Re: Read from file fails

[Bart Samwel <bart@samwel.tk>]

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