When a signal handler is invoked, you usually want it to be able to
finish without being interrupted by another signal. From the moment the
handler starts until the moment it finishes, you must block signals that
might confuse it or corrupt its data.
When a handler function is invoked on a signal, that signal is
automatically blocked (in addition to any other signals that are already
in the process's signal mask) during the time the handler is running.
If you set up a handler for SIGTSTP, for instance, then the
arrival of that signal forces further SIGTSTP signals to wait
during the execution of the handler.
However, by default, other kinds of signals are not blocked; they can
arrive during handler execution.
The reliable way to block other kinds of signals during the execution of
the handler is to use the sa_mask member of the sigaction
This is more reliable than blocking the other signals explicitly in the
code for the handler. If you block signals explicitly in the handler,
you can't avoid at least a short interval at the beginning of the
handler where they are not yet blocked.
You cannot remove signals from the process's current mask using this
mechanism. However, you can make calls to sigprocmask within
your handler to block or unblock signals as you wish.
In any case, when the handler returns, the system restores the mask that
was in place before the handler was entered. If any signals that become
unblocked by this restoration are pending, the process will receive
those signals immediately, before returning to the code that was
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