atoi源代码

isspace(int x)
{
 if(x==' '||x=='\t'||x=='\n'||x=='\f'||x=='\b'||x=='\r')
  return 1;
 else  
  return 0;
}
isdigit(int x)
{
 if(x<='9'&&x>='0')         
  return 1;x` 
 else 
  return 0;

}
int atoi(const char *nptr)
{
        int c;              /* current char */
        int total;         /* current total */
        int sign;           /* if '-', then negative, otherwise positive */

        /* skip whitespace */
        while ( isspace((int)(unsigned char)*nptr) )
            ++nptr;

        c = (int)(unsigned char)*nptr++;
        sign = c;           /* save sign indication */
        if (c == '-' || c == '+')
            c = (int)(unsigned char)*nptr++;    /* skip sign */

        total = 0;

        while (isdigit(c)) {
            total = 10 * total + (c - '0');     /* accumulate digit */
            c = (int)(unsigned char)*nptr++;    /* get next char */
        }

        if (sign == '-')
            return -total;
        else
            return total;   /* return result, negated if necessary */
}