/*

 ** 2007 October 14

 **

 ** The author disclaims copyright to this source code.  In place of

 ** a legal notice, here is a blessing:

 **

 **    May you do good and not evil.

 **    May you find forgiveness for yourself and forgive others.

 **    May you share freely, never taking more than you give.

 **

 *************************************************************************

 ** This file contains the C functions that implement a memory

 ** allocation subsystem for use by SQLite. 

 **

 ** This version of the memory allocation subsystem omits all

 ** use of malloc(). The SQLite user supplies a block of memory

 ** before calling sqlite3_initialize() from which allocations

 ** are made and returned by the xMalloc() and xRealloc() 

 ** implementations. Once sqlite3_initialize() has been called,

 ** the amount of memory available to SQLite is fixed and cannot

 ** be changed.

 **

 ** This version of the memory allocation subsystem is included

 ** in the build only if SQLITE_ENABLE_MEMSYS3 is defined.

 **

 ** $Id: mem3.c,v 1.23 2008/09/02 17:52:52 danielk1977 Exp $

 */

 #include "sqliteInt.h"

 /*

 ** This version of the memory allocator is only built into the library

 ** SQLITE_ENABLE_MEMSYS3 is defined. Defining this symbol does not

 ** mean that the library will use a memory-pool by default, just that

 ** it is available. The mempool allocator is activated by calling

 ** sqlite3_config().

 */

 #ifdef SQLITE_ENABLE_MEMSYS3

 /*

 ** Maximum size (in Mem3Blocks) of a "small" chunk.

 */

 #define MX_SMALL 10

 /*

 ** Number of freelist hash slots

 */

 #define N_HASH  61

 /*

 ** A memory allocation (also called a "chunk") consists of two or 

 ** more blocks where each block is 8 bytes.  The first 8 bytes are 

 ** a header that is not returned to the user.

 **

 ** A chunk is two or more blocks that is either checked out or

 ** free.  The first block has format u.hdr.  u.hdr.size4x is 4 times the

 ** size of the allocation in blocks if the allocation is free.

 ** The u.hdr.size4x&1 bit is true if the chunk is checked out and

 ** false if the chunk is on the freelist.  The u.hdr.size4x&2 bit

 ** is true if the previous chunk is checked out and false if the

 ** previous chunk is free.  The u.hdr.prevSize field is the size of

 ** the previous chunk in blocks if the previous chunk is on the

 ** freelist. If the previous chunk is checked out, then

 ** u.hdr.prevSize can be part of the data for that chunk and should

 ** not be read or written.

 **

 ** We often identify a chunk by its index in mem3.aPool[].  When

 ** this is done, the chunk index refers to the second block of

 ** the chunk.  In this way, the first chunk has an index of 1.

 ** A chunk index of 0 means "no such chunk" and is the equivalent

 ** of a NULL pointer.

 **

 ** The second block of free chunks is of the form u.list.  The

 ** two fields form a double-linked list of chunks of related sizes.

 ** Pointers to the head of the list are stored in mem3.aiSmall[] 

 ** for smaller chunks and mem3.aiHash[] for larger chunks.

 **

 ** The second block of a chunk is user data if the chunk is checked 

 ** out.  If a chunk is checked out, the user data may extend into

 ** the u.hdr.prevSize value of the following chunk.

 */

 typedef struct Mem3Block Mem3Block;

 struct Mem3Block {

   union {

     struct {

       u32 prevSize;   /* Size of previous chunk in Mem3Block elements */

       u32 size4x;     /* 4x the size of current chunk in Mem3Block elements */

     } hdr;

     struct {

       u32 next;       /* Index in mem3.aPool[] of next free chunk */

       u32 prev;       /* Index in mem3.aPool[] of previous free chunk */

     } list;

   } u;

 };

 /*

 ** All of the static variables used by this module are collected

 ** into a single structure named "mem3".  This is to keep the

 ** static variables organized and to reduce namespace pollution

 ** when this module is combined with other in the amalgamation.

 */

 static SQLITE_WSD struct Mem3Global {

   /*

   ** Memory available for allocation. nPool is the size of the array

   ** (in Mem3Blocks) pointed to by aPool less 2.

   */

   u32 nPool;

   Mem3Block *aPool;

   /*

   ** True if we are evaluating an out-of-memory callback.

   */

   int alarmBusy;

   /*

   ** Mutex to control access to the memory allocation subsystem.

   */

   sqlite3_mutex *mutex;

   /*

   ** The minimum amount of free space that we have seen.

   */

   u32 mnMaster;

   /*

   ** iMaster is the index of the master chunk.  Most new allocations

   ** occur off of this chunk.  szMaster is the size (in Mem3Blocks)

   ** of the current master.  iMaster is 0 if there is not master chunk.

   ** The master chunk is not in either the aiHash[] or aiSmall[].

   */

   u32 iMaster;

   u32 szMaster;

   /*

   ** Array of lists of free blocks according to the block size 

   ** for smaller chunks, or a hash on the block size for larger

   ** chunks.

   */

   u32 aiSmall[MX_SMALL-1];   /* For sizes 2 through MX_SMALL, inclusive */

   u32 aiHash[N_HASH];        /* For sizes MX_SMALL+1 and larger */

 } mem3 = { 97535575 };

 #define mem3 GLOBAL(struct Mem3Global, mem3)

 /*

 ** Unlink the chunk at mem3.aPool[i] from list it is currently

 ** on.  *pRoot is the list that i is a member of.

 */

 static void memsys3UnlinkFromList(u32 i, u32 *pRoot){

   u32 next = mem3.aPool[i].u.list.next;

   u32 prev = mem3.aPool[i].u.list.prev;

   assert( sqlite3_mutex_held(mem3.mutex) );

   if( prev==0 ){

     *pRoot = next;

   }else{

     mem3.aPool[prev].u.list.next = next;

   }

   if( next ){

     mem3.aPool[next].u.list.prev = prev;

   }

   mem3.aPool[i].u.list.next = 0;

   mem3.aPool[i].u.list.prev = 0;

 }

 /*

 ** Unlink the chunk at index i from 

 ** whatever list is currently a member of.

 */

 static void memsys3Unlink(u32 i){

   u32 size, hash;

   assert( sqlite3_mutex_held(mem3.mutex) );

   assert( (mem3.aPool[i-1].u.hdr.size4x & 1)==0 );

   assert( i>=1 );

   size = mem3.aPool[i-1].u.hdr.size4x/4;

   assert( size==mem3.aPool[i+size-1].u.hdr.prevSize );

   assert( size>=2 );

   if( size <= MX_SMALL ){

     memsys3UnlinkFromList(i, &mem3.aiSmall[size-2]);

   }else{

     hash = size % N_HASH;

     memsys3UnlinkFromList(i, &mem3.aiHash[hash]);

   }

 }

 /*

 ** Link the chunk at mem3.aPool[i] so that is on the list rooted

 ** at *pRoot.

 */

 static void memsys3LinkIntoList(u32 i, u32 *pRoot){

   assert( sqlite3_mutex_held(mem3.mutex) );

   mem3.aPool[i].u.list.next = *pRoot;

   mem3.aPool[i].u.list.prev = 0;

   if( *pRoot ){

     mem3.aPool[*pRoot].u.list.prev = i;

   }

   *pRoot = i;

 }

 /*

 ** Link the chunk at index i into either the appropriate

 ** small chunk list, or into the large chunk hash table.

 */

 static void memsys3Link(u32 i){

   u32 size, hash;

   assert( sqlite3_mutex_held(mem3.mutex) );

   assert( i>=1 );

   assert( (mem3.aPool[i-1].u.hdr.size4x & 1)==0 );

   size = mem3.aPool[i-1].u.hdr.size4x/4;

   assert( size==mem3.aPool[i+size-1].u.hdr.prevSize );

   assert( size>=2 );

   if( size <= MX_SMALL ){

     memsys3LinkIntoList(i, &mem3.aiSmall[size-2]);

   }else{

     hash = size % N_HASH;

     memsys3LinkIntoList(i, &mem3.aiHash[hash]);

   }

 }

 /*

 ** If the STATIC_MEM mutex is not already held, obtain it now. The mutex

 ** will already be held (obtained by code in malloc.c) if

 ** sqlite3GlobalConfig.bMemStat is true.

 */

 static void memsys3Enter(void){

   if( sqlite3GlobalConfig.bMemstat==0 && mem3.mutex==0 ){

     mem3.mutex = sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_MEM);

   }

   sqlite3_mutex_enter(mem3.mutex);

 }

 static void memsys3Leave(void){

   sqlite3_mutex_leave(mem3.mutex);

 }

 /*

 ** Called when we are unable to satisfy an allocation of nBytes.

 */

 static void memsys3OutOfMemory(int nByte){

   if( !mem3.alarmBusy ){

     mem3.alarmBusy = 1;

     assert( sqlite3_mutex_held(mem3.mutex) );

     sqlite3_mutex_leave(mem3.mutex);

     sqlite3_release_memory(nByte);

     sqlite3_mutex_enter(mem3.mutex);

     mem3.alarmBusy = 0;

   }

 }

 /*

 ** Chunk i is a free chunk that has been unlinked.  Adjust its 

 ** size parameters for check-out and return a pointer to the 

 ** user portion of the chunk.

 */

 static void *memsys3Checkout(u32 i, int nBlock){

   u32 x;

   assert( sqlite3_mutex_held(mem3.mutex) );

   assert( i>=1 );

   assert( mem3.aPool[i-1].u.hdr.size4x/4==nBlock );

   assert( mem3.aPool[i+nBlock-1].u.hdr.prevSize==nBlock );

   x = mem3.aPool[i-1].u.hdr.size4x;

   mem3.aPool[i-1].u.hdr.size4x = nBlock*4 | 1 | (x&2);

   mem3.aPool[i+nBlock-1].u.hdr.prevSize = nBlock;

   mem3.aPool[i+nBlock-1].u.hdr.size4x |= 2;

   return &mem3.aPool[i];

 }

 /*

 ** Carve a piece off of the end of the mem3.iMaster free chunk.

 ** Return a pointer to the new allocation.  Or, if the master chunk

 ** is not large enough, return 0.

 */

 static void *memsys3FromMaster(int nBlock){

   assert( sqlite3_mutex_held(mem3.mutex) );

   assert( mem3.szMaster>=nBlock );

   if( nBlock>=mem3.szMaster-1 ){

     /* Use the entire master */

     void *p = memsys3Checkout(mem3.iMaster, mem3.szMaster);

     mem3.iMaster = 0;

     mem3.szMaster = 0;

     mem3.mnMaster = 0;

     return p;

   }else{

     /* Split the master block.  Return the tail. */

     u32 newi, x;

     newi = mem3.iMaster + mem3.szMaster - nBlock;

     assert( newi > mem3.iMaster+1 );

     mem3.aPool[mem3.iMaster+mem3.szMaster-1].u.hdr.prevSize = nBlock;

     mem3.aPool[mem3.iMaster+mem3.szMaster-1].u.hdr.size4x |= 2;

     mem3.aPool[newi-1].u.hdr.size4x = nBlock*4 + 1;

     mem3.szMaster -= nBlock;

     mem3.aPool[newi-1].u.hdr.prevSize = mem3.szMaster;

     x = mem3.aPool[mem3.iMaster-1].u.hdr.size4x & 2;

     mem3.aPool[mem3.iMaster-1].u.hdr.size4x = mem3.szMaster*4 | x;

     if( mem3.szMaster < mem3.mnMaster ){

       mem3.mnMaster = mem3.szMaster;

     }

     return (void*)&mem3.aPool[newi];

   }

 }

 /*

 ** *pRoot is the head of a list of free chunks of the same size

 ** or same size hash.  In other words, *pRoot is an entry in either

 ** mem3.aiSmall[] or mem3.aiHash[].  

 **

 ** This routine examines all entries on the given list and tries

 ** to coalesce each entries with adjacent free chunks.  

 **

 ** If it sees a chunk that is larger than mem3.iMaster, it replaces 

 ** the current mem3.iMaster with the new larger chunk.  In order for

 ** this mem3.iMaster replacement to work, the master chunk must be

 ** linked into the hash tables.  That is not the normal state of

 ** affairs, of course.  The calling routine must link the master

 ** chunk before invoking this routine, then must unlink the (possibly

 ** changed) master chunk once this routine has finished.

 */

 static void memsys3Merge(u32 *pRoot){

   u32 iNext, prev, size, i, x;

   assert( sqlite3_mutex_held(mem3.mutex) );

   for(i=*pRoot; i>0; i=iNext){

     iNext = mem3.aPool[i].u.list.next;

     size = mem3.aPool[i-1].u.hdr.size4x;

     assert( (size&1)==0 );

     if( (size&2)==0 ){

       memsys3UnlinkFromList(i, pRoot);

       assert( i > mem3.aPool[i-1].u.hdr.prevSize );

       prev = i - mem3.aPool[i-1].u.hdr.prevSize;

       if( prev==iNext ){

         iNext = mem3.aPool[prev].u.list.next;

       }

       memsys3Unlink(prev);

       size = i + size/4 - prev;

       x = mem3.aPool[prev-1].u.hdr.size4x & 2;

       mem3.aPool[prev-1].u.hdr.size4x = size*4 | x;

       mem3.aPool[prev+size-1].u.hdr.prevSize = size;

       memsys3Link(prev);

       i = prev;

     }else{

       size /= 4;

     }

     if( size>mem3.szMaster ){

       mem3.iMaster = i;

       mem3.szMaster = size;

     }

   }

 }

 /*

 ** Return a block of memory of at least nBytes in size.

 ** Return NULL if unable.

 **

 ** This function assumes that the necessary mutexes, if any, are

 ** already held by the caller. Hence "Unsafe".

 */

 static void *memsys3MallocUnsafe(int nByte){

   u32 i;

   int nBlock;

   int toFree;

   assert( sqlite3_mutex_held(mem3.mutex) );

   assert( sizeof(Mem3Block)==8 );

   if( nByte<=12 ){

     nBlock = 2;

   }else{

     nBlock = (nByte + 11)/8;

   }

   assert( nBlock>=2 );

   /* STEP 1:

   ** Look for an entry of the correct size in either the small

   ** chunk table or in the large chunk hash table.  This is

   ** successful most of the time (about 9 times out of 10).

   */

   if( nBlock <= MX_SMALL ){

     i = mem3.aiSmall[nBlock-2];

     if( i>0 ){

       memsys3UnlinkFromList(i, &mem3.aiSmall[nBlock-2]);

       return memsys3Checkout(i, nBlock);

     }

   }else{

     int hash = nBlock % N_HASH;

     for(i=mem3.aiHash[hash]; i>0; i=mem3.aPool[i].u.list.next){

       if( mem3.aPool[i-1].u.hdr.size4x/4==nBlock ){

         memsys3UnlinkFromList(i, &mem3.aiHash[hash]);

         return memsys3Checkout(i, nBlock);

       }

     }

   }

   /* STEP 2:

   ** Try to satisfy the allocation by carving a piece off of the end

   ** of the master chunk.  This step usually works if step 1 fails.

   */

   if( mem3.szMaster>=nBlock ){

     return memsys3FromMaster(nBlock);

   }

   /* STEP 3:  

   ** Loop through the entire memory pool.  Coalesce adjacent free

   ** chunks.  Recompute the master chunk as the largest free chunk.

   ** Then try again to satisfy the allocation by carving a piece off

   ** of the end of the master chunk.  This step happens very

   ** rarely (we hope!)

   */

   for(toFree=nBlock*16; toFree<(mem3.nPool*16); toFree *= 2){

     memsys3OutOfMemory(toFree);

     if( mem3.iMaster ){

       memsys3Link(mem3.iMaster);

       mem3.iMaster = 0;

       mem3.szMaster = 0;

     }

     for(i=0; i<N_HASH; i++){

       memsys3Merge(&mem3.aiHash[i]);

     }

     for(i=0; i<MX_SMALL-1; i++){

       memsys3Merge(&mem3.aiSmall[i]);

     }

     if( mem3.szMaster ){

       memsys3Unlink(mem3.iMaster);

       if( mem3.szMaster>=nBlock ){

         return memsys3FromMaster(nBlock);

       }

     }

   }

   /* If none of the above worked, then we fail. */

   return 0;

 }

 /*

 ** Free an outstanding memory allocation.

 **

 ** This function assumes that the necessary mutexes, if any, are

 ** already held by the caller. Hence "Unsafe".

 */

 void memsys3FreeUnsafe(void *pOld){

   Mem3Block *p = (Mem3Block*)pOld;

   int i;

   u32 size, x;

   assert( sqlite3_mutex_held(mem3.mutex) );

   assert( p>mem3.aPool && p<&mem3.aPool[mem3.nPool] );

   i = p - mem3.aPool;

   assert( (mem3.aPool[i-1].u.hdr.size4x&1)==1 );

   size = mem3.aPool[i-1].u.hdr.size4x/4;

   assert( i+size<=mem3.nPool+1 );

   mem3.aPool[i-1].u.hdr.size4x &= ~1;

   mem3.aPool[i+size-1].u.hdr.prevSize = size;

   mem3.aPool[i+size-1].u.hdr.size4x &= ~2;

   memsys3Link(i);

   /* Try to expand the master using the newly freed chunk */

   if( mem3.iMaster ){

     while( (mem3.aPool[mem3.iMaster-1].u.hdr.size4x&2)==0 ){

       size = mem3.aPool[mem3.iMaster-1].u.hdr.prevSize;

       mem3.iMaster -= size;

       mem3.szMaster += size;

       memsys3Unlink(mem3.iMaster);

       x = mem3.aPool[mem3.iMaster-1].u.hdr.size4x & 2;

       mem3.aPool[mem3.iMaster-1].u.hdr.size4x = mem3.szMaster*4 | x;

       mem3.aPool[mem3.iMaster+mem3.szMaster-1].u.hdr.prevSize = mem3.szMaster;

     }

     x = mem3.aPool[mem3.iMaster-1].u.hdr.size4x & 2;

     while( (mem3.aPool[mem3.iMaster+mem3.szMaster-1].u.hdr.size4x&1)==0 ){

       memsys3Unlink(mem3.iMaster+mem3.szMaster);

       mem3.szMaster += mem3.aPool[mem3.iMaster+mem3.szMaster-1].u.hdr.size4x/4;

       mem3.aPool[mem3.iMaster-1].u.hdr.size4x = mem3.szMaster*4 | x;

       mem3.aPool[mem3.iMaster+mem3.szMaster-1].u.hdr.prevSize = mem3.szMaster;

     }

   }

 }

 /*

 ** Return the size of an outstanding allocation, in bytes.  The

 ** size returned omits the 8-byte header overhead.  This only

 ** works for chunks that are currently checked out.

 */

 static int memsys3Size(void *p){

   Mem3Block *pBlock;

   if( p==0 ) return 0;

   pBlock = (Mem3Block*)p;

   assert( (pBlock[-1].u.hdr.size4x&1)!=0 );

   return (pBlock[-1].u.hdr.size4x&~3)*2 - 4;

 }

 /*

 ** Round up a request size to the next valid allocation size.

 */

 static int memsys3Roundup(int n){

   if( n<=12 ){

     return 12;

   }else{

     return ((n+11)&~7) - 4;

   }

 }

 /*

 ** Allocate nBytes of memory.

 */

 static void *memsys3Malloc(int nBytes){

   sqlite3_int64 *p;

   assert( nBytes>0 );          /* malloc.c filters out 0 byte requests */

   memsys3Enter();

   p = memsys3MallocUnsafe(nBytes);

   memsys3Leave();

   return (void*)p; 

 }

 /*

 ** Free memory.

 */

 void memsys3Free(void *pPrior){

   assert( pPrior );

   memsys3Enter();

   memsys3FreeUnsafe(pPrior);

   memsys3Leave();

 }

 /*

 ** Change the size of an existing memory allocation

 */

 void *memsys3Realloc(void *pPrior, int nBytes){

   int nOld;

   void *p;

   if( pPrior==0 ){

     return sqlite3_malloc(nBytes);

   }

   if( nBytes<=0 ){

     sqlite3_free(pPrior);

     return 0;

   }

   nOld = memsys3Size(pPrior);

   if( nBytes<=nOld && nBytes>=nOld-128 ){

     return pPrior;

   }

   memsys3Enter();

   p = memsys3MallocUnsafe(nBytes);

   if( p ){

     if( nOld<nBytes ){

       memcpy(p, pPrior, nOld);

     }else{

       memcpy(p, pPrior, nBytes);

     }

     memsys3FreeUnsafe(pPrior);

   }

   memsys3Leave();

   return p;

 }

 /*

 ** Initialize this module.

 */

 static int memsys3Init(void *NotUsed){

   if( !sqlite3GlobalConfig.pHeap ){

     return SQLITE_ERROR;

   }

   /* Store a pointer to the memory block in global structure mem3. */

   assert( sizeof(Mem3Block)==8 );

   mem3.aPool = (Mem3Block *)sqlite3GlobalConfig.pHeap;

   mem3.nPool = (sqlite3GlobalConfig.nHeap / sizeof(Mem3Block)) - 2;

   /* Initialize the master block. */

   mem3.szMaster = mem3.nPool;

   mem3.mnMaster = mem3.szMaster;

   mem3.iMaster = 1;

   mem3.aPool[0].u.hdr.size4x = (mem3.szMaster<<2) + 2;

   mem3.aPool[mem3.nPool].u.hdr.prevSize = mem3.nPool;

   mem3.aPool[mem3.nPool].u.hdr.size4x = 1;

   return SQLITE_OK;

 }

 /*

 ** Deinitialize this module.

 */

 static void memsys3Shutdown(void *NotUsed){

   return;

 }

 /*

 ** Open the file indicated and write a log of all unfreed memory 

 ** allocations into that log.

 */

 void sqlite3Memsys3Dump(const char *zFilename){

 #ifdef SQLITE_DEBUG

   FILE *out;

   int i, j;

   u32 size;

   if( zFilename==0 || zFilename[0]==0 ){

     out = stdout;

   }else{

     out = fopen(zFilename, "w");

     if( out==0 ){

       fprintf(stderr, "** Unable to output memory debug output log: %s **\n",

                       zFilename);

       return;

     }

   }

   memsys3Enter();

   fprintf(out, "CHUNKS:\n");

   for(i=1; i<=mem3.nPool; i+=size/4){

     size = mem3.aPool[i-1].u.hdr.size4x;

     if( size/4<=1 ){

       fprintf(out, "%p size error\n", &mem3.aPool[i]);

       assert( 0 );

       break;

     }

     if( (size&1)==0 && mem3.aPool[i+size/4-1].u.hdr.prevSize!=size/4 ){

       fprintf(out, "%p tail size does not match\n", &mem3.aPool[i]);

       assert( 0 );

       break;

     }

     if( ((mem3.aPool[i+size/4-1].u.hdr.size4x&2)>>1)!=(size&1) ){

       fprintf(out, "%p tail checkout bit is incorrect\n", &mem3.aPool[i]);

       assert( 0 );

       break;

     }

     if( size&1 ){

       fprintf(out, "%p %6d bytes checked out\n", &mem3.aPool[i], (size/4)*8-8);

     }else{

       fprintf(out, "%p %6d bytes free%s\n", &mem3.aPool[i], (size/4)*8-8,

                   i==mem3.iMaster ? " **master**" : "");

     }

   }

   for(i=0; i<MX_SMALL-1; i++){

     if( mem3.aiSmall[i]==0 ) continue;

     fprintf(out, "small(%2d):", i);

     for(j = mem3.aiSmall[i]; j>0; j=mem3.aPool[j].u.list.next){

       fprintf(out, " %p(%d)", &mem3.aPool[j],

               (mem3.aPool[j-1].u.hdr.size4x/4)*8-8);

     }

     fprintf(out, "\n"); 

   }

   for(i=0; i<N_HASH; i++){

     if( mem3.aiHash[i]==0 ) continue;

     fprintf(out, "hash(%2d):", i);

     for(j = mem3.aiHash[i]; j>0; j=mem3.aPool[j].u.list.next){

       fprintf(out, " %p(%d)", &mem3.aPool[j],

               (mem3.aPool[j-1].u.hdr.size4x/4)*8-8);

     }

     fprintf(out, "\n"); 

   }

   fprintf(out, "master=%d\n", mem3.iMaster);

   fprintf(out, "nowUsed=%d\n", mem3.nPool*8 - mem3.szMaster*8);

   fprintf(out, "mxUsed=%d\n", mem3.nPool*8 - mem3.mnMaster*8);

   sqlite3_mutex_leave(mem3.mutex);

   if( out==stdout ){

     fflush(stdout);

   }else{

     fclose(out);

   }

 #endif

 }

 /*

 ** This routine is the only routine in this file with external 

 ** linkage.

 **

 ** Populate the low-level memory allocation function pointers in

 ** sqlite3GlobalConfig.m with pointers to the routines in this file. The

 ** arguments specify the block of memory to manage.

 **

 ** This routine is only called by sqlite3_config(), and therefore

 ** is not required to be threadsafe (it is not).

 */

 const sqlite3_mem_methods *sqlite3MemGetMemsys3(void){

   static const sqlite3_mem_methods mempoolMethods = {

      memsys3Malloc,

      memsys3Free,

      memsys3Realloc,

      memsys3Size,

      memsys3Roundup,

      memsys3Init,

      memsys3Shutdown,

      0

   };

   return &mempoolMethods;

 }

 #endif /* SQLITE_ENABLE_MEMSYS3 */