Messages in this thread | | | Date | Sat, 18 Jun 2011 11:51:38 -0700 | Subject | Re: Does Linux select() violate POSIX? | From | Nemo Publius <> |
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On Sat, Jun 18, 2011 at 11:33 AM, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
>> This is simply not true for any POSIX-compliant operating system. >> Which in this case happens to include every Unix ever written since >> the beginning of time, apart from Linux. > > Actually no - there are lots of device cases where instantaneously it is > true that a read would not block but the condition then changes again.
Well, not to be contentious, but... Can you identify any Unix other than Linux where this is allowed to happen? I am pretty sure BSD (for example) takes pains to avoid it.
> An obvious simple example beyond that is a socket with two readers.
With any "test something, then assume result of test" sequence, obviously I can have race conditions with multiple processes or threads. I mean, had I asked, "I call write() and then lseek() to where I started and then read() on a file; am I guaranteed to read back what I wrote?" And you said no, because some other process could write in the meantime... I would say that is technically true but not at all what I was asking.
This is the same thing. Of course I am talking about select() followed by recv() without any intervening user-space operations on the descriptor.
> Linux follows Posix generally, but nobody writes portable code that does > blocking reads on a poll/select interface because there are a bazillion > ways it can then block - events read by other tasks, discards due to > memory exhaustion, events that are cleared the other end, etc.
Only if I wrote my program that way... Or if I am running on Linux.
> It's a design decision and a huge performance win. It's one of the areas > where POSIX read in its strictest form cripples your performance.
A fair answer. :-)
So in short, Linux deliberately chooses non-compliance here because (a) it is a "huge performance win" and (b) there is an easy work-around that (c) you usually want to be doing anyway. That answers my question.
Thank you for taking the time to reply, Alan. I was hoping for an "authoritative" response, and yours certainly qualifies.
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