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Re^8: sysread/syswrite wrappers

by vsespb (Chaplain)
on Oct 12, 2016 at 15:41 UTC ( #1173861=note: print w/replies, xml ) Need Help??


in reply to Re^7: sysread/syswrite wrappers
in thread sysread/syswrite wrappers

Yes, they do!
It's not proof, because code which happens to work without race condition does not proof, that there is no race condition.
The *ONLY REASON* print and readline are said to "not work with select", is because they will block if they receive a partial message, thus preventing the code from getting back to the select
You don't give proof for that.
Now my proof is select man page and this: http://perl.plover.com/FAQs/Buffering.html
For efficiency, Perl uses a trick called buffering. The first time you ask to read from the file, Perl has to make a system call. Since system calls are expensive, it plans ahead. If you tried to read a little bit of text, you will probably want to read the rest later. The blocks on your disk are probably about 8K bytes, and your computer hardware is probably designed to transfer an entire block of data from the disk at once. So instead of asking the system for a little bit of text, Perl actually asks the system for an entire blockful, which hardly takes longer to get than a little bit would have. Then it stores this block of data in a region of memory that is called a buffer, and gives you back the one line you asked for. The next time you ask for a line, Perl already has the line you want in memory in the 8K buffer. It doesn't have to make another system call; it just gives you the next line out of the buffer. Eventually you read up to the end of the buffer, and then Perl makes another system call to get another bufferful of data.
You can see this when executing eof() of FH with data and running select then. Select won't return "can read", while there was data in FH. But readline will return this data. That's because readline reads not only from file, but from buffer as well.

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Re^9: sysread/syswrite wrappers
by BrowserUk (Pope) on Oct 12, 2016 at 15:53 UTC
    hat's because readline reads not only from file, but from buffer as well

    It's a difference, but it doesn't mean "they don't work". The effect of the pipe buffering -- and file system caching -- means you get the same effects with sysread.

    Even sockets have buffers in the tcp stack.


    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority". I knew I was on the right track :)
    In the absence of evidence, opinion is indistinguishable from prejudice.
      No, it means "they don't work", because if you have some data buffered, select() will return you that no data available for read. So you have data to read, but select tells that you don't. It means they don't work at all.
      File system caching is different, it's cached on OS level, so select will know about that and return "you can read".
        So you have data to read, but select tells that you don't.

        Hm. This is a simple server, using non-block sockets, select and readline.

        It reads one line every 1/10th second ensuring the client will have finished and closed its end of the socket long before the server has read the first buffer load:

        And here a simple client that connects, writes 1 full 4k buffer load, then 2k and a distinct last line.

        If the select was returning eof before the buffer was emptied, you wouldn't see the distinct last line; but I always do!:

        Perhaps you can tell me what I'm doing 'wrong' to make it work? Or maybe the behaviour you describe is just a *nix thing.


        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority". I knew I was on the right track :)
        In the absence of evidence, opinion is indistinguishable from prejudice.

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