Re: [fuse-devel] Device Node Emulation

[Miklos S. <mi...@sz...>]

> I'm trying to use FUSE for some software that emulates hardware devices.
> Originally I was going to have my program make a bunch of FIFO's can other
> programs could open, pretending they were device nodes.  This didn't work
> because I need bidirectional communication, so I found FUSE and thought it
> would probably be able to do what I need.
> 
> Unfortunately I'm running in to some issues.  What I would like is for my
> program to create a bunch of character specials that another program can
> open and then read and write to.  All the data read by the client is put
> there by my program, and all the data written by the client is then read by
> my program.  I tried making a character special, which some major and minor
> number, but when I try to read from that file I get permission denied.
> Watching the debugging output from my program never even shows an OPEN
> request, only a LOOKUP.

That's because I/O on all special files (fifo, socket, char/block
device) is handled by the kernel.

The solution is just to create a regular file, and set the 'direct_io'
flag in fuse_file_info in the open() method.

> I have a hunch that the kernel probably intercepts all attempts to open a
> special file, which is why I'm not seeing it.  Still, in this case if I set
> my major and minor number to 1, 9, I would expect to read random data from
> that file, since that is the device id for /dev/urandom.

Exactly.

> Is what I'm trying to do possible?  How might I approach this?  I tried
> using FIFO's as well, but when I read from these, nothing happens, and again
> I never see an OPEN request on the FUSE output.  I had some success with
> regular files, but I'd rather not have to keep track of the file size and
> such.

Yeah, that's what 'direct_io' is for, you'll get all read/write
requests regardless of st_size.

Miklos