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87dfec1942
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37d5cfd532
2 changed files with 17 additions and 226 deletions
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@ -2,13 +2,11 @@
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#define GC_MALLOC_H
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#include <stdint.h>
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#include <stdio.h>
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#include <unistd.h>
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// Generally, inline functions are preferable to macros resembling functions.
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// https://www.kernel.org/doc/html/v4.10/process/coding-style.html#data-structures
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#define ALIGN 8 /* Number of bytes to which chunk size is aligned */
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#define OVERHEAD 1 /* Number of 8-byte segments for chunk size & flags */
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#define MAGIC 2 /* Number of 8-byte segments from top of chunk to fields */
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#define MIN 3 /* Minimum number of 8-byte segments for data in a chunk */
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@ -16,54 +14,38 @@
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/*
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* Metadata and user data for allocated memory
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*
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* footer: size of previous block or user data of previous block if allocated
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* footer: user data for previous chunk
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* size: size of chunk in bytes
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* merge: flag to identify when the chunk can be coalesced,
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* possible values 0 (false, don't merge) or non-zero (true, do merge)
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* free: flag to identify when the chunk is free,
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* possible values 0 (false, allocated) or non-zero (true, chunk is free)
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* merge: flag to identify when the chunk can be coalesced
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* free: flag to identify when the chunk is free
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* next: pointer to next chunk in the free list
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* prev: pointer to previous chunk in the free list
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*/
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struct gc_chunk {
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uint64_t footer;
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uint32_t size;
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uint16_t merge; /* 1 = do merge, initialize to 0 */
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uint16_t free; /* 0 = allocated, initialize to 1 */
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uint16_t merge;
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uint16_t free;
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struct gc_chunk *next;
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struct gc_chunk *prev;
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};
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// Gets a pointer to the next chunk
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struct gc_chunk *find_next_chunk(struct gc_chunk *curr);
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struct gc_chunk *get_next_chunk(struct gc_chunk *curr);
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// Gets a pointer to the previous chunk, ensuring that current can be coalesced
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struct gc_chunk *find_prev_chunk(struct gc_chunk *curr);
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struct gc_chunk *get_prev_chunk(struct gc_chunk *curr);
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// Updates chunk footer information (remember that the current chunk's footer
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// data is actually part of the next chunk)
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// Updates chunk footer information
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void set_footer(struct gc_chunk *curr);
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// Gets footer information for the current chunk - useful for debugging
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uint64_t get_footer(struct gc_chunk *curr);
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// Creates a new chunk with sentinel
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struct gc_chunk *init_gc_chunk(void);
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// Gets a pointer to a suitable chunk (first fit) from the free list, otherwise
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// creates a new chunk and returns it's pointer
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struct gc_chunk *find_free_chunk(size_t size);
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// Coalesces chunks when merge flag is set
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void merge_chunk(struct gc_chunk *curr);
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// Gets footer information for the current chunk
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int get_footer(struct gc_chunk *curr);
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// Performs allocations of SIZE bytes
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void *gc_malloc(size_t size);
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// Frees allocations for re-use
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// Frees blocks of memory allocated by 'gc_alloc'
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void gc_free(void *ptr);
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// Completely de-allocates the heap (break glass in case of emergency)
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void destroy_heap(struct gc_chunk *freelist);
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#endif /* GC_MALLOC_H */
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203
src/gc_malloc.c
203
src/gc_malloc.c
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@ -1,18 +1,9 @@
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#include <assert.h>
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#include <stdint.h>
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#include <stdio.h>
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#include <sys/mman.h>
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#include <unistd.h>
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#include "gc_malloc.h"
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/* Static Variables */
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// Doubly linked list to keep track of free chunks
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static struct gc_chunk *freelist = NULL;
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/* Static Inline Functions */
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// Hides chunk header
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static inline void *hide(struct gc_chunk *curr)
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{
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@ -25,209 +16,27 @@ static inline struct gc_chunk *magic(void *mem)
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return (struct gc_chunk *)((uint64_t *)mem - MAGIC);
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}
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/*
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* Aligns the amount of memory requested by the user with chunk size
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*
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* Example: User requests 1 byte
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* LHS
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* requested size = 0b00000001
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* alignment - 1 = 0b00000111
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* => size + (alignment - 1) = 0b00001000
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* RHS
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* ~(alignment - 1) = 0b11111000
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* LHS & RHS = 0b00001000
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*/
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// TODO: should probably be 16? would need to change chunk struct
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static inline uint32_t align(uint32_t size)
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// Aligns the amount of memory requested by the user with chunk size
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static inline size_t align(size_t size)
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{
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size = (size + (ALIGN - 1)) & ~(ALIGN - 1);
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const size_t alignment = 8;
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size = (size + (alignment - 1)) & ~(alignment - 1);
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// REVIEW: unnecessary with bit-wise math above?
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if (size <= MIN)
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return MIN;
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else
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return size;
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}
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/* Debug Functions */
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// Useful for debugging after find or merge are called
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static void chk_freelist(void)
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{
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struct gc_chunk *next = freelist;
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struct gc_chunk *prev = NULL;
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while (next != NULL) {
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assert(next->free);
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assert(next->merge == 0);
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assert(next->size >= MIN);
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assert(get_footer(next) == next->size);
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assert(next->prev == prev);
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struct gc_chunk *after = find_next_chunk(next);
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assert(after->merge == 1);
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assert(after->free == 0);
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prev = next;
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next = next->next;
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}
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}
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/* gc_malloc Functions */
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struct gc_chunk *find_next_chunk(struct gc_chunk *curr)
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struct gc_chunk *get_next_chunk(struct gc_chunk *curr)
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{
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unsigned size = curr->size;
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return (struct gc_chunk *)((uint64_t *)curr + MAGIC + (size - 1));
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}
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struct gc_chunk *find_prev_chunk(struct gc_chunk *curr)
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struct gc_chunk *get_prev_chunk(struct gc_chunk *curr)
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{
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assert(curr->merge);
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unsigned size = curr->footer;
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return (struct gc_chunk *)((uint64_t *)curr - (size - 1) - MAGIC);
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}
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void set_footer(struct gc_chunk *curr)
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{
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struct gc_chunk *next = find_next_chunk(curr);
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next->footer = curr->size;
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}
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uint64_t get_footer(struct gc_chunk *curr)
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{
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struct gc_chunk *next = find_next_chunk(curr);
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return next->footer;
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}
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struct gc_chunk *init_gc_chunk(void)
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{
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const size_t len = getpagesize();
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struct gc_chunk *chunk =
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(struct gc_chunk *)mmap(NULL, len, PROT_READ | PROT_WRITE,
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MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
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if (chunk == MAP_FAILED)
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return NULL;
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chunk->size = len - MAGIC - OVERHEAD;
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chunk->merge = 0;
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chunk->free = 1;
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chunk->next = NULL;
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chunk->prev = NULL;
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struct gc_chunk *sentinel = find_next_chunk(chunk);
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sentinel->footer = chunk->size;
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sentinel->size = 0;
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sentinel->merge = 1;
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sentinel->free = 1;
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sentinel->next = NULL;
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sentinel->prev = chunk;
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chk_freelist(); // TODO: Remove this line after debugging
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return chunk;
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}
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struct gc_chunk *find_free_chunk(size_t size)
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{
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// HACK: The first loop will run forever if we can never get a big enough chunk
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const size_t page = getpagesize();
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if (size > page) {
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fprintf(stderr,
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"ERROR: find_free_chunk: Requested size is larger than a page!");
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return NULL;
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}
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struct gc_chunk *found;
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while (1) {
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found = freelist;
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while (found != NULL) {
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if (found->size >= size) {
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// found, TODO: split when appropriate
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// remaining block needs to be at least MIN + OVERHEAD
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break;
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}
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found = found->next;
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}
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if (found == NULL) {
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found = init_gc_chunk();
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if (found == NULL) {
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return found;
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}
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freelist = found;
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break;
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} else {
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break;
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}
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}
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return found;
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}
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// FIXME: need to clean up todos before this will work
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void merge_chunk(struct gc_chunk *curr)
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{
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if (curr->merge) {
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struct gc_chunk *prev = find_prev_chunk(curr);
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chk_freelist(); // TODO: Remove this line after debugging
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struct gc_chunk *next = find_next_chunk(curr);
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chk_freelist(); // TODO: Remove this line after debugging
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if (next->free) {
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// TODO Case 1: merge prev and next
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} else {
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// TODO Case 2: merge with prev
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}
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set_footer(prev);
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} else {
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struct gc_chunk *next = find_next_chunk(curr);
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chk_freelist(); // TODO: Remove this line after debugging
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if (next->free) {
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// TODO Case 3: merge with after
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} else {
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// TODO Case 4: can't merge anything
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}
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curr->free = 1;
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set_footer(curr);
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}
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}
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void *gc_malloc(size_t size)
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{
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if (size == 0)
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return NULL;
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uint32_t bytes = align(size);
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struct gc_chunk *curr = find_free_chunk(bytes);
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chk_freelist(); // TODO: Remove this line after debugging
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if (curr == NULL)
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return curr;
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return hide(curr);
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}
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void gc_free(void *ptr)
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{
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if (ptr != NULL) {
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struct gc_chunk *curr = magic(ptr);
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merge_chunk(curr);
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chk_freelist(); // TODO: Remove this line after debugging
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}
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}
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void destroy_heap(struct gc_chunk *freelist)
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{
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if (freelist == NULL) {
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fprintf(stderr, "WARN: destroy_heap: Free list is empty?");
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return;
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}
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struct gc_chunk *curr = freelist;
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while (curr->size != 0) {
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struct gc_chunk *next = curr->next;
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munmap(curr, curr->size);
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curr = next;
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}
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}
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