SQLite

**
** lockMask might contain multiple bits but all bits are guaranteed
** to be contiguous.
**
** Locks block if the UNIX_SHM_MUTEX bit is set and are non-blocking
** otherwise.
*/
static int unixShmSystemLock(
  unixShmFile *pFile,   /* Apply locks to this open shared-memory segment */
  int lockType,         /* F_UNLCK, F_RDLCK, or F_WRLCK */
  u8 lockMask           /* Which bytes to lock or unlock */
){
  struct flock f;       /* The posix advisory locking structure */
  int lockOp;           /* The opcode for fcntl() */
  int i;                /* Offset into the locking byte range */
  int rc;               /* Result code form fcntl() */
  u8 mask;              /* Mask of bits in lockMask */

  /* Access to the unixShmFile object is serialized by the caller */
  assert( sqlite3_mutex_held(pFile->mutex) );

  /* Initialize the locking parameters */
  memset(&f, 0, sizeof(f));
  f.l_type = lockType;
  f.l_whence = SEEK_SET;
  if( (lockMask & UNIX_SHM_MUTEX)!=0 && lockType!=F_UNLCK ){
    lockOp = F_SETLKW;

  unixShmFile *pFile,   /* The underlying shared-memory file */
  unixShm *p,           /* The connection to be unlocked */
  u8 unlockMask         /* Mask of locks to be unlocked */
){
  int rc;      /* Result code */
  unixShm *pX; /* For looping over all sibling connections */
  u8 allMask;  /* Union of locks held by connections other than "p" */

  /* Access to the unixShmFile object is serialized by the caller */
  assert( sqlite3_mutex_held(pFile->mutex) );

  /* We never try to unlock locks that we do not hold */
  assert( ((p->exclMask|p->sharedMask) & unlockMask)==unlockMask );

  /* Compute locks held by sibling connections */
  for(pX=pFile->pFirst; pX; pX=pX->pNext){
    if( pX==p ) continue;
    assert( (pX->exclMask & unlockMask)==0 );
    allMask |= pX->sharedMask;
  }

  /* Unlock the system-level locks */
  if( (unlockMask & allMask)!=unlockMask ){
    rc = unixShmSystemLock(pFile, F_UNLCK, unlockMask & ~allMask);
  }else{
    rc = SQLITE_OK;
  }

  /* Undo the local locks */
  if( rc==SQLITE_OK ){
    p->exclMask &= ~unlockMask;

  unixShmFile *pFile,   /* The underlying shared-memory file */
  unixShm *p,           /* The connection to get the shared locks */
  u8 readMask           /* Mask of shared locks to be acquired */
){
  int rc;        /* Result code */
  unixShm *pX;   /* For looping over all sibling connections */
  u8 allShared;  /* Union of locks held by connections other than "p" */

  /* Access to the unixShmFile object is serialized by the caller */
  assert( sqlite3_mutex_held(pFile->mutex) );

  /* Find out which shared locks are already held by sibling connections.
  ** If any sibling already holds an exclusive lock, go ahead and return
  ** SQLITE_BUSY.
  */
  for(pX=pFile->pFirst; pX; pX=pX->pNext){
    if( pX==p ) continue;
    if( (pX->exclMask & readMask)!=0 ) return SQLITE_BUSY;
    allShared |= pX->sharedMask;
  }

  /* Get shared locks at the system level, if necessary */
  if( (~allShared) & readMask ){
    rc = unixShmSystemLock(pFile, F_RDLCK, readMask);
  }else{
    rc = SQLITE_OK;
  }

  /* Get the local shared locks */
  if( rc==SQLITE_OK ){
    p->sharedMask |= readMask;

static int unixShmExclusiveLock(
  unixShmFile *pFile,    /* The underlying shared-memory file */
  unixShm *p,            /* The connection to get the exclusive locks */
  u8 writeMask           /* Mask of exclusive locks to be acquired */
){
  int rc;        /* Result code */
  unixShm *pX;   /* For looping over all sibling connections */

  /* Access to the unixShmFile object is serialized by the caller */
  assert( sqlite3_mutex_held(pFile->mutex) );

  /* Make sure no sibling connections hold locks that will block this
  ** lock.  If any do, return SQLITE_BUSY right away.
  */
  for(pX=pFile->pFirst; pX; pX=pX->pNext){
    if( pX==p ) continue;
    if( (pX->exclMask & writeMask)!=0 ) return SQLITE_BUSY;
    if( (pX->sharedMask & writeMask)!=0 ) return SQLITE_BUSY;
  }

  /* Get the exclusive locks at the system level.  Then if successful
  ** also mark the local connection as being locked.
  */
  rc = unixShmSystemLock(pFile, F_WRLCK, writeMask);
  if( rc==SQLITE_OK ){
    p->sharedMask &= ~writeMask;
    p->exclMask |= writeMask;
  }
  return rc;
}

** the file must be truncated to zero length or have its header cleared.
*/
static int unixShmOpen(
  sqlite3_vfs *pVfs,    /* The VFS */
  const char *zName,    /* Name of file to mmap */
  sqlite3_shm **pShm    /* Write the unixShm object created here */
){
  struct unixShm *p = 0;             /* The connection to be opened */
  struct unixShmFile *pFile = 0;     /* The underlying mmapped file */
  int rc;                            /* Result code */
  struct unixFileId fid;             /* Unix file identifier */
  struct stat sStat;                 /* Result from stat() an fstat() */

  /* Allocate space for the new sqlite3_shm object */
  p = sqlite3_malloc( sizeof(*p) );
  if( p==0 ) return SQLITE_NOMEM;
  memset(p, 0, sizeof(*p));

  /* Look to see if there is an existing unixShmFile that can be used.
  ** If no matching unixShmFile currently exists, create a new one.
  */
  unixEnterMutex();
  rc = stat(zName, &sStat);
  if( rc==0 ){
    memset(&fid, 0, sizeof(fid));
    fid.dev = sStat.st_dev;
    fid.ino = sStat.st_ino;
    for(pFile = unixShmFileList; pFile; pFile=pFile->pNext){

      rc = SQLITE_CANTOPEN;
      goto shm_open_err;
    }
    pFile->fid.dev = sStat.st_dev;
    pFile->fid.ino = sStat.st_ino;
    pFile->size = (int)sStat.st_size;
    pFile->size = (pFile->size/SQLITE_UNIX_SHM_INCR)*SQLITE_UNIX_SHM_INCR;

    /* Check to see if another process is holding the dead-man switch.
    ** If not, truncate the file to zero length. 
    */
    if( unixShmSystemLock(pFile, F_WRLCK, UNIX_SHM_MUTEX) ){
      rc = SQLITE_IOERR_LOCK;
      goto shm_open_err;
    }
    if( unixShmSystemLock(pFile, F_WRLCK, UNIX_SHM_DMS)==SQLITE_OK ){
      if( ftruncate(pFile->h, 0) ){

        rc = SQLITE_IOERR;
        goto shm_open_err;
      }
      pFile->size = 0;
    }
    rc = unixShmSystemLock(pFile, F_RDLCK, UNIX_SHM_DMS);
    if( rc ) goto shm_open_err;
    unixShmSystemLock(pFile, F_UNLCK, UNIX_SHM_MUTEX);
  }

  /* Make the new connection a child of the unixShmFile */
  p->pFile = pFile;
  p->pNext = pFile->pFirst;
#ifdef SQLITE_DEBUG
  p->id = pFile->nextShmId++;
#endif
  pFile->pFirst = p;
  pFile->nRef++;
  *pShm = (sqlite3_shm*)p;
  unixLeaveMutex();
  return SQLITE_OK;

  /* Jump here on any error */
shm_open_err:
  unixShmPurge();
  sqlite3_free(p);
  sqlite3_free(pFile);
  *pShm = 0;
  unixLeaveMutex();
  return rc;