[Haskell-cafe] Re: Hugsvs GHC (again)was: Re: Somerandomnewbiequestions

Glynn Clements glynn at gclements.plus.com
Mon Jan 24 14:57:03 EST 2005

David Roundy wrote:

> > >If you're reading from a random-access file, there's no way it can 
> > >tell you when the file data is buffered, because it doesn't know which 
> > >part of the file you plan to read. The OS may try to guess for 
> > >readahead purposes, but select()'s behavior can't depend on that guess.
> >
> > But surely it does! read only reads the next block... to skip randomly
> > you must seek... therefore the following sequence does this:
> > 
> >       seek
> >       select
> >       read
> > 
> > The select should block until one disk block from the file is in memory,
> > read is defined such that it will return if some data is ready even if it
> > is not as much as you requested.  So in this case if you ask for a
> > complete file, you may just get one block... or more.
> >
> > In other words the API restricts reads to the 'next' block - so seek
> > knows which block needs to be read into memory...
> Wouldn't select always fail, since the block would never be read into
> memory until you call read?

True, unless calling select() automatically triggered read-ahead
(which isn't an unreasonable idea).

In that regard, select/poll is different from non-blocking and
asynchronous I/O. With the latter, you're explicitly asking for data
to be read.

Glynn Clements <glynn at gclements.plus.com>

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