Re: Forking Win32 ...

I need some way to check whether new data is available in the filehandle, without causing perl to wait until new data becomes available .... If anyone can think up a way around this, I'd really appreciate it!

Actually, that's exactly what Tk's "readable" fileEvent bit is supposed to be doing for you - it shouldn't get triggered until there's something to read.

It sounds to me as though you're trying to do something very similar to Chapter 22 of O'Reilly's Mastering Perl/TK. (Where would copyright violation be without the Chinese?) I suggest that you look at that code and see what, if any, of it can be adapted/coerced to your purposes.

Incidentally, I was going to mention the IO::Select package and how to use that (or the four argument form of the select builtin) to determine whether a filehandle has anything waiting to be read. However, Tk should be doing that internally already, so I'm not sure that gets you anything. (Still, I suppose it's worth trying at least once - there's an example of using it over here)

Comment on Re: Forking Win32 ...

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Re: Re: Forking Win32 ... by owenjm (Initiate) on Feb 25, 2004 at 21:59 UTC
Hi, thanks for your ideas ... I know that's what the readable fileevent should be doing for me, that's what's so frustrating! The problem seems to be that fileevent thinks the filehandle is readable as soon as the http request starts in the child, when there is nothing to read in the filehandle and may not be for some time, which then causes <>, sysread, whatever to pause. (Incidentally, it's not a case of the Child pausing all execution in the Parent - if I stop trying to read the filehandle (or try using IO::Select - see below) then the Parent is unblocked and fine) And then the filehandle doesn't become readable again, according to fileevent (so I can't even just ignore the first readable call! Although I'd really hate doing messy stuff like that anyway ....) I've already looked at the Mastering Perl/Tk section - the problem being that the author does not really have a solution: nothing he tries works on Win32, except using a third party module - and I'm specifically trying to avoid using any third party modules here. I actually think this is in some ways a neater solution anyway - setting up a dedicated child to grab http content, as opposed to forking every time you need to get data ... and it so almost works!! I also tried using IO::Select, but without much success - when I tied it in with fileevent (when fileevent thought that the filehandle was readable) I couldn't get it to ever report that the filehandle had readable data (I don't know why! I had it in an infinite loop within the fileevent call, broken only when it reads data, but it never claimed it could read anything) Maybe I'll give that another try ... I guess I could try ignoring fileevent and waitvariable entirely, and rely on IO::Select to do the work ... perhaps that's the best way forwards. Actually, I've just read a bit more and I now think what I need to do is make the filehandle non-blocking using fcntl (see the Tk::fileevent manpage) - activestate claims to have the fcntl module in their activeperl distro, so when I get to work I'll give that a try ...	[reply]
Re^3: Forking Win32 ... by YorHel (Initiate) on Jun 08, 2004 at 17:59 UTC
Well... The fcntl module is in the activeperl distro, but a non-blocking read is still impossible with that module on Win32, It just have some limitations :'(	[reply]