signal

#include <ctype.h>

double atof(char s[])
{
    double val, power;
    int i, sign;

    for(i=0, isspace(s[i]); i++)
        ;
    sign = (s[i] == '-') ? -1 : 1;
    if (s[i] == '+' || s[i] == '-')
        i++;
    for (val = 0.0; isdigit(s[i]); i++)
        val = 10.0 * val + (s[i] - '0');
    if (s[i] == '.')
        i++;
    for ( power = 1.0; isdigit(s[i]; i++)) {
        val = 10.0 * val + (s[i] - '0');
        power *= 10.0;
    }
    return sign*val / power;
}