Question

Suppose I have a structure, and a pointer to a memory location p, how do I make sure that while creating an instance ‘a’ of the structure, it is placed into a memory chunk starting at p?

So I am passed a chunk of memory and I want to create a node at the beginning of it, so I need to make sure that the node is created at the beginning of it. (Note: I cannot use malloc, calloc, free, memcpy, or similar functions because I am writing code for a memory management system).

Answer 1

You don’t really ‘create’ instances of structures in C like that. Assuming that p points to a block of usable memory, you can just treat p as a pointer to your structure type:

typedef struct {int x; long y;} a;

a *p2 = (a*)p;
int z = p2->x;

// or, if you don't want p2:
z = ((a*)p)->x;

Once p is cast (implicitly or explicitly as above), you can initialize the contents of your structure however you wish.

As an example, the following code will initialize a structure as you seem to request:

typedef struct {int x; float y;} tA;
void initA (void *p) {
    tA *p2 = (tA*)p;
    p2->x = 0;
    p2->y = 3.14159;
}
int main (void) {
    char bigmem[100];
    initA (&(bigmem[0]));
    return 0;
}

Don’t get hung up on the main function above, it’s only to illustrate how you can pass an arbitrary memory address to the function. In your real-world case, you will have the memory already allocated somehow.