is called,
    with one exception: values of objects of automatic storage
    duration that changed between the sigsetjmp call and siglongjmp
    call are indeterminate.

    Because it bypasses the usual function call and return
    mechanisms, siglongjmp executes correctly during interrupts,
    signals, and any of their associated functions. However, if you
    invoke siglongjmp from a nested signal handler (for example,
    from a function invoked as a result of a signal raised during the
    handling of another signal), the behavior is undefined.

    The siglongjmp function restores the saved signal mask only if
    you initialize the env argument by a call to sigsetjmp with a
    nonzero savemask argument.

    After siglongjmp is completed, program execution continues
    as if the corresponding call of sigsetjmp just returned the
    value specified by value. The siglongjmp function cannot cause
    sigsetjmp to return 0 (zero); if value is 0, sigsetjmp returns 1

    See also sigsetjmp.