【求助】错误在哪儿

#include<stdio.h>                       /*头文件*/
#define MAX 100                         /*栈的大小*/
typedef struct                          /*===栈的定义===*/
{
   char data[MAX];
   int top;
}SqStack;

int InitStack_sq(SqStack S)           /*===栈的初始化===*/
{  
   S.top=-1;
   return 0;
}

int StackEmpty_sq(SqStack S)           /*===判断栈空===*/
{
   if(-1==S.top) return 1;
   return 0;
}
int StackFull_sq(SqStack S)            /*===判断栈满===*/
{
  if(MAX-1==S.top)
  return 1;
  return 0;
}
int StackLength_sq(SqStack S)          /*===栈的长度===*/
{
  return S.top+1;
}

int Push_sq(SqStack S,char x)        /*===元素进栈===入口参数*/
{  if(StackFull_sq(S))
   return -1;                         /*栈满，返回错误标志*/
   S.top=S.top+1;                              /*  栈顶指针加1  */
   S.data[S.top]=x;             /* e成为新的栈顶  */
   return 0;                          /*  压栈成功    */
}
int Pop_sq(SqStack S,char y)         /*===元素出栈===*/
{  if (StackEmpty_sq (S))
   return -1;                         /*栈空，返回错误标志*/   
   y=S.data[S.top];
   S.top=S.top-1 ;
   return 0;
}

void main()
{SqStack M;
 char m='c';
 int Push_sq(SqStack S,char x);
 Push_sq(M,m);
 printf("%s",M.data[MAX]);
 getch();
}