The C Standard specifies some translation limits. I cannot understand the following two:
12 pointer, array, and function declarators (in any combinations) modifying an arithmetic, structure, union, or void type in a declaration
63 nesting levels of parenthesized declarators within a full declarator
Can someone explain and give simple examples for:
C 2018 2.7.6 specifies declarators, including showing the grammatical definition of a declarator as:
declarator: pointeropt direct-declarator
direct-declarator:
identifier
(declarator)
direct-declarator[…]
direct-declarator(…)pointer:
*type-qualifier-listopt
(The syntax used for this grammar is briefly discussed in 6.1. Even more briefly, “direct-declarator:” means, when the compiler is looking for a direct-declarator, it will accept any of the options listed on the following lines. The subscript “opt” means an item is optional. I have consolidated multiple options in the original to “…” since the details are unimportant in this question. For example, array declarators may be empty or may contain an assignment expression and may have static, and so on.)
In int x;, x is a declarator. The declaration causes x to be declared as an int.
In int *x, *x is a declarator. It causes x to be declared as an int *. So it has modified the declaration.
In int (*x[3][4])(float, char);, x is a declarator (because it is a direct-declarator that is an identifier). It is inside an array declarator (a term shown in 6.7.6.2 to refer to the options with [ and ] above), x[3]. That is inside another array declarator, x[3][4]. That is a direct-declarator inside a pointer declarator (shown in 6.7.6.1 to refer to the pointer option above), *x[3][4]. That is inside a parenthesized declarator, (*x[3][4]) (a term that is not explicitly defined but is clear). And that is inside a function declarator (6.7.6.3), (*x[3][4])(float, char). Thus, this declaration has five nested levels of declarators modifying the int type in the declaration, four of which are pointer, array, or function declarators (one is a parenthesized declarator). It declares x to be an array of 3 arrays of 4 pointers to a function taking float and char and returning int.
At this point, the “63 nesting levels of parenthesized declarators within a full declarator” should be clear. int (((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((((x))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))); has 63 nested levels of parenthesized declarators within a full declarator. This does not normally arise in direct drafting of a declaration, but construction of declarations by use of various macros might give rise to multiple levels of parentheses.
The standard is treating pointer, array, and function declarators as something the compiler must keep track of, and is therefore subject to a modest limit of 12 nested levels, whereas parentheses are a superficial grammatical construction that need be tracked only during parsing and can then be discarded internally, and so are less burdensome on a compiler and have a more generous limit.