block_layout.c

// ...
struct Block_descriptor_1 {
    unsigned long int reserved;
    unsigned long int size;
};
// ...
struct Block_layout {
    void *isa;
    volatile int32_t flags; // contains ref count
    int32_t reserved; 
    void (*invoke)(void *, ...);
    struct Block_descriptor_1 *descriptor;
    // imported variables
};
// ...