infix to postfix with bracket

/****************************
 *  Umang B Bhatt            *
 *   bhatt.umang7@gmail.com  *
 *****************************/

/**
 * program for infix to postfix with bracket
 */

#include<stdio.h>
#define MAX 100


/* 
 * this will not catch errors
 * eg if you enter (a*b]+c then it will output ab*c+
 */

/* OP:
 * Enter expression: (a*(c+d)+(e+f)*g)/(j+i)
 * postfix of entered (infix) expression is:
 * acd+*ef+g*+ji+/
 */


// personally, i believe that declaring TOP and S here is bad practice

void main()
{
    int TOP = -1, PTOP = 0;

    char s[MAX];
    char post[MAX] = {'\0'};

    int pre(char);
    int push(char[], int, char);

    int pop(char *, int *);

    char c[MAX] = {'\0'}, symb, x;

    int i;
    printf("Enter expression: ");
    gets(c);

    /*
     * gets will give error when compiling on modern compiler
     */

    // x = pop(s,&TOP);
    // TOP = push(s,TOP,symb);

    for (i = 0; c[i] != '\0'; i++)
    {

        symb = c[i];
        if (isalpha(symb) > 0)
        {

            post[PTOP] = symb;
            PTOP++;
        }
        else if (symb == '+' || symb == '-' || symb == '*' || symb == '/' || symb == '\\' || symb == '^' || symb == '$')
        {

            while ((pre(symb)) <= (pre(s[TOP])) && TOP >= 0)
            {

                x = pop(s, &TOP);
                post[PTOP] = x;

                PTOP++;
            }
            TOP = push(s, TOP, symb);
        }

        else if (symb == '(' || symb == '[' || symb == '{')
        {

            TOP = push(s, TOP, symb);
        }
        else

        {
            x = '\0';
            while (x != '(' && x != '{' && x != '[')
            {

                x = pop(s, &TOP);
                if (x != '(' && x != '{' && x != '[')
                {

                    post[PTOP] = x;
                    PTOP++;
                }
            }
        }
    }
    while (TOP >= 0)
    {

        x = pop(s, &TOP);
        post[PTOP] = x;

        PTOP++;
    }
    post[PTOP] = '\0';
    printf("\npostfix of entered (infix) expression is:\n%s\n", post);


    // getch();  // win not work on linux
}

int push(char *s, int top, char ele)
{

    int i;
    if (top >= MAX)
    {

        printf("\nStack Overflow");
    }
    else
    {
        s[++top] = ele;
    }

    return top;
}

int pop(char *a, int *top)
{

    if ((*top) >= 0)
    {
        (*top) = (*top) - 1;

        return a[(*top) + 1];
    }
    else
    {

        printf("Stack underflow\n");
        return 0;
    }
}

int pre(char x)
{

    int a;
    switch (x)
    {
    case '(':

    case '{':
    case '[':
        a = -5;

        break;
    case '+':
    case '-':
        a = 2;

        break;
    case '/':
    case '*':
    case '\\':

    case '%':
        a = 5;
        break;

    case '$':
    case '^':
        a = 10;

        break;
    }
    return a;
}