typedef struct
{
    ...
} Foo;

void do_smth(Foo *obj, unsigned char *data)
{
    ...
}


void loop(int handle, void (*handler)(unsigned char *, const unsigned char *), unsigned char *user)
{
    ...
}

void handler_func(unsigned char *user, const unsigned char *data)
{
    Foo *obj;
    obj = (Foo *)user;

    do_smth(obj, data);
}


int main(void)
{
    Foo *obj = malloc(sizeof(Foo));
    int handle = 1;
    loop(handle, handler_func, (unsigned char *)obj);
    free(obj);
    return 0;
}