Smalltalk VM written in C

This is a very simple Smalltalk VM, written in pure C. It runs images encoded in a custom format (see the minimal kernel image).

It was implemented as an assignment for a university course on runtime systems. It had to be done within ~2 months by myself, all the while being enrolled to several other courses with their own assignments. I did not have any prior knowledge of VMs when I started this project.

Limitations

The VM cannot do much, there are a lot of limitations:

• Blocks/closures are BlueBook-style: they are not reentrant.
• There are only small integers. No large integers, no floats, no fractions.
• Heap size is specified at compile-time, there is no dynamic growing/shrinking.
• The only way of communicating with the external world is through files.
• There is no support for weak references. Symbols will never get garbage-collected.
• Hashing was not implemented. Dictionaries and the symbol table have linear access time.
• And many more…

Building

Makefile is present for standard GNU make, the VM is compiled by executing

make

in the root directory. An executable binary called vm will be created.

Executing

A simple Bash wrapper to launch the VM with the default image is called run.sh. It expects an image file called Smalltalk.image in the current directory.

There are three possible execution modes:

1. Run a script from a file

   ./run.sh tests/hello-world.st

When a value is returned (^) from the script, it will be printed on stdout.

2. Run a script passed as an argument

   ./run.sh -r 'Transcript show: #hello; cr' ./run.sh -r '^ 10 factorial' ./run.sh -r 'Smalltalk keysDo: [ :each | Transcript show: each; cr ]'

Again, a value returned (^) from the script will be printed on stdout.

3. Launch an interactive mode (REPL)

   ./run.sh -i

   or a shortcut

   ./repl.sh

In this mode, each expression result is printed automatically on stdout.

A sample program: grep

A simple implementation of a Unix grep utility was written to demostrate the capabilities of the VM and the kernel. It only provides fixed pattern matching, no support for regular expressions so it acts like GNU grep with the -F option.

The program is embedded directly in the minimal kernel image to avoid its compilation on each run.

A Bash wrapper to launch it is provided in file grep.sh. Basic usage:

./grep.sh PATTERN INPUT-FILE OUTPUT-FILE

If any of the file is - or the argument is omitted, stdin or stdout is used. There are several options implemented: -i, -v, -c, -x, -e, -f; their function is the same as in GNU grep. Running:

./grep.sh -h

provides an overview of these options.

License

The MIT License, see file LICENSE for more information.