pimms / cish

Mostly feature-complete C-interpreter

C. Ish.

Yet another (mostly) functional C interpreter.

I wrote this to use as the central component of a game centered around programming low-level devices, similar to "Shenzen I/O" and "TIS-100", but realized after beginning on the actual "game part" of the game that I don't really like Unity or C#.

Now I don't know what to do with it.

¯\_(ツ)_/¯

Usage

Cish only works with one file at a time. Like with an actual C program, there must be a main function defined. The standard library can be included and used, although it's mostly unimplemented.

Building

Antlr and the Antlr Runtime must be installed on your system. Cish is currently only tested using Antlr v 4.11.1.

brew install antlr antlr4-cpp-runtime

mkdir build
cd build
cmake ..
make

(optionally)
make install

The make-script builds two artifacts - libcish.a and (unless excluded via) CMake options) cish_cli. The library can be used to add C interpretation to your favorite application, and the command line client can be used to combine a subset of C with the speed of a turtle.

Testing

All of Cish is under heavy unit-test barrage from gtest, but there are also multiple cish vs gcc test-programs in the gcc_compare directory. The latter executes a simple program and compares stdout and $?. I've found that if I ever break something, no matter how trivial, it breaks at least one of the many tests.

make check runs the unit-tests, and gcc_compare/compare.sh runs the gcc comparison tests. Note that cish_cli must be on the system path for the compare.sh script to run properly. Most easily achieved by make install.

Major missing features:

• Most of the standard library
• Switch-statements
• Literal arrays
• Break & continue
• enum
• typedef
• Preprocessor macro
• Library-defined global variables (EOF, stdout, errno, etc)
• Ternary expressions
• Performance 🐢

Technical Details

Syntax Parsing

Antlr is used for parsing the AST, and is converted (nearly) one-to-one to an "execution-tree".

Antlr is used to (a) make sure that the program is syntactically valid, and (b) express the program in a manageable structure. Each node in the AST is then visited and (often) converted into either an Expression or a Statement, the sum of which makes up the "execution tree". During this conversion, it is verified that the program makes logical and functional sense. Referred variables must have been declared, expected parameters must be passed, dereferencing an int makes no sense, etc.

Note that because I have no idea what I'm really doing, the "execution tree" I'm referring to is called cish::AST in the code :)

Virtual Machine

Memory Allocation

Cish attempts to emulate the real workings of C as closely as possible, and with it unsafe pointers, wild-west style casting & other neat tricks. The Cish VM does however not have the concept of "stack" or "heap". Instead, everything is allocated in a "heap-like" fashion.

Execution

The execution tree consists of Statement and Expression nodes. The nodes are intrinsically (recursively?) evaluated, so the execution process really only involves finding the node that defines the main()-function and evaluating it. It will in turn evaluate all of its child-nodes, and so on until the program returns.