Date: Sat, 27 Oct 2001 06:39:05 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: Bill Fenner <fenner@research.att.com> Cc: <rwatson@FreeBSD.ORG>, <bright@mu.org>, <arch@FreeBSD.ORG> Subject: Re: Behavior of select() on pipes Message-ID: <20011027061249.K89650-100000@delplex.bde.org> In-Reply-To: <200110252343.QAA28072@windsor.research.att.com>
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On Thu, 25 Oct 2001, Bill Fenner wrote:
> IEEE Std 1003.1-200x says, about select:
>
> A descriptor shall be considered ready for reading when a call
> to an input function with O_NONBLOCK clear would not block,
> whether or not the function would transfer data successfully.
Good. I couldn't find this section when I checked this recently
after further followup to the PR. I still can't find a section
that gives as much detail for poll(). A poll on POLLIN succeeds when
"Data other than high priority data can be read without blocking".
> and about read:
>
> 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.
>
> This combination says to me that select() should consider a FIFO with
shall :-)
> no writers to be readable. This makes sense when it's been open and
> the writer closes it, as you need to read the EOF, but doesn't make
> as much sense when it hasn't been opened for write yet.
>
> Perhaps we can carefully interpret "an empty .. FIFO" to exclude the
> time before the first writer opens it. Maybe during that time it's not
> empty, it's untenanted.
Ugh. It makes sense for the state not to depend on previous activity.
It reduces the effect of races.
Bruce
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