The sigprocmask function is used to examine or change the signal
    mask of the calling process.

    Typically, use the sigprocmask SIG_BLOCK value to block signals
    during a critical section of code, then use the sigprocmask SIG_
    SETMASK value to restore the mask to the previous value returned
    by the sigprocmask SIG_BLOCK value.

    If there are any unblocked signals pending after the call to the
    sigprocmask function, at least one of those signals is delivered
    before the sigprocmask function returns.

    You cannot block SIGKILL or SIGSTOP signals with the sigprocmask
    function. If a program attempts to block one of these signals,
    the sigprocmask function gives no indication of the error.