narwhal-lab/magicskills/c-2-ast

C to AST Skill

Purpose

This Skill converts C source code into its Abstract Syntax Tree (AST) representation.

Use this Skill when you need to:

• Understand the structure of a C program
• Analyze functions, statements, and expressions
• Prepare C code for static analysis or security analysis
• Transform C code into an intermediate representation (AST)
• Feed structured code information into downstream tools or agents

The AST is generated using a Python-based C parser and printed directly to standard output.

When to Use

Apply this Skill when the user asks to:

• “Parse this C code”
• “Convert this C file to an AST”
• “Show me the AST of this C program”
• “Analyze the structure of this C code”
• “Extract syntax tree / abstract syntax tree from C”

This Skill is especially useful in compiler frontends, program analysis, and security research workflows.

Instructions

Follow these steps strictly and in order:

1. Navigate to the Skill scripts directory

• Change working directory to the scripts/ directory under this skill:

`` scripts/ ``

1. Handle the input C source

• If the user provides a path to a `.c` file, use it directly.
• If the user provides inline C code as a string, you must first save it to a C source file using the provided script:

``bash python3 save_c.py --input "<C code string>" --output <output_dir> --filename <filename>.c ``

• Ensure the generated file has a .c extension and is successfully written to disk.

1. Run the AST extraction script

• Convert the C source file to its Abstract Syntax Tree by executing:

``bash python3 c_2_ast.py --input <path_to_c_file> ``

1. Do not modify the source code

• This Skill is strictly read-only.
• Do not rewrite, reformat, optimize, or otherwise alter the C code.

1. Return the AST output verbatim

• The script prints the AST directly to standard output.
• Return the output exactly as produced.
• Do not summarize, paraphrase, or restructure the AST.
• Preserve indentation, hierarchy, and node ordering.

1. If parsing fails

• Report the exact error message produced by the script.
• Suggest likely causes, such as:
• Missing or unsupported header files
• Unsupported C extensions (e.g., compiler-specific syntax)
• Invalid or incomplete C syntax

Output Format

The output is a tree-structured textual AST, for example:

 
FileAST:
FuncDef:
Decl: main
FuncDecl:
TypeDecl:
IdentifierType: ['int']
Compound:
FuncCall:
ID: puts
ExprList:
Constant: string, "Hello"
Return:
Constant: int, 0
 

This output represents the syntactic structure of the C program and can be consumed by downstream tools.

Additional Resources

For deeper understanding of:

• AST node types
• pycparser internal representation
• Meaning of specific syntax tree nodes
• Limitations of the C grammar supported

see the accompanying reference document:

➡️ [reference.md](reference.md)

Only read this file when more detailed or theoretical information is required. For standard AST extraction tasks, this Skill file alone is sufficient.

Notes and Limitations

• The AST follows standard C syntax (C89/C99 subset).
• Some compiler-specific extensions (e.g., GCC attributes) may not be supported.
• Header files are handled using a minimal fake libc environment.
• Macro-heavy or highly platform-specific code may require preprocessing.

Examples

Example 1: Simple AST extraction

User request:

Convert this C file to an AST.

Action:

python3 c_2_AST.py --input example.c

Result:

• The AST is printed directly.

Example 2: Security analysis preparation

User request:

I want to analyze this C program for vulnerabilities.

Action:

• First, extract the AST using this Skill.
• Then, pass the AST to a vulnerability analysis or pattern-matching process.

Best Practices

• Use this Skill before any deep code analysis.
• Treat the AST as an intermediate representation.
• Combine with CFG/DFG or semantic analysis for advanced tasks.

End of Skill.