Question

I’m trying to figure out how to block a signal in Linux kernel 2.4 (user space) from invoking its handler, but keep it available to be handled later, preferably as soon as I re activate the handling of said signal.

The function sigprocmask seem to come up in all my search results, but I can’t find a good, clear description that explains whether the blocked signal gets “saved” to be handled later, and if so where and how do I handle it when I’m ready for it.

Can someone please clarify what’s going on, preferably with a code example?
Thanks in advance.

Answer 1

I really can’t say it better than the signal(7) man page:

A signal may be blocked, which
means that it will not be delivered
until it is later unblocked. Between
the time when it is generated and when
it is delivered a signal is said to be
pending.

Each thread in a process has an
independent signal mask, which
indicates the set of signals that the
thread is currently blocking. A
thread can manipulate its signal mask
using pthread_sigmask(3). In a
traditional single-threaded
application, sigprocmask(2) can be
used to manipulate the signal mask.

So, you can block and unblock the signal with sigprocmask(). If the signal is raised while it’s blocked, the handler won’t be called until it’s unblocked. If a signal is pending when it’s unblocked, the handler for the signal will be called as usual.

Note that a given signal is either pending or not; it can’t be “pending twice” (or more). if the signal is raised twice while it is blocked, it’ll still only be delivered once.