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c programming file i/o select blocking i/o

© December 2004 (various authors)

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From: grante@visi.com (Grant Edwards)
Newsgroups: comp.os.linux.misc
Subject: Re: I/O in application programs.
References: <39EB25B2.53748B60@yahoo.com> 
Message-ID: <V7KG5.1515$FU3.341015@ptah.visi.com> 
Date: Mon, 16 Oct 2000 21:09:09 GMT

In article <39EB25B2.53748B60@yahoo.com>, Fred wrote:

>In section 12.5 of his book, Stevens treats the topic of "I/O
>Multiplexing." Therein Stevens carefully cautions his reader
>that "I/O multiplexing is not yet part of POSIX." In other
>words, reference to the POSIX specification will not answer my
>questions.  I must determine how things work in the real world.
>And because I wish to maximize the portability of my program, I
>must not limit my inquiry to the workings of my specific Unix
>platform.  Rather, I must try to learn how most Unix platforms
>do this kind of work.

I don't think that Unix applications typically try to overlap

[ regarding select ]

>the descriptors that are ready."  From his remarks, I gather
>that the select function will (optionally) suspend execution
>of the application program until such moment as there is a
>change in one or more of these bits.


>So far, so good.  I say this because, although the above-quoted
>passages may appear cryptic when lifted out of context (as I
>have done), they are clear enough when read in combination with
>other things Stevens says about I/O multiplexing.  The problem
>comes a couple of paragraphs later, where he says, "We now need
>to be more specific about what 'ready' means." Stevens then
>proceeds to define "ready" so that the word has no genuine
>meaning.  He says, for instance, that "A descriptor in the
>write set is considered ready if a write to that descriptor
>won't block."

If you're taling about regular file I/O, files are _always_
ready in Unix.  Using select on a file-descriptor that is
associated with a regular file is not a useful thing to do.

File-descriptors associated with things like TCP connections
and serial ports may be not-ready, and select is useful for

>In passing, I wonder how the kernel can know for sure that a
>future I/O operation won't block, without knowing the details
>of the operation.

It can't.  If you try to write a block of data larger than can
be buffered, and you've got non-blocking I/O enabled, then the
driver will take as much as it can.  You have to look at the
return value of write() to see how much data was actually
"consumed" by the driver.  If you try to read more data than
available from a non-blocing descriptor then it will give you
however much it has available.  You've got to check the return
value from read() to see how many bytes you received.

NB: A file descriptor is "ready" if there is at least 1 byte of
    data to be read, or room for at least 1 byte of data.  (Or
    there's an error, or it's close, or ....).

>No doubt it is nice to know that the next call to write
>will not block, but that's not the same as knowing that the
>previous call to write is finished.

How do you define "finished?" Generally, you just leave the
file-descriptors in blocking mode and don't worry about it --
read() won't return until it's read as much data as you
requested, and write() won't return until it's written all of
the data you gave it.

>Similarly it is nice to know that the next call to read will
>not block, but that's not the same as knowing that the previous
>call to read is finished, and much less is it the same as
>knowing that the read did not encounter an EOF condition.  In
>other words, the select function offers a prediction about the
>future, but that is not what I need at steps 3, 8, and 12. What
>I need is not a prediction about the future, but rather a
>statement about the present.  For example, when my program
>arrives at step 3, either the input operation is finished, or
>it's not. If it's finished, I can safely copy data from the
>input buffer to the computational buffer.  If it's not
>finished, I need to go to sleep until such time as it is
>finished. Apparently, neither select nor poll meets this need.

Unless you're writing a multi-threaded program, the easiest
thing to do is leave the descriptors in non-blocking mode
read(), compute(), write().  read() and write() both return the
number of bytes read/written.  If you really want to overlap
the read/compute/write operations, you're going to have to have
multiple threads.  In that case, use blocking read/write calls
in the I/O threads and and semaphores for inter-thread

I'd recommend that you become proficient at single-threaded
Unix application programming before you try to do a
multiple-threaded implimentation.

Grant Edwards                   grante             Yow!  I'm using my X-RAY
                                  at               VISION to obtain a rare
                               visi.com            glimpse of the INNER
                                                   WORKINGS of this POTATO!!

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