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The AMUNIX system provides the interface to the basic Amoeba concepts [AMSYS, AMPRO] like RPC messaging on the top of UNIX like operating systems, for example the open source Linux and FreeBSD operating systems. It consists of these parts:

  1. An UNIX version of the Amoeba thread module called AMUTHR enabling multi-threading normally not a core part of a UNIX operating system. The AMUTHR module is entirely implemented in UNIX user process space and nearly 100% compatible to the Amoeba kernel thread implementation. The Amoeba thread implementation is weaved with the FLIP protocol stack.

  2. The AMUNIX library implements Amoebas basic concepts like capability management or the RPC interface, several server stub functions and many more. It’s mostly derived from the native Amoeba core library.

  3. The FLIPD protocol stack daemon entirely executing in the UNIX user process domain providing access of AMUNIX programs to the (Amoeba) network using the FLIP protocol [KAS93]. The FLIPD is also responsible for loacl communication between Amoeba programs.

  4. A comfortable development environment with predefined makefiles for the Amoeba configuration manager amake similar to UNIX make. With this development environement it’s possible to built libraries and Amoeba executables from C source code.

Figure Figure 12 gives an overview and the relationship between these parts.

Figure 12: AMUNIX: the Amoeba layer on the top of UNIX.

Each AMUNIX executable incorporates the underlying UNIX system, glued in the system C library. On the top of this system library, the AMUTHR module was placed to enable multi-threading. Each AMUNIX process has it’s own encapsulated thread management responsible only for this process. This mechanism differs from the native Amoeba system (VX-Kernel) where all processes share the same thread manger inside the kernel. The interface to the Amoeba world is provided by the AMUNIX layer.

The AMUTHR thread module is nearly identical to theVX- kernel thread implementation. Yes, indead, the source code from the kernel were used nearly unchanged. A thread switch is performed by a small function written in Assembler, consisting of less than 10 lines of code. Only the stack and program code pointers must be changed during a thread switch.

The AMUNIX library differs from the native Amoeba core library only in the thread and the communication backend. Under native Amoeba with the protocol stack inside the kernel, the communication backend (RPC…) is implemented simply with kernel system calls. Under AMUNIX, a UNIX like communication must be established to the FLIPD daemon, an AMUNIX process, too. The communication between AMUNIX processes and the FLIP daemon is realized with generic UNIX sockets.

Using the AMUNIX layer, nearly all Amoeba programs known from the native System can be build for the AMUNIX environment. The programming interface kept unchanged including C header files. Only different libraries must be linked with the AMUNIX executable. And finally, an AMUNIX program can be started from any UNIX shell or forked by another UNIX program.

The first time an AMUNIX thread want to use the RPC interface (e.g. with a trans() call), the AMUNIX layer will try to connect to the FLIP daemon via a public UNIX control socket. After successful connect, the FLIP daemon will establish a new private communication socket only for this particular thread. Additionally for RPC signal transmission, a dedicated signal socket connection is opened if this is the first thread of the process. Each AMUNIX process thread (using RPC) is handled by the FLIPD daemon with an own process thread.

Each AMUNIX process (like a native Amoeba process) must be registered by the FLIP protocol stack. Each Amoeba process gets an unique communication endpoint identifier. As long as the process lives, it keeps this ID number, as well the process migrates to another machine!

Figure Figure 13 shows details of the FLIP daemon operation with AMUNIX processes.

Figure 13: The UNIX implementation of Amoebas FLIP protocol stack in the UNIX user process space.

The network interface of the FLIP daemon needs direct access to the network layer of the underlying operating system to receive and send raw ethernet packets. Therefore FLIPD is connected to the UNIX kernel using some kind of sockets, too. This is the only host system dependent part of the FLIP daemon program. Under Linux, so called raw sockets, and under FreeBSD the packet filter interface is used to connect FLIP to the network. The rest of the FLIP source code (both for the native VX-Kernel and AMUNIX implementation) is operating system independent.



M.F. Kaashoek, R. van Renesse, H. van Staveren, and A.S. Tanenbaum, FLIP: an Internetwork Protocol for Supporting Distributed Systems, ACM Transactions on Computer Systems, pp. 73-106, Feb. 1993.


Amoeba 5.3 System Administration Guide


Amoeba 5.3 Programming Guide