将Float转换为字符串时获取Garbage值？

Question

将float转换为string时，我得到了垃圾值。我不能使用sprintf，因为它需要更多的内存，PIC24单片机的ram大小非常低（128kb）。转换它时会给出一些垃圾值？39.88喜欢这个可能是什么问题。我发布了我的代码......

#include<stdio.h>
#include<string.h>
#include<math.h>
float Distance=0;
char str[15]; 
    unsigned char *ftos(float f,int precision)
{
    memset(str,0,sizeof(str));
    float ff;
    ff = f;
    int a,b,c,k,l=0,m,i=0;
    // check for negative float
    if(f<0.0)
    {
    str[i++]='-';
    f*=-1;
    }
    a=f;    // extracting whole number
    f-=a;   // extracting decimal part
    k = precision;
    // number of digits in whole number
    while(k>0)
    {
        l = pow(10.0f,k);
        m = a/l;
        if(m>0)
        {
            break;
        }
        k--;
    }
    // number of digits in whole number are k+1

    /*
    extracting most significant digit i.e. right most digit , and concatenating to string
    obtained as quotient by dividing number by 10^k where k = (number of digit -1)
    */
    for(l=k+1;l>0;l--)
    {
        b = pow(10.0f,l-1);
        c = a/b;
        str[i++]=c+48;
        a%=b;
    }
    str[i++] = '.';
    /* extracting decimal digits till precision */
    for(l=0;l<precision;l++)
    {
        f*=10.0;
        b = f;
        str[i++]=b+48;
        f-=b;
    }
    str[i]='\0';
    return str;
}
void distance(float lat1, float lon1, float lat2, float lon2)
{
    float dlon=0,dlat=0,a=0,c=0,dist=0;
    dlon = (lon2 - lon1) * (M_PI / 180.0);
    dlat = (lat2 - lat1) * (M_PI / 180.0);
    a = pow(sin(dlat/2.0), 2) + cos(lat1*(M_PI / 180.0)) * cos(lat2*(M_PI / 180.0)) * pow(sin(dlon/2.0), 2);
    c = 2 * atan2(sqrt(a), sqrt(1-a));
    dist = 6367 * c;
    Distance += dist;  
}
int main()
{
    distance(13.00659,80.121212,13.69898,80.256987);
    printf("%s\n",ftos(Distance,6));
}

当我在转换后打印距离值时会给出一些垃圾值......为什么？