Blocking Signals
================
Blocking a signal means telling the operating system to hold it and
deliver it later. Generally, a program does not block signals
indefinitely--it might as well ignore them by setting their actions to
`SIG_IGN'. But it is useful to block signals briefly, to prevent them
from interrupting sensitive operations. For instance:
* You can use the `sigprocmask' function to block signals while you
modify global variables that are also modified by the handlers for
these signals.
* You can set `sa_mask' in your `sigaction' call to block certain
signals while a particular signal handler runs. This way, the
signal handler can run without being interrupted itself by signals.