Re: Read from file fails

[Bart Samwel]

Libor Vanek wrote:

> > > > > OK - how can I "notify" userspace process? Signals are "weak" - I need
> > > > > to send some data (filename etc.) to process. One solution is "on this
> > > > > signal call this syscall and result of this syscall will be data you
> > > > > need" - but I'd prefer to handle this in one "action".
> > > >
> > > > My first thoughts are to make it a blocking call.
> > >
> > > You mean like:
> > > - send signal to user-space process
> > > - wait until user-space process pick ups data (filename etc.), creates 
> > > copy of file (or whatever) and calls another system call that he's finished
> > > - let kernel to continue syscall I blocked
> > > ?
> >
> > No, more like:
> > - user-space process calls syscall, which blocks.
> > - kernel captures a file write event, puts the info in some kind of 
> > queue, wakes up the user-space process and then waits for some kind of 
> > acknowledgement to be returned so that it may continue.
> > - user-space process wakes up, the syscall completes, and passes a 
> > filename etc. to user-space. Copies the file, and calls a syscall to 
> > signify "hey, I'm done with that file". This syscall wakes up the kernel 
> > stuff that was waiting for this acknowledgement.
> > - file write event continues
> > - repeat from start
>
> OK - I'm thinking of using semaphores to "block" system call - is there something why this is not a good idea?

Semaphores are useful to protect a shared resource (and you might want 
to use those to protect your queue of filenames to copy), but not for 
transferring control between threads. I think you might want to look at 
wait_queue, wait_event, wake_up and things like that.

--Bart
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