如何在C中定义数组类型并使其成为const指针（而不是指向const的指针）

Question

I know that I can define array types in C like the following:

typedef int array_t[123];

This however does not make a declaration

void someFunction(array_t myArray)

constant in the sense that

myArray=0;

is forbidden inside the function, but

myArray[0]=0;

is allowed.

Is there a way to achieve this? If I add a "const" in the type definition or the parameter declaration, I end up with the reverse behavior, where array elements are constant, but the array pointer itself is variable.

Answer 1

You can never assign to an array variable, no matter what. While arrays decays to pointers, as long as it hasn't decayed it's no pointer so you can't assign to it.

If you want to set all of the array to a specific value, use memset , or if you want to copy another array use memcpy .

---

After the edit. The important thing to remember here is that if you pass it as an argument, then it's passed as a pointer and not an array.

What you can do is to declare the argument as a constant pointer instead of a pointer to constants.

Like

void someFunction(int * const myArray);

Then you can still modify the elements, but you can't change the actual pointer.

Answer 2

C99 did introduce syntax for declaring constant pointer parameters through "array" syntax

void foo(int a[const 123]) /* same as `int *const a` */
{
  a = NULL; // ERROR
}

As you can see in the above example, const have to be placed inside the [] .

However, that const cannot be embedded into array type "in advance" by using a typedef declaration for an array type. It can only be used directly in function parameter list.

Answer 3

If you absolutely need have to have an typedef , use struct to to encapsulate it. Using typedef on arrays is not a good idea.

typedef struct {
    int arr[123];
} array_t;

And then you can do what Joachim Pileborg suggested with const :

void someFunction(array_t * const myArray)
{
    myArray->arr[0] = ...

Additional advantage to this approach is that you can also transmit other data with your object.