atoi 的内部实现过程
//NULL, "", " ", "0", "1", "12", "123", "-123", "+123", " 12", "12 23", "12ab", "ab12", "222222222222", ''65535", "65536", "-65536", "4294967296"（232）,"4294967295"(232 - 1), "-231 - 1" ， "231 - 1"
char * nptr = "123";
int c; /* current char */
int total; /* current total */
int sign; /* if ''-'', then negative, otherwise positive */
/* skip whitespace */
while ( isspace((int)*nptr) )
nptr++;
c = (int)*nptr++;
sign = c; /* save sign indication */
if (c == '-' || c == '+')
c = (int)*nptr++; /* skip sign */
total = 0;
while (isdigit(c)) {
total = 10 * total + (c - '0'); /* accumulate digit */
//total = 10 * total + (int)c; /* accumulate digit */
c = (int)*nptr++; /* get next char */
}
if (sign == '-')
return -total;
else
return total; /* return result, negated if necessary */
