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Overview
Comment:Improved documentation and addition of source-code evidence marks for the sqlite3_config() interface.
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SHA1: 681031a436fdd4cce426d6cd43cbae6b83167d26
User & Date: drh 2014-11-04 13:41:32
Context
2014-11-04
14:22
Change the definition of SQLITE_CONFIG_SCRATCH so that at most one scratch buffer is used per thread. Use the generic heap memory allocator for the WalIterator object when running a checkpoint. check-in: 391c9b85 user: drh tags: trunk
13:41
Improved documentation and addition of source-code evidence marks for the sqlite3_config() interface. check-in: 681031a4 user: drh tags: trunk
12:11
Add the SQLITE_CONFIG_PCACHE_HDRSZ option for sqlite3_config(). check-in: 6eb03e62 user: drh tags: trunk
Changes
Hide Diffs Unified Diffs Ignore Whitespace Patch

Changes to src/btree.c.

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  /*
  ** Allocate space for memory structures
  */
  szScratch =
       nMaxCells*sizeof(u8*)                       /* apCell */
     + nMaxCells*sizeof(u16)                       /* szCell */
     + pBt->pageSize;                              /* aSpace1 */





  assert( szScratch<=16896 || szScratch<=6*pBt->pageSize );

  apCell = sqlite3ScratchMalloc( szScratch ); 
  if( apCell==0 ){
    rc = SQLITE_NOMEM;
    goto balance_cleanup;
  }
  szCell = (u16*)&apCell[nMaxCells];
  aSpace1 = (u8*)&szCell[nMaxCells];







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  /*
  ** Allocate space for memory structures
  */
  szScratch =
       nMaxCells*sizeof(u8*)                       /* apCell */
     + nMaxCells*sizeof(u16)                       /* szCell */
     + pBt->pageSize;                              /* aSpace1 */

  /* EVIDENCE-OF: R-37926-08392 SQLite will never request a scratch buffer
  ** that is more than 6 times the database page size, except when
  ** performing a checkpoint in WAL mode when the scratch buffer request
  ** size is a small fraction of the size of the WAL file. */
  assert( szScratch<=6*pBt->pageSize );

  apCell = sqlite3ScratchMalloc( szScratch ); 
  if( apCell==0 ){
    rc = SQLITE_NOMEM;
    goto balance_cleanup;
  }
  szCell = (u16*)&apCell[nMaxCells];
  aSpace1 = (u8*)&szCell[nMaxCells];

Changes to src/main.c.

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      /* Retrieve the current mutex implementation */
      *va_arg(ap, sqlite3_mutex_methods*) = sqlite3GlobalConfig.mutex;
      break;
    }
#endif

    case SQLITE_CONFIG_MALLOC: {
      /* Specify an alternative malloc implementation */




      sqlite3GlobalConfig.m = *va_arg(ap, sqlite3_mem_methods*);
      break;
    }
    case SQLITE_CONFIG_GETMALLOC: {
      /* Retrieve the current malloc() implementation */



      if( sqlite3GlobalConfig.m.xMalloc==0 ) sqlite3MemSetDefault();
      *va_arg(ap, sqlite3_mem_methods*) = sqlite3GlobalConfig.m;
      break;
    }
    case SQLITE_CONFIG_MEMSTATUS: {
      /* Enable or disable the malloc status collection */


      sqlite3GlobalConfig.bMemstat = va_arg(ap, int);  /* IMP: R-27464-47829 */
      break;
    }
    case SQLITE_CONFIG_SCRATCH: {
      /* Designate a buffer for scratch memory space */



      sqlite3GlobalConfig.pScratch = va_arg(ap, void*);
      sqlite3GlobalConfig.szScratch = va_arg(ap, int);
      sqlite3GlobalConfig.nScratch = va_arg(ap, int);
      break;
    }
    case SQLITE_CONFIG_PAGECACHE: {
      /* Designate a buffer for page cache memory space */


      sqlite3GlobalConfig.pPage = va_arg(ap, void*);
      sqlite3GlobalConfig.szPage = va_arg(ap, int);
      sqlite3GlobalConfig.nPage = va_arg(ap, int);
      break;
    }
    case SQLITE_CONFIG_PCACHE_HDRSZ: {
      /* Return the total size of all headers added to each page
      ** of the page cache */


      *va_arg(ap, int*) = 
          sqlite3HeaderSizeBtree() +
          sqlite3HeaderSizePcache() +
          sqlite3HeaderSizePcache1();
      break;
    }

................................................................................
    case SQLITE_CONFIG_GETPCACHE: {
      /* now an error */
      rc = SQLITE_ERROR;
      break;
    }

    case SQLITE_CONFIG_PCACHE2: {



      /* Specify an alternative page cache implementation */
      sqlite3GlobalConfig.pcache2 = *va_arg(ap, sqlite3_pcache_methods2*);
      break;
    }
    case SQLITE_CONFIG_GETPCACHE2: {




      if( sqlite3GlobalConfig.pcache2.xInit==0 ){
        sqlite3PCacheSetDefault();
      }
      *va_arg(ap, sqlite3_pcache_methods2*) = sqlite3GlobalConfig.pcache2;
      break;
    }

#if defined(SQLITE_ENABLE_MEMSYS3) || defined(SQLITE_ENABLE_MEMSYS5)
    case SQLITE_CONFIG_HEAP: {
      /* Designate a buffer for heap memory space */


      sqlite3GlobalConfig.pHeap = va_arg(ap, void*);
      sqlite3GlobalConfig.nHeap = va_arg(ap, int);
      sqlite3GlobalConfig.mnReq = va_arg(ap, int);

      if( sqlite3GlobalConfig.mnReq<1 ){
        sqlite3GlobalConfig.mnReq = 1;
      }else if( sqlite3GlobalConfig.mnReq>(1<<12) ){







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      /* Retrieve the current mutex implementation */
      *va_arg(ap, sqlite3_mutex_methods*) = sqlite3GlobalConfig.mutex;
      break;
    }
#endif

    case SQLITE_CONFIG_MALLOC: {
      /* EVIDENCE-OF: R-55594-21030 The SQLITE_CONFIG_MALLOC option takes a
      ** single argument which is a pointer to an instance of the
      ** sqlite3_mem_methods structure. The argument specifies alternative
      ** low-level memory allocation routines to be used in place of the memory
      ** allocation routines built into SQLite. */
      sqlite3GlobalConfig.m = *va_arg(ap, sqlite3_mem_methods*);
      break;
    }
    case SQLITE_CONFIG_GETMALLOC: {
      /* EVIDENCE-OF: R-51213-46414 The SQLITE_CONFIG_GETMALLOC option takes a
      ** single argument which is a pointer to an instance of the
      ** sqlite3_mem_methods structure. The sqlite3_mem_methods structure is
      ** filled with the currently defined memory allocation routines. */
      if( sqlite3GlobalConfig.m.xMalloc==0 ) sqlite3MemSetDefault();
      *va_arg(ap, sqlite3_mem_methods*) = sqlite3GlobalConfig.m;
      break;
    }
    case SQLITE_CONFIG_MEMSTATUS: {
      /* EVIDENCE-OF: R-61275-35157 The SQLITE_CONFIG_MEMSTATUS option takes
      ** single argument of type int, interpreted as a boolean, which enables
      ** or disables the collection of memory allocation statistics. */
      sqlite3GlobalConfig.bMemstat = va_arg(ap, int);
      break;
    }
    case SQLITE_CONFIG_SCRATCH: {
      /* EVIDENCE-OF: R-08404-60887 There are three arguments to
      ** SQLITE_CONFIG_SCRATCH: A pointer an 8-byte aligned memory buffer from
      ** which the scratch allocations will be drawn, the size of each scratch
      ** allocation (sz), and the maximum number of scratch allocations (N). */
      sqlite3GlobalConfig.pScratch = va_arg(ap, void*);
      sqlite3GlobalConfig.szScratch = va_arg(ap, int);
      sqlite3GlobalConfig.nScratch = va_arg(ap, int);
      break;
    }
    case SQLITE_CONFIG_PAGECACHE: {
      /* EVIDENCE-OF: R-31408-40510 There are three arguments to
      ** SQLITE_CONFIG_PAGECACHE: A pointer to 8-byte aligned memory, the size
      ** of each page buffer (sz), and the number of pages (N). */
      sqlite3GlobalConfig.pPage = va_arg(ap, void*);
      sqlite3GlobalConfig.szPage = va_arg(ap, int);
      sqlite3GlobalConfig.nPage = va_arg(ap, int);
      break;
    }
    case SQLITE_CONFIG_PCACHE_HDRSZ: {
      /* EVIDENCE-OF: R-39100-27317 The SQLITE_CONFIG_PCACHE_HDRSZ option takes
      ** a single parameter which is a pointer to an integer and writes into
      ** that integer the number of extra bytes per page required for each page
      ** in SQLITE_CONFIG_PAGECACHE. */
      *va_arg(ap, int*) = 
          sqlite3HeaderSizeBtree() +
          sqlite3HeaderSizePcache() +
          sqlite3HeaderSizePcache1();
      break;
    }

................................................................................
    case SQLITE_CONFIG_GETPCACHE: {
      /* now an error */
      rc = SQLITE_ERROR;
      break;
    }

    case SQLITE_CONFIG_PCACHE2: {
      /* EVIDENCE-OF: R-63325-48378 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. */
      sqlite3GlobalConfig.pcache2 = *va_arg(ap, sqlite3_pcache_methods2*);
      break;
    }
    case SQLITE_CONFIG_GETPCACHE2: {
      /* EVIDENCE-OF: R-22035-46182 The SQLITE_CONFIG_GETPCACHE2 option takes a
      ** single argument which is a pointer to an sqlite3_pcache_methods2
      ** object. SQLite copies of the current page cache implementation into
      ** that object. */
      if( sqlite3GlobalConfig.pcache2.xInit==0 ){
        sqlite3PCacheSetDefault();
      }
      *va_arg(ap, sqlite3_pcache_methods2*) = sqlite3GlobalConfig.pcache2;
      break;
    }

#if defined(SQLITE_ENABLE_MEMSYS3) || defined(SQLITE_ENABLE_MEMSYS5)
    case SQLITE_CONFIG_HEAP: {
      /* EVIDENCE-OF: R-19854-42126 There are three arguments to
      ** SQLITE_CONFIG_HEAP: An 8-byte aligned pointer to the memory, the
      ** number of bytes in the memory buffer, and the minimum allocation size. */
      sqlite3GlobalConfig.pHeap = va_arg(ap, void*);
      sqlite3GlobalConfig.nHeap = va_arg(ap, int);
      sqlite3GlobalConfig.mnReq = va_arg(ap, int);

      if( sqlite3GlobalConfig.mnReq<1 ){
        sqlite3GlobalConfig.mnReq = 1;
      }else if( sqlite3GlobalConfig.mnReq>(1<<12) ){

Changes to src/sqlite.h.in.

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** ^If SQLite is compiled with
** the [SQLITE_THREADSAFE | SQLITE_THREADSAFE=0] compile-time option then
** it is not possible to set the Serialized [threading mode] and
** [sqlite3_config()] will return [SQLITE_ERROR] if called with the
** SQLITE_CONFIG_SERIALIZED configuration option.</dd>
**
** [[SQLITE_CONFIG_MALLOC]] <dt>SQLITE_CONFIG_MALLOC</dt>
** <dd> ^(This option takes a single argument which is a pointer to an
** instance of the [sqlite3_mem_methods] structure.  The argument specifies

** alternative low-level memory allocation routines to be used in place of
** the memory allocation routines built into SQLite.)^ ^SQLite makes
** its own private copy of the content of the [sqlite3_mem_methods] structure
** before the [sqlite3_config()] call returns.</dd>
**
** [[SQLITE_CONFIG_GETMALLOC]] <dt>SQLITE_CONFIG_GETMALLOC</dt>
** <dd> ^(This option takes a single argument which is a pointer to an
** instance of the [sqlite3_mem_methods] structure.  The [sqlite3_mem_methods]

** structure is filled with the currently defined memory allocation routines.)^
** This option can be used to overload the default memory allocation
** routines with a wrapper that simulations memory allocation failure or
** tracks memory usage, for example. </dd>
**
** [[SQLITE_CONFIG_MEMSTATUS]] <dt>SQLITE_CONFIG_MEMSTATUS</dt>
** <dd> ^This option takes single argument of type int, interpreted as a 
** boolean, which enables or disables the collection of memory allocation 
** statistics. ^(When memory allocation statistics are disabled, the 
** following SQLite interfaces become non-operational:
**   <ul>
**   <li> [sqlite3_memory_used()]
**   <li> [sqlite3_memory_highwater()]
**   <li> [sqlite3_soft_heap_limit64()]
**   <li> [sqlite3_status()]
**   </ul>)^
** ^Memory allocation statistics are enabled by default unless SQLite is
** compiled with [SQLITE_DEFAULT_MEMSTATUS]=0 in which case memory
** allocation statistics are disabled by default.
** </dd>
**
** [[SQLITE_CONFIG_SCRATCH]] <dt>SQLITE_CONFIG_SCRATCH</dt>
** <dd> ^This option specifies a static memory buffer that SQLite can use for
** scratch memory.  ^(There are three arguments:  A pointer an 8-byte

** aligned memory buffer from which the scratch allocations will be
** drawn, the size of each scratch allocation (sz),
** and the maximum number of scratch allocations (N).)^
** The first argument must be a pointer to an 8-byte aligned buffer
** of at least sz*N bytes of memory.
** ^SQLite will not use more than two scratch buffers per thread and not
** more than one scratch buffer per thread when not performing
................................................................................
** SQLITE_CONFIG_SCRATCH, SQLite avoids unnecessary large
** [sqlite3_malloc|heap allocations].
** This can help [Robson proof|prevent memory allocation failures] due to heap
** fragmentation in low-memory embedded systems.
** </dd>
**
** [[SQLITE_CONFIG_PAGECACHE]] <dt>SQLITE_CONFIG_PAGECACHE</dt>
** <dd> ^This option specifies a static memory buffer that SQLite can use for
** the database page cache with the default page cache implementation.  

** This configuration should not be used if an application-define page
** cache implementation is loaded using the [SQLITE_CONFIG_PCACHE2]
** configuration option.
** ^There are three arguments to this option: A pointer to 8-byte aligned
** memory, the size of each page buffer (sz), and the number of pages (N).
** The sz argument should be the size of the largest database page
** (a power of two between 512 and 32768) plus some extra bytes for each
** page header.  ^The number of extra bytes needed by the page header
** can be determined using the [SQLITE_CONFIG_PCACHE_HDRSZ] option 
** to [sqlite3_config()].
** ^It is harmless, apart from the wasted memory,
................................................................................
** undefined.
** ^SQLite will use the memory provided by the first argument to satisfy its
** memory needs for the first N pages that it adds to cache.  ^If additional
** page cache memory is needed beyond what is provided by this option, then
** SQLite goes to [sqlite3_malloc()] for the additional storage space.</dd>
**
** [[SQLITE_CONFIG_HEAP]] <dt>SQLITE_CONFIG_HEAP</dt>
** <dd> ^This option specifies a static memory buffer that SQLite will use
** for all of its dynamic memory allocation needs beyond those provided
** for by [SQLITE_CONFIG_SCRATCH] and [SQLITE_CONFIG_PAGECACHE].

** ^There are three arguments: An 8-byte aligned pointer to the memory,
** the number of bytes in the memory buffer, and the minimum allocation size.
** ^If the first pointer (the memory pointer) is NULL, then SQLite reverts
** to using its default memory allocator (the system malloc() implementation),
** undoing any prior invocation of [SQLITE_CONFIG_MALLOC].  ^If the
** memory pointer is not NULL and either [SQLITE_ENABLE_MEMSYS3] or
** [SQLITE_ENABLE_MEMSYS5] are defined, then the alternative memory
** allocator is engaged to handle all of SQLites memory allocation needs.
** The first pointer (the memory pointer) must be aligned to an 8-byte
** boundary or subsequent behavior of SQLite will be undefined.
** The minimum allocation size is capped at 2**12. Reasonable values
** for the minimum allocation size are 2**5 through 2**8.</dd>
**
** [[SQLITE_CONFIG_MUTEX]] <dt>SQLITE_CONFIG_MUTEX</dt>
** <dd> ^(This option takes a single argument which is a pointer to an
** instance of the [sqlite3_mutex_methods] structure.  The argument specifies
** alternative low-level mutex routines to be used in place
** the mutex routines built into SQLite.)^  ^SQLite makes a copy of the
** content of the [sqlite3_mutex_methods] structure before the call to
** [sqlite3_config()] returns. ^If SQLite is compiled with
** 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_MUTEX configuration option will
** return [SQLITE_ERROR].</dd>
**
** [[SQLITE_CONFIG_GETMUTEX]] <dt>SQLITE_CONFIG_GETMUTEX</dt>
** <dd> ^(This option takes a single argument which is a pointer to an
** instance of the [sqlite3_mutex_methods] structure.  The
** [sqlite3_mutex_methods]
** structure is filled with the currently defined mutex routines.)^
** This option can be used to overload the default mutex allocation
** routines with a wrapper used to track mutex usage for performance
** profiling or testing, for example.   ^If SQLite is compiled with
** 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> ^(This option takes two arguments that determine the default
** memory allocation for the lookaside memory allocator on each
** [database connection].  The first argument is the
** size of each lookaside buffer slot and the second is the number of
** slots allocated to each database connection.)^  ^(This option sets the
** <i>default</i> lookaside size. The [SQLITE_DBCONFIG_LOOKASIDE]
** verb to [sqlite3_db_config()] can be used to change the lookaside
** configuration on individual connections.)^ </dd>
**
** [[SQLITE_CONFIG_PCACHE2]] <dt>SQLITE_CONFIG_PCACHE2</dt>
** <dd> ^(This 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
** object and uses it for page cache memory allocations.</dd>
**
** [[SQLITE_CONFIG_GETPCACHE2]] <dt>SQLITE_CONFIG_GETPCACHE2</dt>
** <dd> ^(This option takes a single argument which is a pointer to an
** [sqlite3_pcache_methods2] object.  SQLite copies of the current
** page cache implementation into that object.)^ </dd>
**
** [[SQLITE_CONFIG_LOG]] <dt>SQLITE_CONFIG_LOG</dt>
** <dd> The SQLITE_CONFIG_LOG option is used to configure the SQLite
** global [error log].
** (^The SQLITE_CONFIG_LOG option takes two arguments: a pointer to a
** function with a call signature of void(*)(void*,int,const char*), 
................................................................................
** log message after formatting via [sqlite3_snprintf()].
** The SQLite logging interface is not reentrant; the logger function
** supplied by the application must not invoke any SQLite interface.
** In a multi-threaded application, the application-defined logger
** function must be threadsafe. </dd>
**
** [[SQLITE_CONFIG_URI]] <dt>SQLITE_CONFIG_URI
** <dd>^(This option takes a single argument of type int. If non-zero, then
** URI handling is globally enabled. If the parameter is zero, then URI handling
** is globally disabled.)^ ^If URI handling is globally enabled, all filenames
** passed to [sqlite3_open()], [sqlite3_open_v2()], [sqlite3_open16()] or
** specified as part of [ATTACH] commands are interpreted as URIs, regardless
** of whether or not the [SQLITE_OPEN_URI] flag is set when the database
** connection is opened. ^If it is globally disabled, filenames are
** only interpreted as URIs if the SQLITE_OPEN_URI flag is set when the
** database connection is opened. ^(By default, URI handling is globally
** disabled. The default value may be changed by compiling with the
** [SQLITE_USE_URI] symbol defined.)^
**
** [[SQLITE_CONFIG_COVERING_INDEX_SCAN]] <dt>SQLITE_CONFIG_COVERING_INDEX_SCAN
** <dd>^This option takes a single integer argument which is interpreted as
** a boolean in order to enable or disable the use of covering indices for

** full table scans in the query optimizer.  ^The default setting is determined
** by the [SQLITE_ALLOW_COVERING_INDEX_SCAN] compile-time option, or is "on"
** if that compile-time option is omitted.
** The ability to disable the use of covering indices for full table scans
** is because some incorrectly coded legacy applications might malfunction
** when the optimization is enabled.  Providing the ability to
** disable the optimization allows the older, buggy application code to work
** without change even with newer versions of SQLite.
................................................................................
** exceed the compile-time maximum mmap size set by the
** [SQLITE_MAX_MMAP_SIZE] compile-time option.)^
** ^If either argument to this option is negative, then that argument is
** changed to its compile-time default.
**
** [[SQLITE_CONFIG_WIN32_HEAPSIZE]]
** <dt>SQLITE_CONFIG_WIN32_HEAPSIZE
** <dd>^This option is only available if SQLite is compiled for Windows
** with the [SQLITE_WIN32_MALLOC] pre-processor macro defined.
** SQLITE_CONFIG_WIN32_HEAPSIZE takes a 32-bit unsigned integer value
** that specifies the maximum size of the created heap.
** </dl>
**
** [[SQLITE_CONFIG_PCACHE_HDRSZ]]
** <dt>SQLITE_CONFIG_PCACHE_HDRSZ
** <dd>^This option takes a single parameter which is a pointer to an integer
** and writes into that integer the number of extra bytes per page required
** for each page in [SQLITE_CONFIG_PAGECACHE].  The amount of
** extra space required can change depending on the compiler,
** target platform, and SQLite version.
** </dl>
*/
#define SQLITE_CONFIG_SINGLETHREAD  1  /* nil */
#define SQLITE_CONFIG_MULTITHREAD   2  /* nil */
#define SQLITE_CONFIG_SERIALIZED    3  /* nil */







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** ^If SQLite is compiled with
** the [SQLITE_THREADSAFE | SQLITE_THREADSAFE=0] compile-time option then
** it is not possible to set the Serialized [threading mode] and
** [sqlite3_config()] will return [SQLITE_ERROR] if called with the
** SQLITE_CONFIG_SERIALIZED configuration option.</dd>
**
** [[SQLITE_CONFIG_MALLOC]] <dt>SQLITE_CONFIG_MALLOC</dt>
** <dd> ^(The SQLITE_CONFIG_MALLOC option takes a single argument which is 
** a pointer to an instance of the [sqlite3_mem_methods] structure.
** The argument specifies
** alternative low-level memory allocation routines to be used in place of
** the memory allocation routines built into SQLite.)^ ^SQLite makes
** its own private copy of the content of the [sqlite3_mem_methods] structure
** before the [sqlite3_config()] call returns.</dd>
**
** [[SQLITE_CONFIG_GETMALLOC]] <dt>SQLITE_CONFIG_GETMALLOC</dt>
** <dd> ^(The SQLITE_CONFIG_GETMALLOC option takes a single argument which
** is a pointer to an instance of the [sqlite3_mem_methods] structure.
** The [sqlite3_mem_methods]
** structure is filled with the currently defined memory allocation routines.)^
** This option can be used to overload the default memory allocation
** routines with a wrapper that simulations memory allocation failure or
** tracks memory usage, for example. </dd>
**
** [[SQLITE_CONFIG_MEMSTATUS]] <dt>SQLITE_CONFIG_MEMSTATUS</dt>
** <dd> ^The SQLITE_CONFIG_MEMSTATUS option takes single argument of type int,
** interpreted as a boolean, which enables or disables the collection of
** memory allocation statistics. ^(When memory allocation statistics are disabled, the 
** following SQLite interfaces become non-operational:
**   <ul>
**   <li> [sqlite3_memory_used()]
**   <li> [sqlite3_memory_highwater()]
**   <li> [sqlite3_soft_heap_limit64()]
**   <li> [sqlite3_status()]
**   </ul>)^
** ^Memory allocation statistics are enabled by default unless SQLite is
** compiled with [SQLITE_DEFAULT_MEMSTATUS]=0 in which case memory
** allocation statistics are disabled by default.
** </dd>
**
** [[SQLITE_CONFIG_SCRATCH]] <dt>SQLITE_CONFIG_SCRATCH</dt>
** <dd> ^The SQLITE_CONFIG_SCRATCH option specifies a static memory buffer
** that SQLite can use for scratch memory.  ^(There are three arguments
** to SQLITE_CONFIG_SCRATCH:  A pointer an 8-byte
** aligned memory buffer from which the scratch allocations will be
** drawn, the size of each scratch allocation (sz),
** and the maximum number of scratch allocations (N).)^
** The first argument must be a pointer to an 8-byte aligned buffer
** of at least sz*N bytes of memory.
** ^SQLite will not use more than two scratch buffers per thread and not
** more than one scratch buffer per thread when not performing
................................................................................
** SQLITE_CONFIG_SCRATCH, SQLite avoids unnecessary large
** [sqlite3_malloc|heap allocations].
** This can help [Robson proof|prevent memory allocation failures] due to heap
** fragmentation in low-memory embedded systems.
** </dd>
**
** [[SQLITE_CONFIG_PAGECACHE]] <dt>SQLITE_CONFIG_PAGECACHE</dt>
** <dd> ^The SQLITE_CONFIG_PAGECACHE option specifies a static memory buffer
** that SQLite can use for the database page cache with the default page
** cache implementation.  
** This configuration should not be used if an application-define page
** cache implementation is loaded using the [SQLITE_CONFIG_PCACHE2]
** configuration option.
** ^There are three arguments to SQLITE_CONFIG_PAGECACHE: A pointer to 8-byte aligned
** memory, the size of each page buffer (sz), and the number of pages (N).
** The sz argument should be the size of the largest database page
** (a power of two between 512 and 32768) plus some extra bytes for each
** page header.  ^The number of extra bytes needed by the page header
** can be determined using the [SQLITE_CONFIG_PCACHE_HDRSZ] option 
** to [sqlite3_config()].
** ^It is harmless, apart from the wasted memory,
................................................................................
** undefined.
** ^SQLite will use the memory provided by the first argument to satisfy its
** memory needs for the first N pages that it adds to cache.  ^If additional
** page cache memory is needed beyond what is provided by this option, then
** SQLite goes to [sqlite3_malloc()] for the additional storage space.</dd>
**
** [[SQLITE_CONFIG_HEAP]] <dt>SQLITE_CONFIG_HEAP</dt>
** <dd> ^The SQLITE_CONFIG_HEAP option specifies a static memory buffer 
** that SQLite will use for all of its dynamic memory allocation needs
** beyond those provided for by [SQLITE_CONFIG_SCRATCH] and [SQLITE_CONFIG_PAGECACHE].
** ^There are three arguments to SQLITE_CONFIG_HEAP:
** An 8-byte aligned pointer to the memory,
** the number of bytes in the memory buffer, and the minimum allocation size.
** ^If the first pointer (the memory pointer) is NULL, then SQLite reverts
** to using its default memory allocator (the system malloc() implementation),
** undoing any prior invocation of [SQLITE_CONFIG_MALLOC].  ^If the
** memory pointer is not NULL and either [SQLITE_ENABLE_MEMSYS3] or
** [SQLITE_ENABLE_MEMSYS5] are defined, then the alternative memory
** allocator is engaged to handle all of SQLites memory allocation needs.
** The first pointer (the memory pointer) must be aligned to an 8-byte
** boundary or subsequent behavior of SQLite will be undefined.
** The minimum allocation size is capped at 2**12. Reasonable values
** for the minimum allocation size are 2**5 through 2**8.</dd>
**
** [[SQLITE_CONFIG_MUTEX]] <dt>SQLITE_CONFIG_MUTEX</dt>
** <dd> ^(The SQLITE_CONFIG_MUTEX option takes a single argument which is a
** pointer to an instance of the [sqlite3_mutex_methods] structure.
** The argument specifies alternative low-level mutex routines to be used in place
** the mutex routines built into SQLite.)^  ^SQLite makes a copy of the
** content of the [sqlite3_mutex_methods] structure before the call to
** [sqlite3_config()] returns. ^If SQLite is compiled with
** 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_MUTEX configuration option will
** return [SQLITE_ERROR].</dd>
**
** [[SQLITE_CONFIG_GETMUTEX]] <dt>SQLITE_CONFIG_GETMUTEX</dt>
** <dd> ^(The SQLITE_CONFIG_GETMUTEX option takes a single argument which
** is a pointer to an instance of the [sqlite3_mutex_methods] structure.  The
** [sqlite3_mutex_methods]
** structure is filled with the currently defined mutex routines.)^
** This option can be used to overload the default mutex allocation
** routines with a wrapper used to track mutex usage for performance
** profiling or testing, for example.   ^If SQLite is compiled with
** 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 and the second is the number of
** slots allocated to each database connection.)^  ^(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.)^ </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>
**
** [[SQLITE_CONFIG_GETPCACHE2]] <dt>SQLITE_CONFIG_GETPCACHE2</dt>
** <dd> ^(The SQLITE_CONFIG_GETPCACHE2 option takes a single argument which
** is a pointer to an [sqlite3_pcache_methods2] object.  SQLite copies of the current
** page cache implementation into that object.)^ </dd>
**
** [[SQLITE_CONFIG_LOG]] <dt>SQLITE_CONFIG_LOG</dt>
** <dd> The SQLITE_CONFIG_LOG option is used to configure the SQLite
** global [error log].
** (^The SQLITE_CONFIG_LOG option takes two arguments: a pointer to a
** function with a call signature of void(*)(void*,int,const char*), 
................................................................................
** log message after formatting via [sqlite3_snprintf()].
** The SQLite logging interface is not reentrant; the logger function
** supplied by the application must not invoke any SQLite interface.
** In a multi-threaded application, the application-defined logger
** function must be threadsafe. </dd>
**
** [[SQLITE_CONFIG_URI]] <dt>SQLITE_CONFIG_URI
** <dd>^(The SQLITE_CONFIG_URI option takes a single argument of type int.
** If non-zero, then URI handling is globally enabled. If the parameter is zero,
** then URI handling is globally disabled.)^ ^If URI handling is globally enabled,
** all filenames passed to [sqlite3_open()], [sqlite3_open_v2()], [sqlite3_open16()] or
** specified as part of [ATTACH] commands are interpreted as URIs, regardless
** of whether or not the [SQLITE_OPEN_URI] flag is set when the database
** connection is opened. ^If it is globally disabled, filenames are
** only interpreted as URIs if the SQLITE_OPEN_URI flag is set when the
** database connection is opened. ^(By default, URI handling is globally
** disabled. The default value may be changed by compiling with the
** [SQLITE_USE_URI] symbol defined.)^
**
** [[SQLITE_CONFIG_COVERING_INDEX_SCAN]] <dt>SQLITE_CONFIG_COVERING_INDEX_SCAN
** <dd>^The SQLITE_CONFIG_COVERING_INDEX_SCAN option takes a single integer
** argument which is interpreted as a boolean in order to enable or disable
** the use of covering indices for full table scans in the query optimizer.
** ^The default setting is determined
** by the [SQLITE_ALLOW_COVERING_INDEX_SCAN] compile-time option, or is "on"
** if that compile-time option is omitted.
** The ability to disable the use of covering indices for full table scans
** is because some incorrectly coded legacy applications might malfunction
** when the optimization is enabled.  Providing the ability to
** disable the optimization allows the older, buggy application code to work
** without change even with newer versions of SQLite.
................................................................................
** exceed the compile-time maximum mmap size set by the
** [SQLITE_MAX_MMAP_SIZE] compile-time option.)^
** ^If either argument to this option is negative, then that argument is
** changed to its compile-time default.
**
** [[SQLITE_CONFIG_WIN32_HEAPSIZE]]
** <dt>SQLITE_CONFIG_WIN32_HEAPSIZE
** <dd>^The SQLITE_CONFIG_WIN32_HEAPSIZE option is only available if SQLite is
** compiled for Windows with the [SQLITE_WIN32_MALLOC] pre-processor macro defined.
** ^SQLITE_CONFIG_WIN32_HEAPSIZE takes a 32-bit unsigned integer value
** that specifies the maximum size of the created heap.
** </dl>
**
** [[SQLITE_CONFIG_PCACHE_HDRSZ]]
** <dt>SQLITE_CONFIG_PCACHE_HDRSZ
** <dd>^The SQLITE_CONFIG_PCACHE_HDRSZ option takes a single parameter which
** is a pointer to an integer and writes into that integer the number of extra
** bytes per page required for each page in [SQLITE_CONFIG_PAGECACHE]. The amount of
** extra space required can change depending on the compiler,
** target platform, and SQLite version.
** </dl>
*/
#define SQLITE_CONFIG_SINGLETHREAD  1  /* nil */
#define SQLITE_CONFIG_MULTITHREAD   2  /* nil */
#define SQLITE_CONFIG_SERIALIZED    3  /* nil */