What does the (*) in (int (*)[30]) mean?

Question

What does (int (*)[30]) mean in C? For instance, in:

int (*b)[30] = (int (*) [30]) malloc(30 * sizeof(int [20]));

Answer 1

It means, roughly, "is a pointer".

int (*b)[30]

This means "b is a pointer to an array of 30 integers".

(int (*) [30])

This means "cast to a pointer to an array of 30 integers".

Answer 2

int (*b)[30] = (int (*) [30]) malloc(30 * sizeof(int [20]));

Breaking it down:

      b        -- b
    (*b)       -- is a pointer
    (*b)[30]   -- to a 30-element array
int (*b)[30]   -- of int.

In both declarations and expressions, postfix operators like [] have higher precedence than unary operators like *, so T *a[] is interpreted as T *(a[]); IOW, a is an array of pointer to T. To designate a as a pointer to an array, we have to force the grouping T (*a)[].

Simlilarly, the cast expression (int (*) [30]) means "treat the pointer value returned by malloc as a pointer to a 30-element array of int". Note that, technically speaking, the cast expression is superfluous and should be removed.

The malloc call itself seems very wrong. You're allocating 30 instances of a 20-element array of int, but assigning the result to a pointer to a 30-element array of int; that's going to cause problems. Assuming you're trying to allocate a N x 30 matrix of int, the following would be safer:

int (*b)[30] = malloc(N * sizeof *b); 

The type of the expression *b is int [30], so sizeof *b is the same as sizeof (int [30]).

Answer 3

How to parse C declarations and types: unwind them from outside in.

• int (*b)[30].
• (*b)[30] is an int.
• (*b) is an int array of length 30.
• b is a pointer to an int array of length 30.

The nameless version int (*) [30] is entirely identical, just the name has been omitted.

If you have a copy of The C Programming Language, there's a program in there called cdecl that can transform such declarations into English. There's been various modifications of it over time, for example cutils in Debian supports the nameless form, and cdecl.org is online.