Deallocating only a portion of the memory allocated with malloc

Question

I need to create a vector of which I do not know the number of components at the moment of the declaration. For example, suppose that the vector will contain the prime numbers below a given n and I cannot know how many there are before actually evaluating them.

Whenever I allocate memory with malloc, I know that the memory can later be freed using free:

int main(int argc, char** argv)
{
    int* p=malloc(4*sizeof(int));
    for(int j=0; j<4; j++)
    {
        *(p+j)=2*j;
    }
    printf("%d %d %d %d\n", *p, *(p+1), *(p+2), *(p+3));
    free(p);
    return 0;
}

My idea was to allocate more memory than I what I need, and then to free only a portion of it. So, naively I would like to do something like int* p=malloc(1000*sizeof(int)) (supposing that I know for sure that p will have less than 1000 components) and then use free(p+j) where j is the number of meaningful components that I have given to p after the computation. But that is not allowed.

Of course, there is always the possibility of making a list instead, using struct, and create the array only at the end, but I don't like that solution, it seems convoluted to me.

What is the right way to create an array when I only know an upper bound for its size at the time of declaration, in case the size of the array will only be determined at the end?

Does this answer your question? [dynamic memory allocation in c , free some part of memory that is allocated before using malloc()](https://stackoverflow.com/questions/23040343/dynamic-memory-allocation-in-c-free-some-part-of-memory-that-is-allocated-befo). `realloc` may be the option you are looking for. — Ted Lyngmo, Jan 20 '23 at 19:50
Suppose you guessed `N` primes and did `int *p = malloc(N * sizeof *p)`. If you only use `M` of the elements, you can later do `p = realloc(p, M * sizeof *p);` — Weather Vane, Jan 20 '23 at 19:51
If you want to reduce the size of a block of memory that was previously allocated with `malloc`, you can use `realloc`, giving it the new, smaller size. The initial portion is retained, but note that the address may change. — Tom Karzes, Jan 20 '23 at 19:52

score 5 · Accepted Answer · edited Jan 20 '23 at 20:17

Using free(p+j) is not valid because you can only pass to free a pointer that was returned by malloc, calloc, realloc or aligned_alloc.

And speaking of realloc, that's exactly what you would use to resize a piece of allocated memory:

void *t = realloc(p, j * sizeof *p);
if (t) p = t;

Note that you'll want to save the returned pointer in a separate variable in case realloc fails, in which case the original pointer is still valid.

Deallocating only a portion of the memory allocated with malloc

1 Answers1