get pending signals
sigpending() function returns a mask of the signals
pending for delivery to the calling process in the location indicated by
set. Signals may be pending because they are currently
masked, or transiently before delivery (although the latter case is not
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable
errno is set to indicate the error.
sigpending() function does not currently detect any
sigpending() function is defined by
IEEE Std 1003.1-2008 (“POSIX.1”).