SQLite

static int multiplexStrlen30(const char *z){
  const char *z2 = z;
  if( z==0 ) return 0;
  while( *z2 ){ z2++; }
  return 0x3fffffff & (int)(z2 - z);
}

/*
** Create a temporary file name in zBuf.  zBuf must be big enough to
** hold at pOrigVfs->mxPathname characters.  This function departs
** from the traditional temporary name generation in the os_win
** and os_unix VFS in several ways, but is necessary so that 
** the file name is known for temporary files (like those used 
** during vacuum.)
**
** N.B. This routine assumes your underlying VFS is ok with using
** "/" as a directory seperator.  This is the default for UNIXs
** and is allowed (even mixed) for most versions of Windows.
*/
static int multiplexGetTempname(sqlite3_vfs *pOrigVfs, int nBuf, char *zBuf){
  static char zChars[] =
    "abcdefghijklmnopqrstuvwxyz"
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    "0123456789";
  int i,j;
  int attempts = 0;
  int exists = 0;
  int rc = SQLITE_ERROR;

  /* Check that the output buffer is large enough for 
  ** pVfs->mxPathname characters.
  */
  if( pOrigVfs->mxPathname <= nBuf ){
    char *zTmp = sqlite3_malloc(pOrigVfs->mxPathname);
    if( zTmp==0 ) return SQLITE_NOMEM;

    /* sqlite3_temp_directory should always be less than
    ** pVfs->mxPathname characters.
    */
    sqlite3_snprintf(pOrigVfs->mxPathname,
                     zTmp,
                     "%s/",
                     sqlite3_temp_directory ? sqlite3_temp_directory : ".");
    rc = pOrigVfs->xFullPathname(pOrigVfs, zTmp, nBuf, zBuf);
    sqlite3_free(zTmp);
    if( rc ) return rc;

    /* Check that the output buffer is large enough for the temporary file 
    ** name.
    */
    j = multiplexStrlen30(zBuf);
    if( (j + 8 + 1 + 3 + 1) <= nBuf ){
      /* Make 3 attempts to generate a unique name. */
      do {
        attempts++;
        sqlite3_randomness(8, &zBuf[j]);
        for(i=0; i<8; i++){
          unsigned char uc = (unsigned char)zBuf[j+i];
          zBuf[j+i] = (char)zChars[uc%(sizeof(zChars)-1)];
        }
        memcpy(&zBuf[j+i], ".tmp", 5);
        rc = pOrigVfs->xAccess(pOrigVfs, zBuf, SQLITE_ACCESS_EXISTS, &exists);
      } while ( (rc==SQLITE_OK) && exists && (attempts<3) );
      if( rc==SQLITE_OK && exists ){
        rc = SQLITE_ERROR;
      }
    }
  }

  return rc;
}

/* Compute the filename for the iChunk-th chunk
*/
static int multiplexSubFilename(multiplexGroup *pGroup, int iChunk){
  if( iChunk>=pGroup->nReal ){
    struct multiplexReal *p;
    p = sqlite3_realloc(pGroup->aReal, (iChunk+1)*sizeof(*p));
    if( p==0 ){
      return SQLITE_NOMEM;
    }
    memset(&p[pGroup->nReal], 0, sizeof(p[0])*(iChunk+1-pGroup->nReal));
    pGroup->aReal = p;
    pGroup->nReal = iChunk+1;
  }
  if( pGroup->aReal[iChunk].z==0 ){
    char *z;
    int n = pGroup->nName;
    pGroup->aReal[iChunk].z = z = sqlite3_malloc( n+4 );
    if( z==0 ){
      return SQLITE_NOMEM;
    }
    memcpy(z, pGroup->zName, n+1);

  sqlite3_vfs *pOrigVfs = gMultiplex.pOrigVfs;   /* Real VFS */
  int nName;
  int sz;
  char *zToFree = 0;

  UNUSED_PARAMETER(pVfs);
  memset(pConn, 0, pVfs->szOsFile);


  /* We need to create a group structure and manage
  ** access to this group of files.
  */
  multiplexEnter();
  pMultiplexOpen = (multiplexConn*)pConn;

  /* If the second argument to this function is NULL, generate a 
  ** temporary file name to use.  This will be handled by the
  ** original xOpen method.  We just need to allocate space for
  ** it.
  */
  if( !zName ){
    zName = zToFree = sqlite3_malloc( pOrigVfs->mxPathname + 10 );
    if( zName==0 ){
      rc = SQLITE_NOMEM;
    }else{
      rc = multiplexGetTempname(pOrigVfs, pOrigVfs->mxPathname, zToFree);
    }
  }

  if( rc==SQLITE_OK ){
    /* allocate space for group */
    nName = multiplexStrlen30(zName);
    sz = sizeof(multiplexGroup)                             /* multiplexGroup */
       + nName + 1;                                         /* zName */
    pGroup = sqlite3_malloc( sz );
    if( pGroup==0 ){
      rc = SQLITE_NOMEM;
    }
  }

  if( rc==SQLITE_OK ){
    /* assign pointers to extra space allocated */
    char *p = (char *)&pGroup[1];
    pMultiplexOpen->pGroup = pGroup;
    memset(pGroup, 0, sz);
    pGroup->bEnabled = -1;
    pGroup->szChunk = SQLITE_MULTIPLEX_CHUNK_SIZE;



    if( flags & SQLITE_OPEN_URI ){
      const char *zChunkSize;
      zChunkSize = sqlite3_uri_parameter(zName, "chunksize");
      if( zChunkSize ){
        unsigned int n = 0;
        int i;
        for(i=0; zChunkSize[i]>='0' && zChunkSize[i]<='9'; i++){
          n = n*10 + zChunkSize[i] - '0';
        }
        if( n>0 ){
          pGroup->szChunk = (n+0xffff)&~0xffff;
        }else{
          /* A zero or negative chunksize disabled the multiplexor */
          pGroup->bEnabled = 0;
        }
      }
    }
    pGroup->zName = p;
    /* save off base filename, name length, and original open flags  */
    memcpy(pGroup->zName, zName, nName+1);
    pGroup->nName = nName;

    pGroup->flags = flags;
    rc = multiplexSubFilename(pGroup, 1);
    if( rc==SQLITE_OK ){
      pSubOpen = multiplexSubOpen(pGroup, 0, &rc, pOutFlags);
    }
    if( pSubOpen ){
      int exists, rc2, rc3;

    for(i=0; rc==SQLITE_OK; i++){
      sqlite3_file *pSubOpen = 0;
      int exists = 0;
      rc = multiplexSubFilename(pGroup, i);
      if( rc!=SQLITE_OK ) break;
      if( pGroup->nReal>i && pGroup->aReal[i].p!=0 ){
        exists = 1;
      }else{
        const char *zReal = pGroup->aReal[i].z;
        rc = pOrigVfs->xAccess(pOrigVfs, zReal, SQLITE_ACCESS_EXISTS, &exists);
      }
      if( exists==0 ){
        /* stop at first "gap" or IO error. */
        break;
      }

  UNUSED_PARAMETER(objv);

  pResult = Tcl_NewObj();
  multiplexEnter();
  for(pGroup=gMultiplex.pGroups; pGroup; pGroup=pGroup->pNext){
    pGroupTerm = Tcl_NewObj();


    pGroup->zName[pGroup->nName] = '\0';
    Tcl_ListObjAppendElement(interp, pGroupTerm,
          Tcl_NewStringObj(pGroup->zName, -1));



    Tcl_ListObjAppendElement(interp, pGroupTerm,
          Tcl_NewIntObj(pGroup->nName));
    Tcl_ListObjAppendElement(interp, pGroupTerm,
          Tcl_NewIntObj(pGroup->flags));

    /* count number of chunks with open handles */
    for(i=0; i<pGroup->nReal; i++){