Is there a way to get around this with a loophole?

I'm trying to see if it is possible to make an array in C that has no definite size. I came up with this idea when I woke up so I programmed it, and surprise surprise! It made an error... Here is my C code:

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

struct arraylistInt
{
    long length;
    int (*arrayPtr)[];
    void (*increaseLenArraylistIntPtr)(struct arraylistInt, long);
    void (*setArraylistIntPtr)(struct arraylistInt, long, int);
    int (*getArraylistIntPtr)(struct arraylistInt, long);
    void (*initArrayListIntPtr)(struct arraylistInt, long);
};

void increaseLenArraylistInt(struct arraylistInt self, long sizeInc)
{
    int localArray = *self.arrayPtr;
    free(self.arrayPtr);
    self.length += sizeInc;
    self.arrayPtr = (int*) malloc(4 * self.length);
    for(long i = 0; i < (self.length - sizeInc); i++)
    {
        *self.arrayPtr[i - 1] = localArray[i - 1];
    };
    for(long i = self.size - sizeInc; i < self.size; i++)
    {
        (*self.arrayPtr)[i - 1] = 0;
    }
}

int getArraylistInt(struct arraylistInt self, long index)
{
    return (*self.arrPtr)[index];
}

void setArraylistInt(struct arraylistInt self, long index, int value)
{
    if(index <= self.length)
    {
        (*self.arrayPtr)[index] = value;
    }
    else if(index - self.length == 1)
    {
        (*self.increaseLenArraylistIntPtr)(self, 1);
        self.arrayList[self.length - 1];
    }
    else
    {
        printf('index size exceeds threshold, line 41.');
    };
}

void initArrayListInt(struct arraylistInt self, long defaultLength)
{
    self.length = defaultLength;
    self.arrayPtr = (int*) malloc(self.length * 4);
    self.increaseLenArraylistIntPtr = &increaseLenArraylistInt;

}

int main()
{
    printf("Hello world!\n");
    struct arraylistInt newArraylist;
    newArraylist.initArrayListIntPtr = &initArrayListInt;
    (*newArraylist.initArrayListIntPtr)(newArraylist, 2);
    newArraylist.setArraylistIntPtr(newArraylist, 1, 43);
    printf('The value at index 1 is: %i', (*newArraylist.getArraylistIntPtr)(newArraylist, 1));
    return 0;
}

When I run it, I get this error:

22   error: invalid use of array with unspecified bounds

I know the cause, but I can't think of any loopholes that can fix this. Is there an actual solution? Thanks in advance btw.