Do UNIX signals produce interrupts? Post: 302942825

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Top Forums UNIX for Dummies Questions & Answers Do UNIX signals produce interrupts? Post 302942825 by Tru69 on Friday 1st of May 2015 02:48:22 PM

05-01-2015

Registered User

Do UNIX signals produce interrupts?

Hi folks!

I have been reading Vahalia's Unix Internals book, which states the following in the chapter dedicated to signals:

Quote:

The receiving process becomes aware of the signal when the kernel calls the issig() function [...] The kernel calls issig() only at the following times:

Before returning to user mode from a syscall/interrupt
Just before blocking on an interruptible event
Immediatly after waking from an interruptible event

Given that, my understanding is that processes running in user mode don't become aware of signals until they switch to kernel mode, where the issig() function is called at some points.

However, he later gives an example where a user would type ctrl+c at a terminal, sending a SIGINT signal to the foreground process. If it happens that this process is running on another CPU, a special "cross-processor" interrupt is needed so that the target sees the signal. But why is that? Do signals actually produce interrupts (traps) to the signaled processes so they immediately see them? Otherwise, wouldn't it be enough to just set a flag in this process' structure, which would be then checked in one of that three situations described before?

Thanks in advance!

Tru69

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sigpending

sigpending(2)							System Calls Manual						     sigpending(2)

NAME

       sigpending - examine pending signals

SYNOPSIS

DESCRIPTION

       stores the set of signals that are blocked from delivery and are pending to the calling thread, at the location pointed to by set.

RETURN VALUE

       Upon successful completion, returns 0. Otherwise -1 is returned and is set to indicate the error.

ERRORS

       fails if the following condition is encountered:

       set	      points to an invalid address.  The reliable detection of this error is implementation-dependent.

APPLICATION USAGE

   Threads Considerations
       The  set  of  signals  returned	by  is the union of the signals pending on the process and calling thread.  A signal may be pending on the
       process if all threads block the signal.

       The set of signals returned by is only advisory.  Since other threads may be executing at the time of the call, a  signal  pending  on  the
       process may be delivered to a thread after this system call returns.

       For more information regarding signals and threads, refer to signal(5).

   LWP (Lightweight Processes) Considerations
       The set of signals returned by is the union of the signals pending on the process and calling LWP.

AUTHOR

       was derived from the IEEE POSIX 1003.1-1988 Standard.

SEE ALSO

       sigaction(2), sigsuspend(2), sigprocmask(2), sigsetops(3C), signal(5).

CHANGE HISTORY

       First release in Issue 3.

STANDARDS CONFORMANCE

       : AES, SVID3, XPG3, XPG4, FIPS 151-2, POSIX.1

																     sigpending(2)