Stacks: postfix to infix

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C

I wrote the following code to convert a postfix expression to a fully paranthesised infix expression, using stacks.

#include<stdio.h>
#include<stdlib.h>
#include<string.h>

typedef struct node
{
    char *data;
    struct node *point;
}NODE;
NODE *start = NULL;

void push(char *);
char *pop();

int main()
{
    char c;
    char op[2];
    char *op1;
    char *op2;
    while((c = getchar()) != '\n')
    {
        op[0] = c;
        op[1] = '\0';
        printf("%s\n", op);
        if(isalnum(c))
        {
            push(op);
        }
        else
        {
            op2 = pop();
            op1 = pop();
            op1 = strcat(op1, op_);
            op1 = strcat(op1, op2);
            op1 = strcat("(", op1);
            op1 = strcat(op1, ")");
            push(op1);
            printf("%s\n%s\n", op1, op2);
        }
    }
    printf("%s\n", pop());
    return 0;
}

void push(char *value)
{
    NODE *top = malloc(sizeof(NODE));
    if(top == NULL)
    {
        fputs("Error: no memory\n", stderr);
        abort();
    }
    else
    {
        (top->data) = strcpy(top->data, value);
        top->point = start;
        start = top;
    }
}

char *pop()
{
    if(start == NULL)
    {
        fputs("Error: stack underflow\n", stderr);
        abort();
    }
    else
    {
        NODE *top = start;
        char *value = top->data;
        start = top->point;
        free(top);
        return value;
    }
}

The highlighted statement in the pop() function is the one causing problems. I have used the same statement in other stack programs that I wrote, yet I am facing problems here. Kindly help me out.
Thanks in advance.