The most recent version of FUSD is v1.10, released 19 August 2003. You can get the software in various ways:
You can read the 35-page FUSD User Manual in various formats:
FUSD was written by Jeremy Elson (jelson@circlemud.org) and Lewis Girod, originally at Sensoria Corporation. The public fork is now maintained by those authors from the University of California, Los Angeles. If you have bug reports, patches, suggestions, or any other comments, please feel free to contact the authors.
FUSD has two SourceForge-host mailing lists: a low-traffic list for announcements (fusd-announce) and a list for general discussion (fusd-devel). Subscription information for both lists is available at the SourceForge's FUSD mailing list page.
A user-space device driver can do many of the things that kernel drivers can't, such as perform a long-running computation, block while waiting for an event, or read files from the file system. Unlike kernel drivers, a user-space device driver can use other device drivers--that is, access the network, talk to a serial port, get interactive input from the user, pop up GUI windows, or read from disks. User-space drivers implemented using FUSD can be much easier to debug; it is impossible for them to crash the machine, are easily traceable using tools such as gdb, and can be killed and restarted without rebooting even if they become corrupted. FUSD drivers don't have to be in C--Perl, Python, or any other language that knows how to read from and write to a file descriptor can work with FUSD. User-space drivers can be swapped out, whereas kernel drivers lock physical memory.
FUSD drivers are conceptually similar to kernel drivers: a set of callback functions called in response to system calls made on file descriptors by user programs. FUSD's C library provides a device registration function, similar to the kernel's devfs_register_chrdev() function, to create new devices. fusd_register() accepts the device na me and a structure full of pointers. Those pointers are callback functions which are called in response to certain user system calls--for example, when a process tries to open, close, read from, or write to the device file. The callback functions should conform to the standard definitions of POSIX system call behavior. In many ways, the user-space FUSD callback functions are identical to their kernel counterparts.
The proxying of kernel system calls that makes this kind of program possible is implemented by FUSD, using a combination of a kernel module and cooperating user-space library. The kernel module implements a character device, /dev/fusd, which is used as a control channel between the two. fusd_register() uses this channel to send a message to the FUSD kernel module, telling the name of the device the user wants to register. The kernel module, in turn, registers that device with the kernel proper using devfs. devfs and the kernel don't know anything unusual is happening; it appears from their point of view that the registered devices are simply being implemented by the FUSD module.
Later, when kernel makes a callback due to a system call (e.g. when the character device file is opened or read), the FUSD kernel module's callback blocks the calling process, marshals the arguments of the callback into a message and sends it to user-space. Once there, the library half of FUSD unmarshals it and calls whatever user-space callback the FUSD driver passed to fusd_register(). When that user-space callback returns a value, the process happens in reverse: the return value and its side-effects are marshaled by the library and sent to the kernel. The FUSD kernel module unmarshals this message, matches it up with a corresponding outstanding request, and completes the system call. The calling process is completely unaware of this trickery; it simply enters the kernel once, blocks, unblocks, and returns from the system call--just as it would for any other blocking call.
One of the primary design goals of FUSD is stability. It should not be possible for a FUSD driver to corrupt or crash the kernel, either due to error or malice. Of course, like any other program a buggy driver may corrupt itself (e.g., due to a buffer overrun). However, strict error checking is implemented at the user-kernel boundary which prevents drivers from corrupting the kernel or any other user-space process--including the errant driver's own clients, and other FUSD drivers.
For a more detailed description of what FUSD is, and how to use it, please take a look at the comprehensive FUSD User's Manual.
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Jeremy Elson