Show HN: Cdecl-dump - represent C declarations visually (opens in new tab)

(github.com)

35 pointsbluetomcat3mo ago13 comments

A small tool that parses C declarations and outputs a simple visual representation at each stage, as it encounters arrays, pointers or functions.

The program uses a table-driven lexer and a hand-written, shift-reduce parser. No external dependencies apart from the standard library.

13 comments

xvilka3mo ago

We use the tree-sitter[1] for parsing C declarations in Rizin[2] (see the "td" command, for example). See our custom grammar[3] (modified mainstream tree-sitter-c). The custom grammar was sadly necessary, due to the inability of Tree-Sitter to have the alternate roots[4].

P.S. Please add a license for your code.

[1] https://tree-sitter.github.io/

[2] https://github.com/rizinorg/rizin/tree/dev/librz/type/parser

[3] https://github.com/rizinorg/rizin-grammar-c/

[4] https://github.com/tree-sitter/tree-sitter/issues/711

coherentpony3mo ago

I don’t understand what the visualisation screenshot in the README is trying to communicate to me.

bluetomcatOP3mo ago

It starts from the identifier. At every stage, it outputs a sub-expression which is the “mirrored use” and corresponds to the boxed representation below it. When it reaches the top of the expression, it prints the final type of the expression which is the lone specifier-qualifier list.

As per the screenshot, “arr” is an array of 4 elements. Consequently, “arr[0]” is an array of 8 elements. Then, “arr[0][0]” is a pointer. And so on, until we arrive at the specifier-qualifier list.

coherentpony3mo ago

Ok that helps. Thank you.

pcfwik3mo ago

Since this is about C declarations: for anyone who (like me) had the misfortune of learning the so-called "spiral rule" in college rather than being taught how declarations in C work, below are some links that explain the "declaration follows use" idea that (AFAIK) is the true philosophy behind C declaration syntax (and significantly easier to remember/read/write).

TL;DR: you declare a variable in C _in exactly the same way you would use it:_ if you know how to use a variable, then you know how to read and write a declaration for it.

https://eigenstate.org/notes/c-decl https://news.ycombinator.com/item?id=12775966

userbinator3mo ago

if you know how to use a variable, then you know how to read and write a declaration for it.

In other words, the precedence of operators in a declaration have exactly the same precedence as in its use.

nitrix3mo ago

That is correct.

  int x, *p, arr[5], fn(), (*pfn)();

Using x, or dereferencing p, or subscripting the array arr, or declaring a function that can be called with fn, or dereferencing the function pointer pfn then calling it, all these things would produce an int.

It's the intended way to read/write declarations/expressions. As a consequence, asterisks ends up placed near the identifiers. The confused ones will think it's a stylistic choice and won't understand any of this.

pwdisswordfishy3mo ago

Blame Stroustrup.

https://www.stroustrup.com/bs_faq2.html#whitespace

saagarjha3mo ago

Of course, the correct way to use a function pointer is to call it.

1 more reply

any13mo ago

> It's the intended way to read/write declarations/expressions. As a consequence, asterisks ends up placed near the identifiers.

You know you don't always have to use things as they were intended?

> The confused ones will think it's a stylistic choice and won't understand any of this.

Well, I've written it both ways, and the compiler never seems to mind. :)

Maybe I should start putting space on both sides of the asterisk; seems like it would be a good way to annoy even more people.

j / k navigate · click thread line to collapse

13 comments

xvilka3mo ago

P.S. Please add a license for your code.

[1] https://tree-sitter.github.io/

[2] https://github.com/rizinorg/rizin/tree/dev/librz/type/parser

[3] https://github.com/rizinorg/rizin-grammar-c/

[4] https://github.com/tree-sitter/tree-sitter/issues/711

coherentpony3mo ago

I don’t understand what the visualisation screenshot in the README is trying to communicate to me.

bluetomcatOP3mo ago

coherentpony3mo ago

Ok that helps. Thank you.

pcfwik3mo ago

TL;DR: you declare a variable in C _in exactly the same way you would use it:_ if you know how to use a variable, then you know how to read and write a declaration for it.

https://eigenstate.org/notes/c-decl https://news.ycombinator.com/item?id=12775966

userbinator3mo ago

if you know how to use a variable, then you know how to read and write a declaration for it.

In other words, the precedence of operators in a declaration have exactly the same precedence as in its use.

nitrix3mo ago

That is correct.

  int x, *p, arr[5], fn(), (*pfn)();

pwdisswordfishy3mo ago

Blame Stroustrup.

https://www.stroustrup.com/bs_faq2.html#whitespace

saagarjha3mo ago

Of course, the correct way to use a function pointer is to call it.

1 more reply

any13mo ago

> It's the intended way to read/write declarations/expressions. As a consequence, asterisks ends up placed near the identifiers.

You know you don't always have to use things as they were intended?

> The confused ones will think it's a stylistic choice and won't understand any of this.

Well, I've written it both ways, and the compiler never seems to mind. :)

Maybe I should start putting space on both sides of the asterisk; seems like it would be a good way to annoy even more people.

j / k navigate · click thread line to collapse