《STL源代码剖析》---stl_alloc.h阅读笔记

这一节是讲空间的配置与释放，但不涉及对象的构造和析构，仅仅是解说对象构造前空前的申请以及对象析构后空间怎么释放。

SGI版本号的STL对空间的的申请和释放做了例如以下考虑：

1、向堆申请空间

2、考虑了多线程。可是这节目的仅仅是解说空间配置与释放，因此忽略了多线程。集中学习空间的申请和释放。

3、内存不足时的应变措施

4、考虑到了内存碎片的问题。多次申请释放小块内存可能会造成内存碎片。

在C++中。内存的申请和释放是通过operator new函数和operator delete函数，这两个函数相当于C语言中的malloc和free，可是SGI使用的是malloc和free来完毕空间配置与释放的。原因一方面可能是历史原因。还有一方面是C++并没有提供对应于realloc()函数的类似函数。这样就不能直接用C++的set_new_hanlder()来处理内存分配失败的情况，所以仿真一个set_malloc_hanlder()函数。

为了解决内存碎片问题。SGI设计了双层级配置器。

第一级配置器直接使用malloc和free函数，当申请内存大于128bytes，就觉得“足够大”，使用第一级配置器。

第二级配置器使用了memory pool的方式。在pool中维护着已经开辟“小内存”。使用一个free list维护，当使用时。直接从free list中取。使用完后，还给free list。当free list中内存不足时。从memory pool中取内存填入free list中。

SGI维护着16个free lists，这16个list各自管理大小分别为8、16、24、32、40、48、56、64、72、80、88、96、104、112、120、128bytes大小的内存，当申请内存小鱼128bytes时，自己主动向上添加到8的倍数以便于在free
lists中取。

以下是源码和加的一些凝视

G++ 2.91.57，cygnus\cygwin-b20\include\g++\stl_alloc.h 完整列表
/*
* Copyright (c) 1996-1997
* Silicon Graphics Computer Systems, Inc.
*
* Permission to use, copy, modify, distribute and sell this software
* and its documentation for any purpose is hereby granted without fee,
* provided that the above copyright notice appear in all copies and
* that both that copyright notice and this permission notice appear
* in supporting documentation.  Silicon Graphics makes no
* representations about the suitability of this software for any
* purpose.  It is provided "as is" without express or implied warranty.
*/

/* NOTE: This is an internal header file, included by other STL headers.
*   You should not attempt to use it directly.
*/

#ifndef __SGI_STL_INTERNAL_ALLOC_H
#define __SGI_STL_INTERNAL_ALLOC_H

#ifdef __SUNPRO_CC
#  define __PRIVATE public
// Extra access restrictions prevent us from really making some things
// private.
#else
#  define __PRIVATE private
#endif

#ifdef __STL_STATIC_TEMPLATE_MEMBER_BUG
#  define __USE_MALLOC
#endif

#if 0
#   include <new>
#   define __THROW_BAD_ALLOC throw bad_alloc
#elif !defined(__THROW_BAD_ALLOC)//定义内存申请出错处理
#   include <iostream.h>
#   define __THROW_BAD_ALLOC cerr << "out of memory" << endl; exit(1)
#endif

#ifndef __ALLOC
#   define __ALLOC alloc
#endif
#ifdef __STL_WIN32THREADS
#   include <windows.h>
#endif

#include <stddef.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#ifndef __RESTRICT
#  define __RESTRICT
#endif
/*
假设编译器不支持多线程，那么就不用多线程
__STL_PTHREADS gcc编译器，POSIX接口
__STL_WIN32THREADS msvc编译器
*/
#if !defined(__STL_PTHREADS) && !defined(_NOTHREADS) \
&& !defined(__STL_SGI_THREADS) && !defined(__STL_WIN32THREADS)
#   define _NOTHREADS  //不支持多线程
#endif
/*
假设是gcc编译器，那么增加对相互排斥锁的支持
*/
# ifdef __STL_PTHREADS
// POSIX Threads
// This is dubious, since this is likely to be a high contention
// lock.   Performance may not be adequate.
#   include <pthread.h>
#   define __NODE_ALLOCATOR_LOCK \
if (threads) pthread_mutex_lock(&__node_allocator_lock)
#   define __NODE_ALLOCATOR_UNLOCK \
if (threads) pthread_mutex_unlock(&__node_allocator_lock)
#   define __NODE_ALLOCATOR_THREADS true
#   define __VOLATILE volatile  // Needed at -O3 on SGI
# endif
/*
msvc编译器
*/
# ifdef __STL_WIN32THREADS
// The lock needs to be initialized by constructing an allocator
// objects of the right type.  We do that here explicitly for alloc.
#   define __NODE_ALLOCATOR_LOCK \
EnterCriticalSection(&__node_allocator_lock)
#   define __NODE_ALLOCATOR_UNLOCK \
LeaveCriticalSection(&__node_allocator_lock)
#   define __NODE_ALLOCATOR_THREADS true
#   define __VOLATILE volatile  // may not be needed
# endif /* WIN32THREADS */
/*SGI专用*/
# ifdef __STL_SGI_THREADS
// This should work without threads, with sproc threads, or with
// pthreads.  It is suboptimal in all cases.
// It is unlikely to even compile on nonSGI machines.

extern "C" {
extern int __us_rsthread_malloc;
}
// The above is copied from malloc.h.  Including <malloc.h>
// would be cleaner but fails with certain levels of standard
// conformance.
#   define __NODE_ALLOCATOR_LOCK if (threads && __us_rsthread_malloc) \
{ __lock(&__node_allocator_lock); }
#   define __NODE_ALLOCATOR_UNLOCK if (threads && __us_rsthread_malloc) \
{ __unlock(&__node_allocator_lock); }
#   define __NODE_ALLOCATOR_THREADS true
#   define __VOLATILE volatile  // Needed at -O3 on SGI
# endif
/*不支持多线程*/
# ifdef _NOTHREADS
//  Thread-unsafe
#   define __NODE_ALLOCATOR_LOCK
#   define __NODE_ALLOCATOR_UNLOCK
#   define __NODE_ALLOCATOR_THREADS false
#   define __VOLATILE
# endif

__STL_BEGIN_NAMESPACE

#if defined(__sgi) && !defined(__GNUC__) && (_MIPS_SIM != _MIPS_SIM_ABI32)
#pragma set woff 1174
#endif

// malloc-based allocator. 通常比稍後介紹的 default alloc 速度慢，
// 一般而言是 thread-safe，並且對於空間的運用比较高效（efficient）。
#ifdef __STL_STATIC_TEMPLATE_MEMBER_BUG
# ifdef __DECLARE_GLOBALS_HERE
void (* __malloc_alloc_oom_handler)() = 0;
// g++ 2.7.2 不支持 static template data members.
# else
extern void (* __malloc_alloc_oom_handler)();
# endif
#endif

/*
以下就是第一级配置器，没有template參数，inst没实用
*/
template <int inst>
class __malloc_alloc_template {

private:
/*
oom是指out of memory
定义函数指针。用来处理内存申请失败的情况，C++ 的 set_new_handler()
*/
static void *oom_malloc(size_t);

static void *oom_realloc(void *, size_t);
/*
在以下set_malloc_handler函数设置，用于内存申请失败时的处理，C++ 的 set_new_handler()
*/
#ifndef __STL_STATIC_TEMPLATE_MEMBER_BUG//假设编译器支持静态模板类
static void (* __malloc_alloc_oom_handler)();
#endif

public:

static void * allocate(size_t n)
{
void *result = malloc(n);	// 第一级配置器直接用malloc申请内存
//当malloc申请失败，使用oom_malloc
if (0 == result) result = oom_malloc(n);
return result;
}

static void deallocate(void *p, size_t /* n */)
{
free(p);	// 第一级配置器直接用free释放内存
}

static void * reallocate(void *p, size_t /* old_sz */, size_t new_sz)
{
void * result = realloc(p, new_sz);	// 第一级配置器直接使用 realloc()
if (0 == result) result = oom_realloc(p, new_sz);
return result;
}

/*
设置内存申请失败的错误处理函数，相似 C++ 的 set_new_handler()。

这个是客端设置的。而不是编译器设置。假设不设置。则内存配置失败立即终止
*/
static void (* set_malloc_handler(void (*f)()))()
{
void (* old)() = __malloc_alloc_oom_handler;
__malloc_alloc_oom_handler = f;
return(old);
}

};

// malloc_alloc out-of-memory handling。初始值为0
#ifndef __STL_STATIC_TEMPLATE_MEMBER_BUG
template <int inst>
void (* __malloc_alloc_template<inst>::__malloc_alloc_oom_handler)() = 0;
#endif

template <int inst>
void * __malloc_alloc_template<inst>::oom_malloc(size_t n)
{
void (* my_malloc_handler)();
void *result;

for (;;) {	// 不断尝试释放、配置、再释放、再配置……
my_malloc_handler = __malloc_alloc_oom_handler;
//假设没有设置out-of-memory handling处理函数。则抛出异常，终止
if (0 == my_malloc_handler) { __THROW_BAD_ALLOC; }
(*my_malloc_handler)();		// 调用处理函数。企图释放内存
result = malloc(n);			//再次尝试配置内存
if (result) return(result);
}
}
//和上面的函数相似
template <int inst>
void * __malloc_alloc_template<inst>::oom_realloc(void *p, size_t n)
{
void (* my_malloc_handler)();
void *result;

for (;;) {	// 不断尝试释放、配置、再释放、再配置……
my_malloc_handler = __malloc_alloc_oom_handler;
if (0 == my_malloc_handler) { __THROW_BAD_ALLOC; }
(*my_malloc_handler)();
result = realloc(p, n);
if (result) return(result);
}
}
//将參数inst设为0
typedef __malloc_alloc_template<0> malloc_alloc;
/*
不管alloc是第一级配置器还是第二级配置器，SGI还为它包装一个接口。使之
符合STL规范
*/
template<class T, class Alloc>
class simple_alloc {

public:
/*
以下四个函数都是单纯的转调用，调用传递给配置器（可能是第一级，也可能是第二级）。
依据sizeof(T)或n*sizeof(T)的大小
*/
static T *allocate(size_t n)
{ return 0 == n? 0 : (T*) Alloc::allocate(n * sizeof (T)); }
static T *allocate(void)
{ return (T*) Alloc::allocate(sizeof (T)); }
static void deallocate(T *p, size_t n)
{ if (0 != n) Alloc::deallocate(p, n * sizeof (T)); }
static void deallocate(T *p)
{ Alloc::deallocate(p, sizeof (T)); }
};

// Allocator adaptor to check size arguments for debugging.
// Reports errors using assert.  Checking can be disabled with
// NDEBUG, but it's far better to just use the underlying allocator
// instead when no checking is desired.
// There is some evidence that this can confuse Purify.
template <class Alloc>
class debug_alloc {

private:
/*
extra用于记录配置内存大小，同一时候保证字节对齐
*/
enum {extra = 8};       // Size of space used to store size.  Note
// that this must be large enough to preserve
// alignment.

public:

static void * allocate(size_t n)
{
char *result = (char *)Alloc::allocate(n + extra);
*(size_t *)result = n;//内存分配前面记录分配内存的大小
return result + extra;//返回内存不包含extra大小
}

static void deallocate(void *p, size_t n)
{
char * real_p = (char *)p - extra;//释放时，要加上extra大小内存
assert(*(size_t *)real_p == n);//推断前面的数据是否被改动，假设改动则说明有越界
Alloc::deallocate(real_p, n + extra);
}

static void * reallocate(void *p, size_t old_sz, size_t new_sz)
{
char * real_p = (char *)p - extra;
assert(*(size_t *)real_p == old_sz);
char * result = (char *)
Alloc::reallocate(real_p, old_sz + extra, new_sz + extra);
*(size_t *)result = new_sz;
return result + extra;
}

};

# ifdef __USE_MALLOC

typedef malloc_alloc alloc;	// 令 alloc 为第一级配置器
typedef malloc_alloc single_client_alloc;

# else

// Default node allocator.
// With a reasonable compiler, this should be roughly as fast as the
// original STL class-specific allocators, but with less fragmentation.
// Default_alloc_template parameters are experimental and MAY
// DISAPPEAR in the future.  Clients should just use alloc for now.
/*
翻译：默认的内存配置器。在合适的编译器上，它的性能（SGI版本号的）应该和STL原版
的配置器性能大致同样，可是SGi版本号的使内存碎片更少。

默认的内存配置器仅仅是实验性的且以后可能会消失。

客端如今应该仅仅是用alloc
*/
//
// Important implementation properties:
// 1. If the client request an object of size > __MAX_BYTES, the resulting
//    object will be obtained directly from malloc.
// 2. In all other cases, we allocate an object of size exactly
//    ROUND_UP(requested_size).  Thus the client has enough size
//    information that we can return the object to the proper free list
//    without permanently losing part of the object.
//
/*
翻译：实现中的特性
1、当客端请求内存大小size>__MAX_BYTES时。对象直接调用malloc
2、否则，把size ROUND_UP为8的整数倍。从free list中
*/
// The first template parameter specifies whether more than one thread
// may use this allocator.  It is safe to allocate an object from
// one instance of a default_alloc and deallocate it with another
// one.  This effectively transfers its ownership to the second one.
// This may have undesirable effects on reference locality.
// The second parameter is unreferenced and serves only to allow the
// creation of multiple default_alloc instances.
// Node that containers built on different allocator instances have
// different types, limiting the utility of this approach.
/*
翻译：模板的第一个參数来指定是否有多于一个线程在使用这个alloctor。
在一个实例中配置内存，在还有一个实例中释放是安全的。这样能够有效的转换内存
使用权。

这可能会在引用区域产生意想不到的影响。

第二个參数是非引用的，仅用于创建多个default_alloc实例。
注意：使用不同的allocator创建的容器有不同特性。这限制了通用性。
*/
#ifdef __SUNPRO_CC
// breaks if we make these template class members:
enum {__ALIGN = 8};			// 小型区块的上上调界
enum {__MAX_BYTES = 128};		// 小型区块的上限
enum {__NFREELISTS = __MAX_BYTES/__ALIGN};	// free-lists 个数，共16个
#endif

// 以下是第二级配置器。
// 注意，没有模板參数，inst没实用，第一个參数用于多线程。
template <bool threads, int inst>
class __default_alloc_template {

private:
/*
实际上。我们应该使用static const int x = N来代替enum { x = N }，
可是眼下支持该性能的编译器不多
*/
# ifndef __SUNPRO_CC
enum {__ALIGN = 8};
enum {__MAX_BYTES = 128};
enum {__NFREELISTS = __MAX_BYTES/__ALIGN};
# endif
//将bytes上调至8的整数倍
static size_t ROUND_UP(size_t bytes) {
return (((bytes) + __ALIGN-1) & ~(__ALIGN - 1));
}
__PRIVATE:
/*
这是free list内的结点。
採用union，尽量降低占用内存。
假设使用free_list_link，则指向同样的union结构，这个供链表free list使用。
假设使用client_data[1]，则给客端使用
*/
union obj {
union obj * free_list_link;
char client_data[1];    /* The client sees this. */
};
private:
# ifdef __SUNPRO_CC
static obj * __VOLATILE free_list[];
// Specifying a size results in duplicate def for 4.1
# else
//__NFREELISTS值为16。相应链表维护内存大小为8、16、…、128
static obj * __VOLATILE free_list[__NFREELISTS];
# endif
//依据bytes大小。在16个链表中选取合适的那个
static  size_t FREELIST_INDEX(size_t bytes) {
return (((bytes) + __ALIGN-1)/__ALIGN - 1);
}

// Returns an object of size n, and optionally adds to size n free list.
//返回一个大小为n的对象。而且可能增加大小为n的其它区块到free list
static void *refill(size_t n);
// Allocates a chunk for nobjs of size "size".  nobjs may be reduced
// if it is inconvenient to allocate the requested number.
/*
配置一大块空间。可容纳nobjs个size大小的区块，假设配置
nobjs个区块有所不便，nobjs可能会降低。

*/
static char *chunk_alloc(size_t size, int &nobjs);

// Chunk allocation state.
static char *start_free;//内存池起始位置
static char *end_free;//内存池结束位置
static size_t heap_size;//在堆上已有内存的大小
//假设支持多SGI线程。则提供锁支持
# ifdef __STL_SGI_THREADS
static volatile unsigned long __node_allocator_lock;
static void __lock(volatile unsigned long *);
static inline void __unlock(volatile unsigned long *);
# endif
//假设支持多线程，则提供相互排斥锁
# ifdef __STL_PTHREADS
static pthread_mutex_t __node_allocator_lock;
# endif
//win32多线程
# ifdef __STL_WIN32THREADS
static CRITICAL_SECTION __node_allocator_lock;
static bool __node_allocator_lock_initialized;

public:
__default_alloc_template() {
// This assumes the first constructor is called before threads
// are started.
//假设构造函数在多线程启动前已经调用
if (!__node_allocator_lock_initialized) {
InitializeCriticalSection(&__node_allocator_lock);
__node_allocator_lock_initialized = true;
}
}
private:
# endif

class lock {
public:
lock() { __NODE_ALLOCATOR_LOCK; }
~lock() { __NODE_ALLOCATOR_UNLOCK; }
};
friend class lock;

public:

/* n must be > 0      */
static void * allocate(size_t n)
{
obj * __VOLATILE * my_free_list;
obj * __RESTRICT result;

if (n > (size_t) __MAX_BYTES) {//假设配置内存大于__MAX_BYTES，使用第一级配置器
return(malloc_alloc::allocate(n));
}
my_free_list = free_list + FREELIST_INDEX(n);//在16个free lists中找到相应的那个
// Acquire the lock here with a constructor call.
// This ensures that it is released in exit or during stack
// unwinding.
#       ifndef _NOTHREADS
/*REFERENCED*/
lock lock_instance;
#       endif
result = *my_free_list;
if (result == 0) {//假设没找可用的free list，那么又一次填充free list
void *r = refill(ROUND_UP(n));
return r;
}
//调整free list
*my_free_list = result -> free_list_link;
return (result);
};

/* p may not be 0 */
static void deallocate(void *p, size_t n)
{
obj *q = (obj *)p;
obj * __VOLATILE * my_free_list;

if (n > (size_t) __MAX_BYTES) {//调用第一级配置器的释放函数
malloc_alloc::deallocate(p, n);
return;
}
//在16个free lists中找到相应的那个
my_free_list = free_list + FREELIST_INDEX(n);
// acquire lock
#       ifndef _NOTHREADS
/*REFERENCED*/
lock lock_instance;
#       endif /* _NOTHREADS */
//回收内存到free list
q -> free_list_link = *my_free_list;
*my_free_list = q;
// lock is released here
}

static void * reallocate(void *p, size_t old_sz, size_t new_sz);

} ;

typedef __default_alloc_template<__NODE_ALLOCATOR_THREADS, 0> alloc;
typedef __default_alloc_template<false, 0> single_client_alloc;

/* We allocate memory in large chunks in order to avoid fragmenting     */
/* the malloc heap too much.                                            */
/* We assume that size is properly aligned.                             */
/* We hold the allocation lock.                                         */
/*
分配内存时分配一大块，防止多次分配小内存造成内存碎片
假设size已经对齐
持有allocation锁
*/
//从内存池中去空间给free list，nobjs是引用调用，原因是可能会改动其值。
//当不够nobjs个区块时。可能适当调小nobjs的值
template <bool threads, int inst>
char*
__default_alloc_template<threads, inst>::chunk_alloc(size_t size, int& nobjs)
{
char * result;
size_t total_bytes = size * nobjs;//要配置的空间大小
size_t bytes_left = end_free - start_free;//内存池大小

if (bytes_left >= total_bytes) {//内存池空间满足需求
result = start_free;
start_free += total_bytes;
return(result);
} else if (bytes_left >= size) {
//内存池空间不够，可是足够供应一个（含）以上的块
nobjs = bytes_left/size;
total_bytes = size * nobjs;
result = start_free;
start_free += total_bytes;
return(result);
} else {
//内存池剩余空间大小连一个块大小都无法提供
size_t bytes_to_get = 2 * total_bytes + ROUND_UP(heap_size >> 4);
// Try to make use of the left-over piece.
//尝试内存池中的參与零头还有利用价值
if (bytes_left > 0) {
obj * __VOLATILE * my_free_list =//找到相应的free list
free_list + FREELIST_INDEX(bytes_left);
//调整free list。将内存池空间编入
((obj *)start_free) -> free_list_link = *my_free_list;
*my_free_list = (obj *)start_free;
}
//配置heap，用来补充内存池
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.
/*
尝试用现有的，这不会造成破坏。我们不尝试配置较小的区块，
由于这样做将会在多线程机器上造成灾难
*/
//搜索适当free list，适当是指“尚未用区块，且足够大”的free list
for (i = size; i <= __MAX_BYTES; i += __ALIGN) {
my_free_list = free_list + FREELIST_INDEX(i);
p = *my_free_list;
if (0 != p) {//free内有尚未用区块
//调整free list。释出未用区块
*my_free_list = p -> free_list_link;
start_free = (char *)p;
end_free = start_free + i;
//递归调用自己，修正nobjs
return(chunk_alloc(size, nobjs));
// Any leftover piece will eventually make it to the
// right free list.
//不论什么參与的零头将会被编入适当地free list中备用
}
}
//假设出现意外，到处无可用内存
end_free = 0;	// In case of exception.
//调用第一级配置器，看看out-of-memory机制是否能尽点力
start_free = (char *)malloc_alloc::allocate(bytes_to_get);
// This should either throw an
// exception or remedy the situation.  Thus we assume it
// succeeded.
//这里可能会抛出异常，或内存不足情况得到改善。
}
heap_size += bytes_to_get;
end_free = start_free + bytes_to_get;
//递归调用自己。修正nobjs
return(chunk_alloc(size, nobjs));
}
}

/* 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.                                         */
/*
这个函数和上一个差点儿相同，没有第二个參数nobjs。

这个函数在函数体中设置了其大小为20.
返回大小为n的对象，且增加到free list中
我们假设n已经对齐
持有allocation锁
*/
template <bool threads, int inst>
void* __default_alloc_template<threads, inst>::refill(size_t n)
{
int nobjs = 20;
char * chunk = chunk_alloc(n, nobjs);
obj * __VOLATILE * my_free_list;
obj * result;
obj * current_obj, * next_obj;
int i;
//假设仅仅够一个块的大小
if (1 == nobjs) return(chunk);
my_free_list = free_list + FREELIST_INDEX(n);

/* Build free list in chunk */
//在chunk中建立free list
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);
}
/*
扩展现有内存，又一次分配内存，要把旧内存内容复制到新内存
*/
template <bool threads, int inst>
void*
__default_alloc_template<threads, inst>::reallocate(void *p,
size_t old_sz,
size_t new_sz)
{
void * result;
size_t copy_sz;
//假设就内存和新内存都大于_MAX_BYTES。直接调用realloc
if (old_sz > (size_t) __MAX_BYTES && new_sz > (size_t) __MAX_BYTES) {
return(realloc(p, new_sz));
}
//内存大小没变化（没变化是指经过上调为8的整数倍后没变化），直接返回
if (ROUND_UP(old_sz) == ROUND_UP(new_sz)) return(p);
result = allocate(new_sz);//分配新内存
copy_sz = new_sz > old_sz?

old_sz : new_sz;
memcpy(result, p, copy_sz);//拷贝旧内存的数据到新内存
deallocate(p, old_sz);//释放就内存
return(result);
}

#ifdef __STL_PTHREADS
template <bool threads, int inst>
pthread_mutex_t
__default_alloc_template<threads, inst>::__node_allocator_lock
= PTHREAD_MUTEX_INITIALIZER;
#endif

#ifdef __STL_WIN32THREADS
template <bool threads, int inst> CRITICAL_SECTION
__default_alloc_template<threads, inst>::__node_allocator_lock;

template <bool threads, int inst> bool
__default_alloc_template<threads, inst>::__node_allocator_lock_initialized
= false;
#endif

#ifdef __STL_SGI_THREADS
__STL_END_NAMESPACE
#include <mutex.h>
#include <time.h>
__STL_BEGIN_NAMESPACE
// Somewhat generic lock implementations.  We need only test-and-set
// and some way to sleep.  These should work with both SGI pthreads
// and sproc threads.  They may be useful on other systems.
template <bool threads, int inst>
volatile unsigned long
__default_alloc_template<threads, inst>::__node_allocator_lock = 0;

#if __mips < 3 || !(defined (_ABIN32) || defined(_ABI64)) || defined(__GNUC__)
#   define __test_and_set(l,v) test_and_set(l,v)
#endif

template <bool threads, int inst>
void
__default_alloc_template<threads, inst>::__lock(volatile unsigned long *lock)
{
const unsigned low_spin_max = 30;  // spin cycles if we suspect uniprocessor
const unsigned high_spin_max = 1000; // spin cycles for multiprocessor
static unsigned spin_max = low_spin_max;
unsigned my_spin_max;
static unsigned last_spins = 0;
unsigned my_last_spins;
static struct timespec ts = {0, 1000};
unsigned junk;
#   define __ALLOC_PAUSE junk *= junk; junk *= junk; junk *= junk; junk *= junk
int i;

if (!__test_and_set((unsigned long *)lock, 1)) {
return;
}
my_spin_max = spin_max;
my_last_spins = last_spins;
for (i = 0; i < my_spin_max; i++) {
if (i < my_last_spins/2 || *lock) {
__ALLOC_PAUSE;
continue;
}
if (!__test_and_set((unsigned long *)lock, 1)) {
// got it!
// Spinning worked.  Thus we're probably not being scheduled
// against the other process with which we were contending.
// Thus it makes sense to spin longer the next time.
last_spins = i;
spin_max = high_spin_max;
return;
}
}
// We are probably being scheduled against the other process.  Sleep.
spin_max = low_spin_max;
for (;;) {
if (!__test_and_set((unsigned long *)lock, 1)) {
return;
}
nanosleep(&ts, 0);
}
}

template <bool threads, int inst>
inline void
__default_alloc_template<threads, inst>::__unlock(volatile unsigned long *lock)
{
#   if defined(__GNUC__) && __mips >= 3
asm("sync");
*lock = 0;
#   elif __mips >= 3 && (defined (_ABIN32) || defined(_ABI64))
__lock_release(lock);
#   else
*lock = 0;
// This is not sufficient on many multiprocessors, since
// writes to protected variables and the lock may be reordered.
#   endif
}
#endif

//内存池的起始地址、结束地址以及大小的初始化
template <bool threads, int inst>
char *__default_alloc_template<threads, inst>::start_free = 0;

template <bool threads, int inst>
char *__default_alloc_template<threads, inst>::end_free = 0;

template <bool threads, int inst>
size_t __default_alloc_template<threads, inst>::heap_size = 0;

template <bool threads, int inst>
__default_alloc_template<threads, inst>::obj * __VOLATILE
__default_alloc_template<threads, inst> ::free_list[
# ifdef __SUNPRO_CC
__NFREELISTS
# else
__default_alloc_template<threads, inst>::__NFREELISTS
# endif//free list的初始化
] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, };
// The 16 zeros are necessary to make version 4.1 of the SunPro
// compiler happy.  Otherwise it appears to allocate too little
// space for the array.

# ifdef __STL_WIN32THREADS
// Create one to get critical section initialized.
// We do this onece per file, but only the first constructor
// does anything.
static alloc __node_allocator_dummy_instance;
# endif

#endif /* ! __USE_MALLOC */

#if defined(__sgi) && !defined(__GNUC__) && (_MIPS_SIM != _MIPS_SIM_ABI32)
#pragma reset woff 1174
#endif

__STL_END_NAMESPACE

#undef __PRIVATE

#endif /* __SGI_STL_INTERNAL_ALLOC_H */

// Local Variables:
// mode:C++
// End: