SQLite

/*
** Set up the lookaside buffers for a database connection.
** Return SQLITE_OK on success. 
** If lookaside is already active, return SQLITE_BUSY.
**
** The sz parameter is the number of bytes in each lookaside slot.
** The cnt parameter is the number of slots.  If pBuf is NULL the
** space for the lookaside memory is obtained from sqlite3_malloc()
** or similar.  If pBuf is not NULL then it is sz*cnt bytes of memory
** to use for the lookaside memory.
*/
static int setupLookaside(
  sqlite3 *db,    /* Database connection being configured */
  void *pBuf,     /* Memory to use for lookaside.  May be NULL */
  int sz,         /* Desired size of each lookaside memory slot */
  int cnt         /* Number of slots to allocate */
){
#ifndef SQLITE_OMIT_LOOKASIDE
  void *pStart;          /* Start of the lookaside buffer */
  sqlite3_int64 szAlloc; /* Total space set aside for lookaside memory */
  int nBig;              /* Number of full-size slots */
  int nSm;               /* Number smaller LOOKASIDE_SMALL-byte slots */
 
  if( sqlite3LookasideUsed(db,0)>0 ){
    return SQLITE_BUSY;
  }
  /* Free any existing lookaside buffer for this handle before
  ** allocating a new one so we don't have to have space for
  ** both at the same time.
  */
  if( db->lookaside.bMalloced ){
    sqlite3_free(db->lookaside.pStart);
  }
  /* The size of a lookaside slot after ROUNDDOWN8 needs to be larger
  ** than a pointer and small enough to fit in a u16.
  */
  sz = ROUNDDOWN8(sz);
  if( sz<=(int)sizeof(LookasideSlot*) ) sz = 0;
  if( sz>65528 ) sz = 65528;
  /* Count must be at least 1 to be useful, but not so large as to use
  ** more than 0x7fff0000 total bytes for lookaside. */
  if( cnt<1 ) cnt = 0;
  if( sz>0 && cnt>(0x7fff0000/sz) ) cnt = 0x7fff0000/sz;
  szAlloc = (i64)sz*(i64)cnt;
  if( szAlloc==0 ){
    sz = 0;
    pStart = 0;
  }else if( pBuf==0 ){
    sqlite3BeginBenignMalloc();
    pStart = sqlite3Malloc( szAlloc );
    sqlite3EndBenignMalloc();
    if( pStart ) szAlloc = sqlite3MallocSize(pStart);
  }else{
    pStart = pBuf;
  }
#ifndef SQLITE_OMIT_TWOSIZE_LOOKASIDE
  if( sz>=LOOKASIDE_SMALL*3 ){
    nBig = szAlloc/(3*LOOKASIDE_SMALL+sz);
    nSm = (szAlloc - (i64)sz*(i64)nBig)/LOOKASIDE_SMALL;
  }else if( sz>=LOOKASIDE_SMALL*2 ){
    nBig = szAlloc/(LOOKASIDE_SMALL+sz);
    nSm = (szAlloc - (i64)sz*(i64)nBig)/LOOKASIDE_SMALL;
  }else
#endif /* SQLITE_OMIT_TWOSIZE_LOOKASIDE */
  if( sz>0 ){
    nBig = szAlloc/sz;
    nSm = 0;
  }else{
    nBig = nSm = 0;

** the [SQLITE_THREADSAFE | SQLITE_THREADSAFE=0] compile-time option then
** the entire mutexing subsystem is omitted from the build and hence calls to
** [sqlite3_config()] with the SQLITE_CONFIG_GETMUTEX configuration option will
** return [SQLITE_ERROR].</dd>
**
** [[SQLITE_CONFIG_LOOKASIDE]] <dt>SQLITE_CONFIG_LOOKASIDE</dt>
** <dd> ^(The SQLITE_CONFIG_LOOKASIDE option takes two arguments that determine
** the default size of [lookaside memory] on each [database connection].
** The first argument is the
** size of each lookaside buffer slot ("sz") and the second is the number of
** slots allocated to each database connection ("cnt").)^
** ^(SQLITE_CONFIG_LOOKASIDE sets the <i>default</i> lookaside size.
** The [SQLITE_DBCONFIG_LOOKASIDE] option to [sqlite3_db_config()] can
** be used to change the lookaside configuration on individual connections.)^
** The [-DSQLITE_DEFAULT_LOOKASIDE] option can be used to change the
** default lookaside configuration at compile-time.
** </dd>
**
** [[SQLITE_CONFIG_PCACHE2]] <dt>SQLITE_CONFIG_PCACHE2</dt>
** <dd> ^(The SQLITE_CONFIG_PCACHE2 option takes a single argument which is
** a pointer to an [sqlite3_pcache_methods2] object.  This object specifies
** the interface to a custom page cache implementation.)^
** ^SQLite makes a copy of the [sqlite3_pcache_methods2] object.</dd>
**

** non-zero [error code] if a discontinued or unsupported configuration option
** is invoked.
**
** <dl>
** [[SQLITE_DBCONFIG_LOOKASIDE]]
** <dt>SQLITE_DBCONFIG_LOOKASIDE</dt>
** <dd> The SQLITE_DBCONFIG_LOOKASIDE option is used to adjust the 
** configuration of the [lookaside memory allocator] within a database
** connection.
** The arguments to the SQLITE_DBCONFIG_LOOKASIDE option are <i>not</i>
** in the [DBCONFIG arguments|usual format].
** The SQLITE_DBCONFIG_LOOKASIDE option takes three arguments, not two,
** so that a call to [sqlite3_db_config()] that uses SQLITE_DBCONFIG_LOOKASIDE
** should have a total of five parameters.
** <ol>
** <li><p>The first argument ("buf") is a
** pointer to a memory buffer to use for lookaside memory.

** The first argument may be NULL in which case SQLite will allocate the
** lookaside buffer itself using [sqlite3_malloc()].
** <li><P>The second argument ("sz") is the
** size of each lookaside buffer slot.  Lookaside is disabled if "sz"
** is less than 8.  The "sz" argument should be a multiple of 8 less than
** 65536.  If "sz" does not meet this constraint, it is reduced in size until
** it does.
** <li><p>The third argument ("cnt") is the number of slots. Lookaside is disabled
** if "cnt"is less than 1.  The "cnt" value will be reduced, if necessary, so
** that the product of "sz" and "cnt" does not exceed 2,147,418,112.  The "cnt"
** parameter is usually chosen so that the product of "sz" and "cnt" is less
** than 1,000,000.
** </ol>
** <p>If the "buf" argument is not NULL, then it must
** point to a memory buffer with a size that is greater than
** or equal to the product of "sz" and "cnt".
** The buffer must be aligned to an 8-byte boundary.

** The lookaside memory
** configuration for a database connection can only be changed when that
** connection is not currently using lookaside memory, or in other words

** when the value returned by [SQLITE_DBSTATUS_LOOKASIDE_USED] is zero.
** Any attempt to change the lookaside memory configuration when lookaside
** memory is in use leaves the configuration unchanged and returns
** [SQLITE_BUSY].
** If the "buf" argument is NULL and an attempt
** to allocate memory based on "sz" and "cnt" fails, then
** lookaside is silently disabled.
** <p>
** The [SQLITE_CONFIG_LOOKASIDE] configuration option can be used to set the
** default lookaside configuration at initialization.  The
** [-DSQLITE_DEFAULT_LOOKASIDE] option can be used to set the default lookaside
** configuration at compile-time.  Typical values for lookaside are 1200 for
** "sz" and 40 to 100 for "cnt".
** </dd>
**
** [[SQLITE_DBCONFIG_ENABLE_FKEY]]
** <dt>SQLITE_DBCONFIG_ENABLE_FKEY</dt>
** <dd> ^This option is used to enable or disable the enforcement of
** [foreign key constraints].  This is the same setting that is
** enabled or disabled by the [PRAGMA foreign_keys] statement.
** The first argument is an integer which is 0 to disable FK enforcement,