Re: FIFO issues

[Ken Brown <kbrown@cornell.edu>]

On 9/19/2022 6:05 PM, Enrico Forestieri wrote:
> Ken Brown wrote:
> 
>  > I did an internet search on this issue and found the following, which 
> describes the
> 
>  > situation we're discussing:
> 
>> https://stackoverflow.com/questions/14594508/fifo-pipe-is-always-readable-in-select 
> 
>  > According to that post, select on Linux will wait for a writer the first time 
> it's
> 
>  > called to check read readiness for a FIFO opened for reading with O_NONBLOCK 
> set.
> 
>  > But if the writer then closes the FIFO, subsequent calls to select will 
> always find
> 
>  > the FIFO read ready (and read will return 0). This behavior is not 
> documented, as far as
> 
>  > I can tell, and in fact it contradicts the existing documentation (both POSIX 
> and Linux).
> 
>  > So I don't think someone trying to write a portable program should rely on it.
> 
> 
> Please, note that this code was working on cygwin the way it works on linux 
> until some
> 
> time ago, maybe last year, I am not sure. I also found this stackoverflow 
> discussion:
> 
> https://stackoverflow.com/questions/28851639/select-with-non-blocking-reads
> 
> I tried the code also on Solaris and NetBSD and it works exactly as on linux, so 
> I think
> 
> it is portable.

Then I guess I'm wrong.  I'm really puzzled, because it seems that none of these 
platforms agree with POSIX, which says the following in its 'read' documentation:

     When attempting to read from an empty pipe or FIFO:

         If no process has the pipe open for writing, read() shall return 0 to
         indicate end-of-file.

It seems that there's an exception: If no process has ever had the FIFO open for 
writing since it was opened for reading, then the FIFO is not considered to be 
at end-of-file.

I'll look into fixing this.  But I'd be more confident about it if I could find 
some documentation of the existing behavior.

Ken