Expression Parser

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

int eval(char **_s);
int feed_signed_operand(char **_s);

int feed_num(char **_s)
{
        char *s = *_s;
        int num = 0;

        while (isdigit(*s))
        {
                num = num * 10 + *s - '0';
                s++;
        }
        *_s = s;
        return num;
}

int feed_single_operand(char **_s)
{
        char *s = *_s;
        int  num = 0;


        if (*s == '(')
        {
                s++;
                num = eval(&s);
                if (*s == ')')
                        s++;
                else {
                        printf("missing ) at %s\n", s);
                        exit(0);
                }
        }
        else if (isdigit(*s))
                num = feed_num(&s);
        else {
                printf("error at %s\n", s);
                exit(0);
        }

        *_s = s;
        return num;
}

int feed_powered_operand(char **_s)
{
        char *s = *_s;
        int num = 0;

        num = feed_single_operand(&s);
        if (*s == '^')
        {
                int num2;
                s++;

                num2 = feed_signed_operand(&s);
                num = (int) pow(num, num2);
        }

        *_s = s;
        return num;
}

int feed_signed_operand(char **_s)
{
        char *s = *_s;
        int multBy = 1, num = 0;

        while (*s == '-' || *s == '+')
        {
                if (*s == '-')
                        multBy *= -1;
                s++;
        }

        num = feed_powered_operand(&s);

        num *= multBy;
        *_s = s;
        return num;
}

int feed_mul_dip_expr(char **_s)
{
        char *s = *_s;
        int num;
        int res = 1;
        char c = 0;

        while (1)
        {
                num = feed_signed_operand(&s);
                if (c == '*' || c == 0)
                        res *= num;
                else if (c == '/')
                        res /= num;

                if (*s == '*' || *s == '/')
                        c = *s++;
                else
                        break;
        }

        *_s = s;
        return res;
}


int eval(char **_s)
{
        char *s = *_s;
        int num;
        int res = 0;
        char c = 0;


        while (1)
        {
                num = feed_mul_dip_expr(&s);
                if (c == '+' || res == 0)
                        res += num;
                else if (c == '-')
                        res -= num;

                if (*s == '+' || *s == '-')
                        c = *s++;
                else
                        break;
        }

        *_s = s;
        return res;
}


int main()
{
#define E "-22/-11+2^3*2^(1+2^((-1+3)*2))"
//#define E "-(2^(-++-1))"
        char buff[10000],*s;
        int res;

        strcpy(buff, E);
        s = buff;
        res = eval(&s);
        printf("%s =  %d\n", buff, res);
}