atoi的模拟实现

//atoi的模拟实现
#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <ctype.h>
//解决空指针
//空白字符
//非数字字符
//溢出
enum state
{
	VALID,
	INVALID
}state=INVALID;
my_atoi(const char* str)
{
	assert(str);
	long long ret = 0;
	if (*str == '\0')
	{
		return 0;
	}
	while (isspace(*str))
	{
		str++;
	}
	int flag = 1;
	if (*str == '+')
	{
		str++;
	}
	else if (*str == '-')
	{
		str++;
		flag = -1;
	}
	while (*str)
	{
		if (isdigit(*str))
		{
			ret = ret * 10 + flag*(*str - '0');
			if (ret > INT_MAX)
			{
				return INT_MAX;
			}
			else if (ret < INT_MIN)
			{
				return INT_MIN;
			}
		}
		else
		{
			return (int)ret;
		}
		str++;
	}
	state = VALID;
	return (int)ret;
}
int main()
{  
	int ret=my_atoi("1234");
	if (state == VALID)
		printf("%d\n", ret);
	else
		printf("字符串非法,%d\n", ret);
	return 0;
}