C Syntax for a dynamic-allocated array within a struct within a dynam array within a struct

Question

I have a C syntax question which is driving me crazy. I've defined some container structs:

typedef struct AStruct {
    int importantData1;
    int importantData2;
} A;

typedef struct BStruct {
    A* arr1;                // dynamic array of A structs
} B;

typedef struct CStruct {
    B* arr2;                // dynamic array of B structs
} C;

As you can guess, each A struct hold a part of my program's data. Each B struct holds an array of A's and each C struct holds an array of B's. I anticipate the need for only one C struct. So total amount of data with my C struct will be:

total data in C  ==  (num of B's) * (num of A's)

The tricky part is I don't know how many A's and B's I'll need at compile time. This means I can't hardwire the array sizes; they must be dynamically allocated.

There's gotta be a way to do this. Here's my clunky attempt:

void makeContainers(C* x, int numB, int numA){
    x->arr2 = (B*)malloc(numB * sizeof(B));   // allocate an array of empty B structs
    B* Bptr;
    int i;
    for(i=0; i<numB; i++){
            Bptr = x->arr2[i];                      // Bptr is one of the B's in C's arr2
            Bptr = (A*)malloc(numA * sizeof(A));    // Allocate an array of empty A stucts
    }
}

int main() {
    C* bigContainer = (C*)malloc(sizeof(C));    // Allocate the C struct
    makeContainers(bigContainer, 8, 8);         // populate C with B's and A's

    return 0;
}

This looked great on paper... but the compiler hates the Bptr = x->arr2[i]; line. (Compiling with GCC on Linux) Here's the error:

[Linux]$ gcc -Wall setsInSets.c
setsInSets.c: In function ‘makeContainers’:
setsInSets.c:23:8: error: incompatible types when assigning to type ‘struct B *’ from type ‘B’
   Bptr = x->arr2[i];   // Bptr is one of the B's in C's arr2
        ^

The "to type struct B from type B" part confuses me. Not sure what the compiler is trying to tell me here.

Also... This is kinda a C 101-type question, but I'm not certain I completely understand what the first line in makeContainers() is doing:

x->arr2 = (B*)malloc(numB * sizeof(B));

Here I am allocating memory for an array of B structs... but does that mean when this command is finished, I literally have an array of "empty" B structs ready to go? Or have I just set aside the memory, but the memory itself contains just "garbage"? In other words, do I have to walk the x->arr2 array and malloc() an individual B struct?

I know this is probably a duplicate question - but I ask it anyway because I think the "struct within a dynamic array within a struct within a dynamic array" nature of this question is pretty specific.

Thanks! -Pete

`x->arr2[i]` evaluates to a `B` object, not a pointer. Maybe you meant to write `Bptr = x->arr2`? — Jezor, Nov 14 '16 at 19:32
[Don't cast malloc in C](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — Barmar, Nov 14 '16 at 19:32
Those are not arrays at all. They are structs so you can't do `x->arr2[i]` but if you want to access, for example, `importantData1`, you can actually do `x->arr2->arr1->importantData1` — Nick Rameau, Nov 14 '16 at 19:36
`Bptr = x->arr2[i];`should be `Bptr = x->arr2 + i;` or `Bptr = &x->arr2[i];` — alk, Nov 14 '16 at 20:00
@Jezor Yes! You're totally right. I see it now. Its so easy to misunderstand what your C pointer really dereferences to. Thank you! — Pete, Nov 14 '16 at 23:43
@NickRameau Thanks Nick. You are right about how it access importantData1. Once I saw your answer, I realized what I'd done wrong. — Pete, Nov 14 '16 at 23:45

score 3 · Accepted Answer · answered Nov 14 '16 at 19:39

x->arr2 is pointer to an array of B structs

x->arr2[i] is dereferencing one element in the array i.e. a B struct

To set a pointer you need the address of the struct

B* Bptr = &(x->arry[i]);

or more conveniently

B* Bptr = x->arry + i;

I would suggest you write the size of the array in your struct as well, it may come in handy later.

e.g.

typedef struct BStruct {
    size_t elements;        // to know how many A structs 
    A* arr1; 
} B;

EDIT:

This part of the code

Bptr = x->arr2 + i;         // Bptr is one of the B's in C's arr2
Bptr = malloc(numA * sizeof(A));

makes no sense, you first assign Bptr, then assigning it to another address.

This is very, very clear, thank you. My syntax works now. Adding the size into the struct is a stroke of genius, I've already implemented it. Nice! — Pete, Nov 14 '16 at 23:47

C Syntax for a dynamic-allocated array within a struct within a dynam array within a struct

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