c_declaration_decoder

A small decoder program as an initiative to understand how compiler decodes the declaration in C programming language based on the precedence rule

We assume that the declaration provided if syntactically correct, our decoder doesn't support syntax validations (as of now).

How Declaration is formed!

Definition of Declarator in C

How many	Name in C	How it looks in C
zero or more	pointers	*, * const
exactly one	direct_declarator	identifier, identifier[size], identifier(args...), (declarator)
zero or one	initializer	= initial_value

Declaration in C

Below is the regular expression of a declaration in C.

<type-specifier> <type-qualifier>? <declarator>;

Thumb Rule

Since this is just for learning and experimenting we can't be having a decoder supporting all the type and storage-class specifiers. Below are the supported type and storage-class specifiers we will support.

Basic Data Type Specifier

• int
• char
• void

A declaration can have exactly one basic type, and it's always on the far left of the expression.

Derived Types

• * a pointer to
• [] array of
• () function returning

Type-specifier

• const

Precedence Rule (operator precedence)

Always start with the variable name:

foo is ...

and always end with the basic type:

foo is ... int

The middle part is filled based on operator precedence rules:-

• parentheses grouping together parts of a declaration.
• the postfix operator: parentheses () indicating a function, and square brackets [] indicating an array.
• the prefix operator: the * denoting "pointer to".

Decoder FSM

Approach

Tokenize

Tokenise the whole string input tokens like TYPE (for char, int, const), POINTER (for *), IDENTIFIER etc.

I am using ply lexer library of python to save my time and efforts :) which do the task of tokenising for me.

Make sense out of tokens

Why Py

I prefer using python here because we are dealing with complex string operations like tokenizing, slicing, subarray matching etc.

References

• http://unixwiz.net/techtips/reading-cdecl.html
• Expert C Programming - by Peter van der Linden
• https://medium.com/@bartobri/untangling-complex-c-declarations-9b6a0cf88c96
• https://user.ceng.metu.edu.tr/~ceng140/c_decl.pdf