How Can I Obtain a Human-Readable Abstract Syntax Tree (AST) from C Code?

Get Human Readable AST from C Code

In the realm of C software development, it can be beneficial to have a human-readable representation of a program's Abstract Syntax Tree (AST). ASTs provide a structured view of a program's syntax, enabling easier understanding and analysis.

Parsing C Code

To obtain an AST from C code, several tools are available. One notable option is the Clang compiler. Clang provides a feature called "-emit-llvm", which can generate an AST in a human-readable format. However, this feature has been deprecated and removed from recent versions of Clang.

Alternative Tool

An alternative tool that can generate ASTs from C code is the DMS Software Reengineering Toolkit. DMS provides a comprehensive C parser that supports modern C standards, including C 11 and C 17.

DMS Approach

To use DMS to get an AST from C code, you can follow the following steps:

1. Install DMS.
2. Create a C file with the code you want to parse.
3. Run the following command:

run ..\DomainParser ++AST "<path_to_C++_file>"

Replace with the actual path to your C file. DMS will generate an AST and display it in a human-readable form.

Examples

Consider the following simple Fibonacci program in C :

int fib(int n) {
    if ( n == 0 || n == 1 ) 
        return n;
    int fib1 = 0; 
    int fib2 = 1;
    int fib = 0;
    for ( int i = 2; i < n; i++ ) 
    {
        fib = fib1 + fib2;
        fib1 = fib2;
        fib2 = fib;
    }
    return fib;
}

Running DMS on this program will produce the following AST:

(translation_unit
 (function_definition
  (function_head
   (simple_type_specifier 
    ('int')
   )
   (noptr_declarator
    (IDENTIFIER ['fib'])
    ('(')
    (parameter_declaration
     (simple_type_specifier ('int'))
     (IDENTIFIER ['n'])
    )
    (')')
    (function_qualifiers)
   )
  )
  (compound_statement
   ('{'
   (statement_seq
    (statement_seq
     (statement_seq
      (statement_seq
       (selection_statement
        ('if')
        ('(')
         (logical_or_expression
          (equality_expression
           (IDENTIFIER ['n'])
           ('==')
           (INT_LITERAL [0])
          )
          ('||')
          (equality_expression
           (IDENTIFIER ['n'])
           ('==')
           (INT_LITERAL [1])
          )
         )
        (')')
        (jump_statement
         ('return')
         (IDENTIFIER ['n'])
        )
       )
       (simple_declaration
        (simple_type_specifier $('int'))
        (init_declarator
         (IDENTIFIER ['fib1'])
         (initializer
          ('=')
          (INT_LITERAL [0])
         )
        )
       )
      )
      (simple_declaration
       (simple_type_specifier $('int'))
       (init_declarator
        (IDENTIFIER ['fib2'])
        (initializer
         ('=')
         (INT_LITERAL [1])
        )
       )
      )
     )
     (simple_declaration
      (simple_type_specifier $('int'))
      (init_declarator
       (IDENTIFIER ['fib'])
       (initializer
        ('=')
        (INT_LITERAL [0])
       )
      )
     )
    )
    (iteration_statement
     ('for')
     ('(')
      (simple_declaration
       (simple_type_specifier $('int'))
       (init_declarator
        (IDENTIFIER ['i'])
        (initializer
         ('=')
         (INT_LITERAL [2])
        )
       )
      )
      (relational_expression
       (IDENTIFIER ['i'])
       ('<')
       (IDENTIFIER ['n'])
      )
      (';'
       (postfix_expression
        (IDENTIFIER ['i'])
        ('++')
       )
      ')'
     (compound_statement
      ('{'
       (statement_seq
        (statement_seq
         (expression_statement
          (assignment_expression
           (IDENTIFIER ['fib'])
           ('=')
           (additive_expression
            (IDENTIFIER ['fib1'])
            ('+')
            (IDENTIFIER ['fib2'])
           )
          )
         )
         (expression_statement
          (assignment_expression
           (IDENTIFIER ['fib1'])
           ('=')
           (IDENTIFIER ['fib2'])
          )
         )
        )
        (expression_statement
         (assignment_expression
          (IDENTIFIER ['fib2'])
          ('=')
          (IDENTIFIER ['fib'])
         )
        )
       )
      }'))
     )
    )
    (jump_statement
     ('return')
     (IDENTIFIER ['fib'])
    )
   }'))
)

This AST provides a detailed representation of the program's structure and semantics, making it easier to understand and analyze the program's behavior.

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