C struct hack at work

Question

Is this how one can use the the "extra" memory allocated while using the C struct hack?

Questions:

I have a C struct hack implementation below. My question is how can I use the "extra" memory that I have allocated with the hack. Can someone please give me an example on using that extra memory ?

#include<stdio.h>
#include<stdlib.h>

int main()
{

    struct mystruct {

        int len;
        char chararray[1];

    };

    struct mystruct *ptr = malloc(sizeof(struct mystruct) + 10 - 1);
    ptr->len=10;


    ptr->chararray[0] = 'a';
    ptr->chararray[1] = 'b';
    ptr->chararray[2] = 'c';
    ptr->chararray[3] = 'd';
    ptr->chararray[4] = 'e';
    ptr->chararray[5] = 'f';
    ptr->chararray[6] = 'g';
    ptr->chararray[7] = 'h';
    ptr->chararray[8] = 'i';
    ptr->chararray[9] = 'j';


}

Answer 1

Yes, that is (and was) the standard way in C to create and process a variably-sized struct.

That example is a bit verbose. Most programmers would handle it more deftly:

struct mystruct {
        int len;
        char chararray[1];  // some compilers would allow [0] here
    };
    char *msg = "abcdefghi";
    int n = strlen (msg);

    struct mystruct *ptr = malloc(sizeof(struct mystruct) + n + 1);

    ptr->len = n;
    strcpy (ptr->chararray, msg);
}