Postfix expression evaluation

#include<stdio.h>
#include<stdlib.h>
#include<ctype.h>
struct node
{
    int data;
    struct node *next;
};

void push(int, struct node **);
int pop(struct node **);
int eval(int, int, char);


void main()
{
    char c;
    struct node *start=NULL;

    while((c=getchar())!='\n')
    {
        if(isdigit(c))
        {
            push((c-'0'),&start);
        }

        if(c=='*' || c=='/' || c=='+' || c=='-')
        {

            push(eval(pop(&start),pop(&start), c),&start);
        }
    }

    printf("\nAnswer is: %d", pop(&start));

}
void push(int element, struct node **start)
{

        struct node *new;
        new=(struct node *) (malloc(sizeof(struct node)));

    new->data=element;

        new->next=NULL;
        if((*start)==NULL)
        {
                *start=new;

                return;
        }
        new->next=*start;

        *start=new;
}

int pop(struct node **start)
 {
    struct node *p;
    int x;

        p=(*start);
        (*start)=(p->next);
    x=p->data;
        free(p);
        return(x);

 }

int eval(int top, int bot, char c)

{
        printf("\n %d %d %c",top,bot,c);
    switch(c)
    {
    //  printf("\n %d %d %c",top,bot,c);
        case '+':
            return (bot+top);
        case '/':
            return (bot/top);
        case '*':
            return (bot*top);
        case '-':
            return (bot-top);
    }
}