mem3.c

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/*
** 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().  All dynamically allocatable memory is
** contained in a static array, mem.aPool[].  The size of this
** fixed memory pool is SQLITE_MEMORY_SIZE bytes.
**
** This version of the memory allocation subsystem is used if
** and only if SQLITE_MEMORY_SIZE is defined.
**
** $Id: mem3.c,v 1.7 2007/11/29 18:36:49 drh Exp $
*/

/*
** This version of the memory allocator is used only when 
** SQLITE_MEMORY_SIZE is defined.
*/
#if defined(SQLITE_MEMORY_SIZE)
#include "sqliteInt.h"

#ifdef SQLITE_MEMDEBUG
# error  cannot define both SQLITE_MEMDEBUG and SQLITE_MEMORY_SIZE
#endif

/*
** 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.size is the
** size of the allocation in blocks if the allocation is free.
** If the allocation is checked out, u.hdr.size is the negative
** of the size.  Similarly, u.hdr.prevSize is the size of the
** immediately previous allocation.
**
** We often identify a chunk by its index in mem.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 mem.aiSmall[] 
** for smaller chunks and mem.aiHash[] for larger chunks.
**
** The second block of a chunk is user data if the chunk is checked 
** out.
*/
typedef struct Mem3Block Mem3Block;
struct Mem3Block {
  union {
    struct {
      int prevSize;   /* Size of previous chunk in Mem3Block elements */
      int size;       /* Size of current chunk in Mem3Block elements */
    } hdr;
    struct {
      int next;       /* Index in mem.aPool[] of next free chunk */
      int prev;       /* Index in mem.aPool[] of previous free chunk */
    } list;
  } u;
};

/*
** All of the static variables used by this module are collected
** into a single structure named "mem".  This is to keep the
** static variables organized and to reduce namespace pollution
** when this module is combined with other in the amalgamation.
*/
static struct {
  /*
  ** 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.
  */
  int 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[].
  */
  int iMaster;
  int 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.
  */
  int aiSmall[MX_SMALL-1];   /* For sizes 2 through MX_SMALL, inclusive */
  int aiHash[N_HASH];        /* For sizes MX_SMALL+1 and larger */

  /*
  ** Memory available for allocation
  */
  Mem3Block aPool[SQLITE_MEMORY_SIZE/sizeof(Mem3Block)+2];
} mem;

/*
** Unlink the chunk at mem.aPool[i] from list it is currently
** on.  *pRoot is the list that i is a member of.
*/
static void memsys3UnlinkFromList(int i, int *pRoot){
  int next = mem.aPool[i].u.list.next;
  int prev = mem.aPool[i].u.list.prev;
  assert( sqlite3_mutex_held(mem.mutex) );
  if( prev==0 ){
    *pRoot = next;
  }else{
    mem.aPool[prev].u.list.next = next;
  }
  if( next ){
    mem.aPool[next].u.list.prev = prev;
  }
  mem.aPool[i].u.list.next = 0;
  mem.aPool[i].u.list.prev = 0;
}

/*
** Unlink the chunk at index i from 
** whatever list is currently a member of.
*/
static void memsys3Unlink(int i){
  int size, hash;
  assert( sqlite3_mutex_held(mem.mutex) );
  size = mem.aPool[i-1].u.hdr.size;
  assert( size==mem.aPool[i+size-1].u.hdr.prevSize );
  assert( size>=2 );
  if( size <= MX_SMALL ){
    memsys3UnlinkFromList(i, &mem.aiSmall[size-2]);
  }else{
    hash = size % N_HASH;
    memsys3UnlinkFromList(i, &mem.aiHash[hash]);
  }
}

/*
** Link the chunk at mem.aPool[i] so that is on the list rooted
** at *pRoot.
*/
static void memsys3LinkIntoList(int i, int *pRoot){
  assert( sqlite3_mutex_held(mem.mutex) );
  mem.aPool[i].u.list.next = *pRoot;
  mem.aPool[i].u.list.prev = 0;
  if( *pRoot ){
    mem.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(int i){
  int size, hash;
  assert( sqlite3_mutex_held(mem.mutex) );
  size = mem.aPool[i-1].u.hdr.size;
  assert( size==mem.aPool[i+size-1].u.hdr.prevSize );
  assert( size>=2 );
  if( size <= MX_SMALL ){
    memsys3LinkIntoList(i, &mem.aiSmall[size-2]);
  }else{
    hash = size % N_HASH;
    memsys3LinkIntoList(i, &mem.aiHash[hash]);
  }
}

/*
** Enter the mutex mem.mutex. Allocate it if it is not already allocated.
**
** Also:  Initialize the memory allocation subsystem the first time
** this routine is called.
*/
static void memsys3Enter(void){
  if( mem.mutex==0 ){
    mem.mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_STATIC_MEM);
    mem.aPool[0].u.hdr.size = SQLITE_MEMORY_SIZE/8;
    mem.aPool[SQLITE_MEMORY_SIZE/8].u.hdr.prevSize = SQLITE_MEMORY_SIZE/8;
    mem.iMaster = 1;
    mem.szMaster = SQLITE_MEMORY_SIZE/8;
    mem.mnMaster = mem.szMaster;
  }
  sqlite3_mutex_enter(mem.mutex);
}

/*
** Return the amount of memory currently checked out.
*/
sqlite3_int64 sqlite3_memory_used(void){
  sqlite3_int64 n;
  memsys3Enter();
  n = SQLITE_MEMORY_SIZE - mem.szMaster*8;
  sqlite3_mutex_leave(mem.mutex);  
  return n;
}

/*
** Return the maximum amount of memory that has ever been
** checked out since either the beginning of this process
** or since the most recent reset.
*/
sqlite3_int64 sqlite3_memory_highwater(int resetFlag){
  sqlite3_int64 n;
  memsys3Enter();
  n = SQLITE_MEMORY_SIZE - mem.mnMaster*8;
  if( resetFlag ){
    mem.mnMaster = mem.szMaster;
  }
  sqlite3_mutex_leave(mem.mutex);  
  return n;
}

/*
** Change the alarm callback.
**
** This is a no-op for the static memory allocator.  The purpose
** of the memory alarm is to support sqlite3_soft_heap_limit().
** But with this memory allocator, the soft_heap_limit is really
** a hard limit that is fixed at SQLITE_MEMORY_SIZE.
*/
int sqlite3_memory_alarm(
  void(*xCallback)(void *pArg, sqlite3_int64 used,int N),
  void *pArg,
  sqlite3_int64 iThreshold
){
  return SQLITE_OK;
}

/*
** Called when we are unable to satisfy an allocation of nBytes.
*/
static void memsys3OutOfMemory(int nByte){
  if( !mem.alarmBusy ){
    mem.alarmBusy = 1;
    assert( sqlite3_mutex_held(mem.mutex) );
    sqlite3_mutex_leave(mem.mutex);
    sqlite3_release_memory(nByte);
    sqlite3_mutex_enter(mem.mutex);
    mem.alarmBusy = 0;
  }
}

/*
** 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 = (Mem3Block*)p;
  assert( pBlock[-1].u.hdr.size<0 );
  return (-1-pBlock[-1].u.hdr.size)*8;
}

/*
** 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(int i, int nBlock){
  assert( sqlite3_mutex_held(mem.mutex) );
  assert( mem.aPool[i-1].u.hdr.size==nBlock );
  assert( mem.aPool[i+nBlock-1].u.hdr.prevSize==nBlock );
  mem.aPool[i-1].u.hdr.size = -nBlock;
  mem.aPool[i+nBlock-1].u.hdr.prevSize = -nBlock;
  return &mem.aPool[i];
}

/*
** Carve a piece off of the end of the mem.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(mem.mutex) );
  assert( mem.szMaster>=nBlock );
  if( nBlock>=mem.szMaster-1 ){
    /* Use the entire master */