Convert string to integer

unsigned int atoi(char* s, unsigned char len);

unsigned int atoi(char* s, unsigned char len){
unsigned int val=0;
unsigned int flag=0;
unsigned char i;
for(i=0;i<len;i++)
{
    if(s[i] >= '0' && s[i] <= '9'){val = val * 10 + s[i] -'0';}
    else if(s[0] == '-' && i==0){flag = 1;}
    else{break;}
}
if(flag == 1)
val = val * -1;
return val;
}

example of the function being used:

char c[10] = "123";
int i = atoi(c, 3);
printf("%d", i);