Weird C stack memory overrides - c++

Question

I am implementing a version of malloc and free for practice. So, I have a static char array of a fixed length (10000). Then, I implemented a struct memblock that holds information like size of the block, if it is free...
The way I am implementing malloc is such that I put small blocks (< 8 bytes) to the front of the char array and larger ones to the other end. So, I am basically using two linked lists to link the blocks in front and blocks in back. However, I am having weird problems with initializing the lists (on first call of malloc).
This is my code:
#define MEMSIZE 10000 // this is the maximum size of the char * array
#define BLOCKSIZE sizeof(memblock) // size of the memblock struct
static char memory[MEMSIZE]; // array to store all the memory
static int init; // checks if memory is initialized
static memblock root; // general ptr that deals with both smallroot and bigroot
static memblock smallroot, bigroot; // pointers to storage of small memory blocks and bigger blocks
void initRoots(size_t size, char* fileName, int lineNum)
{
smallroot = (memblock)memory;
smallroot->prev = smallroot->next = 0;
smallroot->size = MEMSIZE - 2 * BLOCKSIZE;
smallroot->isFree = 1;
smallroot->file = fileName;
smallroot->lineNum = lineNum;
bigroot = (memblock)(((char *)memory) + MEMSIZE - BLOCKSIZE - 1);
bigroot->prev = bigroot->next = 0;
bigroot->size = MEMSIZE - 2 * BLOCKSIZE;
bigroot->isFree = 1;
bigroot->file = fileName;
bigroot->lineNum = lineNum;
init = 1;
}
I used GDB to see where I am getting a Seg Fault. It happens when bigroot->next = 0; is executed. This somehow sets smallroot to 0. What is more weird? If I set bigroot->next = 0x123, then smallroot becomes 0x1. If I set 0x1234, then it becomes 0x12. It is setting smallroot to the value of bigroot->next's value excluding its last two bits. I really don't understand how this is happening!
This is the definition of memblock:
typedef struct memblock_* memblock;
struct memblock_ {
struct memblock_ *prev, *next; // pointers to next and previous blocks
/* size: size of allocated memory
isFree: 0 if not free, 1 if free
lineNum: line number of user's file where malloc was invoked
*/
size_t size, isFree, lineNum;
char* file; // user's file name where the block was malloced
};

Answer 1

#define BLOCKSIZE sizeof(memblock) // size of the memblock struct
You want:
#define BLOCKSIZE sizeof(*memblock) // size of the memblock_ struct
Also the -1 here is bogus (creates mis-aligned pointer):
bigroot = (memblock)(((char *)memory) + MEMSIZE - BLOCKSIZE - 1);
Actually, I am storing the pointer to the memblock in the memory array. The values of memblock are stored in stack.
No, they are not. The smallroot and bigroot clearly point into the array itself.