原创：实现atoi函数

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

int my_atoi(char *str)
{
    if(NULL == str)
    {
        printf("argc is null\n");
        return -1;        
    }
    while(' ' == *str)
        str++;
    int flag = 1;
    if('-' == *str)
    {
        flag = -1;
        str++;
    }
    if('+' == *str)
    {
        str++;
    }
    unsigned int sum = 0;
    while(*str)
    {
        if(*str >= '0' && *str <= '9')
        {
            int tmp = *str - '0';
            sum *= 10;
            sum += tmp;
            //此处判断溢出的目的是避免sum在计算过程中溢出 
            if((1 == flag) && (sum > INT_MAX))
            {
                printf("int  正数溢出\n");
                return -1;
            }
            //计算最小int的绝对值，注意要使用unsigned int否则会溢出 
            unsigned int  absIntMin = (unsigned int)INT_MAX + 1;
            if((-1 == flag) && (sum > absIntMin))
            {
                printf("int 负数溢出\n");
                return -1;
            }
        }
        else
        {
            printf("str (%s) exists illegal character\n", str);
            return -1;
        }
        str++;
    }
    return flag * sum;
}

#include <float.h>
#include <limits.h>
extern int my_atoi(char *str);
int main(int argc, char *argv[])
{
    char a[20] = {'\0'};
    while(1)
    {
        scanf("%s", a);
        printf("count = %d\n", my_atoi(a));
    }
    return 0;
}