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183 lines
5.0 KiB
183 lines
5.0 KiB
/*
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* Yet Another Malloc
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* yamalloc.c
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*/
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/* Feature test macros */
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#define _DEFAULT_SOURCE // for sbrk
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/* Includes */
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#include <unistd.h>
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#include <stdio.h>
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#include <stdbool.h>
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#include "yamalloc.h"
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/* Constants */
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#define YA_SZ_WORD (sizeof(intptr_t))
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#define YA_SZ_DWORD (2 * YA_SZ_WORD)
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#define YA_SZ_CHUNK 8192
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/* Macros */
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#define YA_IS_ALLOC(block) ((block)[-1] & 1)
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#define YA_SZ_BLOCK(size) ((size) & -2)
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#define YA_ROUND_DIV(n, m) (((n) + ((m)-1)) / (m))
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#define YA_ROUND(n, m) (YA_ROUND_DIV(n,m) * (m))
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/* Globals */
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intptr_t *heap_start = NULL; // space for 2 words before
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intptr_t *heap_end = NULL; // first block outside heap
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/* Local declarations */
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intptr_t *heap_init();
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intptr_t *heap_extend(intptr_t size);
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intptr_t *block_join(intptr_t *block);
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void block_split(intptr_t *block, intptr_t size);
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intptr_t *block_find(intptr_t size);
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/* Function definitions */
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#ifdef YA_DEBUG
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void ya_print_blocks() {
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intptr_t *block;
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intptr_t block_size;
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for (block = heap_start; block < heap_end; block += block_size) {
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block_size = YA_SZ_BLOCK(block[-1]);
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printf("Block at %p size = %ld alloc = %d\n",
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block, block_size, YA_IS_ALLOC(block));
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}
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}
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#endif
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intptr_t *block_join_prev(intptr_t *block) {
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intptr_t prev_size = YA_SZ_BLOCK(block[-2]);
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intptr_t *prev = block - prev_size;
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if (prev <= heap_start || YA_IS_ALLOC(prev)) {
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return block;
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}
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intptr_t block_size = YA_SZ_BLOCK(block[-1]);
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intptr_t size = prev_size + block_size;
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prev[-1] = size;
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block[block_size - 2] = size;
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ya_debug("block_join_prev: joining %p:%ld and %p:%ld -> %p:%ld\n",
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block, block_size, prev, prev_size, prev, size);
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return prev;
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}
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intptr_t *block_join_next(intptr_t *block) {
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intptr_t block_size = YA_SZ_BLOCK(block[-1]);
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intptr_t *next = block + block_size;
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if (next >= heap_end || YA_IS_ALLOC(next)) {
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return block;
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}
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intptr_t next_size = YA_SZ_BLOCK(next[-1]);
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intptr_t size = next_size + block_size;
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block[-1] = size;
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next[next_size - 2] = size;
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ya_debug("block_join_next: joining %p:%ld and %p:%ld -> %p:%ld\n",
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block, block_size, next, next_size, block, size);
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return block;
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}
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intptr_t *block_join(intptr_t *block) {
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if (block > heap_start) {
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block = block_join_prev(block);
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}
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return block_join_next(block);
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}
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void block_split(intptr_t *block, intptr_t size) {
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intptr_t old_size = YA_SZ_BLOCK(block[-1]);
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block[-1] = size;
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block[size - 2] = size;
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block[size - 1] = old_size - size;
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block[old_size - 2] = old_size - size;
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}
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intptr_t *block_find(intptr_t size) {
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intptr_t *block;
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intptr_t block_size;
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for (block = heap_start; block < heap_end; block += block_size) {
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block_size = YA_SZ_BLOCK(block[-1]);
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if (!YA_IS_ALLOC(block) && size <= block_size) {
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return block;
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}
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}
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// could not find block, extend heap
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return heap_extend(size);
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}
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intptr_t *heap_init() {
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intptr_t size_w = YA_SZ_CHUNK / YA_SZ_WORD;
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void *ptr = sbrk(YA_SZ_WORD * (size_w + 2));
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if (ptr == (void*) -1) {
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heap_start = NULL;
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heap_end = NULL;
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return NULL;
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}
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heap_start = ptr; // cast to intptr_t *
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heap_start += 2; // space for the first block[-1] + dword alignment
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heap_end = heap_start + size_w;
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heap_start[-1] = size_w;
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heap_end[-2] = size_w;
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ya_debug("heap_init: start = %p, end = %p, size_w = %ld\n",
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heap_start, heap_end, size_w);
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return heap_start;
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}
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intptr_t *heap_extend(intptr_t size_w) {
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intptr_t size = YA_ROUND(size_w * YA_SZ_WORD, YA_SZ_CHUNK);
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size_w = size / YA_SZ_WORD;
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ya_debug("heap_extend: size_w = %ld\n", size_w);
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void *ptr = sbrk(size);
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if (ptr == (void *) - 1) {
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return NULL;
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}
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intptr_t *block = ptr; // == old heap_end
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heap_end = block + size_w;
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block[-1] = size_w;
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heap_end[-2] = size_w;
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ya_debug("heap_extend: old end = %p, new end = %p, size_w = %ld\n",
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block, heap_end, size_w);
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return block_join(ptr);
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}
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void *malloc(size_t size) {
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if (size == 0) {
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return NULL;
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}
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if (heap_start == NULL || heap_end == NULL) {
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if (!heap_init()) {
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return NULL;
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}
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}
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intptr_t size_w = YA_ROUND_DIV(size, YA_SZ_WORD); // size in words
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size_w = 2 + YA_ROUND(size_w, 2); // round to dword and make space for tags
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ya_debug("malloc: size = %ld, size_w = %ld\n", size, size_w);
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intptr_t *block = block_find(size_w);
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intptr_t block_size = YA_SZ_BLOCK(block[-1]);
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if (size_w < block_size) {
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block_split(block, size_w);
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}
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block[-1] |= 1;
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block[block_size - 2] |= 1;
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return block;
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}
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void free(void *ptr) {
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intptr_t *block = ptr;
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if (block < heap_start || block > heap_end || !YA_IS_ALLOC(block)) {
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return; // TODO: provoke segfault
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}
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intptr_t block_size = YA_SZ_BLOCK(block[-1]);
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block[-1] &= -2; // erase allocated bit
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block[block_size - 2] &= -2;
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block_join(block);
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}
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