An educational software system of a tiny self-compiling C compiler, a tiny self-executing RISC-V emulator, and a tiny self-hosting RISC-V hypervisor.
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Selfie is a project of the Computational Systems Group at the Department of Computer Sciences of the University of Salzburg in Austria.

The Selfie Project provides an educational platform for teaching undergraduate and graduate students the design and implementation of programming languages and runtime systems. The focus is on the construction of compilers, libraries, operating systems, and even virtual machine monitors. The common theme is to identify and resolve self-reference in systems code which is seen as the key challenge when teaching systems engineering, hence the name.

Slides providing an overview of selfie and an introduction to its design and implementation are made available incrementally here. There is also a free book in early draft form called Selfie: Computer Science for Everyone using selfie even more ambitiously reaching out to everyone with an interest in learning about computer science.

Selfie is a self-contained 64-bit, 10-KLOC C implementation of:

  1. a self-compiling compiler called starc that compiles a tiny but still fast subset of C called C Star (C*) to a tiny and easy-to-teach subset of RISC-V called RISC-U,
  2. a self-executing emulator called mipster that executes RISC-U code including itself when compiled with starc,
  3. a self-hosting hypervisor called hypster that provides RISC-U virtual machines that can host all of selfie, that is, starc, mipster, and hypster itself,
  4. a prototypical symbolic execution engine called monster that executes RISC-U code symbolically,
  5. a simple SAT solver that reads CNF DIMACS files, and
  6. a tiny C* library called libcstar utilized by selfie.

Selfie is implemented in a single (!) file and kept minimal for simplicity. There is also a simple in-memory linker, a RISC-U disassembler, a profiler, and a debugger with replay as well as minimal operating system support built into the emulator. Selfie generates ELF binaries that are compatible with the official RISC-V toolchain, in particular the spike emulator and the pk kernel.

For further information and support please refer to

Supported Platforms

Selfie runs on Mac, Linux, and Windows machines and possibly other systems that have a terminal and a C compiler installed. However, even if there is no C compiler installed on your machine or you only have access to a web browser you can still run selfie.

Installing Selfie

There are at least three ways to install and run selfie, from real simple to a bit more difficult:

  1. If you have access to a Mac, Linux, or Windows machine download and install docker. Then, open a terminal window and type docker run -it cksystemsteaching/selfie. Besides simplicity, the key advantage of using docker is that you can run selfie out of the box natively on your machine but also on spike and pk which are both pre-installed in the selfie docker image.

  2. Instead of using docker, you may also just download and unzip selfie, and then open a terminal window to run selfie on your machine. However, for this to work you need to have a C compiler installed on your machine. We recommend using clang or gcc.

  3. If you only have access to a web browser, create a github account, unless you already have one, and fork selfie into your github account. Then, create a cloud9 student account, connect it to your github account, verify your email address and set a password (important!), and finally clone your fork of selfie into a new cloud9 workspace.

At this point we assume that you have a system that supports running selfie. Below we use the make command assuming it is installed on your system which is usually the case. However, we also show the command invoked by make so that you can always invoke that command manually if your system does not have make installed.

The next step is to produce a selfie binary. To do that type make in your terminal. With docker, the system will respond make: 'selfie' is up to date since there is already a selfie binary pre-installed. Without docker, make will invoke the C compiler on your machine or in the cloud9 workspace:

cc -w -O3 -m64 -D'main(a,b)=main(int argc, char** argv)' -Duint64_t='unsigned long long' selfie.c -o selfie

and then compile selfie.c into an executable called selfie as directed by the -o option. The executable contains the C* compiler, the mipster emulator, and the hypster hypervisor. The -w option suppresses warnings that can be ignored here. The -O3 option instructs the compiler to generate optimized code. The -m64 option makes the compiler generate a 64-bit executable. The -D'main(a,b)=main(int argc, char** argv)' and -Duint64_t='unsigned long long' options are needed to bootstrap the code. The char data type is not available in C* but may be required by the compiler. The uint64_t data type is undefined since C* does not support including the necessary declarations.

Running Selfie

Once you have successfully compiled selfie.c you may invoke selfie without any arguments as follows:

$ ./selfie
./selfie { -c { source } | -o binary | -s assembly | -l binary | -sat dimacs } [ ( -m | -d | -r | -n | -y | -min | -mob ) 0-64 ... ]

In this case, selfie responds with its usage pattern.

The order in which the options are provided matters for taking full advantage of self-referentiality.

The -c option invokes the C* compiler on the given list of source files compiling and linking them into RISC-U code that is stored internally. For example, selfie may be used to compile its own source code selfie.c as follows:

$ ./selfie -c selfie.c

The -o option writes RISC-U code produced by the most recent compiler invocation to the given binary file. For example, selfie may be instructed to compile itself and then output the generated RISC-U code into a RISC-U binary file called selfie.m:

$ ./selfie -c selfie.c -o selfie.m

The -s option writes RISC-U assembly of the RISC-U code produced by the most recent compiler invocation including approximate source line numbers to the given assembly file. Similarly as before, selfie may be instructed to compile itself and then output the generated RISC-U code into a RISC-U assembly file called selfie.s:

$ ./selfie -c selfie.c -s selfie.s

The -l option loads RISC-U code from the given binary file. The -o and -s options can also be used after the -l option. However, in this case the -s option does not generate approximate source line numbers. For example, the previously generated RISC-U binary file selfie.m may be loaded as follows:

$ ./selfie -l selfie.m

The -m option invokes the mipster emulator to execute RISC-U code most recently loaded or produced by a compiler invocation. The emulator creates a machine instance with 0-64 MB of memory. The source or binary name of the RISC-U code and any remaining ... arguments are passed to the main function of the code. For example, the following invocation executes selfie.m using mipster:

$ ./selfie -l selfie.m -m 1

This is in fact semantically equivalent to executing selfie without any arguments:

$ ./selfie

The -d option is similar to the -m option except that mipster outputs each executed instruction, its approximate source line number, if available, and the relevant machine state. Alternatively, the -r option limits the amount of output created with the -d option by having mipster merely replay code execution when runtime errors such as division by zero occur. In this case, mipster outputs only the instructions that were executed right before the error occurred. The -min and -mob options invoke special versions of the mipster emulator used for teaching.

If you are using docker you can also execute selfie.m directly on spike and pk as follows:

$ spike pk selfie.m

which is again semantically equivalent to executing selfie without any arguments.

The -y option invokes the hypster hypervisor to execute RISC-U code similar to the mipster emulator. The difference to mipster is that hypster creates RISC-U virtual machines rather than a RISC-U emulator to execute the code. See below for an example.

The -n option invokes the symbolic execution engine which interprets the 0-64 value as fuzzing parameter. Value 0 means that code is executed symbolically but without any fuzzing of its input. In other words, code execution uses the symbolic execution engine but is effectively concrete. The 64 value, on the other hand, means that all input read from files is fuzzed to the extent that any machine word read from files may represent any 64-bit value. Note that the current implementation is incomplete and buggy.

The -sat option invokes the SAT solver on the SAT instance loaded from the dimacs file. The current implementation is naive and only works on small instances.


Here is an example of how to perform self-compilation of selfie.c and then check if the RISC-U code selfie1.m generated for selfie.c by executing the ./selfie binary is equivalent to the code selfie2.m generated by executing the just generated selfie1.m binary:

$ ./selfie -c selfie.c -o selfie1.m -m 2 -c selfie.c -o selfie2.m
$ diff -s selfie1.m selfie2.m
Files selfie1.m and selfie2.m are identical

Note that at least 2MB of memory is required for this to work.


The following example shows how to perform self-execution of the mipster emulator. In this case we invoke mipster to invoke itself to execute selfie:

$ ./selfie -c selfie.c -o selfie.m -m 2 -l selfie.m -m 1

which is again semantically equivalent to executing selfie without any arguments but this time with selfie printing its usage pattern much slower since there is a mipster running on top of another mipster.


The previous example can also be done by running hypster on mipster. This is significantly faster and requires less memory since hypster does not create a second emulator instance on top of the first emulator instance. Instead, hypster creates a virtual machine to execute selfie that runs concurrently to hypster on the first emulator instance:

$ ./selfie -c selfie.c -o selfie.m -m 1 -l selfie.m -y 1

We may even run hypster on hypster on mipster which is still reasonably fast since there is still only one emulator instance involved and hypster itself does not add much overhead:

$ ./selfie -c selfie.c -o selfie.m -m 2 -l selfie.m -y 1 -l selfie.m -y 1


To compile any C* source code and execute it right away in a single invocation of selfie without generating a RISC-U binary use:

$ ./selfie -c any-cstar-file.c -m 1 "arguments for any-cstar-file.c"

Equivalently, you may also use a selfie-compiled version of selfie and have the mipster emulator execute selfie to compile any C* source code and then execute it right away with hypster on the same emulator instance:

$ ./selfie -c selfie.c -m 1 -c any-cstar-file.c -y 1 "arguments for any-cstar-file.c"

You may also generate RISC-U binaries both ways which will then be identical:

$ ./selfie -c any-cstar-file.c -o any-cstar-file1.m
$ ./selfie -c selfie.c -m 1 -c any-cstar-file.c -o any-cstar-file2.m
$ diff -s any-cstar-file1.m any-cstar-file2.m
Files any-cstar-file1.m and any-cstar-file2.m are identical

This can also be done in a single invocation of selfie:

$ ./selfie -c any-cstar-file.c -o any-cstar-file1.m -c selfie.c -m 1 -c any-cstar-file.c -o any-cstar-file2.m
$ diff -s any-cstar-file1.m any-cstar-file2.m
Files any-cstar-file1.m and any-cstar-file2.m are identical

The generated RISC-U binaries can then be loaded and executed as follows:

$ ./selfie -l any-cstar-file1.m -m 1 "arguments for any-cstar-file1.m"


To compile and link any C* source code from multiple source files use:

$ ./selfie -c any-cstar-file1.c any-cstar-file2.c ... -m 1

For example, to make the source code of selfie.c available as library code in any C* source code use:

$ ./selfie -c any-cstar-file.c selfie.c -m 1

Note that multiple definitions of symbols are ignored by the compiler with a warning.


Selfie's console messages always begin with the name of the source or binary file currently running. The mipster emulator also shows the amount of memory allocated for its machine instance and how execution terminated (exit code).

As discussed before, RISC-U assembly for selfie and any other C* file is generated as follows:

$ ./selfie -c selfie.c -s selfie.s

If the assembly code is generated from a binary generated by the compiler (and not loaded from a file) approximate source line numbers are included in the assembly file.

Verbose debugging information is printed with the -d option, for example:

$ ./selfie -c selfie.c -d 1

Similarly, if the executed binary is generated by the compiler (and not loaded from a file) approximate source line numbers are included in the debug information.