How to determine the number of array elemnts in C?

Question

I have a function in C (I am using gcc 4.8.0) which receives an array as its argument. Is there a way to accurately determine the number of elements in the array, without passing an additional argument array_count?

Answer 1

There is no way to determine in general the number of array elements passed as parameter.

When you pass a array as argument in C, you only pass the pointer to that array, that is the pointer to the first element (indexed 0) of the memory zone holding that array.

Very often, programmers have the convention of passing, as another argument, the size of that array. For example, the standard qsort(3) library function is expecting, as its second nmemb argument, the number of elements in the array to be sorted.

Alternatively, you might use flexible array members, e.g. pass (in C99, not in earlier C standard) address of structures like

struct my_flexarray_st {
   int size; // allocated size
   int len;  // used length, should be less than size
   double arr[]; /// only len numbers are meaningful
};

Whatever method you use, you need to understand that array sizes are conventionally known by functions when passed by argument. So please, document that convention. You could even have the (bad) convention that all arrays have some global variable as their dimension.

For heap allocated memory zones, the standard gives you malloc, calloc and friends to get such fresh zones, but no way to query their allocated sizes. Some C libraries have non-standard extensions to query that (but I don't recommend using them).

In recent C++11 (which is not the same as the C language) you might be interested by std::vector and std::array template containers.

Answer 2

Arrays decay into pointers in function arguments so size cannot be determined.

An lvalue [see question 2.5] of type array-of-T which appears in an expression decays (with three exceptions) into a pointer to its first element; the type of the resultant pointer is pointer-to-T because an array is not a "modifiable lvalue,"

(The exceptions are when the array is the operand of a sizeof or & operator, or is a literal string initializer for a character array.)

Answer 3

You can't , unless the array is static (i.e. not dynamic-allocated) , then you can use sizeof operator.

Answer 4

Not possible. This is why you either use a data structure like a linked list where you can actually determine the length or you require a length argument in your function. Even with the required length argument alone there is no way to know it is true. So, your API should also require a standard sentinel value to terminate the array. Generally this could be NULL or something but depends on your array type.

Answer 5

This is doable in C. You can query size of array if you are in the same scope of its definition via sizeof. If you are in scope of function which takes array as parameter you need to declare the parameter as pointer to array (not just as array - in this case array will decay to pointer to first element), in this case size of array would be saved during argument passing:

void foo(int (*param)[3] ) { assert (sizeof(*param) == sizeof(int)*3; }

However, if you mean by "array" pointer to some dynamically allocated memory with not-known at compile time size, then you definetely need to pass size separately.

Answer 6

you can do a while look

function(arr[])
{
   /* not this is pseduo code */
    int i = 0;
    while (*arr[i++] != null)
    {

    }

// i is number of elements
}