Question

I have this structure:

typedef struct
{
int data[10];
} small_structure;

and this code:

small_structure *s_struct;
void * chunk;

chunk = malloc(1000);
s_struct = chunk;

Is it ok to do something like this? Ignore the fact that this is wasting memory.

Answer 1

Yes, that’s fine. malloc will return you suitably aligned memory. Just assigning any arbitrary void * pointer to a small_structure * variable is not OK, however. That means your specific example is fine, but something like:

int function(void *p)
{
    small_structure *s = p;
    return s->data[0];
}

is not! If p isn’t suitably aligned for a small_structure * pointer, you’ve just caused undefined behaviour.