/* Coalesce 'oldBlock' with any preceeding or following free blocks. */
static void coalesceFreeBlock(BlockInfo* oldBlock) {
BlockInfo *blockCursor;
BlockInfo *newBlock;
BlockInfo *freeBlock;
// size of old block
size_t oldSize = SIZE(oldBlock->sizeAndTags);
// running sum to be size of final coalesced block
size_t newSize = oldSize;
// Coalesce with any preceding free block
blockCursor = oldBlock;
while ((blockCursor->sizeAndTags & TAG_PRECEDING_USED)==0) {
// While the block preceding this one in memory (not the
// prev. block in the free list) is free:
// Get the size of the previous block from its boundary tag.
size_t size = SIZE(*((size_t*)UNSCALED_POINTER_SUB(blockCursor, WORD_SIZE)));
// Use this size to find the block info for that block.
freeBlock = (BlockInfo*)UNSCALED_POINTER_SUB(blockCursor, size);
// Remove that block from free list.
removeFreeBlock(freeBlock);
// Count that block's size and update the current block pointer.
newSize += size;
blockCursor = freeBlock;
}
newBlock = blockCursor;
// Coalesce with any following free block.
// Start with the block following this one in memory
blockCursor = (BlockInfo*)UNSCALED_POINTER_ADD(oldBlock, oldSize);
while ((blockCursor->sizeAndTags & TAG_USED)==0) {
// While the block is free:
size_t size = SIZE(blockCursor->sizeAndTags);
// Remove it from the free list.
removeFreeBlock(blockCursor);
// Count its size and step to the following block.
newSize += size;
blockCursor = (BlockInfo*)UNSCALED_POINTER_ADD(blockCursor, size);
}
// If the block actually grew, remove the old entry from the free
// list and add the new entry.
if (newSize != oldSize) {
// Remove the original block from the free list
removeFreeBlock(oldBlock);
// Save the new size in the block info and in the boundary tag
// and tag it to show the preceding block is used (otherwise, it
// would have become part of this one!).
newBlock->sizeAndTags = newSize | TAG_PRECEDING_USED;
// The boundary tag of the preceding block is the word immediately
// preceding block in memory where we left off advancing blockCursor.
*(size_t*)UNSCALED_POINTER_SUB(blockCursor, WORD_SIZE) = newSize | TAG_PRECEDING_USED;
// Put the new block in the free list.
insertFreeBlock(newBlock);
}
return;
}
/* Get more heap space of size at least reqSize. */
static void requestMoreSpace(size_t reqSize) {
size_t pagesize = mem_pagesize();
size_t numPages = (reqSize + pagesize - 1) / pagesize;
BlockInfo *newBlock;
size_t totalSize = numPages * pagesize;
size_t prevLastWordMask;
void* mem_sbrk_result = mem_sbrk(totalSize);
if ((size_t)mem_sbrk_result == -1) {
printf("ERROR: mem_sbrk failed in requestMoreSpace\n");
exit(0);
}
newBlock = (BlockInfo*)UNSCALED_POINTER_SUB(mem_sbrk_result, WORD_SIZE);
/* initialize header, inherit TAG_PRECEDING_USED status from the
previously useless last word however, reset the fake TAG_USED
bit */
printf("newBlock: %p\n", (void *) newBlock);
prevLastWordMask = newBlock->sizeAndTags & TAG_PRECEDING_USED;
newBlock->sizeAndTags = totalSize | prevLastWordMask;
// Initialize boundary tag.
((BlockInfo*)UNSCALED_POINTER_ADD(newBlock, totalSize - WORD_SIZE))->sizeAndTags =
totalSize | prevLastWordMask;
/* initialize "new" useless last word
the previous block is free at this moment
but this word is useless, so its use bit is set
This trick lets us do the "normal" check even at the end of
the heap and avoid a special check to see if the following
block is the end of the heap... */
*((size_t*)UNSCALED_POINTER_ADD(newBlock, totalSize)) = TAG_USED;
// Add the new block to the free list and immediately coalesce newly
// allocated memory space
insertFreeBlock(newBlock);
coalesceFreeBlock(newBlock);
}
/* Free the block referenced by ptr. */
void mm_free (void *ptr) {
size_t payloadSize;
BlockInfo * blockInfo;
BlockInfo * followingBlock;
blockInfo = (BlockInfo *)UNSCALED_POINTER_SUB(ptr, WORD_SIZE);//header of chosen block to free
blockInfo->sizeAndTags = blockInfo->sizeAndTags & (~TAG_USED);//turn off the tag used tag
payloadSize = SIZE(blockInfo->sizeAndTags);//get the payload of the block soon to be freed
followingBlock = (BlockInfo *)UNSCALED_POINTER_ADD(blockInfo, payloadSize + WORD_SIZE);//move to the block that follows the block to be freed
followingBlock->sizeAndTags = (followingBlock->sizeAndTags) & (~TAG_PRECEDING_USED);//turn off its preceding tag used flag
insertFreeBlock(blockInfo);//insert block that was freed back to the list
coalesceFreeBlock(blockInfo);//coalesce, if possible
}
/* Free the block referenced by ptr. */
void mm_free (void *ptr) {
printf("Begin free\n");
size_t payloadSize;
BlockInfo * blockInfo;
BlockInfo * footer;
BlockInfo * nextBlock;
size_t precedingBlockUseTag;
blockInfo = (BlockInfo *) UNSCALED_POINTER_SUB(ptr, WORD_SIZE);
payloadSize = SIZE(blockInfo->sizeAndTags);
precedingBlockUseTag = blockInfo->sizeAndTags & TAG_PRECEDING_USED;
blockInfo->sizeAndTags = (payloadSize & ~TAG_USED) | precedingBlockUseTag;
footer = (BlockInfo *) UNSCALED_POINTER_ADD(blockInfo, payloadSize - WORD_SIZE);
footer->sizeAndTags = blockInfo->sizeAndTags;
nextBlock = (BlockInfo *) UNSCALED_POINTER_ADD(ptr, payloadSize);
set_next_block(nextBlock, precedingBlockUseTag);
insertFreeBlock(blockInfo);
coalesceFreeBlock(blockInfo);
}
/* Initialize the allocator. */
int mm_init () {
// Head of the free list.
BlockInfo *firstFreeBlock;
// Initial heap size: WORD_SIZE byte heap-header (stores pointer to head
// of free list), MIN_BLOCK_SIZE bytes of space, WORD_SIZE byte heap-footer.
size_t initSize = WORD_SIZE+MIN_BLOCK_SIZE+WORD_SIZE;
size_t totalSize;
void* mem_sbrk_result = mem_sbrk(initSize);
// printf("mem_sbrk returned %p\n", mem_sbrk_result);
if ((ssize_t)mem_sbrk_result == -1) {
printf("ERROR: mem_sbrk failed in mm_init, returning %p\n",
mem_sbrk_result);
exit(1);
}
firstFreeBlock = (BlockInfo*)UNSCALED_POINTER_ADD(mem_heap_lo(), WORD_SIZE);
// Total usable size is full size minus heap-header and heap-footer words
// NOTE: These are different than the "header" and "footer" of a block!
// The heap-header is a pointer to the first free block in the free list.
// The heap-footer is used to keep the data structures consistent (see
// requestMoreSpace() for more info, but you should be able to ignore it).
totalSize = initSize - WORD_SIZE - WORD_SIZE;
// The heap starts with one free block, which we initialize now.
firstFreeBlock->sizeAndTags = totalSize | TAG_PRECEDING_USED;
firstFreeBlock->next = NULL;
firstFreeBlock->prev = NULL;
// boundary tag
*((size_t*)UNSCALED_POINTER_ADD(firstFreeBlock, totalSize - WORD_SIZE)) = totalSize | TAG_PRECEDING_USED;
// Tag "useless" word at end of heap as used.
// This is the is the heap-footer.
*((size_t*)UNSCALED_POINTER_SUB(mem_heap_hi(), WORD_SIZE - 1)) = TAG_USED;
// set the head of the free list to this new free block.
FREE_LIST_HEAD = firstFreeBlock;
printf("Heap after initialization\n");
examine_heap();
return 0;
}
//changes the size allocated to a block, if possible
void* mm_realloc(void* ptr, size_t size) {
size_t oldsize;
BlockInfo * blockInfo,*block;
void * copyptr;
if(ptr == NULL && (size > 0)){
copyptr = mm_malloc(size);
mm_free(ptr);
}
if(size == 0){
mm_free(ptr);
copyptr = NULL;
}
//code for iterating through the freelist. upon each call of realloc, search the freelist, and coalesce all unused blocks
//likely inefficent, but sufficient for the project. prevents us from getting a multitude of free blocks that are too big
//to be used, which causes mem_sbrk to fail and the program to quit.
for(block = (BlockInfo *)UNSCALED_POINTER_ADD(mem_heap_lo(), WORD_SIZE); /* first block on heap */
SIZE(block->sizeAndTags) != 0 && block < mem_heap_hi();
block = (BlockInfo *)UNSCALED_POINTER_ADD(block, SIZE(block->sizeAndTags))) {
/* and allocated/free specific data */
if ((block->sizeAndTags & TAG_USED) == 0) {
coalesceFreeBlock(block);
}
}
copyptr = mm_malloc(size);//call to mm_malloc for the requested size
blockInfo = (BlockInfo *)UNSCALED_POINTER_SUB(ptr,WORD_SIZE); //pointer to the header of ptr
oldsize = SIZE(blockInfo->sizeAndTags);//get the size of ptr
if(size < oldsize){//if the size of ptr is greater than requested
oldsize = size;// change the value of bytes to be copied to the requested size
}
memcpy(copyptr, ptr, oldsize); //copy "n = oldsize" bytes from ptr to copyptr
mm_free(ptr); //return ptr to the freelist
return copyptr; // return copyptr
}
