Stack PUSH and POP in C

void STACKinit(int);

int STACKempty();
void STACKpush(Item);
Item STACKpop();

#include <stdio.h>
#include <string.h>
#include "Item.h"
#include "STACK.h"
main(int argc, char *argv[])
  { char *a = argv[1]; int i, N = strlen(a);
    STACKinit(N);
    for (i = 0; i < N; i++)
      {
        if (a[i] == '+')
          STACKpush(STACKpop()+STACKpop());
        if (a[i] == '*')
          STACKpush(STACKpop()*STACKpop());
        if ((a[i] >= '0') && (a[i] <= '9')) 
          STACKpush(0);
        while ((a[i] >= '0') && (a[i] <= '9')) 
          STACKpush(10*STACKpop() + (a[i++]-'0')); 
      }
    printf("%d \n", STACKpop());
  }