What does it mean?

gistfile1.c

typedef union {
    int a;
    short b;
    char c[1];
    long d;
#ifdef HAVE_U64_TYPEDEF
    u64 e;
#endif
    float f;
    double g;
} PROPERLY_ALIGNED_TYPE;

typedef struct memblock_struct MEMBLOCK;
struct memblock_struct {
    unsigned size;
    union {
      MEMBLOCK *next;
      PROPERLY_ALIGNED_TYPE aligned;
    } u;
};

MEMBLOCK* mb = (MEMBLOCK*)((char*)p - ((size_t) &((MEMBLOCK*)0)->u.aligned.c));