The Global Layer Unix (GLUnix [97]) system uses software-based fault isolation (SFI) to move operating system functionality into user level libraries. User applications are linked with the operating system libraries, and system calls are converted to function calls into these libraries. Proponents of GLUnix claim that the cost of SFI is offset by not having to trap into the kernel for every system call.
The GLUnix libraries will provide a single system view of a network of workstations, even though the individual workstations run a standard operating system. This approach is limited by the functionality exported by the underlying operating system. Furthermore, to send and receive messages, the services of the operating system kernel are required. Since the goal of GLUnix is to use vendor supplied operating systems and use them unchanged by building additional functionality in a layer that sits on top of the operating system, there is no possibility of inserting user handlers into the kernel, unless the underlying operating system supports that.