How can SIGINT be safely delivered on the main thread?

Raymond Chen

Raymond


Commenter

AnotherMatt
wonders why Win32 console programs deliver console
notifications on a different thread.
Why doesn’t it deliver them on the main thread?



Actually, my question is the reverse.
Why does unix deliver it on the main thread?
It makes it

nearly impossible to do anything of consequence
inside the signal handler
.
The main thread might be inside the heap manager (holding the heap
critical section) when the signal is raised.
If the signal handler tried to access the heap, it would deadlock
with itself if you’re lucky, or just corrupt the heap if you aren’t.



For example, consider this signal handler:



void catch_int(int sig_num)
{
/* re-set the signal handler again to catch_int, for next time */
signal(SIGINT, catch_int);
/* and print the message */
printf(“Don’t do that”);
fflush(stdout);
}


What happens if the signal is raised while the main program is
executing its own fflush, say after it had already
flushed half the buffer?
If two threads called fflush, the second caller would
wait for the first to complete.
But here, it’s all coming from within the same thread;
the second caller can’t wait for the first caller to return,
since the first caller can’t run until the second caller returns!



(Note also that this signal handler potentially modifies errno,
which can lead to “impossible” bugs in the main program.)



Win32 doesn’t believe in interrupt user-mode code with other
user-mode code asynchronously because it makes it impossible to reason
about the state of the process.
Delivering the console notification on a second
thread means that if the second thread tries to access the heap
while the first thread is inside the heap manager,
the second thread will dutifully wait for the heap to stabilize
before it goes ahead and starts mucking with it.

Raymond Chen
Raymond Chen

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