Static allocation of opaque data types

Question

Very often malloc() is absolutely not allowed when programming for embedded systems. Most of the time I\'m pretty able to deal with this, but one thing irritates me: it keep

Answer 1

This is an old question, but since it's also biting me, I wanted to provide here a possible answer (which I'm using).

So here is an example :

// file.h
typedef struct { size_t space[3]; } publicType;
int doSomething(publicType* object);

// file.c
typedef struct { unsigned var1; int var2; size_t var3; } privateType;

int doSomething(publicType* object)
{
    privateType* obPtr  = (privateType*) object;
    (...)
}

Advantages : publicType can be allocated on stack.

Note that correct underlying type must be selected in order to ensure proper alignment (i.e. don't use char). Note also that sizeof(publicType) >= sizeof(privateType). I suggest a static assert to make sure this condition is always checked. As a final note, if you believe your structure may evolve later on, don't hesitate to make the public type a bit bigger, to keep room for future expansions without breaking ABI.

Disadvantage : The casting from public to private type can trigger strict aliasing warnings.

I discovered later on that this method has similarities with struct sockaddr within BSD socket, which meets basically the same problem with strict aliasing warnings.