How to retrieve the structure holding a pointer with container_of macro

Question

I'm using the macro container_of defined as:

#define container_of(ptr, type, member) ((type *)((char *)(1 ? (ptr) : &((type *)0)->member) - offsetof(type, member)))

the structure vector:

struct vector {
    uint32_t signature;
    size_t element_size;
    size_t size;
    void *data;
};

And finally the function vectorSize receiving a pointer to some data . This function is not working because only the data member inside struct vector v have the right value, the others have garbage, resulting in failing the if statement SIGNATURE.

size_t vectorSize(void *vec)
{
    void **pdata = &vec;
    struct vector *v = container_of(pdata, struct vector, data);

    if (v->signature != VECTOR_SIGNATURE) return 0;
    return v->size;
}

What is wrong with my setup?

Answer 1

First of all, please don't post code like 1 ? this : that 1 ? this : that . I know, this is experimental code from you whilst digging into the problem. But it shouldn't posted here as it confuses us.

If I understood you right, your function receives a pointer to a struct member and you need a macro delivering the container struct. Try this:

#define CONTAINER_OF(MemberPtr, StrucType, MemberName) ((StrucType*)( (char*)(MemberPtr) - offsetof(StrucType, MemberName)))

Followed by:

size_t vectorSize(void *vec)
{
    struct vector *v = CONTAINER_OF(vec, struct vector, data);

    if (v->signature != VECTOR_SIGNATURE) return 0;
    return v->size;
}

Answer 2

Example usage of the container_of macro:

struct vector *vector_of(void* element) 
{
    return container_of(element, struct vector, data);
}

For your implementation, it seems to me it should look like this:

size_t vector_size(void *vec_ptr)
{
    struct vector *vec = v_ptr;
    return vec->signature != VECTOR_SIGNATURE ? 0 : vec->size;
}

Answer 3

Since data is of type (void *), vec needs to be of (void **). But you should not give the address of vec. Instead, just cast it as a (void **). Like below:

struct vector *v = container_of((void **)vec, struct vector, data);