/*
** When SQLITE_OMIT_WSD is defined, it means that the target platform does
** not support Writable Static Data (WSD) such as global and static variables.
** All variables must either be on the stack or dynamically allocated from
** the heap.  When WSD is unsupported, the variable declarations scattered
** throughout the SQLite code must become constants instead.  The SQLITE_WSD
** macro is used for this purpose.  And instead of referencing the variable
** directly, we use its constant as a key to lookup the run-time allocated
** buffer that holds real variable.  The constant is also the initializer
** for the run-time allocated buffer.
**
** In the usual case where WSD is supported, the SQLITE_WSD and GLOBAL
** macros become no-ops and have zero performance impact.
*/

       如果目標平臺不支持靜態變量和全局變量，所有變量都需要從棧或堆里分配，這時候全局變量的聲明都是const，sqlite以變量的地址作為關鍵字來映射堆中的一片地址空間，從而對這個地址的讀寫來代替對靜態變量的讀寫。

使用時需要打開SQLITE_OMIT_WSD宏定義，以全局變量sqlite3Config為例，首先把sqlite3Config定義成const變量，再傳入變量的地址和變量的長度，返回堆中映射的地址。

#ifdef SQLITE_OMIT_WSD
  #define SQLITE_WSD const
  #define GLOBAL(t,v) (*(t*)sqlite3_wsd_find((void*)&(v), sizeof(v)))//傳入地址和長度
  #define sqlite3GlobalConfig GLOBAL(struct Sqlite3Config, sqlite3Config)
  int sqlite3_wsd_init(int N, int J);
  void *sqlite3_wsd_find(void *K, int L);
#else
  #define SQLITE_WSD
  #define GLOBAL(t,v) v
  #define sqlite3GlobalConfig sqlite3Config
#endif
SQLITE_WSD struct Sqlite3Config sqlite3Config//定義const變量

實現時，首先要有一個pGlobal來管理堆中申請的空間，pGlobal的類型是ProcessLocalStorage，結構定義如下：

#define PLS_HASHSIZE 43

typedef struct ProcessLocalStorage ProcessLocalStorage;
typedef struct ProcessLocalVar ProcessLocalVar;

struct ProcessLocalStorage {
  ProcessLocalVar *aData[PLS_HASHSIZE];
  int nFree;
  u8 *pFree;
};

struct ProcessLocalVar {
  void *pKey;
  ProcessLocalVar *pNext;
};

static ProcessLocalStorage *pGlobal = 0; 

其中pGlobal-> aData映射傳進來的靜態變量的實際地址。 

初始化時首先申請總的空間N + sizeof(ProcessLocalStorage) + J*sizeof(ProcessLocalVar);其中N是變量的總空間，J是變量的個數

int sqlite3_wsd_init(int N, int J){
  if( !pGlobal ){
    int nMalloc = N + sizeof(ProcessLocalStorage) + J*sizeof(ProcessLocalVar);
    pGlobal = (ProcessLocalStorage *)malloc(nMalloc);
    if( pGlobal ){
      memset(pGlobal, 0, sizeof(ProcessLocalStorage));
      pGlobal->nFree = nMalloc - sizeof(ProcessLocalStorage);
      pGlobal->pFree = (u8 *)&pGlobal[1];
    }
  }

  return pGlobal ? SQLITE_OK : SQLITE_NOMEM;
}

其中pGlobal->pFree是未使用的空間，已經使用的空間作為靜態變量的映射存在hash表里，相同hash值的變量通過鏈表連接在一起。

使用時只要調用函數void *sqlite3_wsd_find(void *K, int L)就可以了，其中K是變量的原始地址，L是變量所占的空間長度。

void *sqlite3_wsd_find(void *K, int L){
  int i;
  int iHash = 0;
  ProcessLocalVar *pVar;

  /* Calculate a hash of K */
  /*把變量的地址K通過一定的算法得到hash值 iHash */
  for(i=0; i<sizeof(void*); i++){
    iHash = (iHash<<3) + ((unsigned char *)&K)[i];
  }
  iHash = iHash%PLS_HASHSIZE;

  /* Search the hash table for K. */
  /*在iHash的hash元素鏈表里查找關鍵字是K的元素*/
  for(pVar=pGlobal->aData[iHash]; pVar && pVar->pKey!=K; pVar=pVar->pNext);

  /* If no entry for K was found, create and populate a new one. */
  if( !pVar ){
    int nByte = ROUND8(sizeof(ProcessLocalVar) + L);//申請的空間總長度
    assert( pGlobal->nFree>=nByte );
    pVar = (ProcessLocalVar *)pGlobal->pFree;//在pFree里獲取
    pVar->pKey = K;
    pVar->pNext = pGlobal->aData[iHash];//插入到hash值是iHash的鏈表中
    pGlobal->aData[iHash] = pVar;//更新頭結點
    pGlobal->nFree -= nByte;
    pGlobal->pFree += nByte;//未使用的空間地址向后移動
    memcpy(&pVar[1], K, L);//得到初始值
  }

  return (void *)&pVar[1];// 去除頭部ProcessLocalVar，返回映射地址
}