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 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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