Oops!

#include <limits.h>

#define INT_MAX_DIV_BY_10 (INT_MAX / 10)
#define INT_MAX_REM_BY_10 (INT_MAX % 10)
#define INT_MIN_DIV_BY_10 (INT_MIN / 10)
#define INT_MIN_REM_BY_10 (INT_MIN % 10)

int myAtoi(char * str){
    char *p = str, c;
    int s = 1;
    int d;
    int v = 0;

    while (c = *p++) {
        if (c == ' ')
            continue;

        if ((c == '+' || c == '-') && *p >= '0' && *p <= '9') {
            if (c == '-')
                s = -1;
            break;
        } else if (c >= '0' && c <= '9') {
            p--; // point back to currect digit
            break;
        } else {
            return 0; // invalid
        }
    }

    // handle empty string or it contains only whitespaces
    if (!c)
        return 0;

    // at this time p must point to a digit
    while (c = *p++) {
        if (c < '0' || c > '9')
            return v;

        d = c - '0';

        if (s == 1) { // exceeds INT_MAX?
            if (v < INT_MAX_DIV_BY_10 || (v == INT_MAX_DIV_BY_10 && d < INT_MAX_REM_BY_10))
                v = 10 * v + d;
            else
                return INT_MAX;
        } else { // exceeds INT_MIN?
            if (v > INT_MIN_DIV_BY_10 || (v == INT_MIN_DIV_BY_10 && -d > INT_MIN_REM_BY_10))
                v = 10 * v - d;
            else
                return INT_MIN;
        }
    }

    return v;
}