src/os_win.c

/*
** 2004 May 22
**
** 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 code that is specific to windows.
*/
#include "sqliteInt.h"
#include "os.h"
#if OS_WIN               /* This file is used for windows only */

#include <winbase.h>

#ifdef __CYGWIN__
# include <sys/cygwin.h>
#endif

/*
** Macros used to determine whether or not to use threads.
*/
#if defined(THREADSAFE) && THREADSAFE
# define SQLITE_W32_THREADS 1
#endif

/*
** Include code that is common to all os_*.c files
*/
#include "os_common.h"

/*
** Do not include any of the File I/O interface procedures if the
** SQLITE_OMIT_DISKIO macro is defined (indicating that there database
** will be in-memory only)
*/
#ifndef SQLITE_OMIT_DISKIO

/*
** The following variable is (normally) set once and never changes
** thereafter.  It records whether the operating system is Win95
** or WinNT.
**
** 0:   Operating system unknown.
** 1:   Operating system is Win95.
** 2:   Operating system is WinNT.
**
** In order to facilitate testing on a WinNT system, the test fixture
** can manually set this value to 1 to emulate Win98 behavior.
*/
int sqlite3_os_type = 0;

/*
** Return true (non-zero) if we are running under WinNT, Win2K or WinXP.
** 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.
*/
static int isNT(void){
  if( sqlite3_os_type==0 ){
    OSVERSIONINFO sInfo;
    sInfo.dwOSVersionInfoSize = sizeof(sInfo);
    GetVersionEx(&sInfo);
    sqlite3_os_type = sInfo.dwPlatformId==VER_PLATFORM_WIN32_NT ? 2 : 1;
  }
  return sqlite3_os_type==2;
}

/*
** Convert a UTF-8 string to UTF-32.  Space to hold the returned string
** is obtained from sqliteMalloc.
*/
static WCHAR *utf8ToUnicode(const char *zFilename){
  int nByte;
  WCHAR *zWideFilename;

  if( !isNT() ){
    return 0;
  }
  nByte = MultiByteToWideChar(CP_UTF8, 0, zFilename, -1, NULL, 0)*sizeof(WCHAR);
  zWideFilename = sqliteMalloc( nByte*sizeof(zWideFilename[0]) );
  if( zWideFilename==0 ){
    return 0;
  }
  nByte = MultiByteToWideChar(CP_UTF8, 0, zFilename, -1, zWideFilename, nByte);
  if( nByte==0 ){
    sqliteFree(zWideFilename);
    zWideFilename = 0;
  }
  return zWideFilename;
}

/*
** Convert UTF-32 to UTF-8.  Space to hold the returned string is
** obtained from sqliteMalloc().
*/
static char *unicodeToUtf8(const WCHAR *zWideFilename){
  int nByte;
  char *zFilename;

  nByte = WideCharToMultiByte(CP_UTF8, 0, zWideFilename, -1, 0, 0, 0, 0);
  zFilename = sqliteMalloc( nByte );
  if( zFilename==0 ){
    return 0;
  }
  nByte = WideCharToMultiByte(CP_UTF8, 0, zWideFilename, -1, zFilename, nByte,
                              0, 0);
  if( nByte == 0 ){
    sqliteFree(zFilename);
    zFilename = 0;
  }
  return zFilename;
}


/*
** Delete the named file
*/
int sqlite3OsDelete(const char *zFilename){
  WCHAR *zWide = utf8ToUnicode(zFilename);
  if( zWide ){
    DeleteFileW(zWide);
    sqliteFree(zWide);
  }else{
    DeleteFileA(zFilename);
  }
  TRACE2("DELETE \"%s\"\n", zFilename);
  return SQLITE_OK;
}

/*
** Return TRUE if the named file exists.
*/
int sqlite3OsFileExists(const char *zFilename){
  int exists = 0;
  WCHAR *zWide = utf8ToUnicode(zFilename);
  if( zWide ){
    exists = GetFileAttributesW(zWide) != 0xffffffff;
    sqliteFree(zWide);
  }else{
    exists = GetFileAttributesA(zFilename) != 0xffffffff;
  }
  return exists;
}

/*
** Attempt to open a file for both reading and writing.  If that
** fails, try opening it read-only.  If the file does not exist,
** try to create it.
**
** On success, a handle for the open file is written to *id
** and *pReadonly is set to 0 if the file was opened for reading and
** writing or 1 if the file was opened read-only.  The function returns
** SQLITE_OK.
**
** On failure, the function returns SQLITE_CANTOPEN and leaves
** *id and *pReadonly unchanged.
*/
int sqlite3OsOpenReadWrite(
  const char *zFilename,
  OsFile *id,
  int *pReadonly
){
  HANDLE h;
  WCHAR *zWide = utf8ToUnicode(zFilename);
  assert( !id->isOpen );
  if( zWide ){
    h = CreateFileW(zWide,
       GENERIC_READ | GENERIC_WRITE,
       FILE_SHARE_READ | FILE_SHARE_WRITE,
       NULL,
       OPEN_ALWAYS,
       FILE_ATTRIBUTE_NORMAL | FILE_FLAG_RANDOM_ACCESS,
       NULL
    );
    if( h==INVALID_HANDLE_VALUE ){
      h = CreateFileW(zWide,
         GENERIC_READ,
         FILE_SHARE_READ,
         NULL,
         OPEN_ALWAYS,
         FILE_ATTRIBUTE_NORMAL | FILE_FLAG_RANDOM_ACCESS,
         NULL
      );
      if( h==INVALID_HANDLE_VALUE ){
        sqliteFree(zWide);
        return SQLITE_CANTOPEN;
      }
      *pReadonly = 1;
    }else{
      *pReadonly = 0;
    }
    sqliteFree(zWide);
  }else{
    h = CreateFileA(zFilename,
       GENERIC_READ | GENERIC_WRITE,
       FILE_SHARE_READ | FILE_SHARE_WRITE,
       NULL,
       OPEN_ALWAYS,
       FILE_ATTRIBUTE_NORMAL | FILE_FLAG_RANDOM_ACCESS,
       NULL
    );
    if( h==INVALID_HANDLE_VALUE ){
      h = CreateFileA(zFilename,
         GENERIC_READ,
         FILE_SHARE_READ,
         NULL,
         OPEN_ALWAYS,
         FILE_ATTRIBUTE_NORMAL | FILE_FLAG_RANDOM_ACCESS,
         NULL
      );
      if( h==INVALID_HANDLE_VALUE ){
        return SQLITE_CANTOPEN;
      }
      *pReadonly = 1;
    }else{
      *pReadonly = 0;
    }
  }
  id->h = h;
  id->locktype = NO_LOCK;
  id->sharedLockByte = 0;
  id->isOpen = 1;
  OpenCounter(+1);
  TRACE3("OPEN R/W %d \"%s\"\n", h, zFilename);
  return SQLITE_OK;
}


/*
** Attempt to open a new file for exclusive access by this process.
** The file will be opened for both reading and writing.  To avoid
** a potential security problem, we do not allow the file to have
** previously existed.  Nor do we allow the file to be a symbolic
** link.
**
** If delFlag is true, then make arrangements to automatically delete
** the file when it is closed.
**
** On success, write the file handle into *id and return SQLITE_OK.
**
** On failure, return SQLITE_CANTOPEN.
*/
int sqlite3OsOpenExclusive(const char *zFilename, OsFile *id, int delFlag){
  HANDLE h;
  int fileflags;
  WCHAR *zWide = utf8ToUnicode(zFilename);
  assert( !id->isOpen );
  if( delFlag ){
    fileflags = FILE_ATTRIBUTE_TEMPORARY | FILE_FLAG_RANDOM_ACCESS 
                     | FILE_FLAG_DELETE_ON_CLOSE;
  }else{
    fileflags = FILE_FLAG_RANDOM_ACCESS;
  }
  if( zWide ){
    h = CreateFileW(zWide,
       GENERIC_READ | GENERIC_WRITE,
       0,
       NULL,
       CREATE_ALWAYS,
       fileflags,
       NULL
    );
    sqliteFree(zWide);
  }else{
    h = CreateFileA(zFilename,
       GENERIC_READ | GENERIC_WRITE,
       0,
       NULL,
       CREATE_ALWAYS,
       fileflags,
       NULL
    );
  }
  if( h==INVALID_HANDLE_VALUE ){
    return SQLITE_CANTOPEN;
  }
  id->h = h;
  id->locktype = NO_LOCK;
  id->sharedLockByte = 0;
  id->isOpen = 1;
  OpenCounter(+1);
  TRACE3("OPEN EX %d \"%s\"\n", h, zFilename);
  return SQLITE_OK;
}

/*
** Attempt to open a new file for read-only access.
**
** On success, write the file handle into *id and return SQLITE_OK.
**
** On failure, return SQLITE_CANTOPEN.
*/
int sqlite3OsOpenReadOnly(const char *zFilename, OsFile *id){
  HANDLE h;
  WCHAR *zWide = utf8ToUnicode(zFilename);
  assert( !id->isOpen );
  if( zWide ){
    h = CreateFileW(zWide,
       GENERIC_READ,
       0,
       NULL,
       OPEN_EXISTING,
       FILE_ATTRIBUTE_NORMAL | FILE_FLAG_RANDOM_ACCESS,
       NULL
    );
    sqliteFree(zWide);
  }else{
    h = CreateFileA(zFilename,
       GENERIC_READ,
       0,
       NULL,
       OPEN_EXISTING,
       FILE_ATTRIBUTE_NORMAL | FILE_FLAG_RANDOM_ACCESS,
       NULL
    );
  }
  if( h==INVALID_HANDLE_VALUE ){
    return SQLITE_CANTOPEN;
  }
  id->h = h;
  id->locktype = NO_LOCK;
  id->sharedLockByte = 0;
  id->isOpen = 1;
  OpenCounter(+1);
  TRACE3("OPEN RO %d \"%s\"\n", h, zFilename);
  return SQLITE_OK;
}

/*
** Attempt to open a file descriptor for the directory that contains a
** file.  This file descriptor can be used to fsync() the directory
** in order to make sure the creation of a new file is actually written
** to disk.
**
** This routine is only meaningful for Unix.  It is a no-op under
** windows since windows does not support hard links.
**
** On success, a handle for a previously open file is at *id is
** updated with the new directory file descriptor and SQLITE_OK is
** returned.
**
** On failure, the function returns SQLITE_CANTOPEN and leaves
** *id unchanged.
*/
int sqlite3OsOpenDirectory(
  const char *zDirname,
  OsFile *id
){
  return SQLITE_OK;
}

/*
** If the following global variable points to a string which is the
** name of a directory, then that directory will be used to store
** temporary files.
*/
char *sqlite3_temp_directory = 0;

/*
** Create a temporary file name in zBuf.  zBuf must be big enough to
** hold at least SQLITE_TEMPNAME_SIZE characters.
*/
int sqlite3OsTempFileName(char *zBuf){
  static char zChars[] =
    "abcdefghijklmnopqrstuvwxyz"
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    "0123456789";
  int i, j;
  char zTempPath[SQLITE_TEMPNAME_SIZE];
  if( sqlite3_temp_directory ){
    strncpy(zTempPath, sqlite3_temp_directory, SQLITE_TEMPNAME_SIZE-30);
    zTempPath[SQLITE_TEMPNAME_SIZE-30] = 0;
  }else if( isNT() ){
    char *zMulti;
    WCHAR zWidePath[SQLITE_TEMPNAME_SIZE];
    GetTempPathW(SQLITE_TEMPNAME_SIZE-30, zWidePath);
    zMulti = unicodeToUtf8(zWidePath);
    if( zMulti ){
      strncpy(zTempPath, zMulti, SQLITE_TEMPNAME_SIZE-30);
      zTempPath[SQLITE_TEMPNAME_SIZE-30] = 0;
      sqliteFree(zMulti);
    }
  }else{
    GetTempPathA(SQLITE_TEMPNAME_SIZE-30, zTempPath);
  }
  for(i=strlen(zTempPath); i>0 && zTempPath[i-1]=='\\'; i--){}
  zTempPath[i] = 0;
  for(;;){
    sprintf(zBuf, "%s\\"TEMP_FILE_PREFIX, zTempPath);
    j = strlen(zBuf);
    sqlite3Randomness(15, &zBuf[j]);
    for(i=0; i<15; i++, j++){
      zBuf[j] = (char)zChars[ ((unsigned char)zBuf[j])%(sizeof(zChars)-1) ];
    }
    zBuf[j] = 0;
    if( !sqlite3OsFileExists(zBuf) ) break;
  }
  TRACE2("TEMP FILENAME: %s\n", zBuf);
  return SQLITE_OK; 
}

/*
** Close a file.
*/
int sqlite3OsClose(OsFile *id){
  if( id->isOpen ){
    TRACE2("CLOSE %d\n", id->h);
    CloseHandle(id->h);
    OpenCounter(-1);
    id->isOpen = 0;
  }
  return SQLITE_OK;
}

/*
** Read data from a file into a buffer.  Return SQLITE_OK if all
** bytes were read successfully and SQLITE_IOERR if anything goes
** wrong.
*/
int sqlite3OsRead(OsFile *id, void *pBuf, int amt){
  DWORD got;
  assert( id->isOpen );
  SimulateIOError(SQLITE_IOERR);
  TRACE3("READ %d lock=%d\n", id->h, id->locktype);
  if( !ReadFile(id->h, pBuf, amt, &got, 0) ){
    got = 0;
  }
  if( got==(DWORD)amt ){
    return SQLITE_OK;
  }else{
    return SQLITE_IOERR;
  }
}

/*
** Write data from a buffer into a file.  Return SQLITE_OK on success
** or some other error code on failure.
*/
int sqlite3OsWrite(OsFile *id, const void *pBuf, int amt){
  int rc = 0;
  DWORD wrote;
  assert( id->isOpen );
  SimulateIOError(SQLITE_IOERR);
  SimulateDiskfullError;
  TRACE3("WRITE %d lock=%d\n", id->h, id->locktype);
  assert( amt>0 );
  while( amt>0 && (rc = WriteFile(id->h, pBuf, amt, &wrote, 0))!=0 && wrote>0 ){
    amt -= wrote;
    pBuf = &((char*)pBuf)[wrote];
  }
  if( !rc || amt>(int)wrote ){
    return SQLITE_FULL;
  }
  return SQLITE_OK;
}

/*
** Move the read/write pointer in a file.
*/
int sqlite3OsSeek(OsFile *id, i64 offset){
  LONG upperBits = offset>>32;
  LONG lowerBits = offset & 0xffffffff;
  DWORD rc;
  assert( id->isOpen );
#ifdef SQLITE_TEST
  if( offset ) SimulateDiskfullError
#endif
  SEEK(offset/1024 + 1);
  rc = SetFilePointer(id->h, lowerBits, &upperBits, FILE_BEGIN);
  TRACE3("SEEK %d %lld\n", id->h, offset);
  if( rc==INVALID_SET_FILE_POINTER && GetLastError()!=NO_ERROR ){
    return SQLITE_FULL;
  }
  return SQLITE_OK;
}

/*
** Make sure all writes to a particular file are committed to disk.
*/
int sqlite3OsSync(OsFile *id, int dataOnly){
  assert( id->isOpen );
  TRACE3("SYNC %d lock=%d\n", id->h, id->locktype);
  if( FlushFileBuffers(id->h) ){
    return SQLITE_OK;
  }else{
    return SQLITE_IOERR;
  }
}

/*
** Sync the directory zDirname. This is a no-op on operating systems other
** than UNIX.
*/
int sqlite3OsSyncDirectory(const char *zDirname){
  SimulateIOError(SQLITE_IOERR);
  return SQLITE_OK;
}

/*
** Truncate an open file to a specified size
*/
int sqlite3OsTruncate(OsFile *id, i64 nByte){
  LONG upperBits = nByte>>32;
  assert( id->isOpen );
  TRACE3("TRUNCATE %d %lld\n", id->h, nByte);
  SimulateIOError(SQLITE_IOERR);
  SetFilePointer(id->h, nByte, &upperBits, FILE_BEGIN);
  SetEndOfFile(id->h);
  return SQLITE_OK;
}

/*
** Determine the current size of a file in bytes
*/
int sqlite3OsFileSize(OsFile *id, i64 *pSize){
  DWORD upperBits, lowerBits;
  assert( id->isOpen );
  SimulateIOError(SQLITE_IOERR);
  lowerBits = GetFileSize(id->h, &upperBits);
  *pSize = (((i64)upperBits)<<32) + lowerBits;
  return SQLITE_OK;
}

/*
** Acquire a reader lock.
** Different API routines are called depending on whether or not this
** is Win95 or WinNT.
*/
static int getReadLock(OsFile *id){
  int res;
  if( isNT() ){
    OVERLAPPED ovlp;
    ovlp.Offset = SHARED_FIRST;
    ovlp.OffsetHigh = 0;
    ovlp.hEvent = 0;
    res = LockFileEx(id->h, LOCKFILE_FAIL_IMMEDIATELY, 0, SHARED_SIZE,0,&ovlp);
  }else{
    int lk;
    sqlite3Randomness(sizeof(lk), &lk);
    id->sharedLockByte = (lk & 0x7fffffff)%(SHARED_SIZE - 1);
    res = LockFile(id->h, SHARED_FIRST+id->sharedLockByte, 0, 1, 0);
  }
  return res;
}

/*
** Undo a readlock
*/
static int unlockReadLock(OsFile *id){
  int res;
  if( isNT() ){
    res = UnlockFile(id->h, SHARED_FIRST, 0, SHARED_SIZE, 0);
  }else{
    res = UnlockFile(id->h, SHARED_FIRST + id->sharedLockByte, 0, 1, 0);
  }
  return res;
}

#ifndef SQLITE_OMIT_PAGER_PRAGMAS
/*
** Check that a given pathname is a directory and is writable 
**
*/
int sqlite3OsIsDirWritable(char *zDirname){
  int fileAttr;
  WCHAR *zWide;
  if( zDirname==0 ) return 0;
  if( !isNT() && strlen(zDirname)>MAX_PATH ) return 0;
  zWide = utf8ToUnicode(zDirname);
  if( zWide ){
    fileAttr = GetFileAttributesW(zWide);
    sqliteFree(zWide);
  }else{
    fileAttr = GetFileAttributesA(zDirname);
  }
  if( fileAttr == 0xffffffff ) return 0;
  if( (fileAttr & FILE_ATTRIBUTE_DIRECTORY) != FILE_ATTRIBUTE_DIRECTORY ){
    return 0;
  }
  return 1;
}
#endif /* SQLITE_OMIT_PAGER_PRAGMAS */

/*
** Lock the file with the lock specified by parameter locktype - one
** of the following:
**
**     (1) SHARED_LOCK
**     (2) RESERVED_LOCK
**     (3) PENDING_LOCK
**     (4) EXCLUSIVE_LOCK
**
** Sometimes when requesting one lock state, additional lock states
** are inserted in between.  The locking might fail on one of the later
** transitions leaving the lock state different from what it started but
** still short of its goal.  The following chart shows the allowed
** transitions and the inserted intermediate states:
**
**    UNLOCKED -> SHARED
**    SHARED -> RESERVED
**    SHARED -> (PENDING) -> EXCLUSIVE
**    RESERVED -> (PENDING) -> EXCLUSIVE
**    PENDING -> EXCLUSIVE
**
** This routine will only increase a lock.  The sqlite3OsUnlock() routine
** erases all locks at once and returns us immediately to locking level 0.
** It is not possible to lower the locking level one step at a time.  You
** must go straight to locking level 0.
*/
int sqlite3OsLock(OsFile *id, int locktype){
  int rc = SQLITE_OK;    /* Return code from subroutines */
  int res = 1;           /* Result of a windows lock call */
  int newLocktype;       /* Set id->locktype to this value before exiting */
  int gotPendingLock = 0;/* True if we acquired a PENDING lock this time */

  assert( id->isOpen );
  TRACE5("LOCK %d %d was %d(%d)\n",
          id->h, locktype, id->locktype, id->sharedLockByte);

  /* If there is already a lock of this type or more restrictive on the
  ** OsFile, do nothing. Don't use the end_lock: exit path, as
  ** sqlite3OsEnterMutex() hasn't been called yet.
  */
  if( id->locktype>=locktype ){
    return SQLITE_OK;
  }

  /* Make sure the locking sequence is correct
  */
  assert( id->locktype!=NO_LOCK || locktype==SHARED_LOCK );
  assert( locktype!=PENDING_LOCK );
  assert( locktype!=RESERVED_LOCK || id->locktype==SHARED_LOCK );

  /* Lock the PENDING_LOCK byte if we need to acquire a PENDING lock or
  ** a SHARED lock.  If we are acquiring a SHARED lock, the acquisition of
  ** the PENDING_LOCK byte is temporary.
  */
  newLocktype = id->locktype;
  if( id->locktype==NO_LOCK
   || (locktype==EXCLUSIVE_LOCK && id->locktype==RESERVED_LOCK)
  ){
    int cnt = 3;
    while( cnt-->0 && (res = LockFile(id->h, PENDING_BYTE, 0, 1, 0))==0 ){
      /* Try 3 times to get the pending lock.  The pending lock might be
      ** held by another reader process who will release it momentarily.
      */
      TRACE2("could not get a PENDING lock. cnt=%d\n", cnt);
      Sleep(1);
    }
    gotPendingLock = res;
  }

  /* Acquire a shared lock
  */
  if( locktype==SHARED_LOCK && res ){
    assert( id->locktype==NO_LOCK );
    res = getReadLock(id);
    if( res ){
      newLocktype = SHARED_LOCK;
    }
  }

  /* Acquire a RESERVED lock
  */
  if( locktype==RESERVED_LOCK && res ){
    assert( id->locktype==SHARED_LOCK );
    res = LockFile(id->h, RESERVED_BYTE, 0, 1, 0);
    if( res ){
      newLocktype = RESERVED_LOCK;
    }
  }

  /* Acquire a PENDING lock
  */
  if( locktype==EXCLUSIVE_LOCK && res ){
    newLocktype = PENDING_LOCK;
    gotPendingLock = 0;
  }

  /* Acquire an EXCLUSIVE lock
  */
  if( locktype==EXCLUSIVE_LOCK && res ){
    assert( id->locktype>=SHARED_LOCK );
    res = unlockReadLock(id);
    TRACE2("unreadlock = %d\n", res);
    res = LockFile(id->h, SHARED_FIRST, 0, SHARED_SIZE, 0);
    if( res ){
      newLocktype = EXCLUSIVE_LOCK;
    }else{
      TRACE2("error-code = %d\n", GetLastError());
    }
  }

  /* If we are holding a PENDING lock that ought to be released, then
  ** release it now.
  */
  if( gotPendingLock && locktype==SHARED_LOCK ){
    UnlockFile(id->h, PENDING_BYTE, 0, 1, 0);
  }

  /* Update the state of the lock has held in the file descriptor then
  ** return the appropriate result code.
  */
  if( res ){
    rc = SQLITE_OK;
  }else{
    TRACE4("LOCK FAILED %d trying for %d but got %d\n", id->h,
           locktype, newLocktype);
    rc = SQLITE_BUSY;
  }
  id->locktype = newLocktype;
  return rc;
}

/*
** This routine checks if there is a RESERVED lock held on the specified
** file by this or any other process. If such a lock is held, return
** non-zero, otherwise zero.
*/
int sqlite3OsCheckReservedLock(OsFile *id){
  int rc;
  assert( id->isOpen );
  if( id->locktype>=RESERVED_LOCK ){
    rc = 1;
    TRACE3("TEST WR-LOCK %d %d (local)\n", id->h, rc);
  }else{
    rc = LockFile(id->h, RESERVED_BYTE, 0, 1, 0);
    if( rc ){
      UnlockFile(id->h, RESERVED_BYTE, 0, 1, 0);
    }
    rc = !rc;
    TRACE3("TEST WR-LOCK %d %d (remote)\n", id->h, rc);
  }
  return rc;
}

/*
** Lower the locking level on file descriptor id to locktype.  locktype
** must be either NO_LOCK or SHARED_LOCK.
**
** If the locking level of the file descriptor is already at or below
** the requested locking level, this routine is a no-op.
**
** It is not possible for this routine to fail if the second argument
** is NO_LOCK.  If the second argument is SHARED_LOCK then this routine
** might return SQLITE_IOERR;
*/
int sqlite3OsUnlock(OsFile *id, int locktype){
  int type;
  int rc = SQLITE_OK;
  assert( id->isOpen );
  assert( locktype<=SHARED_LOCK );
  TRACE5("UNLOCK %d to %d was %d(%d)\n", id->h, locktype,
          id->locktype, id->sharedLockByte);
  type = id->locktype;
  if( type>=EXCLUSIVE_LOCK ){
    UnlockFile(id->h, SHARED_FIRST, 0, SHARED_SIZE, 0);
    if( locktype==SHARED_LOCK && !getReadLock(id) ){
      /* This should never happen.  We should always be able to
      ** reacquire the read lock */
      rc = SQLITE_IOERR;
    }
  }
  if( type>=RESERVED_LOCK ){
    UnlockFile(id->h, RESERVED_BYTE, 0, 1, 0);
  }
  if( locktype==NO_LOCK && type>=SHARED_LOCK ){
    unlockReadLock(id);
  }
  if( type>=PENDING_LOCK ){
    UnlockFile(id->h, PENDING_BYTE, 0, 1, 0);
  }
  id->locktype = locktype;
  return rc;
}

/*
** Turn a relative pathname into a full pathname.  Return a pointer
** to the full pathname stored in space obtained from sqliteMalloc().
** The calling function is responsible for freeing this space once it
** is no longer needed.
*/
char *sqlite3OsFullPathname(const char *zRelative){
  char *zNotUsed;
  char *zFull;
  WCHAR *zWide;
  int nByte;
#ifdef __CYGWIN__
  nByte = strlen(zRelative) + MAX_PATH + 1001;
  zFull = sqliteMalloc( nByte );
  if( zFull==0 ) return 0;
  if( cygwin_conv_to_full_win32_path(zRelative, zFull) ) return 0;
#else
  zWide = utf8ToUnicode(zRelative);
  if( zWide ){
    WCHAR *zTemp, *zNotUsedW;
    nByte = GetFullPathNameW(zWide, 0, 0, &zNotUsedW) + 1;
    zTemp = sqliteMalloc( nByte*sizeof(zTemp[0]) );
    if( zTemp==0 ) return 0;
    GetFullPathNameW(zWide, nByte, zTemp, &zNotUsedW);
    sqliteFree(zWide);
    zFull = unicodeToUtf8(zTemp);
    sqliteFree(zTemp);
  }else{
    nByte = GetFullPathNameA(zRelative, 0, 0, &zNotUsed) + 1;
    zFull = sqliteMalloc( nByte*sizeof(zFull[0]) );
    if( zFull==0 ) return 0;
    GetFullPathNameA(zRelative, nByte, zFull, &zNotUsed);
  }
#endif
  return zFull;
}

#endif /* SQLITE_OMIT_DISKIO */
/***************************************************************************
** Everything above deals with file I/O.  Everything that follows deals
** with other miscellanous aspects of the operating system interface
****************************************************************************/

/*
** Get information to seed the random number generator.  The seed
** is written into the buffer zBuf[256].  The calling function must
** supply a sufficiently large buffer.
*/
int sqlite3OsRandomSeed(char *zBuf){
  /* We have to initialize zBuf to prevent valgrind from reporting
  ** errors.  The reports issued by valgrind are incorrect - we would
  ** prefer that the randomness be increased by making use of the
  ** uninitialized space in zBuf - but valgrind errors tend to worry
  ** some users.  Rather than argue, it seems easier just to initialize
  ** the whole array and silence valgrind, even if that means less randomness
  ** in the random seed.
  **
  ** When testing, initializing zBuf[] to zero is all we do.  That means
  ** that we always use the same random number sequence.* This makes the
  ** tests repeatable.
  */
  memset(zBuf, 0, 256);
  GetSystemTime((LPSYSTEMTIME)zBuf);
  return SQLITE_OK;
}

/*
** Sleep for a little while.  Return the amount of time slept.
*/
int sqlite3OsSleep(int ms){
  Sleep(ms);
  return ms;
}

/*
** Static variables used for thread synchronization
*/
static int inMutex = 0;
#ifdef SQLITE_W32_THREADS
  static CRITICAL_SECTION cs;
#endif

/*
** The following pair of routine implement mutual exclusion for
** multi-threaded processes.  Only a single thread is allowed to
** executed code that is surrounded by EnterMutex() and LeaveMutex().
**
** SQLite uses only a single Mutex.  There is not much critical
** code and what little there is executes quickly and without blocking.
*/
void sqlite3OsEnterMutex(){
#ifdef SQLITE_W32_THREADS
  static int isInit = 0;
  while( !isInit ){
    static long lock = 0;
    if( InterlockedIncrement(&lock)==1 ){
      InitializeCriticalSection(&cs);
      isInit = 1;
    }else{
      Sleep(1);
    }
  }
  EnterCriticalSection(&cs);
#endif
  assert( !inMutex );
  inMutex = 1;
}
void sqlite3OsLeaveMutex(){
  assert( inMutex );
  inMutex = 0;
#ifdef SQLITE_W32_THREADS
  LeaveCriticalSection(&cs);
#endif
}

/*
** The following variable, if set to a non-zero value, becomes the result
** returned from sqlite3OsCurrentTime().  This is used for testing.
*/
#ifdef SQLITE_TEST
int sqlite3_current_time = 0;
#endif

/*
** Find the current time (in Universal Coordinated Time).  Write the
** current time and date as a Julian Day number into *prNow and
** return 0.  Return 1 if the time and date cannot be found.
*/
int sqlite3OsCurrentTime(double *prNow){
  FILETIME ft;
  /* FILETIME structure is a 64-bit value representing the number of 
     100-nanosecond intervals since January 1, 1601 (= JD 2305813.5). 
  */
  double now;
  GetSystemTimeAsFileTime( &ft );
  now = ((double)ft.dwHighDateTime) * 4294967296.0; 
  *prNow = (now + ft.dwLowDateTime)/864000000000.0 + 2305813.5;
#ifdef SQLITE_TEST
  if( sqlite3_current_time ){
    *prNow = sqlite3_current_time/86400.0 + 2440587.5;
  }
#endif
  return 0;
}

#endif /* OS_WIN */