allocating flexible array members as a struct

Question

I understand more about pointers and stuff but I have no idea what I am doing wrong here. if i Have char *(*data)[] That would just be interpreted as "a pointer to an array of char pointers", right? Then I have a struct like this, typedef'd to be myStruct, redundant as it may be, but that's aside the point:

    typedef struct myStruct myStruct;
    struct myStruct{
      int size;
      char *name;
      myStruct *(*array)[];
}

Having looked around the site for similar posts, I got something like this:

    //let's say allocating 5 spaces for this case
    myStruct *a = malloc(sizeof(myStruct)+ sizeof(struct myStruct *)*5);

I am sure that the number I allocated the struct with is the size of the array. I can't quite get my head wrapped around this, how does it work if it's a struct? The plan here is to have this struct, and it contains an array of 5 myStruct's. Do I have to allocate them separately as well? like this?

    a->array[0] = malloc( .... ) 

I tried and it keeps giving me an error saying Invalid use of array with unspecified bounds. What am I doing wrong or how Can i fix this? Thank you

Answer 1

From your comment, it sounds like you want a pointer to an array of pointers to structures, rather than a pointer to an array of structures, since "pointer to an array of char *" also has two levels of indirection.

Here's the difference:

1. A pointer to a structure:

2. A pointer to an array of structures:

3. A pointer to an array of pointers to structures:

Assuming you want #3, you can do it like so (in "traditional" C):

typedef struct myStruct myStruct;

struct myStruct
{
    int        size;
    char      *name;
    myStruct **array;
};

myStruct *allocate_node(char *name, int size)
{
    myStruct *p_node;
    if (size < 0)
        size = 0;
    p_node = calloc(1, sizeof(myStruct));
    p_node->name = name;
    p_node->size = size;
    p_node->array = calloc(1, size * sizeof(myStruct *));

    return p_node;
}

void expand_node_child_array(myStruct *p_node, int size_to_add)
{
    if (size_to_add < 1 || p_node == NULL)
        return;
    if (p_node->array == NULL)
    {
        p_node->size = size_to_add;
        p_node->array = calloc(1, size_to_add * sizeof(myStruct *));
    }
    else
    {
        p_node->array = realloc(p_node->array, (p_node->size + size_to_add) * sizeof(myStruct *));
        memset(p_node->array + p_node->size * sizeof(myStruct *), 0, size_to_add * sizeof(myStruct *));
        p_node->size += size_to_add;
    }
}

myStruct *get_child_node(myStruct *p_node, int index)
{
    if (index < 0 || index >= p_node->size)
        return 0;
    return p_node->array[index];
}

int set_child_node(myStruct *p_node, int index, myStruct *p_child)
{
    if (index < 0 || index >= p_node->size)
        return FALSE;
    p_node->array[index] = p_child;
    return TRUE;
}

void free_node(myStruct **pp_node)
{
    // Free p_node and the array but DO NOT free the children
    if (pp_node == NULL || *pp_node == NULL)
        return;
    if ((*pp_node)->array != NULL)
        free((*pp_node)->array);
    free((*pp_node));
    *pp_node = NULL;
}

void free_node_and_children(myStruct **pp_node)
{
    int iChild;

    if (pp_node == NULL || *pp_node == NULL)
        return;
    for (iChild = 0; iChild < (*pp_node)->size; iChild++)
    {
        myStruct *p_child = get_child_node((*pp_node), iChild);
        if (p_child != NULL)
            free_node_and_children(&p_child);
        set_child_node((*pp_node), iChild, NULL);
    }
    free_node(pp_node);
}

Update

A flexible array, under the C99 standard syntax, is a variable-length array that appears at the tail of a structure and whose actual length is set at run time. It looks like this in memory:

Assuming your compiler supports this syntax (not all do), you declare it like this:

struct myStruct
{
    Type  array_of_type[]; /* AT THE END OF THE STRUCT ONLY */
};

And the code for "myStruct" becomes:

typedef struct myStruct myStruct;

struct myStruct
{
    int        size;
    char      *name;
    myStruct  *array[];
};

myStruct *allocate_node(char *name, int size)
{
    myStruct *p_node;
    if (size < 0)
        size = 0;
    p_node = calloc(1, sizeof(myStruct) + size * sizeof(myStruct *));
    p_node->name = name;
    p_node->size = size;

    return p_node;
}

myStruct *get_child_node(myStruct *p_node, int index)
{
    if (index < 0 || index >= p_node->size)
        return NULL;
    return p_node->array[index];
}

int set_child_node(myStruct *p_node, int index, myStruct *p_child)
{
    if (index < 0 || index >= p_node->size)
        return FALSE;
    p_node->array[index] = p_child;
    return TRUE;
}

void free_node(myStruct **pp_node)
{
    if (pp_node == NULL || *pp_node == NULL)
        return;
    free((*pp_node));
    *pp_node = NULL;
}

void free_node_and_children(myStruct **pp_node)
{
    int iChild;

    if (pp_node == NULL || *pp_node == NULL)
        return;
    for (iChild = 0; iChild < (*pp_node)->size; iChild++)
    {
        myStruct *p_child = get_child_node((*pp_node), iChild);
        if (p_child != NULL)
            free_node_and_children(&p_child);
        set_child_node((*pp_node), iChild, NULL);
    }
    free_node(pp_node);
}

If you compiler does not, see here for some workarounds.

With flexible arrays, expanding the array would require re-allocating the node itself and fixing all references to it, something not required in the "pointer to array of pointers" design.

The syntax you are using:

    myStruct  *(*array)[];

should be read as "a pointer to array(s) of unknown size of pointers to structures", rather than

    myStruct  **array;

which is "a pointer to pointer(s) to structures", or (e.g.):

    myStruct *(*array)[4];

which is "a pointer to array(s) of length 4 of pointers.

Your syntax actually produces memory map #3, however accessing the individual elements is a bit more awkward because you have to explicitly get a pointer to the zeroth element of the "array of unknown size", which is (*p_node->array). Thus the functions from #3 are modified as follows:

void expand_node_child_array(myStruct *p_node, int size_to_add)
{
    if (size_to_add < 1 || p_node == NULL)
        return;
    if (p_node->array == NULL)
    {
        p_node->size = size_to_add;
        p_node->array = calloc(1, size_to_add * sizeof(myStruct *));
    }
    else
    {
        p_node->array = realloc(p_node->array, (p_node->size + size_to_add) * sizeof(myStruct *));
        memset((*p_node->array) + p_node->size * sizeof(myStruct *), 0, size_to_add * sizeof(myStruct *));
        p_node->size += size_to_add;
    }
}

myStruct *get_child_node(myStruct *p_node, int index)
{
    if (index < 0 || index >= p_node->size)
        return NULL;
    return (*p_node->array)[index];
}

int set_child_node(myStruct *p_node, int index, myStruct *p_child)
{
    if (index < 0 || index >= p_node->size)
        return FALSE;
    (*p_node->array)[index] = p_child;
    return TRUE;
}

And finally, the test code for either architecture:

void dump_nodes_recursive(myStruct *p_node, int level)
{
    if (p_node == NULL)
    {
        printf("%*s", 4*level, " ");
        printf("NULL\n");
    }
    else
    {
        int iChild;
        printf("%*s", 4*level, " ");
        printf("Node: Name=\"%s\", array size=%d\n", p_node->name, p_node->size);
        for (iChild = 0; iChild < p_node->size; iChild++)
        {
            myStruct *p_child = get_child_node(p_node, iChild);
            printf("%*s", 4*level, " ");
            printf("Child [%d]:\n", iChild);
            dump_nodes_recursive(p_child, level+1);
        }
    }
}

void dump_nodes(myStruct *p_node)
{
    dump_nodes_recursive(p_node, 0);
}

void test_my_struct()
{
    myStruct *p_top = allocate_node("top", 4);
    myStruct *p_child0 = allocate_node("child0", 1);
    myStruct *p_child1 = allocate_node("child1", 5);
    myStruct *p_child2 = allocate_node("child2", 0);
    myStruct *p_child3 = allocate_node("child3", 0);
    myStruct *p_child00 = allocate_node("child00", 0);

    set_child_node(p_top, 0, p_child0);
    set_child_node(p_top, 1, p_child1);
    set_child_node(p_top, 2, p_child2);

    set_child_node(p_top, 3, p_child3);

    set_child_node(p_child0, 0, p_child00);

    dump_nodes(p_top);

    free_node_and_children(&p_top);
}

Answer 2

myStruct *(*array)[]; is not a flexible array member, since it's not an array type. It is a pointer which happens to be pointing to an incomplete array type.

The general pattern for flexible array member is:

struct myStruct {
    int size;
    char *name;
    Type array[];
};

where in your case, Type would be defined by typedef MyStruct * Type;. I'm assuming you want an array which will contain 5 pointers. (This is the same effect as having myStruct *array[]; in the struct, of course).

(If you do actually want your struct to contain a single pointer, which points to an array of 5 elements; then flexible array member is not the right technique to use).

Your malloc is correct for this definition I have just given. It allocates a contiguous bloc of memory and you can use array as if it were actually an array of 5 objects, except you can't do sizeof on it to find the size.