mutex_w32.c

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/*
** 2007 August 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 mutexes for win32
**
** $Id: mutex_w32.c,v 1.12 2008/11/10 20:01:41 shane Exp $
*/
#include "sqliteInt.h"

/*
** The code in this file is only used if we are compiling multithreaded
** on a win32 system.
*/
#ifdef SQLITE_MUTEX_W32

/*
** Each recursive mutex is an instance of the following structure.
*/
struct sqlite3_mutex {
  CRITICAL_SECTION mutex;    /* Mutex controlling the lock */
  int id;                    /* Mutex type */
  int nRef;                  /* Number of enterances */
  DWORD owner;               /* Thread holding this mutex */
};

/*
** Return true (non-zero) if we are running under WinNT, Win2K, WinXP,
** or WinCE.  Return false (zero) for Win95, Win98, or WinME.
**
** Here is an interesting observation:  Win95, Win98, and WinME lack
** the LockFileEx() API.  But we can still statically link against that
** API as long as we don't call it win running Win95/98/ME.  A call to
** this routine is used to determine if the host is Win95/98/ME or
** WinNT/2K/XP so that we will know whether or not we can safely call
** the LockFileEx() API.
**
** mutexIsNT() is only used for the TryEnterCriticalSection() API call,
** which is only available if your application was compiled with 
** _WIN32_WINNT defined to a value >= 0x0400.  Currently, the only
** call to TryEnterCriticalSection() is #ifdef'ed out, so #ifdef 
** this out as well.
*/
#if 0
#if SQLITE_OS_WINCE
# define mutexIsNT()  (1)
#else
  static int mutexIsNT(void){
    static int osType = 0;
    if( osType==0 ){
      OSVERSIONINFO sInfo;
      sInfo.dwOSVersionInfoSize = sizeof(sInfo);
      GetVersionEx(&sInfo);
      osType = sInfo.dwPlatformId==VER_PLATFORM_WIN32_NT ? 2 : 1;
    }
    return osType==2;
  }
#endif /* SQLITE_OS_WINCE */
#endif

#ifdef SQLITE_DEBUG
/*
** The sqlite3_mutex_held() and sqlite3_mutex_notheld() routine are
** intended for use only inside assert() statements.
*/
static int winMutexHeld(sqlite3_mutex *p){
  return p->nRef!=0 && p->owner==GetCurrentThreadId();
}
static int winMutexNotheld(sqlite3_mutex *p){
  return p->nRef==0 || p->owner!=GetCurrentThreadId();
}
#endif


/*
** Initialize and deinitialize the mutex subsystem.
*/
static int winMutexInit(void){ return SQLITE_OK; }
static int winMutexEnd(void){ return SQLITE_OK; }

/*
** The sqlite3_mutex_alloc() routine allocates a new
** mutex and returns a pointer to it.  If it returns NULL
** that means that a mutex could not be allocated.  SQLite
** will unwind its stack and return an error.  The argument
** to sqlite3_mutex_alloc() is one of these integer constants:
**
** <ul>
** <li>  SQLITE_MUTEX_FAST               0
** <li>  SQLITE_MUTEX_RECURSIVE          1
** <li>  SQLITE_MUTEX_STATIC_MASTER      2
** <li>  SQLITE_MUTEX_STATIC_MEM         3
** <li>  SQLITE_MUTEX_STATIC_PRNG        4
** </ul>
**
** The first two constants cause sqlite3_mutex_alloc() to create
** a new mutex.  The new mutex is recursive when SQLITE_MUTEX_RECURSIVE
** is used but not necessarily so when SQLITE_MUTEX_FAST is used.
** The mutex implementation does not need to make a distinction
** between SQLITE_MUTEX_RECURSIVE and SQLITE_MUTEX_FAST if it does
** not want to.  But SQLite will only request a recursive mutex in
** cases where it really needs one.  If a faster non-recursive mutex
** implementation is available on the host platform, the mutex subsystem
** might return such a mutex in response to SQLITE_MUTEX_FAST.
**
** The other allowed parameters to sqlite3_mutex_alloc() each return
** a pointer to a static preexisting mutex.  Three static mutexes are
** used by the current version of SQLite.  Future versions of SQLite
** may add additional static mutexes.  Static mutexes are for internal
** use by SQLite only.  Applications that use SQLite mutexes should
** use only the dynamic mutexes returned by SQLITE_MUTEX_FAST or
** SQLITE_MUTEX_RECURSIVE.
**
** Note that if one of the dynamic mutex parameters (SQLITE_MUTEX_FAST
** or SQLITE_MUTEX_RECURSIVE) is used then sqlite3_mutex_alloc()
** returns a different mutex on every call.  But for the static 
** mutex types, the same mutex is returned on every call that has
** the same type number.
*/
static sqlite3_mutex *winMutexAlloc(int iType){
  sqlite3_mutex *p;

  switch( iType ){
    case SQLITE_MUTEX_FAST:
    case SQLITE_MUTEX_RECURSIVE: {
      p = sqlite3MallocZero( sizeof(*p) );
      if( p ){
        p->id = iType;
        InitializeCriticalSection(&p->mutex);
      }
      break;
    }
    default: {
      static sqlite3_mutex staticMutexes[6];
      static int isInit = 0;
      while( !isInit ){
        static long lock = 0;
        if( InterlockedIncrement(&lock)==1 ){
          int i;
          for(i=0; i<sizeof(staticMutexes)/sizeof(staticMutexes[0]); i++){
            InitializeCriticalSection(&staticMutexes[i].mutex);
          }
          isInit = 1;
        }else{
          Sleep(1);
        }
      }
      assert( iType-2 >= 0 );
      assert( iType-2 < sizeof(staticMutexes)/sizeof(staticMutexes[0]) );
      p = &staticMutexes[iType-2];
      p->id = iType;
      break;
    }
  }
  return p;
}


/*
** This routine deallocates a previously
** allocated mutex.  SQLite is careful to deallocate every
** mutex that it allocates.
*/
static void winMutexFree(sqlite3_mutex *p){
  assert( p );
  assert( p->nRef==0 );
  assert( p->id==SQLITE_MUTEX_FAST || p->id==SQLITE_MUTEX_RECURSIVE );
  DeleteCriticalSection(&p->mutex);
  sqlite3_free(p);
}

/*
** The sqlite3_mutex_enter() and sqlite3_mutex_try() routines attempt
** to enter a mutex.  If another thread is already within the mutex,
** sqlite3_mutex_enter() will block and sqlite3_mutex_try() will return
** SQLITE_BUSY.  The sqlite3_mutex_try() interface returns SQLITE_OK
** upon successful entry.  Mutexes created using SQLITE_MUTEX_RECURSIVE can
** be entered multiple times by the same thread.  In such cases the,
** mutex must be exited an equal number of times before another thread
** can enter.  If the same thread tries to enter any other kind of mutex
** more than once, the behavior is undefined.
*/
static void winMutexEnter(sqlite3_mutex *p){
  assert( p->id==SQLITE_MUTEX_RECURSIVE || winMutexNotheld(p) );
  EnterCriticalSection(&p->mutex);
  p->owner = GetCurrentThreadId(); 
  p->nRef++;
}
static int winMutexTry(sqlite3_mutex *p){
  int rc = SQLITE_BUSY;
  assert( p->id==SQLITE_MUTEX_RECURSIVE || winMutexNotheld(p) );
  /*
  ** The sqlite3_mutex_try() routine is very rarely used, and when it
  ** is used it is merely an optimization.  So it is OK for it to always
  ** fail.  
  **
  ** The TryEnterCriticalSection() interface is only available on WinNT.
  ** And some windows compilers complain if you try to use it without
  ** first doing some #defines that prevent SQLite from building on Win98.
  ** For that reason, we will omit this optimization for now.  See
  ** ticket #2685.
  */
#if 0
  if( mutexIsNT() && TryEnterCriticalSection(&p->mutex) ){
    p->owner = GetCurrentThreadId();
    p->nRef++;
    rc = SQLITE_OK;
  }
#endif
  return rc;
}

/*
** The sqlite3_mutex_leave() routine exits a mutex that was
** previously entered by the same thread.  The behavior
** is undefined if the mutex is not currently entered or
** is not currently allocated.  SQLite will never do either.
*/
static void winMutexLeave(sqlite3_mutex *p){
  assert( p->nRef>0 );
  assert( p->owner==GetCurrentThreadId() );
  p->nRef--;
  assert( p->nRef==0 || p->id==SQLITE_MUTEX_RECURSIVE );
  LeaveCriticalSection(&p->mutex);
}

sqlite3_mutex_methods *sqlite3DefaultMutex(void){
  static sqlite3_mutex_methods sMutex = {
    winMutexInit,
    winMutexEnd,
    winMutexAlloc,
    winMutexFree,
    winMutexEnter,
    winMutexTry,
    winMutexLeave,
#ifdef SQLITE_DEBUG
    winMutexHeld,
    winMutexNotheld
#endif
  };

  return &sMutex;
}
#endif /* SQLITE_MUTEX_W32 */