C语言编的计算器的代码 谁有帮忙给个吧

#include <stdio.h>
struct s_node
{
int data;
struct s_node *next;
};
typedef struct s_node s_list;
typedef s_list *link;
link operator=NULL;
link operand=NULL;

link push(link stack,int value)
{
link newnode;

newnode=(link) malloc(sizeof(s_list));
if(!newnode)
{
printf("\nMemory allocation failure!!!");
return NULL;
}
newnode->data=value;
newnode->next=stack;
stack=newnode;
return stack;
}

link pop(link stack,int *value)
{
link top;
if(stack !=NULL)
{
top=stack;
stack=stack->next;
*value=top->data;
free(top);
return stack;
}
else
*value=-1;
}

int empty(link stack)
{
if(stack==NULL)
return 1;
else
return 0;

}

int is_operator(char operator)
{