Re: Memory leak in select

[Peter Rosin <peda@lysator.liu.se>]

Den 2011-04-18 17:24 skrev Christopher Faylor:
> On Mon, Apr 18, 2011 at 04:32:10PM +0200, Peter Rosin wrote:
>> Den 2011-04-18 14:23 skrev Peter Rosin:
>>> Den 2011-04-18 13:43 skrev Peter Rosin:
>>>> Hi!
>>>>
>>>> Using the following STC, I'm seeing what appears to be a memory
>>>> leak in select(2).
>>>>
>>> ----------------8<---(selectleak.c)---------
>>> #include <sys/time.h>
>>> #include <fcntl.h>
>>>
>>> int
>>> main(void)
>>> {
>>> 	fd_set fdset;
>>>
>>> 	long flags = fcntl(0, F_GETFL);
>>> 	fcntl(0, F_SETFL, flags | O_NONBLOCK);
>>>
>>> 	for (;;) {
>>> 		int res;
>>> 		char buf[20];
>>>
>>> 		FD_ZERO(&fdset);
>>> 		FD_SET(0, &fdset);
>>> 		res = select(1, &fdset, NULL, NULL, NULL);
>>> 		if (!res)
>>> 			continue;
>>> 		if (res < 0)
>>> 			return 1;
>>> 		res = read(0, buf, sizeof(buf));
>>> 		if (!res)
>>> 			break;
>>> 		if (res < 0)
>>> 			return 1;
>>> 	}
>>>
>>> 	return 0;
>>> }
>>> ----------------8<--------------------------
>>
>> Ok, I'm taking a wild swing at this, and my guess is that the call
>> sel.cleanup () in cygwin_select prematurely zeros out the cleanup
>> member of the select_record. The call to sel.poll () then adds
>> "stuff" to the select_record that really should have been cleaned
>> up, but isn't since cleanup has already been executed and then
>> zapped (by select_stuff::cleanup).
>>
>> But what do I know?
> 
> How does sel.poll add "stuff" that should be cleaned up?  That function
> only looks for bits to set.

I added syscall to the strace mask and that places the leak in read(2)
instead. But as stated previously, just look at the symptoms instead and
ignore my finger-pointing.

(I was fooled by the fact that the leak was after the sel.cleanup call,
but my limited experience with strace made me miss the fact that read(2)
was entered before the leak. So, I mistakedly placed the leak in the only
part left in cygwin_select, namely sel.poll)

Cheers,
Peter

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