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tclIOGT.c

/*
 * tclIOGT.c --
 *
 *    Implements a generic transformation exposing the underlying API at the
 *    script level. Contributed by Andreas Kupries.
 *
 * Copyright (c) 2000 Ajuba Solutions
 * Copyright (c) 1999-2000 Andreas Kupries (a.kupries@westend.com)
 *
 * See the file "license.terms" for information on usage and redistribution of
 * this file, and for a DISCLAIMER OF ALL WARRANTIES.
 *
 * CVS: $Id: tclIOGT.c,v 1.20 2007/12/13 15:23:18 dgp Exp $
 */

#include "tclInt.h"
#include "tclIO.h"

/*
 * Forward declarations of internal procedures. First the driver procedures of
 * the transformation.
 */

static int        TransformBlockModeProc(ClientData instanceData,
                      int mode);
static int        TransformCloseProc(ClientData instanceData,
                      Tcl_Interp *interp);
static int        TransformInputProc(ClientData instanceData, char *buf,
                      int toRead, int *errorCodePtr);
static int        TransformOutputProc(ClientData instanceData,
                      const char *buf, int toWrite, int *errorCodePtr);
static int        TransformSeekProc(ClientData instanceData, long offset,
                      int mode, int *errorCodePtr);
static int        TransformSetOptionProc(ClientData instanceData,
                      Tcl_Interp *interp, const char *optionName,
                      const char *value);
static int        TransformGetOptionProc(ClientData instanceData,
                      Tcl_Interp *interp, const char *optionName,
                      Tcl_DString *dsPtr);
static void       TransformWatchProc(ClientData instanceData, int mask);
static int        TransformGetFileHandleProc(ClientData instanceData,
                      int direction, ClientData *handlePtr);
static int        TransformNotifyProc(ClientData instanceData, int mask);
static Tcl_WideInt      TransformWideSeekProc(ClientData instanceData,
                      Tcl_WideInt offset, int mode, int *errorCodePtr);

/*
 * Forward declarations of internal procedures. Secondly the procedures for
 * handling and generating fileeevents.
 */

static void       TransformChannelHandlerTimer(ClientData clientData);

/*
 * Forward declarations of internal procedures. Third, helper procedures
 * encapsulating essential tasks.
 */

typedef struct TransformChannelData TransformChannelData;

static int        ExecuteCallback(TransformChannelData *ctrl,
                      Tcl_Interp *interp, unsigned char *op,
                      unsigned char *buf, int bufLen, int transmit,
                      int preserve);

/*
 * Action codes to give to 'ExecuteCallback' (argument 'transmit'), telling
 * the procedure what to do with the result of the script it calls.
 */

#define TRANSMIT_DONT   0     /* No transfer to do. */
#define TRANSMIT_DOWN   1     /* Transfer to the underlying channel. */
#define TRANSMIT_SELF   2     /* Transfer into our channel. */
#define TRANSMIT_IBUF   3     /* Transfer to internal input buffer. */
#define TRANSMIT_NUM    4     /* Transfer number to 'maxRead'. */

/*
 * Codes for 'preserve' of 'ExecuteCallback'.
 */

#define P_PRESERVE      1
#define P_NO_PRESERVE   0

/*
 * Strings for the action codes delivered to the script implementing a
 * transformation. Argument 'op' of 'ExecuteCallback'.
 */

#define A_CREATE_WRITE  (UCHARP("create/write"))
#define A_DELETE_WRITE  (UCHARP("delete/write"))
#define A_FLUSH_WRITE   (UCHARP("flush/write"))
#define A_WRITE         (UCHARP("write"))

#define A_CREATE_READ   (UCHARP("create/read"))
#define A_DELETE_READ   (UCHARP("delete/read"))
#define A_FLUSH_READ    (UCHARP("flush/read"))
#define A_READ          (UCHARP("read"))

#define A_QUERY_MAXREAD (UCHARP("query/maxRead"))
#define A_CLEAR_READ    (UCHARP("clear/read"))

/*
 * Management of a simple buffer.
 */

typedef struct ResultBuffer ResultBuffer;

static inline void      ResultClear(ResultBuffer *r);
static inline void      ResultInit(ResultBuffer *r);
static inline int ResultEmpty(ResultBuffer *r);
static inline int ResultCopy(ResultBuffer *r, unsigned char *buf,
                      size_t toRead);
static inline void      ResultAdd(ResultBuffer *r, unsigned char *buf,
                      size_t toWrite);

/*
 * This structure describes the channel type structure for Tcl-based
 * transformations.
 */

static Tcl_ChannelType transformChannelType = {
    "transform",        /* Type name. */
    TCL_CHANNEL_VERSION_5,    /* v5 channel */
    TransformCloseProc,       /* Close proc. */
    TransformInputProc,       /* Input proc. */
    TransformOutputProc,      /* Output proc. */
    TransformSeekProc,        /* Seek proc. */
    TransformSetOptionProc,   /* Set option proc. */
    TransformGetOptionProc,   /* Get option proc. */
    TransformWatchProc,       /* Initialize notifier. */
    TransformGetFileHandleProc,     /* Get OS handles out of channel. */
    NULL,               /* close2proc */
    TransformBlockModeProc,   /* Set blocking/nonblocking mode.*/
    NULL,               /* Flush proc. */
    TransformNotifyProc,      /* Handling of events bubbling up. */
    TransformWideSeekProc,    /* Wide seek proc. */
    NULL,               /* Thread action. */
    NULL,               /* Truncate. */
};

/*
 * Possible values for 'flags' field in control structure, see below.
 */

#define CHANNEL_ASYNC (1<<0)  /* Non-blocking mode. */

/*
 * Definition of the structure containing the information about the internal
 * input buffer.
 */

struct ResultBuffer {
    unsigned char *buf;       /* Reference to the buffer area. */
    size_t allocated;         /* Allocated size of the buffer area. */
    size_t used;        /* Number of bytes in the buffer, no more than
                         * number allocated. */
};

/*
 * Additional bytes to allocate during buffer expansion.
 */

#define INCREMENT 512

/*
 * Number of milliseconds to wait before firing an event to flush out
 * information waiting in buffers (fileevent support).
 */

#define FLUSH_DELAY     5

/*
 * Convenience macro to make some casts easier to use.
 */

#define UCHARP(x) ((unsigned char *) (x))

/*
 * Definition of a structure used by all transformations generated here to
 * maintain their local state.
 */

struct TransformChannelData {
    /*
     * General section. Data to integrate the transformation into the channel
     * system.
     */

    Tcl_Channel self;         /* Our own Channel handle. */
    int readIsFlushed;        /* Flag to note whether in.flushProc was
                         * called or not. */
    int flags;                /* Currently CHANNEL_ASYNC or zero. */
    int watchMask;            /* Current watch/event/interest mask. */
    int mode;                 /* Mode of parent channel, OR'ed combination
                         * of TCL_READABLE, TCL_WRITABLE. */
    Tcl_TimerToken timer;     /* Timer for automatic flushing of information
                         * sitting in an internal buffer. Required for
                         * full fileevent support. */

    /*
     * Transformation specific data.
     */

    int maxRead;        /* Maximum allowed number of bytes to read, as
                         * given to us by the Tcl script implementing
                         * the transformation. */
    Tcl_Interp *interp;       /* Reference to the interpreter which created
                         * the transformation. Used to execute the
                         * code below. */
    Tcl_Obj *command;         /* Tcl code to execute for a buffer */
    ResultBuffer result;      /* Internal buffer used to store the result of
                         * a transformation of incoming data. Also
                         * serves as buffer of all data not yet
                         * consumed by the reader. */
};

/*
 *----------------------------------------------------------------------
 *
 * TclChannelTransform --
 *
 *    Implements the Tcl "testchannel transform" debugging command. This is
 *    part of the testing environment. This sets up a tcl script (cmdObjPtr)
 *    to be used as a transform on the channel.
 *
 * Results:
 *    A standard Tcl result.
 *
 * Side effects:
 *    None.
 *
 *----------------------------------------------------------------------
 */

      /* ARGSUSED */
int
TclChannelTransform(
    Tcl_Interp *interp,       /* Interpreter for result. */
    Tcl_Channel chan,         /* Channel to transform. */
    Tcl_Obj *cmdObjPtr)       /* Script to use for transform. */
{
    Channel *chanPtr;         /* The actual channel. */
    ChannelState *statePtr;   /* State info for channel. */
    int mode;                 /* Read/write mode of the channel. */
    TransformChannelData *dataPtr;
    Tcl_DString ds;

    if (chan == NULL) {
      return TCL_ERROR;
    }

    chanPtr = (Channel *) chan;
    statePtr = chanPtr->state;
    chanPtr = statePtr->topChanPtr;
    chan = (Tcl_Channel) chanPtr;
    mode = (statePtr->flags & (TCL_READABLE|TCL_WRITABLE));

    /*
     * Now initialize the transformation state and stack it upon the specified
     * channel. One of the necessary things to do is to retrieve the blocking
     * regime of the underlying channel and to use the same for us too.
     */

    dataPtr = (TransformChannelData *) ckalloc(sizeof(TransformChannelData));

    Tcl_DStringInit(&ds);
    Tcl_GetChannelOption(interp, chan, "-blocking", &ds);
    dataPtr->readIsFlushed = 0;
    dataPtr->flags = 0;
    if (ds.string[0] == '0') {
      dataPtr->flags |= CHANNEL_ASYNC;
    }
    Tcl_DStringFree(&ds);

    dataPtr->self = chan;
    dataPtr->watchMask = 0;
    dataPtr->mode = mode;
    dataPtr->timer = NULL;
    dataPtr->maxRead = 4096;  /* Initial value not relevant. */
    dataPtr->interp = interp;
    dataPtr->command = cmdObjPtr;
    Tcl_IncrRefCount(dataPtr->command);

    ResultInit(&dataPtr->result);

    dataPtr->self = Tcl_StackChannel(interp, &transformChannelType, dataPtr,
          mode, chan);
    if (dataPtr->self == NULL) {
      Tcl_AppendResult(interp, "\nfailed to stack channel \"",
            Tcl_GetChannelName(chan), "\"", NULL);
      Tcl_DecrRefCount(dataPtr->command);
      ResultClear(&dataPtr->result);
      ckfree((char *) dataPtr);
      return TCL_ERROR;
    }

    /*
     * At last initialize the transformation at the script level.
     */

    if ((dataPtr->mode & TCL_WRITABLE) && ExecuteCallback(dataPtr, NULL,
          A_CREATE_WRITE, NULL, 0, TRANSMIT_DONT, P_NO_PRESERVE) != TCL_OK){
      Tcl_UnstackChannel(interp, chan);
      return TCL_ERROR;
    }

    if ((dataPtr->mode & TCL_READABLE) && ExecuteCallback(dataPtr, NULL,
          A_CREATE_READ, NULL, 0, TRANSMIT_DONT, P_NO_PRESERVE) != TCL_OK) {
      ExecuteCallback(dataPtr, NULL, A_DELETE_WRITE, NULL, 0, TRANSMIT_DONT,
            P_NO_PRESERVE);
      Tcl_UnstackChannel(interp, chan);
      return TCL_ERROR;
    }

    return TCL_OK;
}

/*
 *----------------------------------------------------------------------
 *
 * ExecuteCallback --
 *
 *    Executes the defined callback for buffer and operation.
 *
 * Side effects:
 *    As of the executed tcl script.
 *
 * Result:
 *    A standard TCL error code. In case of an error a message is left in
 *    the result area of the specified interpreter.
 *
 *----------------------------------------------------------------------
 */

static int
ExecuteCallback(
    TransformChannelData *dataPtr,
                        /* Transformation with the callback. */
    Tcl_Interp *interp,       /* Current interpreter, possibly NULL. */
    unsigned char *op,        /* Operation invoking the callback. */
    unsigned char *buf,       /* Buffer to give to the script. */
    int bufLen,               /* And its length. */
    int transmit,       /* Flag, determines whether the result of the
                         * callback is sent to the underlying channel
                         * or not. */
    int preserve)       /* Flag. If true the procedure will preserve
                         * the result state of all accessed
                         * interpreters. */
{
    Tcl_Obj *resObj;          /* See below, switch (transmit). */
    int resLen;
    unsigned char *resBuf;
    Tcl_InterpState state = NULL;
    int res = TCL_OK;
    Tcl_Obj *command = Tcl_DuplicateObj(dataPtr->command);

    /*
     * Step 1, create the complete command to execute. Do this by appending
     * operation and buffer to operate upon to a copy of the callback
     * definition. We *cannot* create a list containing 3 objects and then use
     * 'Tcl_EvalObjv', because the command may contain additional prefixed
     * arguments. Feather's curried commands would come in handy here.
     */

    if (preserve == P_PRESERVE) {
      state = Tcl_SaveInterpState(dataPtr->interp, res);
    }

    Tcl_IncrRefCount(command);
    res = Tcl_ListObjAppendElement(dataPtr->interp, command,
          Tcl_NewStringObj((char *) op, -1));
    if (res != TCL_OK) {
      goto cleanup;
    }

    /*
     * Use a byte-array to prevent the misinterpretation of binary data coming
     * through as UTF while at the tcl level.
     */

    res = Tcl_ListObjAppendElement(dataPtr->interp, command,
          Tcl_NewByteArrayObj(buf, bufLen));
    if (res != TCL_OK) {
      goto cleanup;
    }

    /*
     * Step 2, execute the command at the global level of the interpreter used
     * to create the transformation. Destroy the command afterward. If an
     * error occured and the current interpreter is defined and not equal to
     * the interpreter for the callback, then copy the error message into
     * current interpreter. Don't copy if in preservation mode.
     */

    res = Tcl_EvalObjEx(dataPtr->interp, command, TCL_EVAL_GLOBAL);
    Tcl_DecrRefCount(command);
    command = NULL;

    if ((res != TCL_OK) && (interp != NULL) && (dataPtr->interp != interp)
          && (preserve == P_NO_PRESERVE)) {
      Tcl_SetObjResult(interp, Tcl_GetObjResult(dataPtr->interp));
      return res;
    }

    /*
     * Step 3, transmit a possible conversion result to the underlying
     * channel, or ourselves.
     */

    switch (transmit) {
    case TRANSMIT_DONT:
      /* nothing to do */
      break;

    case TRANSMIT_DOWN:
      resObj = Tcl_GetObjResult(dataPtr->interp);
      resBuf = Tcl_GetByteArrayFromObj(resObj, &resLen);
      Tcl_WriteRaw(Tcl_GetStackedChannel(dataPtr->self), (char *) resBuf,
            resLen);
      break;

    case TRANSMIT_SELF:
      resObj = Tcl_GetObjResult(dataPtr->interp);
      resBuf = Tcl_GetByteArrayFromObj(resObj, &resLen);
      Tcl_WriteRaw(dataPtr->self, (char *) resBuf, resLen);
      break;

    case TRANSMIT_IBUF:
      resObj = Tcl_GetObjResult(dataPtr->interp);
      resBuf = Tcl_GetByteArrayFromObj(resObj, &resLen);
      ResultAdd(&dataPtr->result, resBuf, resLen);
      break;

    case TRANSMIT_NUM:
      /*
       * Interpret result as integer number.
       */

      resObj = Tcl_GetObjResult(dataPtr->interp);
      TclGetIntFromObj(dataPtr->interp, resObj, &dataPtr->maxRead);
      break;
    }

    Tcl_ResetResult(dataPtr->interp);
    if (preserve == P_PRESERVE) {
      (void) Tcl_RestoreInterpState(dataPtr->interp, state);
    }
    return res;

  cleanup:
    if (preserve == P_PRESERVE) {
      (void) Tcl_RestoreInterpState(dataPtr->interp, state);
    }
    if (command != NULL) {
      Tcl_DecrRefCount(command);
    }
    return res;
}

/*
 *----------------------------------------------------------------------
 *
 * TransformBlockModeProc --
 *
 *    Trap handler. Called by the generic IO system during option processing
 *    to change the blocking mode of the channel.
 *
 * Side effects:
 *    Forwards the request to the underlying channel.
 *
 * Result:
 *    0 if successful, errno when failed.
 *
 *----------------------------------------------------------------------
 */

static int
TransformBlockModeProc(
    ClientData instanceData,  /* State of transformation. */
    int mode)                 /* New blocking mode. */
{
    TransformChannelData *dataPtr = instanceData;

    if (mode == TCL_MODE_NONBLOCKING) {
      dataPtr->flags |= CHANNEL_ASYNC;
    } else {
      dataPtr->flags &= ~CHANNEL_ASYNC;
    }
    return 0;
}

/*
 *----------------------------------------------------------------------
 *
 * TransformCloseProc --
 *
 *    Trap handler. Called by the generic IO system during destruction of
 *    the transformation channel.
 *
 * Side effects:
 *    Releases the memory allocated in 'Tcl_TransformObjCmd'.
 *
 * Result:
 *    None.
 *
 *----------------------------------------------------------------------
 */

static int
TransformCloseProc(
    ClientData instanceData,
    Tcl_Interp *interp)
{
    TransformChannelData *dataPtr = instanceData;

    /*
     * Important: In this procedure 'dataPtr->self' already points to the
     * underlying channel.
     *
     * There is no need to cancel an existing channel handler, this is already
     * done. Either by 'Tcl_UnstackChannel' or by the general cleanup in
     * 'Tcl_Close'.
     *
     * But we have to cancel an active timer to prevent it from firing on the
     * removed channel.
     */

    if (dataPtr->timer != NULL) {
      Tcl_DeleteTimerHandler(dataPtr->timer);
      dataPtr->timer = NULL;
    }

    /*
     * Now flush data waiting in internal buffers to output and input. The
     * input must be done despite the fact that there is no real receiver for
     * it anymore. But the scripts might have sideeffects other parts of the
     * system rely on (f.e. signaling the close to interested parties).
     */

    if (dataPtr->mode & TCL_WRITABLE) {
      ExecuteCallback(dataPtr, interp, A_FLUSH_WRITE, NULL, 0,
            TRANSMIT_DOWN, P_PRESERVE);
    }

    if ((dataPtr->mode & TCL_READABLE) && !dataPtr->readIsFlushed) {
      dataPtr->readIsFlushed = 1;
      ExecuteCallback(dataPtr, interp, A_FLUSH_READ, NULL, 0, TRANSMIT_IBUF,
            P_PRESERVE);
    }

    if (dataPtr->mode & TCL_WRITABLE) {
      ExecuteCallback(dataPtr, interp, A_DELETE_WRITE, NULL, 0,
            TRANSMIT_DONT, P_PRESERVE);
    }
    if (dataPtr->mode & TCL_READABLE) {
      ExecuteCallback(dataPtr, interp, A_DELETE_READ, NULL, 0,
            TRANSMIT_DONT, P_PRESERVE);
    }

    /*
     * General cleanup.
     */

    ResultClear(&dataPtr->result);
    Tcl_DecrRefCount(dataPtr->command);
    ckfree((char *) dataPtr);
    return TCL_OK;
}

/*
 *----------------------------------------------------------------------
 *
 * TransformInputProc --
 *
 *    Called by the generic IO system to convert read data.
 *
 * Side effects:
 *    As defined by the conversion.
 *
 * Result:
 *    A transformed buffer.
 *
 *----------------------------------------------------------------------
 */

static int
TransformInputProc(
    ClientData instanceData,
    char *buf,
    int toRead,
    int *errorCodePtr)
{
    TransformChannelData *dataPtr = instanceData;
    int gotBytes, read, copied;
    Tcl_Channel downChan;

    /*
     * Should assert(dataPtr->mode & TCL_READABLE);
     */

    if (toRead == 0) {
      /*
       * Catch a no-op.
       */
      return 0;
    }

    gotBytes = 0;
    downChan = Tcl_GetStackedChannel(dataPtr->self);

    while (toRead > 0) {
      /*
       * Loop until the request is satisfied (or no data is available from
       * below, possibly EOF).
       */

      copied = ResultCopy(&dataPtr->result, UCHARP(buf), toRead);
      toRead -= copied;
      buf += copied;
      gotBytes += copied;

      if (toRead == 0) {
          /*
           * The request was completely satisfied from our buffers. We can
           * break out of the loop and return to the caller.
           */

          return gotBytes;
      }

      /*
       * Length (dataPtr->result) == 0, toRead > 0 here. Use the incoming
       * 'buf'! as target to store the intermediary information read from
       * the underlying channel.
       *
       * Ask the tcl level how much data it allows us to read from the
       * underlying channel. This feature allows the transform to signal EOF
       * upstream although there is none downstream. Useful to control an
       * unbounded 'fcopy', either through counting bytes, or by pattern
       * matching.
       */

      ExecuteCallback(dataPtr, NULL, A_QUERY_MAXREAD, NULL, 0,
            TRANSMIT_NUM /* -> maxRead */, P_PRESERVE);

      if (dataPtr->maxRead >= 0) {
          if (dataPtr->maxRead < toRead) {
            toRead = dataPtr->maxRead;
          }
      } /* else: 'maxRead < 0' == Accept the current value of toRead. */
      if (toRead <= 0) {
          return gotBytes;
      }

      /*
       * Get bytes from the underlying channel.
       */

      read = Tcl_ReadRaw(downChan, buf, toRead);
      if (read < 0) {
          /*
           * Report errors to caller. EAGAIN is a special situation. If we
           * had some data before we report that instead of the request to
           * re-try.
           */

          if ((Tcl_GetErrno() == EAGAIN) && (gotBytes > 0)) {
            return gotBytes;
          }

          *errorCodePtr = Tcl_GetErrno();
          return -1;
      } else if (read == 0) {
          /*
           * Check wether we hit on EOF in the underlying channel or not. If
           * not differentiate between blocking and non-blocking modes. In
           * non-blocking mode we ran temporarily out of data. Signal this
           * to the caller via EWOULDBLOCK and error return (-1). In the
           * other cases we simply return what we got and let the caller
           * wait for more. On the other hand, if we got an EOF we have to
           * convert and flush all waiting partial data.
           */

          if (!Tcl_Eof(downChan)) {
            if ((gotBytes == 0) && (dataPtr->flags & CHANNEL_ASYNC)) {
                *errorCodePtr = EWOULDBLOCK;
                return -1;
            }
            return gotBytes;
          }

          if (dataPtr->readIsFlushed) {
            /*
             * Already flushed, nothing to do anymore.
             */

            return gotBytes;
          }

          dataPtr->readIsFlushed = 1;
          ExecuteCallback(dataPtr, NULL, A_FLUSH_READ, NULL, 0,
                TRANSMIT_IBUF, P_PRESERVE);

          if (ResultEmpty(&dataPtr->result)) {
            /*
             * We had nothing to flush.
             */

            return gotBytes;
          }

          continue;           /* at: while (toRead > 0) */
      } /* read == 0 */

      /*
       * Transform the read chunk and add the result to our read buffer
       * (dataPtr->result).
       */

      if (ExecuteCallback(dataPtr, NULL, A_READ, UCHARP(buf), read,
            TRANSMIT_IBUF, P_PRESERVE) != TCL_OK) {
          *errorCodePtr = EINVAL;
          return -1;
      }
    } /* while toRead > 0 */

    return gotBytes;
}

/*
 *----------------------------------------------------------------------
 *
 * TransformOutputProc --
 *
 *    Called by the generic IO system to convert data waiting to be written.
 *
 * Side effects:
 *    As defined by the transformation.
 *
 * Result:
 *    A transformed buffer.
 *
 *----------------------------------------------------------------------
 */

static int
TransformOutputProc(
    ClientData instanceData,
    const char *buf,
    int toWrite,
    int *errorCodePtr)
{
    TransformChannelData *dataPtr = instanceData;

    /*
     * Should assert(dataPtr->mode & TCL_WRITABLE);
     */

    if (toWrite == 0) {
      /*
       * Catch a no-op.
       */

      return 0;
    }

    if (ExecuteCallback(dataPtr, NULL, A_WRITE, UCHARP(buf), toWrite,
          TRANSMIT_DOWN, P_NO_PRESERVE) != TCL_OK) {
      *errorCodePtr = EINVAL;
      return -1;
    }

    return toWrite;
}

/*
 *----------------------------------------------------------------------
 *
 * TransformSeekProc --
 *
 *    This procedure is called by the generic IO level to move the access
 *    point in a channel.
 *
 * Side effects:
 *    Moves the location at which the channel will be accessed in future
 *    operations. Flushes all transformation buffers, then forwards it to
 *    the underlying channel.
 *
 * Result:
 *    -1 if failed, the new position if successful. An output argument
 *    contains the POSIX error code if an error occurred, or zero.
 *
 *----------------------------------------------------------------------
 */

static int
TransformSeekProc(
    ClientData instanceData,  /* The channel to manipulate. */
    long offset,        /* Size of movement. */
    int mode,                 /* How to move. */
    int *errorCodePtr)        /* Location of error flag. */
{
    TransformChannelData *dataPtr = instanceData;
    Tcl_Channel parent = Tcl_GetStackedChannel(dataPtr->self);
    Tcl_ChannelType *parentType = Tcl_GetChannelType(parent);
    Tcl_DriverSeekProc *parentSeekProc = Tcl_ChannelSeekProc(parentType);

    if ((offset == 0) && (mode == SEEK_CUR)) {
      /*
       * This is no seek but a request to tell the caller the current
       * location. Simply pass the request down.
       */

      return parentSeekProc(Tcl_GetChannelInstanceData(parent), offset,
            mode, errorCodePtr);
    }

    /*
     * It is a real request to change the position. Flush all data waiting for
     * output and discard everything in the input buffers. Then pass the
     * request down, unchanged.
     */

    if (dataPtr->mode & TCL_WRITABLE) {
      ExecuteCallback(dataPtr, NULL, A_FLUSH_WRITE, NULL, 0, TRANSMIT_DOWN,
            P_NO_PRESERVE);
    }

    if (dataPtr->mode & TCL_READABLE) {
      ExecuteCallback(dataPtr, NULL, A_CLEAR_READ, NULL, 0, TRANSMIT_DONT,
            P_NO_PRESERVE);
      ResultClear(&dataPtr->result);
      dataPtr->readIsFlushed = 0;
    }

    return parentSeekProc(Tcl_GetChannelInstanceData(parent), offset, mode,
          errorCodePtr);
}

/*
 *----------------------------------------------------------------------
 *
 * TransformWideSeekProc --
 *
 *    This procedure is called by the generic IO level to move the access
 *    point in a channel, with a (potentially) 64-bit offset.
 *
 * Side effects:
 *    Moves the location at which the channel will be accessed in future
 *    operations. Flushes all transformation buffers, then forwards it to
 *    the underlying channel.
 *
 * Result:
 *    -1 if failed, the new position if successful. An output argument
 *    contains the POSIX error code if an error occurred, or zero.
 *
 *----------------------------------------------------------------------
 */

static Tcl_WideInt
TransformWideSeekProc(
    ClientData instanceData,  /* The channel to manipulate. */
    Tcl_WideInt offset,       /* Size of movement. */
    int mode,                 /* How to move. */
    int *errorCodePtr)        /* Location of error flag. */
{
    TransformChannelData *dataPtr = instanceData;
    Tcl_Channel parent = Tcl_GetStackedChannel(dataPtr->self);
    Tcl_ChannelType *parentType     = Tcl_GetChannelType(parent);
    Tcl_DriverSeekProc *parentSeekProc = Tcl_ChannelSeekProc(parentType);
    Tcl_DriverWideSeekProc *parentWideSeekProc =
          Tcl_ChannelWideSeekProc(parentType);
    ClientData parentData = Tcl_GetChannelInstanceData(parent);

    if ((offset == Tcl_LongAsWide(0)) && (mode == SEEK_CUR)) {
      /*
       * This is no seek but a request to tell the caller the current
       * location. Simply pass the request down.
       */

      if (parentWideSeekProc != NULL) {
          return parentWideSeekProc(parentData, offset, mode, errorCodePtr);
      }

      return Tcl_LongAsWide(parentSeekProc(parentData, 0, mode,
            errorCodePtr));
    }

    /*
     * It is a real request to change the position. Flush all data waiting for
     * output and discard everything in the input buffers. Then pass the
     * request down, unchanged.
     */

    if (dataPtr->mode & TCL_WRITABLE) {
      ExecuteCallback(dataPtr, NULL, A_FLUSH_WRITE, NULL, 0, TRANSMIT_DOWN,
            P_NO_PRESERVE);
    }

    if (dataPtr->mode & TCL_READABLE) {
      ExecuteCallback(dataPtr, NULL, A_CLEAR_READ, NULL, 0, TRANSMIT_DONT,
            P_NO_PRESERVE);
      ResultClear(&dataPtr->result);
      dataPtr->readIsFlushed = 0;
    }

    /*
     * If we have a wide seek capability, we should stick with that.
     */

    if (parentWideSeekProc != NULL) {
      return parentWideSeekProc(parentData, offset, mode, errorCodePtr);
    }

    /*
     * We're transferring to narrow seeks at this point; this is a bit complex
     * because we have to check whether the seek is possible first (i.e.
     * whether we are losing information in truncating the bits of the
     * offset). Luckily, there's a defined error for what happens when trying
     * to go out of the representable range.
     */

    if (offset<Tcl_LongAsWide(LONG_MIN) || offset>Tcl_LongAsWide(LONG_MAX)) {
      *errorCodePtr = EOVERFLOW;
      return Tcl_LongAsWide(-1);
    }

    return Tcl_LongAsWide(parentSeekProc(parentData, Tcl_WideAsLong(offset),
          mode, errorCodePtr));
}

/*
 *----------------------------------------------------------------------
 *
 * TransformSetOptionProc --
 *
 *    Called by generic layer to handle the reconfiguration of channel
 *    specific options. As this channel type does not have such, it simply
 *    passes all requests downstream.
 *
 * Side effects:
 *    As defined by the channel downstream.
 *
 * Result:
 *    A standard TCL error code.
 *
 *----------------------------------------------------------------------
 */

static int
TransformSetOptionProc(
    ClientData instanceData,
    Tcl_Interp *interp,
    const char *optionName,
    const char *value)
{
    TransformChannelData *dataPtr = instanceData;
    Tcl_Channel downChan = Tcl_GetStackedChannel(dataPtr->self);
    Tcl_DriverSetOptionProc *setOptionProc;

    setOptionProc = Tcl_ChannelSetOptionProc(Tcl_GetChannelType(downChan));
    if (setOptionProc == NULL) {
      return TCL_ERROR;
    }

    return setOptionProc(Tcl_GetChannelInstanceData(downChan), interp,
          optionName, value);
}

/*
 *----------------------------------------------------------------------
 *
 * TransformGetOptionProc --
 *
 *    Called by generic layer to handle requests for the values of channel
 *    specific options. As this channel type does not have such, it simply
 *    passes all requests downstream.
 *
 * Side effects:
 *    As defined by the channel downstream.
 *
 * Result:
 *    A standard TCL error code.
 *
 *----------------------------------------------------------------------
 */

static int
TransformGetOptionProc(
    ClientData instanceData,
    Tcl_Interp *interp,
    const char *optionName,
    Tcl_DString *dsPtr)
{
    TransformChannelData *dataPtr = instanceData;
    Tcl_Channel downChan = Tcl_GetStackedChannel(dataPtr->self);
    Tcl_DriverGetOptionProc *getOptionProc;

    getOptionProc = Tcl_ChannelGetOptionProc(Tcl_GetChannelType(downChan));
    if (getOptionProc != NULL) {
      return getOptionProc(Tcl_GetChannelInstanceData(downChan), interp,
            optionName, dsPtr);
    } else if (optionName == NULL) {
      /*
       * Request is query for all options, this is ok.
       */

      return TCL_OK;
    }

    /*
     * Request for a specific option has to fail, since we don't have any.
     */

    return TCL_ERROR;
}

/*
 *----------------------------------------------------------------------
 *
 * TransformWatchProc --
 *
 *    Initialize the notifier to watch for events from this channel.
 *
 * Side effects:
 *    Sets up the notifier so that a future event on the channel will be
 *    seen by Tcl.
 *
 * Result:
 *    None.
 *
 *----------------------------------------------------------------------
 */

      /* ARGSUSED */
static void
TransformWatchProc(
    ClientData instanceData,  /* Channel to watch. */
    int mask)                 /* Events of interest. */
{
    TransformChannelData *dataPtr = instanceData;
    Tcl_Channel downChan;

    /*
     * The caller expressed interest in events occuring for this channel. We
     * are forwarding the call to the underlying channel now.
     */

    dataPtr->watchMask = mask;

    /*
     * No channel handlers any more. We will be notified automatically about
     * events on the channel below via a call to our 'TransformNotifyProc'.
     * But we have to pass the interest down now. We are allowed to add
     * additional 'interest' to the mask if we want to. But this
     * transformation has no such interest. It just passes the request down,
     * unchanged.
     */

    downChan = Tcl_GetStackedChannel(dataPtr->self);

    Tcl_GetChannelType(downChan)->watchProc(
          Tcl_GetChannelInstanceData(downChan), mask);

    /*
     * Management of the internal timer.
     */

    if ((dataPtr->timer != NULL) &&
          (!(mask & TCL_READABLE) || ResultEmpty(&dataPtr->result))) {
      /*
       * A pending timer exists, but either is there no (more) interest in
       * the events it generates or nothing is available for reading, so
       * remove it.
       */

      Tcl_DeleteTimerHandler(dataPtr->timer);
      dataPtr->timer = NULL;
    }

    if ((dataPtr->timer == NULL) && (mask & TCL_READABLE)
          && !ResultEmpty(&dataPtr->result)) {
      /*
       * There is no pending timer, but there is interest in readable events
       * and we actually have data waiting, so generate a timer to flush
       * that.
       */

      dataPtr->timer = Tcl_CreateTimerHandler(FLUSH_DELAY,
            TransformChannelHandlerTimer, dataPtr);
    }
}

/*
 *----------------------------------------------------------------------
 *
 * TransformGetFileHandleProc --
 *
 *    Called from Tcl_GetChannelHandle to retrieve OS specific file handle
 *    from inside this channel.
 *
 * Side effects:
 *    None.
 *
 * Result:
 *    The appropriate Tcl_File or NULL if not present.
 *
 *----------------------------------------------------------------------
 */

static int
TransformGetFileHandleProc(
    ClientData instanceData,  /* Channel to query. */
    int direction,            /* Direction of interest. */
    ClientData *handlePtr)    /* Place to store the handle into. */
{
    TransformChannelData *dataPtr = instanceData;

    /*
     * Return the handle belonging to parent channel. IOW, pass the request
     * down and the result up.
     */

    return Tcl_GetChannelHandle(Tcl_GetStackedChannel(dataPtr->self),
          direction, handlePtr);
}

/*
 *----------------------------------------------------------------------
 *
 * TransformNotifyProc --
 *
 *    Handler called by Tcl to inform us of activity on the underlying
 *    channel.
 *
 * Side effects:
 *    May process the incoming event by itself.
 *
 * Result:
 *    None.
 *
 *----------------------------------------------------------------------
 */

static int
TransformNotifyProc(
    ClientData clientData,    /* The state of the notified
                         * transformation. */
    int mask)                 /* The mask of occuring events. */
{
    TransformChannelData *dataPtr = clientData;

    /*
     * An event occured in the underlying channel. This transformation doesn't
     * process such events thus returns the incoming mask unchanged.
     */

    if (dataPtr->timer != NULL) {
      /*
       * Delete an existing timer. It was not fired, yet we are here, so the
       * channel below generated such an event and we don't have to. The
       * renewal of the interest after the execution of channel handlers
       * will eventually cause us to recreate the timer (in
       * TransformWatchProc).
       */

      Tcl_DeleteTimerHandler(dataPtr->timer);
      dataPtr->timer = NULL;
    }
    return mask;
}

/*
 *----------------------------------------------------------------------
 *
 * TransformChannelHandlerTimer --
 *
 *    Called by the notifier (-> timer) to flush out information waiting in
 *    the input buffer.
 *
 * Side effects:
 *    As of 'Tcl_NotifyChannel'.
 *
 * Result:
 *    None.
 *
 *----------------------------------------------------------------------
 */

static void
TransformChannelHandlerTimer(
    ClientData clientData)    /* Transformation to query. */
{
    TransformChannelData *dataPtr = clientData;

    dataPtr->timer = NULL;
    if (!(dataPtr->watchMask&TCL_READABLE) || ResultEmpty(&dataPtr->result)) {
      /*
       * The timer fired, but either is there no (more) interest in the
       * events it generates or nothing is available for reading, so ignore
       * it and don't recreate it.
       */

      return;
    }
    Tcl_NotifyChannel(dataPtr->self, TCL_READABLE);
}

/*
 *----------------------------------------------------------------------
 *
 * ResultClear --
 *
 *    Deallocates any memory allocated by 'ResultAdd'.
 *
 * Side effects:
 *    See above.
 *
 * Result:
 *    None.
 *
 *----------------------------------------------------------------------
 */

static inline void
ResultClear(
    ResultBuffer *r)          /* Reference to the buffer to clear out. */
{
    r->used = 0;

    if (r->allocated) {
      ckfree((char *) r->buf);
      r->buf = NULL;
      r->allocated = 0;
    }
}

/*
 *----------------------------------------------------------------------
 *
 * ResultInit --
 *
 *    Initializes the specified buffer structure. The structure will contain
 *    valid information for an emtpy buffer.
 *
 * Side effects:
 *    See above.
 *
 * Result:
 *    None.
 *
 *----------------------------------------------------------------------
 */

static inline void
ResultInit(
    ResultBuffer *r)          /* Reference to the structure to
                         * initialize. */
{
    r->used = 0;
    r->allocated = 0;
    r->buf = NULL;
}

/*
 *----------------------------------------------------------------------
 *
 * ResultEmpty --
 *
 *    Returns whether the number of bytes stored in the buffer is zero.
 *
 * Side effects:
 *    None.
 *
 * Result:
 *    A boolean.
 *
 *----------------------------------------------------------------------
 */

static inline int
ResultEmpty(
    ResultBuffer *r)          /* The structure to query. */
{
    return r->used == 0;
}

/*
 *----------------------------------------------------------------------
 *
 * ResultCopy --
 *
 *    Copies the requested number of bytes from the buffer into the
 *    specified array and removes them from the buffer afterward. Copies
 *    less if there is not enough data in the buffer.
 *
 * Side effects:
 *    See above.
 *
 * Result:
 *    The number of actually copied bytes, possibly less than 'toRead'.
 *
 *----------------------------------------------------------------------
 */

static inline int
ResultCopy(
    ResultBuffer *r,          /* The buffer to read from. */
    unsigned char *buf,       /* The buffer to copy into. */
    size_t toRead)            /* Number of requested bytes. */
{
    if (r->used == 0) {
      /*
       * Nothing to copy in the case of an empty buffer.
       */

      return 0;
    } else if (r->used == toRead) {
      /*
       * We have just enough. Copy everything to the caller.
       */

      memcpy(buf, r->buf, toRead);
      r->used = 0;
    } else if (r->used > toRead) {
      /*
       * The internal buffer contains more than requested. Copy the
       * requested subset to the caller, and shift the remaining bytes down.
       */

      memcpy(buf, r->buf, toRead);
      memmove(r->buf, r->buf + toRead, r->used - toRead);
      r->used -= toRead;
    } else {
      /*
       * There is not enough in the buffer to satisfy the caller, so take
       * everything.
       */

      memcpy(buf, r->buf, r->used);
      toRead = r->used;
      r->used = 0;
    }
    return toRead;
}

/*
 *----------------------------------------------------------------------
 *
 * ResultAdd --
 *
 *    Adds the bytes in the specified array to the buffer, by appending it.
 *
 * Side effects:
 *    See above.
 *
 * Result:
 *    None.
 *
 *----------------------------------------------------------------------
 */

static inline void
ResultAdd(
    ResultBuffer *r,          /* The buffer to extend. */
    unsigned char *buf,       /* The buffer to read from. */
    size_t toWrite)           /* The number of bytes in 'buf'. */
{
    if (r->used + toWrite > r->allocated) {
      /*
       * Extension of the internal buffer is required.
       */

      if (r->allocated == 0) {
          r->allocated = toWrite + INCREMENT;
          r->buf = UCHARP(ckalloc(r->allocated));
      } else {
          r->allocated += toWrite + INCREMENT;
          r->buf = UCHARP(ckrealloc((char *) r->buf, r->allocated));
      }
    }

    /*
     * Now we may copy the data.
     */

    memcpy(r->buf + r->used, buf, toWrite);
    r->used += toWrite;
}

/*
 * Local Variables:
 * mode: c
 * c-basic-offset: 4
 * fill-column: 78
 * End:
 */

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