segfault infix to postfix

/* convert infix notation to postfix notation */
#include<stdio.h>
#include<string.h>
char stackPop(char *, int *); //stack array and address of top.
void stackPush(char *, char, int *); //stack array, element to push, address of top.
void main()
{
    int i, top = -1;
    char stack[100],infix[100],postfix[100],p;
    printf("Enter the infix expression: ");
    scanf("%s",infix);
    stackPush(stack,'(',&top);
    strcat(")",infix);
    i=0;
    while (top!=-1) //process stops when stack is empty.
    {
        if(infix[i] == '+' || infix[i] == '-' || infix[i] == '*' || infix[i] == '/')
        {
            if (infix[i] == '+' || infix[i] == '-')
            {
                while(stack[top] == '+' || stack[top] == '-' || stack[top] == '*' || stack[top] == '/')
                {
                    p = stackPop(stack, &top);
                    postfix [strlen(postfix)] = p;
                }
            }
            else
            {
                while(stack[top] == '*' || stack[top] == '/')
                {
                    p = stackPop(stack, &top);
                    postfix [strlen(postfix)] = p;
                }
            }
            stackPush(stack,infix[i],&top);
        }
        else if(infix[i] == '(')
            stackPush(stack,infix[i],&top);
        else if(infix[i] == ')')
        {
            p = stackPop(stack,&top);
            while (p!= '(')
            {
                postfix [strlen(postfix)] = p;
                p = stackPop(stack,&top);
            }
        }
        else //for infix[i] == operand
            postfix [strlen(postfix)] = infix[i];
        i++;
    }
    printf("The postfix notation is %s", postfix);
}

void stackPush(char *stack, char item, int *top)
{
    stack[++(*top)]=item;
}

char stackPop(char *stack, int *top)
{
    int item = stack[(*top)--];
    return item;
}