Postfix evaluation using stacks and C

I was on here a while a go with a similar problem but I think with the wrong question. To give a bit of background, I a tasked with creating a C program to solve a postfix expression in the form

8 7 - 9 * =

What I think my problem is, is that my prof gave as some incorrect stack code. I say this because I am constantly getting the stack overflow (lol) error and my stack is nowhere near full. If it helps I'm using visual studio 2005. Here's my code:

 #include <stdio.h> ` #include <stdlib.h> #define STACK_SIZE 20 typedef int Bit; Bit contents[STACK_SIZE]; int top = 0; void make_empty(void); int is_empty(void); int is_full(void); void push(Bit i); int pop(void); void stack_overflow(void); void stack_underflow(void); int main(void) { Bit bit; char operation; int operand; Bit current; int result; while(scanf("%d",&current)!= '=') { push(current); } scanf("%c", &operation); while(operation != '=') { scanf("%d", &operand); printf("%d\n",top); //Pushes any number into the stack if(operand==1||operand==2||operand==3||operand==4||operand==5||operand==6||operand==7||operand==8||operand==9||operand==0) { printf("entered number loop\n"); bit = operand; if(top==20) { stack_overflow(); } push(&bit); } //Performs subtraction operation else if(operation == '-') { printf("entered minus loop\n"); if(top==1) { stack_underflow(); } result = pop() - pop(); bit = result; if(top==20) { stack_overflow(); } push(&bit); } //Performs addition operation else if(operation == '+') { if(top==1) { stack_underflow(); } result = pop() + pop(); bit = result; if(top==20) { stack_overflow(); } push(&bit); } //Performs multiplication operation else if(operation == '*') { if(top==1) { stack_underflow(); } result = pop() * pop(); bit = result; if(top==20) { stack_overflow(); } push(&bit); } //Performs division operation else if(operation == '/') { if(top==1) { stack_underflow(); } result = pop() / pop(); bit = result; if(top==20) { stack_overflow(); } push(&bit); } else if(operation == '=') { if(top==0) { stack_underflow(); } printf("%d\n",pop()); break; } } return 0; } void make_empty(void) { top = 0; } int is_empty(void) { return top == 0; } int is_full(void) { return top == STACK_SIZE; } void push(Bit i) { if (is_full()) stack_overflow(); else contents[top++] = i; } int pop(void) { if (is_empty()) stack_underflow(); else return contents[top--]; } void stack_overflow(void) { printf("Error: stack overflow!\n"); exit(EXIT_FAILURE); } void stack_underflow(void) { printf("Error: stack underflow!\n"); exit(EXIT_FAILURE); } 

Now I realize that my code is a little barbaric right now and for that I apologize. That being said, any help or input at all would be greatly appreciated and thank you all in advance.

Ok, so after taking everything into account, I think I'm getting close. Everything going into the stack properly and everything is being read properly. However, my new implementation includes making everything a character and then converting the integers when they need to be used. Here is my source code once again:

#include <stdio.h> #include <stdlib.h> #define STACK_SIZE 20 typedef int Bit; char contents[STACK_SIZE]; int top = 0; void make_empty(void); int is_empty(void); int is_full(void); void push(char i); char pop(void); void stack_overflow(void); void stack_underflow(void); int main(void) { char current = 'a'; char result = 'a'; char operation = 'a'; char char1; char char2; int number1; int number2; scanf("%c", &current); //While program successfully scanned a number while(current != '=') { //Performs subtraction operation if(current == '-') { printf("entered if 2\n"); char1 = pop(); number1 = char1 - '0'; printf("%d\n", number1); char2 = pop(); number2 = char2 - '0'; printf("%d\n", number2); result = number1 - number2; push(result); } //Performs addition operation else if(current == '+') { printf("entered if 2\n"); char1 = pop(); number1 = char1 - '0'; printf("%d\n", number1); char2 = pop(); number2 = char2 - '0'; printf("%d\n", number2); result = number1 + number2; push(result); } //Performs multiplication operation else if(current == '*') { printf("entered if 2\n"); char1 = pop(); number1 = char1 - '0'; printf("%d\n", number1); char2 = pop(); number2 = char2 - '0'; printf("%d\n", number2); result = number1 * number2; push(result); } //Performs division operation else if(current == '/') { printf("entered if 2\n"); char1 = pop(); number1 = char1 - '0'; printf("%d\n", number1); char2 = pop(); number2 = char2 - '0'; printf("%d\n", number2); result = number1 / number2; push(result); } else { push(current); printf("%c\n", current); } scanf(" %c", &current); } //Prints result printf("%c\n",pop()); return 0; } void make_empty(void) { top = 0; } int is_empty(void) { return top == 0; } int is_full(void) { return top == STACK_SIZE; } void push(char i) { if (is_full()) stack_overflow(); else contents[top++] = i; } char pop(void) { if (is_empty()) stack_underflow(); else return contents[top--]; } void stack_overflow(void) { printf("Error: stack overflow!\n"); exit(EXIT_FAILURE); } void stack_underflow(void) { printf("Error: stack underflow!\n"); exit(EXIT_FAILURE); } 

Please keep in mind that I have been playing around with it quite a bit, so there are random printfs and useless variables all for debugging purposes. Whenever I run it (with example input 3 5 + =) I get:

So again, please excuse my some what messy code as I am quite new to C but any help would be great!