C - return array of unknown size

Question

Suppose I want to write (in C99) the following function:

NAME: primes
INPUT: an integer n > 0
OUTPUT: int array filled with the prime numbers in range [2, n]

How can I return such an array from my function? Is that possible?

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Note that I don't want the caller to allocate an n * sizeof(int) array that I'll fill with 0 (composite) and 1 (primes).

I can't just return a pointer to the array, because the caller has no way to know how long the array is:

int * primes(int n)
{
    int * arr = malloc(n * sizeof(int));
    // do stuff
    return arr;
} 

int main(void)
{
    int * arr = primes(100);
    printf("%lu \n", sizeof arr); // prints 8
}

and I can't change the signature like this:

int (*primes(int n))[LENGTH]  

because LENGTH is unknown at compile time.

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I read somewhere something like "to return a struct with the array is an horrible idea", and, well... that was my last idea.

What's the best practice in cases like this?

Answer 1

If the function that you call must decide the actual number of elements that it needs to allocate, you should pass a pointer to the allocated length along with the rest of the parameters, like this:

size_t actual_length;
int *arr = primes(100, &actual_length);
if (arr == NULL) {
    ... // Report an error
}
for (size_t i = 0 ; i != actual_length ; i++) {
    printf("%d\n", array[i]);
}

The primes would then look like this:

int *primes(int count, size_t *actual_length) {
    size_t primes_needed = ...
    int *res = malloc(sizeof(*res)*primes_needed);
    *actual_length = primes_needed;
    // Do calculations, perhaps some reallocs
    // Don't forget to reassign *actual_length = ... on realloc
    ...
    return res;
}