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Re^4: [Win32] IO::Select's can_read method

by Corion (Pope)
on Dec 09, 2011 at 11:01 UTC ( #942618=note: print w/ replies, xml ) Need Help??


in reply to Re^3: [Win32] IO::Select's can_read method
in thread [Win32] IO::Select's can_read method

The ugly part of Alertable IO is that you will always read (at least) one byte that you can't easily stuff back. I haven't found (by just reading the documentation, not by trying it out) a way that gives you just "you can read n bytes from this filehandle now" or "read as many bytes as are available without blocking" outside of PeekNamedPipe().


Comment on Re^4: [Win32] IO::Select's can_read method
Re^5: [Win32] IO::Select's can_read method
by BrowserUk (Pope) on Dec 09, 2011 at 11:11 UTC
    The ugly part of Alertable IO is that you will always read (at least) one byte that you can't easily stuff back.

    For files, it essentially mean buffering the input within the application. Select for unix must do the same thing, though the buffering is probably done by the OS rather than in user space. For files, 'put back' simply means a negative seek.

    For pipes, the data has to have been received and therefore buffered before any OS can tell it it can read. PeekNamedPipe() just allows to to find out how much (and what, but that's by the by) has been received and buffered.


    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".
    In the absence of evidence, opinion is indistinguishable from prejudice.

    The start of some sanity?

Re^5: [Win32] IO::Select's can_read method
by patcat88 (Deacon) on Dec 09, 2011 at 17:26 UTC
    Cygwin deals with the problem by buffering the data. Attempt to do a non-blocking read, do the async IO for the number of bytes the read wanted, if more than X ms passes without a async response (event obj, APC, or IOCP), declare the read failed with EWOULDBLOCK. Until the read completes, every read will fail with EWOULDBLOCK. Its very simple, simply buffer the data internally.

    Select is not that hard to make file aware, you just have to multiplex the async file kernel event objects with the winsock handles. I think winsock handles can goto waitformultiple unmodified unless this is DOS Windows.
      Thanks for the explanation/pointers/elaboration, guys.

      No further questions at this stage - though that might change as I delve further into the project.

      (There's currently a PDL-Graphics-Gnuplot implementation which works well on nix type systems (and Cygwin) ... and works to a certain extent on Windows. Gnuplot seems pretty cool, and a fully functional Windows port would be good.)

      Cheers,
      Rob

        There is a MinGW compiled binary of gnuplot here that seems to work after a fashion. I'm actually getting rotating 3D plots displayed now, though they're rather jerky and require the task manager to terminate.

        There are several additional executables delivered -- wgnuplot.exe, pgnuplot.exe, and intriguingly, but unexplained as far as I can see, wgnuplot_pipes.exe -- one or more of which might work better than base gnuplot.exe when piping stuff from perl to plot?

        If only OSS came with better documentation :(


        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".
        In the absence of evidence, opinion is indistinguishable from prejudice.

        The start of some sanity?

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