array as an abstract data type

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

struct myArray{
    int total_size;
    int used_size;
    int *ptr;
};

void createArray(struct myArray*a, int tSize, int uSize){
    // (*a).total_size = tSize;
    // (*a).used_size = uSize;
    // (*a).ptr = (int*)malloc(tSize*sizeof(int));
    
    a->total_size = tSize;
    a->used_size = uSize;
    a->ptr = (int*)malloc(tSize*sizeof(int));
}

void show(struct myArray *a){
    for(int i=0; i<a->used_size;i++){
        printf("%d ", (a->ptr)[i]);
    }
}
void setVal(struct myArray *a){
    int n;
    for(int i=0; i<a->used_size;i++){
        printf("enter element %d\n",i);
        scanf("%d",&n);
        (a->ptr)[i]=n;
    }
}

int main() {
    struct myArray marks;
    createArray(&marks,100,20);
    setVal(&marks);
    show(&marks);
    
    return 0;
}