gocgo

gocgo (go-c-go), implemented in Go, includes lexer, parser, and visitors for the C programming language.

This small project started as a demo of ANTLR and the visitor design pattern. It turned out that having a C parser available was valuable for a few other cases on our side, thus, we decided to make it public as it might be of interest to others as well.

gocgo is (currently) suitable for anlaysis of C code. While some transformation is possible, having a flexible transformation API could be added later.

A Quick Example

Below is an example of a visitor that collects C code into a buffer and along the way removes function bodies.

type FuncDeleteVisitor struct {
	*BaseVisitorImpl
	in   bool
	Buff bytes.Buffer
}

func NewFuncDeleteVisitor() *FuncDeleteVisitor {
	return &FuncDeleteVisitor{in: false}
}

func (v *FuncDeleteVisitor) VisitTerminal(n antlr.TerminalNode) error {
	v.Buff.WriteString(n.GetSymbol().GetText())
	v.Buff.WriteString(" ")
	return nil
}

func (v *FuncDeleteVisitor) VisitFunctionDefinition(ctx *parsing.FunctionDefinitionContext) (bool, error) {
	v.in = true
	return true, nil
}

func (v *FuncDeleteVisitor) VisitFunctionDefinitionEnd(ctx *parsing.FunctionDefinitionContext) error {
	v.in = false
	return nil
}

func (v *FuncDeleteVisitor) VisitBlockItemList(ctx *parsing.BlockItemListContext) (bool, error) {
	if v.in {
		return false, nil
	} else {
		return true, nil
	}
}

Usage

We provide a complete example of running the aforementioned visitor as a command in this repo. You can run the following:

go run ./cmd/gocgo/gocgo.go cprogs/for.c 

You can take a look at the command for details, but it is rather simple code.

Architecture and Design

Each visitor should embed a pointer to BaseVisitorImpl, which provides the default implementation of Visit methods.

There is a VisitX method for each type of AST node (e.g., VisitFunctionDefinition for a function definition). Consider this example:

func (v *FuncDeleteVisitor) VisitFunctionDefinition(ctx *parsing.FunctionDefinitionContext) (bool, error) {
	v.in = true
	return true, nil
}

This Visit method is invoked for every function definition in parsed C code. There are two return values. The first value (bool) says if children should be visited. The second value (error) is the error Go interface.

Each VisitX method has a corresponding VisitXEnd method invoked once all children of the node are visited. From our earlier example:

func (v *FuncDeleteVisitor) VisitFunctionDefinitionEnd(ctx *parsing.FunctionDefinitionContext) error {
	v.in = false
	return nil
}

VisitXEnd methods have a single return value (error).

This architecture is common in many tools, e.g., visitors in the Eclipse implementation.

Implementation

gocgo cuts a few steps for you if you were planning to use ANTLR to obtain a parser for the C programming language.

We used the grammar from the ANTLR repo.

If you wish to reproduce the steps to get lexer, parser, and the original visitors, you can run the following command in the root of this repo:

go generate ./...

We made a few changes to simplify the usage of visitors and address some issues (e.g., Issue 4398). Below is the summary of key changes / additions:

• Addressed the issue 4398, by introducing (locally in this repo) VisitChildren.

• Removed the generated c_base_visitor.go, because it limits changing behavior of any Visit method.

• Introduced our own visitor interface (BaseVisitor) and its implementation (BaseVisitorImpl) that enable proper struct embedding. Your visitor can now change behavior of (a subset of) Visit methods.

• Updated API design, such that each Visit method returns bool and error (rather than interface{}). We find the new approach more Go appropriate.

License

BSD-3-Clause license.

Contact

Feel free to get in touch if you have any comments: Milos Gligoric <[email protected]>.