my_array.c

#include <assert.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

struct my_array {
	void *data;
	size_t cap;
	size_t len, elem_size;
};

void my_array_init(struct my_array *array, size_t elem_size) {
	memset(array, 0, sizeof(struct my_array));
	array->elem_size = elem_size;
}

void *my_array_grow(struct my_array *array, size_t n) {
	size_t size = array->len * array->elem_size;
    assert(size <= array->cap);

    size_t new_len = array->len + n;
	size_t new_size = size + n * array->elem_size;

	size_t new_cap = array->cap;
	if (new_cap == 0) {
		new_cap = 16;
	}
	while (new_size > new_cap) {
		new_cap *= 2;
	}

	if (array->cap < new_cap) {
		void *new_data = realloc(array->data, new_cap);
		if (new_data == NULL) {
			return NULL;
		}
		array->data = new_data;
		array->cap = new_cap;
	}

	void *ptr = (char *)array->data + size;
	array->len = new_len;
	return ptr;
}

void my_array_add(struct my_array *array, void *value) {
	void *ptr = my_array_grow(array, 1);
	if (ptr == NULL) {
		return;
	}
	memcpy(ptr, value, array->elem_size);
}

void my_array_add_array(struct my_array *array, void *src, size_t n) {
	void *ptr = my_array_grow(array, n);
	memcpy(ptr, src, n * array->elem_size);
}

void my_array_finish(struct my_array *array) {
	free(array->data);
}

void main() {
    struct my_array a = {0};
    my_array_init(&a, sizeof(int));
    int v = 0;
    my_array_add(&a, &v);
    v = 1;
    my_array_add(&a, &v);
    v = 42;
    my_array_add(&a, &v);

    int *values = a.data;
    for (size_t i = 0; i < a.len; ++i) {
        printf("%d\n", values[i]);
    }
}