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author : Hou Jie (侯捷)
date : 2015/11/11
compiler : DevC++ 5.61 (MinGW with GNU 4.9.2)
說明：這是侯捷 E-learning video "C++內存管理" 的實例程式.
filename : allocc.h
取材自 SGI STL 2.91 <stl_alloc.h>, 移植至 C language.
#include <stdlib.h> for malloc(),realloc()
#include <stddef.h> for size_t
#include <memory.h> for memcpy()
#define THROW_BAD_ALLOC cerr << "out of memory" << endl; exit(1)
#define THROW_BAD_ALLOC exit(1)———————————————-
第1級配置器。
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void (*oom_handler)() = 0;
void* oom_malloc(size_t n)
{
void (*my_malloc_handler)();
void* result;
for (;;) { 不斷嘗試釋放、配置、再釋放、再配置…
my_malloc_handler = oom_handler;
if (0 == my_malloc_handler) { THROW_BAD_ALLOC; }
(*my_malloc_handler)(); 呼叫處理常式，企圖釋放記憶體
result = malloc(n); 再次嘗試配置記憶體
if (result) return(result);
}
}
void* oom_realloc(void *p, size_t n)
{
void (*my_malloc_handler)();
void* result;
for (;;) { 不斷嘗試釋放、配置、再釋放、再配置…
my_malloc_handler = oom_handler;
if (0 == my_malloc_handler) { THROW_BAD_ALLOC; }
(*my_malloc_handler)(); 呼叫處理常式，企圖釋放記憶體。
result = realloc(p, n); 再次嘗試配置記憶體。
if (result) return(result);
}
}
void* malloc_allocate(size_t n)
{
void *result = malloc(n); 直接使用 malloc()
if (0 == result) result = oom_malloc(n);
return result;
}
void malloc_deallocate(void* p, size_t n)
{
free(p); 直接使用 free()
}
void* malloc_reallocate(void *p, size_t old_sz, size_t new_sz)
{
void* result = realloc(p, new_sz); 直接使用 realloc()
if (0 == result) result = oom_realloc(p, new_sz);
return result;
}
void (*set_malloc_handler(void (*f)()))()
{ 類似 C++ 的 set_new_handler().
void (*old)() = oom_handler;
oom_handler = f;
return(old);
}
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第二級配置器
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enum {ALIGN = 8}; 小區塊的上調邊界
enum {MAX_BYTES = 128}; 小區塊的上限
enum {NFREELISTS = MAX_BYTES/ALIGN}; free-lists 個數
union obj { G291[o],CB5[x],VC6[x]
union obj* free_list_link; 這麼寫在 VC6 和 CB5 中也可以，
}; 但以後就得使用 "union obj" 而不能只寫 "obj"
typedef struct obj {
struct obj* free_list_link;
} obj;
char* start_free = 0;
char* end_free = 0;
size_t heap_size = 0;
obj* free_list[NFREELISTS]
= {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
size_t ROUND_UP(size_t bytes) {
return (bytes) + ALIGN-1) & ~(ALIGN - 1;
}
size_t FREELIST_INDEX(size_t bytes) {
return (bytes) + ALIGN-1)/ALIGN - 1);
}
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We allocate memory in large chunks in order to
avoid fragmentingthe malloc heap too much.
We assume that size is properly aligned.
Allocates a chunk for nobjs of size "size".
nobjs may be reduced if it is inconvenient to
allocate the requested number.
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char* chunk_alloc(size_t size, int& nobjs) G291[o],VC6[x],CB5[x]
char* chunk_alloc(size_t size, int* nobjs)
{
char* result;
size_t total_bytes = size * (*nobjs); 原 nobjs 改為 (*nobjs)
size_t bytes_left = end_free - start_free;
if (bytes_left >= total_bytes) {
result = start_free;
start_free += total_bytes;
return(result);
} else if (bytes_left >= size) {
*nobjs = bytes_left / size; 原 nobjs 改為 (*nobjs)
total_bytes = size * (*nobjs); 原 nobjs 改為 (*nobjs)
result = start_free;
start_free += total_bytes;
return(result);
} else {
size_t bytes_to_get =
2 * total_bytes + ROUND_UP(heap_size >> 4);
Try to make use of the left-over piece.
if (bytes_left > 0) {
obj* volatile *my_free_list =
free_list + FREELIST_INDEX(bytes_left);
((obj*)start_free)->free_list_link = *my_free_list;
*my_free_list = (obj*)start_free;
}
start_free = (char*)malloc(bytes_to_get);
if (0 == start_free) {
int i;
obj* volatile *my_free_list, *p;
Try to make do with what we have. That can't
hurt. We do not try smaller requests, since that tends
to result in disaster on multi-process machines.
for (i = size; i <= MAX_BYTES; i += ALIGN) {
my_free_list = free_list + FREELIST_INDEX(i);
p = *my_free_list;
if (0 != p) {
*my_free_list = p -> free_list_link;
start_free = (char*)p;
end_free = start_free + i;
return(chunk_alloc(size, nobjs));
Any leftover piece will eventually make it to the
right free list.
}
}
end_free = 0; In case of exception.
start_free = (char*)malloc_allocate(bytes_to_get);
This should either throw an exception or
remedy the situation. Thus we assume it
succeeded.
}
heap_size += bytes_to_get;
end_free = start_free + bytes_to_get;
return(chunk_alloc(size, nobjs));
}
}
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Returns an object of size n, and optionally adds
to size n free list. We assume that n is properly aligned.
We hold the allocation lock.
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void* refill(size_t n)
{
int nobjs = 20;
char* chunk = chunk_alloc(n,&nobjs);
obj* volatile *my_free_list; obj my_free_list;
obj* result;
obj* current_obj;
obj* next_obj;
int i;
if (1 == nobjs) return(chunk);
my_free_list = free_list + FREELIST_INDEX(n);
Build free list in chunk
result = (obj*)chunk;
*my_free_list = next_obj = (obj*)(chunk + n);
for (i=1; ; ++i) {
current_obj = next_obj;
next_obj = (obj*)((char*)next_obj + n);
if (nobjs-1 == i) {
current_obj->free_list_link = 0;
break;
} else {
current_obj->free_list_link = next_obj;
}
}
return(result);
}
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void* allocate(size_t n) n must be > 0
{
obj* volatile *my_free_list; obj my_free_list;
obj* result;
if (n > (size_t)MAX_BYTES) {
return(malloc_allocate(n));
}
my_free_list = free_list + FREELIST_INDEX(n);
result = *my_free_list;
if (result == 0) {
void* r = refill(ROUND_UP(n));
return r;
}
*my_free_list = result->free_list_link;
return (result);
}
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void deallocate(void *p, size_t n) p may not be 0
{
obj* q = (obj*)p;
obj* volatile *my_free_list; obj** my_free_list;
if (n > (size_t) MAX_BYTES) {
malloc_deallocate(p, n);
return;
}
my_free_list = free_list + FREELIST_INDEX(n);
q->free_list_link = *my_free_list;
*my_free_list = q;
}
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void* reallocate(void *p, size_t old_sz, size_t new_sz)
{
void * result;
size_t copy_sz;
if (old_sz > (size_t) MAX_BYTES && new_sz > (size_t) MAX_BYTES) {
return(realloc(p, new_sz));
}
if (ROUND_UP(old_sz) == ROUND_UP(new_sz)) return(p);
result = allocate(new_sz);
copy_sz = new_sz > old_sz? old_sz : new_sz;
memcpy(result, p, copy_sz);
deallocate(p, old_sz);
return(result);
}
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