How to implement binary128 in C?

I started to try and implement new data types, specifically floating-point values, using C. I came up with this sort of way on implementing binary128 using longer_int which I haven't fully understood.

The way how I started was to implement it via copying the code and modifying it. However, the code is C++, and C doesn't support OOP, so what I have to do is to try and translate the code.

I can't find the C version for longer_int because this question I once owned (the same account but now deleted) is now deleted and I can't view the links.

With the help of these C++ files, how can I implement binary128 using C?

And just so this question won't get closed, I want to know how to translate the OOP format in longer_int.h.
class longer_int {
private:
    char num_str[];
public:
    longer_int();
    longer_int(const int &num);
    longer_int(const char &num[]);
    longer_int(const longer_int &num);

// Sample code from it, now translated to C, but still OOP

Solution

You should begin with something like this: a struct instead of a class, ignore the public/private keywords, the constructors are methods that return the my_int128. There is no overloading in C, so each different constructor must have a different name. Another school of thought would say that you don't need specialized constructors, but simply special method setters that can copy data from an int, from a char[] and from a my_int128 (and I prefer this school of thought). I give an example setter at the end.

#include <stdio.h>
#include <string.h>

typedef struct 
{
    char num_str[16]; /* fixed to int128 */
} my_int128;

/* Don't need this: my_int128 li = { 0 }; is enough!
my_int128 create_int128()
{
    my_int128 li = { 0 };
    return li;
}
*/

my_int128 create_int128_from_num(int num)
{
    my_int128 li = { 0 };
    /* TODO: set li with num */
    return li;
}

my_int128 create_int128_from_char_array(const char num[16])
{
    my_int128 li = { 0 };
    memcpy(li.num_str, num, 16);
    return li;
}

my_int128 create_int128_from_int128_ptr(const my_int128 *num)
{
    return create_int128_from_char_array(num->num_str);
}

The setter:

void set_from_int(my_int128 *li, int num)
{
    /* TODO: set li with num */
}