Error with the malloc command in C - c

Question

my Excercice is to initalise space from the heap in function1(); and to create an array there. In the main I have to print the array. What have I done wrong?
CODE
#include <stdio.h>
#include <stdlib.h>
int functionOne(int size);
int main()
{
int size = 0,i;
scanf("%d",&size);
int *arrsize = functionOne(size);
printf("rueckgabe %d",arrsize);
int arr [*arrsize];
arr[0] = 7;
arr[1] = 2;
arr[2] = 3;
arr[3] = 4;
arr[4] = 5;
for(i=0; i<size; i++)
{
printf("[%d]",arr[i]);
}
}
int functionOne(int size)
{
int *arr;
arr = NULL;
arr = malloc(size * sizeof(int));
return arr;
}

Answer 1

The thing is on the line printf("rueckgabe %d",arrsize); you are printing the address returned by other function - that too using wrong format specifier.
Suppose you allocated memory for 4 int using the other function functionOne and the memory of the allocated chunk is being returned - now when you do *arrsize you are basically accessing the 0th positional int value. But as it is uninitialized - the value of it is indeterminate.
Earlier you were declaring VLA with uninitialized value. This is undefined behavior. Also there is no meaning printing the contents of the variable length array unless you initialize them (i.e.,printf("[%d]",arr[i]);).
Return value of the function functionOne is int but you are returning int*.
There are many other things you can follow in the code, like correcting the indentation - checking the return value of malloc and changing the signature of main from int main() to int main(void).
I have added some demo code showing whatever I have mentioned above:
Edit:
#include <stdio.h>
#include <stdlib.h>
int* functionOne(int size);
int main(void)
{
int size = 0;
if( scanf("%d",&size)!= 1){
fprintf(stderr,"Error in input\n");
exit(EXIT_FAILURE);
}
if( size <= 0){
fprintf(stderr,"Error in input [size]\n");
exit(EXIT_FAILURE);
}
int *arr = functionOne(size);
for(size_t i = 0; i < size; i++){
arr[i]=i;
}
for(size_t i = 0; i<size; i++){
printf("arr[%zu] = %d \n",i,arr[i]);
}
free(arr);
return 0;
}
int* functionOne(int size)
{
int *arr;
arr = malloc(sizeof(int)*size);
if( arr == NULL && size > 0){
perror("malloc");
exit(EXIT_FAILURE);
}
return arr;
}