栈的基本操作，请指点哪里出现错误。

各位大神，请看一下我的顺序栈的建立与输出到底哪出现错误，百思不得其解。我用的是DEV-C。
#include <stdio.h>
#include <stdlib.h>
#include<malloc.h>

#define STACK_INIT_SIZE 100
typedef int SElemType;

 typedef struct {
    SElemType *base;
    SElemType *top;
    int StackSize;
}sqStack;

//创建栈
void InitialStack(sqStack *S)
{
    int a;
    S->base = (SElemType *)malloc(STACK_INIT_SIZE*sizeof(SElemType));
    if(!S->base)
        printf("创建失败！\n");
    else
    {
        S->top = S->base;
        S->StackSize = STACK_INIT_SIZE;
        scanf("%d",&a);
        while(a!=-1&&((S->top )- (S->base )< (S->StackSize)))
        {
            *(S->top)=a;
            S->top++;
                     scanf("%d",&a);
        }
    }   
}
void StackPrintf(sqStack *S){
    SElemType *p;
    p=S->base;
    while(p!=S->top)
        {
        printf(" %d",*p);
        p++;}
}

int main(int argc, char *argv[]) {
   
    sqStack *S;
    printf("\n创建一个栈:\n");
    InitialStack(S);
    //输出栈
    StackPrintf(S);
    return 0;
}