Clockwise/Spiral Rule (1994) (c-faq.com)

This comes up every so often, and while it's sort of almost true and attractive, it isn't actually correct.

The correct rule is "follow the C grammar". An easier to remember and also correct rule is "start at the identifier being declared; work outwards from that point, reading right until you hit a closing parenthesis, then left until you hit the corresponding open parenthesis, then resume reading right..." (this is sometimes called the "right-left rule": https://cseweb.ucsd.edu/~gbournou/CSE131/rt_lt.rule.html).

> "str is an array 10 of pointers to char"

Wow, imagine if it was possible to actually use a language like that do declare the type of the variable like that? Something like

    str: array [0..9] of ^Char; 

or even

    var str [10]*uint8

Just imagine...

  var str: array[10, ptr char]

Being taught this rule in undergrad really hampered my appreciation of C. As I've said in a previous comment, the real key that unlocked understanding C declarations for me is the mantra "declaration follows use." 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. Once I understood this elegant idea, it became hard to enjoy using statically typed languages that eschew it.

It is explained in more detail at this link: https://eigenstate.org/notes/c-decl

This is why I like python.

str is a duck.

This is useful if you're far from the internet. Here's a web site that translates the gibberish to English or vice versa:

https://cdecl.org/

Or

  apt install cdecl

on Linux.

I've written some C code recently, and it came to me perhaps the pointer syntax may not be ideal. I'm not sure what the ideal would be, but I think a different notation for usage and declaration could make it less confusion.

In particular I associate '*' (used as *ptr, i.e. content that ptr points to), with content, as opposed to '&' (from &var, address of var), so again '*' means content thing points to. But in declaration, when you declare 'char *ptr', which is a pointer to a char, you clearly can't read it exactly the same way ("char with content of a pointer"? More like, the content of a pointer is char). So maybe another symbol like @ (denoting "is a pointer"), or just the keyword pointer, might make things clearer, so you'd have 'char pointer ptr' (ptr is a pointer to a char, read backwards) or simply 'char @ ptr'. The shorter '@' would be justified when you have multiple pointer e.g. when working with multidimensional arrays (which are often @@@float, something like that). Just an idea that occurred me ;)

(Although I hadn't thought about pcfwik's principle that it's written as used, that makes somewhat more sense to me)*

Edit: Said otherwise, in usage syntax the convention (or at least my way of thinking) may be left-to-right, "content of" or "address of", while in declaration we read right-to-left, "is an int", or "is a pointer", and it would make sense to me that the symbol for "is a pointer" is different than the symbol for "content of"/"address of".

There is only one iteration through the spiral in any one declarator unless there are parentheses. This is because it's basically:

   [pre] [pre] ... [pre] [core] [post] ... [post]

We have the core of the declarator, usually a name (or empty when omitted). On the left there is a clump of zero or more prefix declarative operators like the pointer *, and on the right postfix ones, like array and function parentheses.

No matter how many there are, you only to around he spiral one time. Let's add the declaration specifiers:

   [spec] ... [spec] [pre] [pre] ... [pre] [core] [post] ... [post]
                                           ------

"We declare "core" to be ..."

  [spec] ... [spec] [pre] [pre] ... [pre] [core] [post] ... [post]
                                          ------      ^
                                             \_______/

                                   

"We declare "core" to be a this, that and other postfix thing ..."

                             ________________________
                             /                        \
  [spec] ... [spec] [pre] [pre] ... [pre] [core] [post] ... [post]
                                           ------      ^
                                              \_______/

"We declare "core"t to be a this, that and other postfix thing, of this pre, pre ...

                              ________________________
                             /                        \
  [spec] ... [spec] [pre] [pre] ... [pre] [core] [post] ... [post]
                  ^________/               ------      ^
                                              \_______/

"We declare "core"t to be a this, that and other postfix thing, of this pre, pre of type/quality given by specs."

For instance:

   const unsigned int * * * x [][][3]
  

"Declare x to be a an array of arrays of arrays of 3 pointers to pointers to pointers, to const unsigned int"

But parentheses override the precedence of postfix versus prefix, so that's when the path follows a spiral with multiple loops, for each nesting level:

    const unsigned int *(*(*x)[][])[3]

Without parentheses, the precedence is as if implicitly there were these parentheses:

    const unsigned int ***(x[][][3])

i.e. postfix "binds tighter" than prefix/unary. That's the whole basis for the spiral: flipping from left to right chasing the sequences of postfix and unary operators, though all the levels of parentheses.

BTW, as a matter of terminology, ISO C does not call type construction punctuators operators; only expressions have operators. In computer science terminology related to programming languages, C declarators are type constructing expressions in which the elements like [] and * are type constructing operators.

My ideal syntax for function pointers is:

  fn signal(signum: int, fp: fn(int -> void)) -> fn(int -> void)

It keeps the parameter and return types inside, makes it obvious that it's a function using the fn keyword (or func or whatever), short and readable.

When you add more parameters you get `fn(int, char -> int)`. That's the only sane way to handle it and it also supports multiple return values if you want them.

Bonus: all type modifiers should be prefix like `?*MyStruct` and control flow should be postfix `task.await.match { ... }`. Every language should either have a pipe operator or let you call any function with method syntax. `x |> f |> g` is better than `g(f(x))`.

D simply reads right to left:

    int[]* p; // pointer to array of int
    int function(char*) fp; // pointer to function with char* parameter returning an int

The spiral rule makes no sense, I never got why it got popular.

Related. Others?

C "clockwise/spiral" rule to understand declarations - https://news.ycombinator.com/item?id=42564861 - Jan 2025 (75 comments)

Clockwise/Spiral Rule - https://news.ycombinator.com/item?id=25494219 - Dec 2020 (16 comments)

The Clockwise/Spiral Rule of C declarations - https://news.ycombinator.com/item?id=12775735 - Oct 2016 (68 comments)

The “Clockwise/Spiral Rule” - https://news.ycombinator.com/item?id=8648287 - Nov 2014 (26 comments)

The Clockwise/Spiral Rule - https://news.ycombinator.com/item?id=6471202 - Sept 2013 (7 comments)

The "Clockwise/Spiral Rule" in C - https://news.ycombinator.com/item?id=5079787 - Jan 2013 (45 comments)

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Learning this rule will mean you miss out on the opportunity to use one of the best words in the English language, which is “boustrophedonically” (meaning alternating left to right with right to left “as an ox ploughs a field”[1]).

C is parsed boustrophedonically.

[1] This is apparently the ancient Greek origin of the word.

I think that in 2026, it's not a good idea to still be using human brainpower to parse gibberish that was hastily decided in the 1960s/1970s for the convenience of writing parsers. Just use tools like cdecl: https://manpages.ubuntu.com/manpages/focal/man1/cdecl.1.html