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SubjectRe: signalfd and thread semantics
On Tue, 17 Jul 2007, Michael Kerrisk wrote:

> Hi Davide,
> Working on the signalfd man page, another question comes up:
> What are the intended semantics for a signalfd file descriptor with respect
> to threads? I have not yet tested the behavior, but in any case, I better
> check what is expected.
> A signal can be directed to the process as a whole (e.g., using kill(2)),
> or to a particular thread (using, e.g., pthread_kill(2), or tgkill(2)).
> So that raises the question: If a thread calls signalfd(), does the
> resulting file descriptor return just those signals directed to [the thread
> and the process as a whole], or will it also receive signals that are
> targeted at other threads in the process? I would hope the former is the
> case, but I'm not sure what has been implemented (or intended).

If thread A calls signalfd(), a read() from the signalfd will return
thread A private (tgkill) signals (only when called by thread A) and
thread A shared (kill) signals (readable from any thread).
So a call to signalfd() virtually attaches the fd to the calling thread
signal context.
This is the reason of the "virtual connection" dropped I was talking about
in the other email. If the signal context the fd is attached to
(struct sighand), goes away, the fd becomes like a disconnected socket
with no peer to read form.

- Davide

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