Passing a array as an argument in C and finding the sum of its elements with no prior knowledge of its length

I am trying to sum the elements of an int array, without having prior knowledge of the the array length.

#include <stdio.h>

int sum(int anArray[]) {
  
  int sum = 0;
  size_t arrayLength = sizeof(anArray) / sizeof(int);
  
  for(int i = 0; i < arrayLength; i++) {
    sum += anArray[i];
  }

  return sum;
}

int main() {
  int myIntegers[3] = {1, 2, 3};

  printf("sum: %d", sum(myIntegers));
  return 0;
}

I get 3 as a result, but I expect 6. I don't understand why this happens.

More particularly

sizeof(anArray)

the above returns 8 for a 3 element array, while I expect it to return 12. Note that sizeof(int) in my machine is 4.