C structure to be dynamically allocated

Question

I’m needing it to be dynamically allocated but I’m not finding the problem.

#include <stdio.h>
#include <stdlib.h>
#define tam 10

typedef struct pilha{
    char dado[tam];
    int pos;
}Pilha;

Pilha *p=(Pilha*)malloc(sizeof(Pilha));

void push(char valor){
    p->dado[p->pos]=valor;
    p->pos++;
}

void pop(){
    p->dado[--p->pos];
}

int size(){
    return p->pos;
}

int main(){

    push("d");
    push("b");
    pop();

    printf("Tamanho da pilha é: %d",size());
    return 0;
}