Casting a void pointer in struct

Question

Here is what I got ..

typedef struct SysInfo_P
{
    int some_P_int;
    char some_P_char[10];
    ...

} SysInfo_P;


typedef struct SysInfo_S
{
    int some_S_int;
    char some_S_char[10];
    ...

} SysInfo_S;


typedef struct Device
{
    struct SomeData *data;
    void *sysInfo;
    ...
} Device;

The idea is to have a base class Device, and cast its void pointer to the appropriate SysInfo_x class during allocation. I use void pointers all the time to provide versatility to common functions, but I can't seem to wrap my head around this ... perhaps it cannot be done.

Here is my futile attempt at casting it .. in my allocator.

self->sysInfo = (SysInfo_S *)malloc(sizeof(*self->properties->sysInfo));
memset(self->sysInfo, 0, sizeof(*self->properties->sysInfo));

Answer 1

What you want is a union:

typedef struct Device
{
    struct SomeData *data;
    union {
        struct SysInfo_S s;
        struct SysInfo_P p;
    } sysInfo;
    ...
} Device;