float to string

//Warning: if the float variable is negative or larger than the 10000,the decimal part digit will be not precise(but the func logic is correct)!!!
//keep 3 decimal precision digit
int FtoStr(float f, char *str)
{
char buf[20] = {0};
char *pos = buf;
long tmp;

if(NULL == str) return -1;
if(f < 0) {
f = -f;
*str++ = '-';
}

tmp = ((long)(f * 10000) % 10) >= 5 ? (f * 1000) + 1 : (f * 1000);
do {
if(pos == buf + 3)//insert the decimal point position
*pos++ = '.';
*pos++ = tmp % 10 + '0';
}while(tmp /= 10);//check whether the copied char is the last char or not

while(pos < buf + 3)//less than 1.0
*pos++ = '0';
if(pos == buf + 3){
*pos++ = '.';
*pos++ = tmp +'0';
}

while(pos > buf)
*str++ = *--pos;
*str = 0;
}

int _tmain(int argc, _TCHAR* argv[])
{
char str[200] = {0};
//float f = 0.23;
float f = 1456.0125678;

FtoStr(f, str);
LogOut("the str is : %s\n", str);

getchar();
}