shared_source/c_tcl_base_7_5_w_mods/tclio.c

/* $Header: /cvsroot/esrg/sfesrg/esrgpcpj/shared/tcl_base/tclio.c,v 1.1.1.1 2001/06/13 04:42:01 dtashley Exp $ */

/* 
 * tclIO.c --
 *
 *      This file provides the generic portions (those that are the same on
 *      all platforms and for all channel types) of Tcl's IO facilities.
 *
 * Copyright (c) 1998 Scriptics Corporation
 * Copyright (c) 1995-1997 Sun Microsystems, Inc.
 *
 * See the file "license.terms" for information on usage and redistribution
 * of this file, and for a DISCLAIMER OF ALL WARRANTIES.
 *
 * RCS: @(#) $Id: tclio.c,v 1.1.1.1 2001/06/13 04:42:01 dtashley Exp $
 */

#include "tclInt.h"
#include "tclPort.h"

/*
 * Make sure that both EAGAIN and EWOULDBLOCK are defined. This does not
 * compile on systems where neither is defined. We want both defined so
 * that we can test safely for both. In the code we still have to test for
 * both because there may be systems on which both are defined and have
 * different values.
 */

#if ((!defined(EWOULDBLOCK)) && (defined(EAGAIN)))
#   define EWOULDBLOCK EAGAIN
#endif
#if ((!defined(EAGAIN)) && (defined(EWOULDBLOCK)))
#   define EAGAIN EWOULDBLOCK
#endif
#if ((!defined(EAGAIN)) && (!defined(EWOULDBLOCK)))
    error one of EWOULDBLOCK or EAGAIN must be defined
#endif

/*
 * The following structure encapsulates the state for a background channel
 * copy.  Note that the data buffer for the copy will be appended to this
 * structure.
 */

typedef struct CopyState {
    struct Channel *readPtr;    /* Pointer to input channel. */
    struct Channel *writePtr;   /* Pointer to output channel. */
    int readFlags;              /* Original read channel flags. */
    int writeFlags;             /* Original write channel flags. */
    int toRead;                 /* Number of bytes to copy, or -1. */
    int total;                  /* Total bytes transferred (written). */
    Tcl_Interp *interp;         /* Interp that started the copy. */
    Tcl_Obj *cmdPtr;            /* Command to be invoked at completion. */
    int bufSize;                /* Size of appended buffer. */
    char buffer[1];             /* Copy buffer, this must be the last
                                 * field. */
} CopyState;

/*
 * struct ChannelBuffer:
 *
 * Buffers data being sent to or from a channel.
 */

typedef struct ChannelBuffer {
    int nextAdded;              /* The next position into which a character
                                 * will be put in the buffer. */
    int nextRemoved;            /* Position of next byte to be removed
                                 * from the buffer. */
    int bufLength;              /* How big is the buffer? */
    struct ChannelBuffer *nextPtr;
                                /* Next buffer in chain. */
    char buf[4];                /* Placeholder for real buffer. The real
                                 * buffer occuppies this space + bufSize-4
                                 * bytes. This must be the last field in
                                 * the structure. */
} ChannelBuffer;

#define CHANNELBUFFER_HEADER_SIZE       (sizeof(ChannelBuffer) - 4)

/*
 * How much extra space to allocate in buffer to hold bytes from previous
 * buffer (when converting to UTF-8) or to hold bytes that will go to
 * next buffer (when converting from UTF-8).
 */
 
#define BUFFER_PADDING      16
 
/*
 * The following defines the *default* buffer size for channels.
 */

#define CHANNELBUFFER_DEFAULT_SIZE      (1024 * 4)

/*
 * Structure to record a close callback. One such record exists for
 * each close callback registered for a channel.
 */

typedef struct CloseCallback {
    Tcl_CloseProc *proc;                /* The procedure to call. */
    ClientData clientData;              /* Arbitrary one-word data to pass
                                         * to the callback. */
    struct CloseCallback *nextPtr;      /* For chaining close callbacks. */
} CloseCallback;

/*
 * The following structure describes the information saved from a call to
 * "fileevent". This is used later when the event being waited for to
 * invoke the saved script in the interpreter designed in this record.
 */

typedef struct EventScriptRecord {
    struct Channel *chanPtr;    /* The channel for which this script is
                                 * registered. This is used only when an
                                 * error occurs during evaluation of the
                                 * script, to delete the handler. */
    Tcl_Obj *scriptPtr;         /* Script to invoke. */
    Tcl_Interp *interp;         /* In what interpreter to invoke script? */
    int mask;                   /* Events must overlap current mask for the
                                 * stored script to be invoked. */
    struct EventScriptRecord *nextPtr;
                                /* Next in chain of records. */
} EventScriptRecord;

/*
 * struct Channel:
 *
 * One of these structures is allocated for each open channel. It contains data
 * specific to the channel but which belongs to the generic part of the Tcl
 * channel mechanism, and it points at an instance specific (and type
 * specific) * instance data, and at a channel type structure.
 */

typedef struct Channel {
    char *channelName;          /* The name of the channel instance in Tcl
                                 * commands. Storage is owned by the generic IO
                                 * code,  is dynamically allocated. */
    int flags;                  /* ORed combination of the flags defined
                                 * below. */
    Tcl_Encoding encoding;      /* Encoding to apply when reading or writing
                                 * data on this channel.  NULL means no
                                 * encoding is applied to data. */
    Tcl_EncodingState inputEncodingState;
                                /* Current encoding state, used when converting
                                 * input data bytes to UTF-8. */
    int inputEncodingFlags;     /* Encoding flags to pass to conversion
                                 * routine when converting input data bytes to
                                 * UTF-8.  May be TCL_ENCODING_START before
                                 * converting first byte and TCL_ENCODING_END
                                 * when EOF is seen. */
    Tcl_EncodingState outputEncodingState;
                                /* Current encoding state, used when converting
                                 * UTF-8 to output data bytes. */
    int outputEncodingFlags;    /* Encoding flags to pass to conversion
                                 * routine when converting UTF-8 to output
                                 * data bytes.  May be TCL_ENCODING_START
                                 * before converting first byte and
                                 * TCL_ENCODING_END when EOF is seen. */
    Tcl_EolTranslation inputTranslation;
                                /* What translation to apply for end of line
                                 * sequences on input? */    
    Tcl_EolTranslation outputTranslation;
                                /* What translation to use for generating
                                 * end of line sequences in output? */
    int inEofChar;              /* If nonzero, use this as a signal of EOF
                                 * on input. */
    int outEofChar;             /* If nonzero, append this to the channel
                                 * when it is closed if it is open for
                                 * writing. */
    int unreportedError;        /* Non-zero if an error report was deferred
                                 * because it happened in the background. The
                                 * value is the POSIX error code. */
    ClientData instanceData;    /* Instance-specific data provided by
                                 * creator of channel. */

    Tcl_ChannelType *typePtr;   /* Pointer to channel type structure. */
    int refCount;               /* How many interpreters hold references to
                                 * this IO channel? */
    CloseCallback *closeCbPtr;  /* Callbacks registered to be called when the
                                 * channel is closed. */
    char *outputStage;          /* Temporary staging buffer used when
                                 * translating EOL before converting from
                                 * UTF-8 to external form. */
    ChannelBuffer *curOutPtr;   /* Current output buffer being filled. */
    ChannelBuffer *outQueueHead;/* Points at first buffer in output queue. */
    ChannelBuffer *outQueueTail;/* Points at last buffer in output queue. */

    ChannelBuffer *saveInBufPtr;/* Buffer saved for input queue - eliminates
                                 * need to allocate a new buffer for "gets"
                                 * that crosses buffer boundaries. */
    ChannelBuffer *inQueueHead; /* Points at first buffer in input queue. */
    ChannelBuffer *inQueueTail; /* Points at last buffer in input queue. */

    struct ChannelHandler *chPtr;/* List of channel handlers registered
                                  * for this channel. */
    int interestMask;           /* Mask of all events this channel has
                                 * handlers for. */
    struct Channel *nextChanPtr;/* Next in list of channels currently open. */
    EventScriptRecord *scriptRecordPtr;
                                /* Chain of all scripts registered for
                                 * event handlers ("fileevent") on this
                                 * channel. */
    int bufSize;                /* What size buffers to allocate? */
    Tcl_TimerToken timer;       /* Handle to wakeup timer for this channel. */
    CopyState *csPtr;           /* State of background copy, or NULL. */
    struct Channel* supercedes; /* Refers to channel this one was stacked upon.
                                   This reference is NULL for normal channels.
                                   See Tcl_StackChannel. */

} Channel;
    
/*
 * Values for the flags field in Channel. Any ORed combination of the
 * following flags can be stored in the field. These flags record various
 * options and state bits about the channel. In addition to the flags below,
 * the channel can also have TCL_READABLE (1<<1) and TCL_WRITABLE (1<<2) set.
 */

#define CHANNEL_NONBLOCKING     (1<<3)  /* Channel is currently in
                                         * nonblocking mode. */
#define CHANNEL_LINEBUFFERED    (1<<4)  /* Output to the channel must be
                                         * flushed after every newline. */
#define CHANNEL_UNBUFFERED      (1<<5)  /* Output to the channel must always
                                         * be flushed immediately. */
#define BUFFER_READY            (1<<6)  /* Current output buffer (the
                                         * curOutPtr field in the
                                         * channel structure) should be
                                         * output as soon as possible even
                                         * though it may not be full. */
#define BG_FLUSH_SCHEDULED      (1<<7)  /* A background flush of the
                                         * queued output buffers has been
                                         * scheduled. */
#define CHANNEL_CLOSED          (1<<8)  /* Channel has been closed. No
                                         * further Tcl-level IO on the
                                         * channel is allowed. */
#define CHANNEL_EOF             (1<<9)  /* EOF occurred on this channel.
                                         * This bit is cleared before every
                                         * input operation. */
#define CHANNEL_STICKY_EOF      (1<<10) /* EOF occurred on this channel because
                                         * we saw the input eofChar. This bit
                                         * prevents clearing of the EOF bit
                                         * before every input operation. */
#define CHANNEL_BLOCKED (1<<11) /* EWOULDBLOCK or EAGAIN occurred
                                         * on this channel. This bit is
                                         * cleared before every input or
                                         * output operation. */
#define INPUT_SAW_CR            (1<<12) /* Channel is in CRLF eol input
                                         * translation mode and the last
                                         * byte seen was a "\r". */
#define INPUT_NEED_NL           (1<<15) /* Saw a '\r' at end of last buffer,
                                         * and there should be a '\n' at
                                         * beginning of next buffer. */
#define CHANNEL_DEAD            (1<<13) /* The channel has been closed by
                                         * the exit handler (on exit) but
                                         * not deallocated. When any IO
                                         * operation sees this flag on a
                                         * channel, it does not call driver
                                         * level functions to avoid referring
                                         * to deallocated data. */
#define CHANNEL_NEED_MORE_DATA  (1<<14) /* The last input operation failed
                                         * because there was not enough data
                                         * to complete the operation.  This
                                         * flag is set when gets fails to
                                         * get a complete line or when read
                                         * fails to get a complete character.
                                         * When set, file events will not be
                                         * delivered for buffered data until
                                         * the state of the channel changes. */

/*
 * For each channel handler registered in a call to Tcl_CreateChannelHandler,
 * there is one record of the following type. All of records for a specific
 * channel are chained together in a singly linked list which is stored in
 * the channel structure.
 */

typedef struct ChannelHandler {
    Channel *chanPtr;           /* The channel structure for this channel. */
    int mask;                   /* Mask of desired events. */
    Tcl_ChannelProc *proc;      /* Procedure to call in the type of
                                 * Tcl_CreateChannelHandler. */
    ClientData clientData;      /* Argument to pass to procedure. */
    struct ChannelHandler *nextPtr;
                                /* Next one in list of registered handlers. */
} ChannelHandler;

/*
 * This structure keeps track of the current ChannelHandler being invoked in
 * the current invocation of ChannelHandlerEventProc. There is a potential
 * problem if a ChannelHandler is deleted while it is the current one, since
 * ChannelHandlerEventProc needs to look at the nextPtr field. To handle this
 * problem, structures of the type below indicate the next handler to be
 * processed for any (recursively nested) dispatches in progress. The
 * nextHandlerPtr field is updated if the handler being pointed to is deleted.
 * The nextPtr field is used to chain together all recursive invocations, so
 * that Tcl_DeleteChannelHandler can find all the recursively nested
 * invocations of ChannelHandlerEventProc and compare the handler being
 * deleted against the NEXT handler to be invoked in that invocation; when it
 * finds such a situation, Tcl_DeleteChannelHandler updates the nextHandlerPtr
 * field of the structure to the next handler.
 */

typedef struct NextChannelHandler {
    ChannelHandler *nextHandlerPtr;     /* The next handler to be invoked in
                                         * this invocation. */
    struct NextChannelHandler *nestedHandlerPtr;
                                        /* Next nested invocation of
                                         * ChannelHandlerEventProc. */
} NextChannelHandler;


/*
 * The following structure describes the event that is added to the Tcl
 * event queue by the channel handler check procedure.
 */

typedef struct ChannelHandlerEvent {
    Tcl_Event header;           /* Standard header for all events. */
    Channel *chanPtr;           /* The channel that is ready. */
    int readyMask;              /* Events that have occurred. */
} ChannelHandlerEvent;

/*
 * The following structure is used by Tcl_GetsObj() to encapsulates the
 * state for a "gets" operation.
 */
 
typedef struct GetsState {
    Tcl_Obj *objPtr;            /* The object to which UTF-8 characters
                                 * will be appended. */
    char **dstPtr;              /* Pointer into objPtr's string rep where
                                 * next character should be stored. */
    Tcl_Encoding encoding;      /* The encoding to use to convert raw bytes
                                 * to UTF-8.  */
    ChannelBuffer *bufPtr;      /* The current buffer of raw bytes being
                                 * emptied. */
    Tcl_EncodingState state;    /* The encoding state just before the last
                                 * external to UTF-8 conversion in
                                 * FilterInputBytes(). */
    int rawRead;                /* The number of bytes removed from bufPtr
                                 * in the last call to FilterInputBytes(). */
    int bytesWrote;             /* The number of bytes of UTF-8 data
                                 * appended to objPtr during the last call to
                                 * FilterInputBytes(). */
    int charsWrote;             /* The corresponding number of UTF-8
                                 * characters appended to objPtr during the
                                 * last call to FilterInputBytes(). */
    int totalChars;             /* The total number of UTF-8 characters
                                 * appended to objPtr so far, just before the
                                 * last call to FilterInputBytes(). */
} GetsState;

/*
 * All static variables used in this file are collected into a single
 * instance of the following structure.  For multi-threaded implementations,
 * there is one instance of this structure for each thread.
 *
 * Notice that different structures with the same name appear in other
 * files.  The structure defined below is used in this file only.
 */

typedef struct ThreadSpecificData {

    /*
     * This variable holds the list of nested ChannelHandlerEventProc 
     * invocations.
     */
    NextChannelHandler *nestedHandlerPtr;

    /*
     * List of all channels currently open.
     */
    Channel *firstChanPtr;
#ifdef oldcode
    /*
     * Has a channel exit handler been created yet?
     */
    int channelExitHandlerCreated;

    /*
     * Has the channel event source been created and registered with the
     * notifier?
     */
    int channelEventSourceCreated;
#endif
    /*
     * Static variables to hold channels for stdin, stdout and stderr.
     */
    Tcl_Channel stdinChannel;
    int stdinInitialized;
    Tcl_Channel stdoutChannel;
    int stdoutInitialized;
    Tcl_Channel stderrChannel;
    int stderrInitialized;

} ThreadSpecificData;

static Tcl_ThreadDataKey dataKey;


/*
 * Static functions in this file:
 */

static ChannelBuffer *  AllocChannelBuffer _ANSI_ARGS_((int length));
static void             ChannelEventScriptInvoker _ANSI_ARGS_((
                            ClientData clientData, int flags));
static void             ChannelTimerProc _ANSI_ARGS_((
                            ClientData clientData));
static int              CheckChannelErrors _ANSI_ARGS_((Channel *chanPtr,
                            int direction));
static int              CheckFlush _ANSI_ARGS_((Channel *chanPtr,
                            ChannelBuffer *bufPtr, int newlineFlag));
static int              CheckForDeadChannel _ANSI_ARGS_((Tcl_Interp *interp,
                            Channel *chan));
static void             CheckForStdChannelsBeingClosed _ANSI_ARGS_((
                            Tcl_Channel chan));
static void             CleanupChannelHandlers _ANSI_ARGS_((
                            Tcl_Interp *interp, Channel *chanPtr));
static int              CloseChannel _ANSI_ARGS_((Tcl_Interp *interp,
                            Channel *chanPtr, int errorCode));
static void             CommonGetsCleanup _ANSI_ARGS_((Channel *chanPtr,
                            Tcl_Encoding encoding));
static int              CopyAndTranslateBuffer _ANSI_ARGS_((
                            Channel *chanPtr, char *result, int space));
static int              CopyData _ANSI_ARGS_((CopyState *csPtr, int mask));
static void             CopyEventProc _ANSI_ARGS_((ClientData clientData,
                            int mask));
static void             CreateScriptRecord _ANSI_ARGS_((
                            Tcl_Interp *interp, Channel *chanPtr,
                            int mask, Tcl_Obj *scriptPtr));
static void             DeleteChannelTable _ANSI_ARGS_((
                            ClientData clientData, Tcl_Interp *interp));
static void             DeleteScriptRecord _ANSI_ARGS_((Tcl_Interp *interp,
                            Channel *chanPtr, int mask));
static void             DiscardInputQueued _ANSI_ARGS_((
                            Channel *chanPtr, int discardSavedBuffers));
static void             DiscardOutputQueued _ANSI_ARGS_((
                            Channel *chanPtr));
static int              DoRead _ANSI_ARGS_((Channel *chanPtr, char *srcPtr,
                            int slen));
static int              DoWrite _ANSI_ARGS_((Channel *chanPtr, char *src,
                            int srcLen));
static int              FilterInputBytes _ANSI_ARGS_((Channel *chanPtr,
                            GetsState *statePtr));
static int              FlushChannel _ANSI_ARGS_((Tcl_Interp *interp,
                            Channel *chanPtr, int calledFromAsyncFlush));
static Tcl_HashTable *  GetChannelTable _ANSI_ARGS_((Tcl_Interp *interp));
static int              GetInput _ANSI_ARGS_((Channel *chanPtr));
static void             PeekAhead _ANSI_ARGS_((Channel *chanPtr,
                            char **dstEndPtr, GetsState *gsPtr));
static int              ReadBytes _ANSI_ARGS_((Channel *chanPtr,
                            Tcl_Obj *objPtr, int charsLeft, int *offsetPtr));
static int              ReadChars _ANSI_ARGS_((Channel *chanPtr,
                            Tcl_Obj *objPtr, int charsLeft, int *offsetPtr,
                            int *factorPtr));
static void             RecycleBuffer _ANSI_ARGS_((Channel *chanPtr,
                            ChannelBuffer *bufPtr, int mustDiscard));
static int              SetBlockMode _ANSI_ARGS_((Tcl_Interp *interp,
                            Channel *chanPtr, int mode));
static void             StopCopy _ANSI_ARGS_((CopyState *csPtr));
static int              TranslateInputEOL _ANSI_ARGS_((Channel *chanPtr,
                            char *dst, CONST char *src, int *dstLenPtr,
                            int *srcLenPtr));
static int              TranslateOutputEOL _ANSI_ARGS_((Channel *chanPtr,
                            char *dst, CONST char *src, int *dstLenPtr,
                            int *srcLenPtr));
static void             UpdateInterest _ANSI_ARGS_((Channel *chanPtr));
static int              WriteBytes _ANSI_ARGS_((Channel *chanPtr,
                            CONST char *src, int srcLen));
static int              WriteChars _ANSI_ARGS_((Channel *chanPtr,
                            CONST char *src, int srcLen));


/*
 *---------------------------------------------------------------------------
 *
 * TclInitIOSubsystem --
 *
 *      Initialize all resources used by this subsystem on a per-process
 *      basis.  
 *
 * Results:
 *      None.
 *
 * Side effects:
 *      Depends on the memory subsystems.
 *
 *---------------------------------------------------------------------------
 */

void
TclInitIOSubsystem()
{
    /*
     * By fetching thread local storage we take care of
     * allocating it for each thread.
     */
    (void) TCL_TSD_INIT(&dataKey);
}   

/*
 *-------------------------------------------------------------------------
 *
 * TclFinalizeIOSubsystem --
 *
 *      Releases all resources used by this subsystem on a per-process 
 *      basis.  Closes all extant channels that have not already been 
 *      closed because they were not owned by any interp.  
 *
 * Results:
 *      None.
 *
 * Side effects:
 *      Depends on encoding and memory subsystems.
 *
 *-------------------------------------------------------------------------
 */

        /* ARGSUSED */
void
TclFinalizeIOSubsystem()
{
    ThreadSpecificData *tsdPtr = TCL_TSD_INIT(&dataKey);
    Channel *chanPtr;                   /* Iterates over open channels. */
    Channel *nextChanPtr;               /* Iterates over open channels. */


    for (chanPtr = tsdPtr->firstChanPtr; chanPtr != (Channel *) NULL;
             chanPtr = nextChanPtr) {
        nextChanPtr = chanPtr->nextChanPtr;

        /*
         * Set the channel back into blocking mode to ensure that we wait
         * for all data to flush out.
         */
        
        (void) Tcl_SetChannelOption(NULL, (Tcl_Channel) chanPtr,
                "-blocking", "on");

        if ((chanPtr == (Channel *) tsdPtr->stdinChannel) ||
                (chanPtr == (Channel *) tsdPtr->stdoutChannel) ||
                (chanPtr == (Channel *) tsdPtr->stderrChannel)) {

            /*
             * Decrement the refcount which was earlier artificially bumped
             * up to keep the channel from being closed.
             */

            chanPtr->refCount--;
        }

        if (chanPtr->refCount <= 0) {

            /*
             * Close it only if the refcount indicates that the channel is not
             * referenced from any interpreter. If it is, that interpreter will
             * close the channel when it gets destroyed.
             */

            (void) Tcl_Close((Tcl_Interp *) NULL, (Tcl_Channel) chanPtr);

        } else {

            /*
             * The refcount is greater than zero, so flush the channel.
             */

            Tcl_Flush((Tcl_Channel) chanPtr);

            /*
             * Call the device driver to actually close the underlying
             * device for this channel.
             */
            
            if (chanPtr->typePtr->closeProc != TCL_CLOSE2PROC) {
                (chanPtr->typePtr->closeProc)(chanPtr->instanceData,
                        (Tcl_Interp *) NULL);
            } else {
                (chanPtr->typePtr->close2Proc)(chanPtr->instanceData,
                        (Tcl_Interp *) NULL, 0);
            }

            /*
             * Finally, we clean up the fields in the channel data structure
             * since all of them have been deleted already. We mark the
             * channel with CHANNEL_DEAD to prevent any further IO operations
             * on it.
             */

            chanPtr->instanceData = (ClientData) NULL;
            chanPtr->flags |= CHANNEL_DEAD;
        }
    }
}


/*
 *----------------------------------------------------------------------
 *
 * Tcl_SetStdChannel --
 *
 *      This function is used to change the channels that are used
 *      for stdin/stdout/stderr in new interpreters.
 *
 * Results:
 *      None
 *
 * Side effects:
 *      None.
 *
 *----------------------------------------------------------------------
 */

void
Tcl_SetStdChannel(channel, type)
    Tcl_Channel channel;
    int type;                   /* One of TCL_STDIN, TCL_STDOUT, TCL_STDERR. */
{
    ThreadSpecificData *tsdPtr = TCL_TSD_INIT(&dataKey);
    switch (type) {
        case TCL_STDIN:
            tsdPtr->stdinInitialized = 1;
            tsdPtr->stdinChannel = channel;
            break;
        case TCL_STDOUT:
            tsdPtr->stdoutInitialized = 1;
            tsdPtr->stdoutChannel = channel;
            break;
        case TCL_STDERR:
            tsdPtr->stderrInitialized = 1;
            tsdPtr->stderrChannel = channel;
            break;
    }
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_GetStdChannel --
 *
 *      Returns the specified standard channel.
 *
 * Results:
 *      Returns the specified standard channel, or NULL.
 *
 * Side effects:
 *      May cause the creation of a standard channel and the underlying
 *      file.
 *
 *----------------------------------------------------------------------
 */
Tcl_Channel
Tcl_GetStdChannel(type)
    int type;                   /* One of TCL_STDIN, TCL_STDOUT, TCL_STDERR. */
{
    Tcl_Channel channel = NULL;
    ThreadSpecificData *tsdPtr = TCL_TSD_INIT(&dataKey);

    /*
     * If the channels were not created yet, create them now and
     * store them in the static variables. 
     */

    switch (type) {
        case TCL_STDIN:
            if (!tsdPtr->stdinInitialized) {
                tsdPtr->stdinChannel = TclpGetDefaultStdChannel(TCL_STDIN);
                tsdPtr->stdinInitialized = 1;

                /*
                 * Artificially bump the refcount to ensure that the channel
                 * is only closed on exit.
                 *
                 * NOTE: Must only do this if stdinChannel is not NULL. It
                 * can be NULL in situations where Tcl is unable to connect
                 * to the standard input.
                 */

                if (tsdPtr->stdinChannel != (Tcl_Channel) NULL) {
                    (void) Tcl_RegisterChannel((Tcl_Interp *) NULL,
                            tsdPtr->stdinChannel);
                }
            }
            channel = tsdPtr->stdinChannel;
            break;
        case TCL_STDOUT:
            if (!tsdPtr->stdoutInitialized) {
                tsdPtr->stdoutChannel = TclpGetDefaultStdChannel(TCL_STDOUT);
                tsdPtr->stdoutInitialized = 1;
                if (tsdPtr->stdoutChannel != (Tcl_Channel) NULL) {
                    (void) Tcl_RegisterChannel((Tcl_Interp *) NULL,
                            tsdPtr->stdoutChannel);
                }
            }
            channel = tsdPtr->stdoutChannel;
            break;
        case TCL_STDERR:
            if (!tsdPtr->stderrInitialized) {
                tsdPtr->stderrChannel = TclpGetDefaultStdChannel(TCL_STDERR);
                tsdPtr->stderrInitialized = 1;
                if (tsdPtr->stderrChannel != (Tcl_Channel) NULL) {
                    (void) Tcl_RegisterChannel((Tcl_Interp *) NULL,
                            tsdPtr->stderrChannel);
                }
            }
            channel = tsdPtr->stderrChannel;
            break;
    }
    return channel;
}


/*
 *----------------------------------------------------------------------
 *
 * Tcl_CreateCloseHandler
 *
 *      Creates a close callback which will be called when the channel is
 *      closed.
 *
 * Results:
 *      None.
 *
 * Side effects:
 *      Causes the callback to be called in the future when the channel
 *      will be closed.
 *
 *----------------------------------------------------------------------
 */

void
Tcl_CreateCloseHandler(chan, proc, clientData)
    Tcl_Channel chan;           /* The channel for which to create the
                                 * close callback. */
    Tcl_CloseProc *proc;        /* The callback routine to call when the
                                 * channel will be closed. */
    ClientData clientData;      /* Arbitrary data to pass to the
                                 * close callback. */
{
    Channel *chanPtr;
    CloseCallback *cbPtr;

    chanPtr = (Channel *) chan;

    cbPtr = (CloseCallback *) ckalloc((unsigned) sizeof(CloseCallback));
    cbPtr->proc = proc;
    cbPtr->clientData = clientData;

    cbPtr->nextPtr = chanPtr->closeCbPtr;
    chanPtr->closeCbPtr = cbPtr;
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_DeleteCloseHandler --
 *
 *      Removes a callback that would have been called on closing
 *      the channel. If there is no matching callback then this
 *      function has no effect.
 *
 * Results:
 *      None.
 *
 * Side effects:
 *      The callback will not be called in the future when the channel
 *      is eventually closed.
 *
 *----------------------------------------------------------------------
 */

void
Tcl_DeleteCloseHandler(chan, proc, clientData)
    Tcl_Channel chan;           /* The channel for which to cancel the
                                 * close callback. */
    Tcl_CloseProc *proc;        /* The procedure for the callback to
                                 * remove. */
    ClientData clientData;      /* The callback data for the callback
                                 * to remove. */
{
    Channel *chanPtr;
    CloseCallback *cbPtr, *cbPrevPtr;

    chanPtr = (Channel *) chan;
    for (cbPtr = chanPtr->closeCbPtr, cbPrevPtr = (CloseCallback *) NULL;
             cbPtr != (CloseCallback *) NULL;
             cbPtr = cbPtr->nextPtr) {
        if ((cbPtr->proc == proc) && (cbPtr->clientData == clientData)) {
            if (cbPrevPtr == (CloseCallback *) NULL) {
                chanPtr->closeCbPtr = cbPtr->nextPtr;
            }
            ckfree((char *) cbPtr);
            break;
        } else {
            cbPrevPtr = cbPtr;
        }
    }
}

/*
 *----------------------------------------------------------------------
 *
 * GetChannelTable --
 *
 *      Gets and potentially initializes the channel table for an
 *      interpreter. If it is initializing the table it also inserts
 *      channels for stdin, stdout and stderr if the interpreter is
 *      trusted.
 *
 * Results:
 *      A pointer to the hash table created, for use by the caller.
 *
 * Side effects:
 *      Initializes the channel table for an interpreter. May create
 *      channels for stdin, stdout and stderr.
 *
 *----------------------------------------------------------------------
 */

static Tcl_HashTable *
GetChannelTable(interp)
    Tcl_Interp *interp;
{
    Tcl_HashTable *hTblPtr;     /* Hash table of channels. */
    Tcl_Channel stdinChan, stdoutChan, stderrChan;

    hTblPtr = (Tcl_HashTable *) Tcl_GetAssocData(interp, "tclIO", NULL);
    if (hTblPtr == (Tcl_HashTable *) NULL) {
        hTblPtr = (Tcl_HashTable *) ckalloc((unsigned) sizeof(Tcl_HashTable));
        Tcl_InitHashTable(hTblPtr, TCL_STRING_KEYS);

        (void) Tcl_SetAssocData(interp, "tclIO",
                (Tcl_InterpDeleteProc *) DeleteChannelTable,
                (ClientData) hTblPtr);

        /*
         * If the interpreter is trusted (not "safe"), insert channels
         * for stdin, stdout and stderr (possibly creating them in the
         * process).
         */

        if (Tcl_IsSafe(interp) == 0) {
            stdinChan = Tcl_GetStdChannel(TCL_STDIN);
            if (stdinChan != NULL) {
                Tcl_RegisterChannel(interp, stdinChan);
            }
            stdoutChan = Tcl_GetStdChannel(TCL_STDOUT);
            if (stdoutChan != NULL) {
                Tcl_RegisterChannel(interp, stdoutChan);
            }
            stderrChan = Tcl_GetStdChannel(TCL_STDERR);
            if (stderrChan != NULL) {
                Tcl_RegisterChannel(interp, stderrChan);
            }
        }

    }
    return hTblPtr;
}

/*
 *----------------------------------------------------------------------
 *
 * DeleteChannelTable --
 *
 *      Deletes the channel table for an interpreter, closing any open
 *      channels whose refcount reaches zero. This procedure is invoked
 *      when an interpreter is deleted, via the AssocData cleanup
 *      mechanism.
 *
 * Results:
 *      None.
 *
 * Side effects:
 *      Deletes the hash table of channels. May close channels. May flush
 *      output on closed channels. Removes any channeEvent handlers that were
 *      registered in this interpreter.
 *
 *----------------------------------------------------------------------
 */

static void
DeleteChannelTable(clientData, interp)
    ClientData clientData;      /* The per-interpreter data structure. */
    Tcl_Interp *interp;         /* The interpreter being deleted. */
{
    Tcl_HashTable *hTblPtr;     /* The hash table. */
    Tcl_HashSearch hSearch;     /* Search variable. */
    Tcl_HashEntry *hPtr;        /* Search variable. */
    Channel *chanPtr;   /* Channel being deleted. */
    EventScriptRecord *sPtr, *prevPtr, *nextPtr;
                                /* Variables to loop over all channel events
                                 * registered, to delete the ones that refer
                                 * to the interpreter being deleted. */
    
    /*
     * Delete all the registered channels - this will close channels whose
     * refcount reaches zero.
     */
    
    hTblPtr = (Tcl_HashTable *) clientData;
    for (hPtr = Tcl_FirstHashEntry(hTblPtr, &hSearch);
             hPtr != (Tcl_HashEntry *) NULL;
             hPtr = Tcl_FirstHashEntry(hTblPtr, &hSearch)) {

        chanPtr = (Channel *) Tcl_GetHashValue(hPtr);

        /*
         * Remove any fileevents registered in this interpreter.
         */
        
        for (sPtr = chanPtr->scriptRecordPtr,
                 prevPtr = (EventScriptRecord *) NULL;
                 sPtr != (EventScriptRecord *) NULL;
                 sPtr = nextPtr) {
            nextPtr = sPtr->nextPtr;
            if (sPtr->interp == interp) {
                if (prevPtr == (EventScriptRecord *) NULL) {
                    chanPtr->scriptRecordPtr = nextPtr;
                } else {
                    prevPtr->nextPtr = nextPtr;
                }

                Tcl_DeleteChannelHandler((Tcl_Channel) chanPtr,
                        ChannelEventScriptInvoker, (ClientData) sPtr);

                Tcl_DecrRefCount(sPtr->scriptPtr);
                ckfree((char *) sPtr);
            } else {
                prevPtr = sPtr;
            }
        }

        /*
         * Cannot call Tcl_UnregisterChannel because that procedure calls
         * Tcl_GetAssocData to get the channel table, which might already
         * be inaccessible from the interpreter structure. Instead, we
         * emulate the behavior of Tcl_UnregisterChannel directly here.
         */

        Tcl_DeleteHashEntry(hPtr);
        chanPtr->refCount--;
        if (chanPtr->refCount <= 0) {
            if (!(chanPtr->flags & BG_FLUSH_SCHEDULED)) {
                (void) Tcl_Close(interp, (Tcl_Channel) chanPtr);
            }
        }
    }
    Tcl_DeleteHashTable(hTblPtr);
    ckfree((char *) hTblPtr);
}

/*
 *----------------------------------------------------------------------
 *
 * CheckForStdChannelsBeingClosed --
 *
 *      Perform special handling for standard channels being closed. When
 *      given a standard channel, if the refcount is now 1, it means that
 *      the last reference to the standard channel is being explicitly
 *      closed. Now bump the refcount artificially down to 0, to ensure the
 *      normal handling of channels being closed will occur. Also reset the
 *      static pointer to the channel to NULL, to avoid dangling references.
 *
 * Results:
 *      None.
 *
 * Side effects:
 *      Manipulates the refcount on standard channels. May smash the global
 *      static pointer to a standard channel.
 *
 *----------------------------------------------------------------------
 */

static void
CheckForStdChannelsBeingClosed(chan)
    Tcl_Channel chan;
{
    Channel *chanPtr = (Channel *) chan;
    ThreadSpecificData *tsdPtr = TCL_TSD_INIT(&dataKey);

    if ((chan == tsdPtr->stdinChannel) && (tsdPtr->stdinInitialized)) {
        if (chanPtr->refCount < 2) {
            chanPtr->refCount = 0;
            tsdPtr->stdinChannel = NULL;
            return;
        }
    } else if ((chan == tsdPtr->stdoutChannel) && (tsdPtr->stdoutInitialized)) {
        if (chanPtr->refCount < 2) {
            chanPtr->refCount = 0;
            tsdPtr->stdoutChannel = NULL;
            return;
        }
    } else if ((chan == tsdPtr->stderrChannel) && (tsdPtr->stderrInitialized)) {
        if (chanPtr->refCount < 2) {
            chanPtr->refCount = 0;
            tsdPtr->stderrChannel = NULL;
            return;
        }
    }
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_RegisterChannel --
 *
 *      Adds an already-open channel to the channel table of an interpreter.
 *      If the interpreter passed as argument is NULL, it only increments
 *      the channel refCount.
 *
 * Results:
 *      None.
 *
 * Side effects:
 *      May increment the reference count of a channel.
 *
 *----------------------------------------------------------------------
 */

void
Tcl_RegisterChannel(interp, chan)
    Tcl_Interp *interp;         /* Interpreter in which to add the channel. */
    Tcl_Channel chan;           /* The channel to add to this interpreter
                                 * channel table. */
{
    Tcl_HashTable *hTblPtr;     /* Hash table of channels. */
    Tcl_HashEntry *hPtr;        /* Search variable. */
    int new;                    /* Is the hash entry new or does it exist? */
    Channel *chanPtr;           /* The actual channel. */

    chanPtr = (Channel *) chan;

    if (chanPtr->channelName == (char *) NULL) {
        panic("Tcl_RegisterChannel: channel without name");
    }
    if (interp != (Tcl_Interp *) NULL) {
        hTblPtr = GetChannelTable(interp);
        hPtr = Tcl_CreateHashEntry(hTblPtr, chanPtr->channelName, &new);
        if (new == 0) {
            if (chan == (Tcl_Channel) Tcl_GetHashValue(hPtr)) {
                return;
            }

            /* Andreas Kupries <a.kupries@westend.com>, 12/13/1998
             * "Trf-Patch for filtering channels"
             *
             * This is the change to 'Tcl_RegisterChannel'.
             *
             * Explanation:
             *          The moment a channel is stacked upon another he
             *          takes the identity of the channel he supercedes,
             *          i.e. he gets the *same* name. Because of this we
             *          cannot check for duplicate names anymore, they
             *          have to be allowed now.
             */

            /* panic("Tcl_RegisterChannel: duplicate channel names"); */
        }
        Tcl_SetHashValue(hPtr, (ClientData) chanPtr);
    }
    chanPtr->refCount++;
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_UnregisterChannel --
 *
 *      Deletes the hash entry for a channel associated with an interpreter.
 *      If the interpreter given as argument is NULL, it only decrements the
 *      reference count.
 *
 * Results:
 *      A standard Tcl result.
 *
 * Side effects:
 *      Deletes the hash entry for a channel associated with an interpreter.
 *
 *----------------------------------------------------------------------
 */

int
Tcl_UnregisterChannel(interp, chan)
    Tcl_Interp *interp;         /* Interpreter in which channel is defined. */
    Tcl_Channel chan;           /* Channel to delete. */
{
    Tcl_HashTable *hTblPtr;     /* Hash table of channels. */
    Tcl_HashEntry *hPtr;        /* Search variable. */
    Channel *chanPtr;           /* The real IO channel. */

    chanPtr = (Channel *) chan;
    
    if (interp != (Tcl_Interp *) NULL) {
        hTblPtr = (Tcl_HashTable *) Tcl_GetAssocData(interp, "tclIO", NULL);
        if (hTblPtr == (Tcl_HashTable *) NULL) {
            return TCL_OK;
        }
        hPtr = Tcl_FindHashEntry(hTblPtr, chanPtr->channelName);
        if (hPtr == (Tcl_HashEntry *) NULL) {
            return TCL_OK;
        }
        if ((Channel *) Tcl_GetHashValue(hPtr) != chanPtr) {
            return TCL_OK;
        }
        Tcl_DeleteHashEntry(hPtr);

        /*
         * Remove channel handlers that refer to this interpreter, so that they
         * will not be present if the actual close is delayed and more events
         * happen on the channel. This may occur if the channel is shared
         * between several interpreters, or if the channel has async
         * flushing active.
         */
    
        CleanupChannelHandlers(interp, chanPtr);
    }

    chanPtr->refCount--;
    
    /*
     * Perform special handling for standard channels being closed. If the
     * refCount is now 1 it means that the last reference to the standard
     * channel is being explicitly closed, so bump the refCount down
     * artificially to 0. This will ensure that the channel is actually
     * closed, below. Also set the static pointer to NULL for the channel.
     */

    CheckForStdChannelsBeingClosed(chan);

    /*
     * If the refCount reached zero, close the actual channel.
     */

    if (chanPtr->refCount <= 0) {

        /*
         * Ensure that if there is another buffer, it gets flushed
         * whether or not we are doing a background flush.
         */

        if ((chanPtr->curOutPtr != NULL) &&
                (chanPtr->curOutPtr->nextAdded >
                        chanPtr->curOutPtr->nextRemoved)) {
            chanPtr->flags |= BUFFER_READY;
        }
        chanPtr->flags |= CHANNEL_CLOSED;
        if (!(chanPtr->flags & BG_FLUSH_SCHEDULED)) {
            if (Tcl_Close(interp, chan) != TCL_OK) {
                return TCL_ERROR;
            }
        }
    }
    return TCL_OK;
}

/*
 *---------------------------------------------------------------------------
 *
 * Tcl_GetChannel --
 *
 *      Finds an existing Tcl_Channel structure by name in a given
 *      interpreter. This function is public because it is used by
 *      channel-type-specific functions.
 *
 * Results:
 *      A Tcl_Channel or NULL on failure. If failed, interp's result
 *      object contains an error message.  *modePtr is filled with the
 *      modes in which the channel was opened.
 *
 * Side effects:
 *      None.
 *
 *---------------------------------------------------------------------------
 */

Tcl_Channel
Tcl_GetChannel(interp, chanName, modePtr)
    Tcl_Interp *interp;         /* Interpreter in which to find or create
                                 * the channel. */
    char *chanName;             /* The name of the channel. */
    int *modePtr;               /* Where to store the mode in which the
                                 * channel was opened? Will contain an ORed
                                 * combination of TCL_READABLE and
                                 * TCL_WRITABLE, if non-NULL. */
{
    Channel *chanPtr;           /* The actual channel. */
    Tcl_HashTable *hTblPtr;     /* Hash table of channels. */
    Tcl_HashEntry *hPtr;        /* Search variable. */
    char *name;                 /* Translated name. */

    /*
     * Substitute "stdin", etc.  Note that even though we immediately
     * find the channel using Tcl_GetStdChannel, we still need to look
     * it up in the specified interpreter to ensure that it is present
     * in the channel table.  Otherwise, safe interpreters would always
     * have access to the standard channels.
     */

    name = chanName;
    if ((chanName[0] == 's') && (chanName[1] == 't')) {
        chanPtr = NULL;
        if (strcmp(chanName, "stdin") == 0) {
            chanPtr = (Channel *)Tcl_GetStdChannel(TCL_STDIN);
        } else if (strcmp(chanName, "stdout") == 0) {
            chanPtr = (Channel *)Tcl_GetStdChannel(TCL_STDOUT);
        } else if (strcmp(chanName, "stderr") == 0) {
            chanPtr = (Channel *)Tcl_GetStdChannel(TCL_STDERR);
        }
        if (chanPtr != NULL) {
            name = chanPtr->channelName;
        }
    }
    
    hTblPtr = GetChannelTable(interp);
    hPtr = Tcl_FindHashEntry(hTblPtr, name);
    if (hPtr == (Tcl_HashEntry *) NULL) {
        Tcl_AppendResult(interp, "can not find channel named \"",
                chanName, "\"", (char *) NULL);
        return NULL;
    }

    chanPtr = (Channel *) Tcl_GetHashValue(hPtr);
    if (modePtr != NULL) {
        *modePtr = (chanPtr->flags & (TCL_READABLE|TCL_WRITABLE));
    }
    
    return (Tcl_Channel) chanPtr;
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_CreateChannel --
 *
 *      Creates a new entry in the hash table for a Tcl_Channel
 *      record.
 *
 * Results:
 *      Returns the new Tcl_Channel.
 *
 * Side effects:
 *      Creates a new Tcl_Channel instance and inserts it into the
 *      hash table.
 *
 *----------------------------------------------------------------------
 */

Tcl_Channel
Tcl_CreateChannel(typePtr, chanName, instanceData, mask)
    Tcl_ChannelType *typePtr;   /* The channel type record. */
    char *chanName;             /* Name of channel to record. */
    ClientData instanceData;    /* Instance specific data. */
    int mask;                   /* TCL_READABLE & TCL_WRITABLE to indicate
                                 * if the channel is readable, writable. */
{
    Channel *chanPtr;           /* The channel structure newly created. */
    CONST char *name;
    ThreadSpecificData *tsdPtr = TCL_TSD_INIT(&dataKey);

    chanPtr = (Channel *) ckalloc((unsigned) sizeof(Channel));
    
    if (chanName != (char *) NULL) {
        chanPtr->channelName = ckalloc((unsigned) (strlen(chanName) + 1));
        strcpy(chanPtr->channelName, chanName);
    } else {
        panic("Tcl_CreateChannel: NULL channel name");
    }

    chanPtr->flags = mask;

    /*
     * Set the channel to system default encoding.
     */

    chanPtr->encoding = NULL;
    name = Tcl_GetEncodingName(NULL);
    if (strcmp(name, "binary") != 0) {
        chanPtr->encoding = Tcl_GetEncoding(NULL, name);
    }
    chanPtr->inputEncodingState = NULL;
    chanPtr->inputEncodingFlags = TCL_ENCODING_START;
    chanPtr->outputEncodingState = NULL;
    chanPtr->outputEncodingFlags = TCL_ENCODING_START;

    /*
     * Set the channel up initially in AUTO input translation mode to
     * accept "\n", "\r" and "\r\n". Output translation mode is set to
     * a platform specific default value. The eofChar is set to 0 for both
     * input and output, so that Tcl does not look for an in-file EOF
     * indicator (e.g. ^Z) and does not append an EOF indicator to files.
     */

    chanPtr->inputTranslation = TCL_TRANSLATE_AUTO;
    chanPtr->outputTranslation = TCL_PLATFORM_TRANSLATION;
    chanPtr->inEofChar = 0;
    chanPtr->outEofChar = 0;

    chanPtr->unreportedError = 0;
    chanPtr->instanceData = instanceData;
    chanPtr->typePtr = typePtr;
    chanPtr->refCount = 0;
    chanPtr->closeCbPtr = (CloseCallback *) NULL;
    chanPtr->curOutPtr = (ChannelBuffer *) NULL;
    chanPtr->outQueueHead = (ChannelBuffer *) NULL;
    chanPtr->outQueueTail = (ChannelBuffer *) NULL;
    chanPtr->saveInBufPtr = (ChannelBuffer *) NULL;
    chanPtr->inQueueHead = (ChannelBuffer *) NULL;
    chanPtr->inQueueTail = (ChannelBuffer *) NULL;
    chanPtr->chPtr = (ChannelHandler *) NULL;
    chanPtr->interestMask = 0;
    chanPtr->scriptRecordPtr = (EventScriptRecord *) NULL;
    chanPtr->bufSize = CHANNELBUFFER_DEFAULT_SIZE;
    chanPtr->timer = NULL;
    chanPtr->csPtr = NULL;
    chanPtr->supercedes = (Channel*) NULL;

    chanPtr->outputStage = NULL;
    if ((chanPtr->encoding != NULL) && (chanPtr->flags & TCL_WRITABLE)) {
        chanPtr->outputStage = (char *)
                ckalloc((unsigned) (chanPtr->bufSize + 2));
    }

    /*
     * Link the channel into the list of all channels; create an on-exit
     * handler if there is not one already, to close off all the channels
     * in the list on exit.
     */

    chanPtr->nextChanPtr = tsdPtr->firstChanPtr;
    tsdPtr->firstChanPtr = chanPtr;

    /*
     * Install this channel in the first empty standard channel slot, if
     * the channel was previously closed explicitly.
     */

    if ((tsdPtr->stdinChannel == NULL) && (tsdPtr->stdinInitialized == 1)) {
        Tcl_SetStdChannel((Tcl_Channel)chanPtr, TCL_STDIN);
        Tcl_RegisterChannel((Tcl_Interp *) NULL, (Tcl_Channel) chanPtr);
    } else if ((tsdPtr->stdoutChannel == NULL) && (tsdPtr->stdoutInitialized == 1)) {
        Tcl_SetStdChannel((Tcl_Channel)chanPtr, TCL_STDOUT);
        Tcl_RegisterChannel((Tcl_Interp *) NULL, (Tcl_Channel) chanPtr);
    } else if ((tsdPtr->stderrChannel == NULL) && (tsdPtr->stderrInitialized == 1)) {
        Tcl_SetStdChannel((Tcl_Channel)chanPtr, TCL_STDERR);
        Tcl_RegisterChannel((Tcl_Interp *) NULL, (Tcl_Channel) chanPtr);
    } 
    return (Tcl_Channel) chanPtr;
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_StackChannel --
 *
 *      Replaces an entry in the hash table for a Tcl_Channel
 *      record. The replacement is a new channel with same name,
 *      it supercedes the replaced channel. Input and output of
 *      the superceded channel is now going through the newly
 *      created channel and allows the arbitrary filtering/manipulation
 *      of the dataflow.
 *
 *      Andreas Kupries <a.kupries@westend.com>, 12/13/1998
 *      "Trf-Patch for filtering channels"
 *
 * Results:
 *      Returns the new Tcl_Channel, which actually contains the
 *      saved information about prevChan.
 *
 * Side effects:
 *    A new channel structure is allocated and linked below
 *    the existing channel.  The channel operations and client
 *    data of the existing channel are copied down to the newly
 *    created channel, and the current channel has its operations
 *    replaced by the new typePtr.
 *
 *----------------------------------------------------------------------
 */

Tcl_Channel
Tcl_StackChannel(interp, typePtr, instanceData, mask, prevChan)
    Tcl_Interp*      interp;       /* The interpreter we are working in */
    Tcl_ChannelType *typePtr;      /* The channel type record for the new
                                    * channel. */
    ClientData       instanceData; /* Instance specific data for the new
                                    * channel. */
    int              mask;         /* TCL_READABLE & TCL_WRITABLE to indicate
                                    * if the channel is readable, writable. */
    Tcl_Channel      prevChan;     /* The channel structure to replace */
{
    ThreadSpecificData *tsdPtr = TCL_TSD_INIT(&dataKey);
    Channel            *chanPtr, *pt;
    int                 interest = 0;

    /*
     * AK, 06/30/1999
     *
     * Tcl_StackChannel differs from Tcl_ReplaceChannel of the
     * original "Trf" patch. Instead of seeing the
     * newly created structure as the *new* channel to cover the specified
     * one use it to *save* the current state of the specified channel and
     * then reinitialize the current structure for the given transformation.
     *
     * Advantages:
     * - No splicing into the (thread-)global list of channels (or the per-
     *   interp hash-tables).
     * - Users of the C-API still have valid channel references even after
     *   the call to this procedure.
     *
     * Disadvantages:
     * - Untested code.
     */

    /*
     * Find the given channel in the list of all channels.
     */

    pt     = (Channel*) tsdPtr->firstChanPtr;

    while (pt != (Channel *) prevChan) {
        pt = pt->nextChanPtr;
    }

    /*
     * 'pt == prevChan' now (or NULL, if not found).
     */

    if (!pt) {
        return (Tcl_Channel) NULL;
    }

    /*
     * Here we check if the given "mask" matches the "flags"
     * of the already existing channel.
     *
     *    | - | R | W | RW |
     *  --+---+---+---+----+    <=>  0 != (chan->mask & prevChan->mask)
     *  - |   |   |   |    |
     *  R |   | + |   | +  |    The superceding channel is allowed to
     *  W |   |   | + | +  |    restrict the capabilities of the
     *  RW|   | + | + | +  |    superceded one !
     *  --+---+---+---+----+
     */

    if ((mask & Tcl_GetChannelMode (prevChan)) == 0) {
        return (Tcl_Channel) NULL;
    }

    chanPtr = (Channel *) ckalloc((unsigned) sizeof(Channel));

    /*
     * If there is some interest in the channel, remove it, break
     * down the whole chain. It will be reconstructed later.
     */

    interest = pt->interestMask;

    pt->interestMask = 0;

    if (interest) {
        (pt->typePtr->watchProc) (pt->instanceData, 0);
    }

    /*
     * Save some of the current state into the new structure,
     * reinitialize the parts which will stay with the transformation.
     *
     * Remarks:
     * - We cannot discard the buffers, and they cannot be used from the
     *   transformation placed later into the 'pt' structure. Save them,
     *   and believe that Tcl_SetChannelOption (buffering, none) will do
     *   the right thing.
     * - encoding and EOL-translation control information is initialized
     *   to values for 'binary'. This is later reinforced via
     *   Tcl_SetChanneloption to get the handling of flags and the event
     *   system right.
     * - The 'interestMask' of the saved channel is cleared, but the
     *   transformations WatchProc is used to establish the connection
     *   between transformation and underlying channel. This should
     *   reestablish the correct mask.
     * - TTO = Transform Takes Over.   The hidden channel no longer
     *         needs to perform this function.
     */

    chanPtr->channelName = (char *) ckalloc (strlen(pt->channelName)+1);
    strcpy (chanPtr->channelName, pt->channelName);

    chanPtr->flags               = pt->flags;           /* Save */

    chanPtr->encoding            = (Tcl_Encoding) NULL; /* == 'binary' */
    chanPtr->inputEncodingState  = (Tcl_EncodingState) NULL;
    chanPtr->inputEncodingFlags  = TCL_ENCODING_START;
    chanPtr->outputEncodingState = (Tcl_EncodingState) NULL;
    chanPtr->outputEncodingFlags = TCL_ENCODING_START;

    chanPtr->inputTranslation    = TCL_TRANSLATE_LF; /* == 'binary' */
    chanPtr->outputTranslation   = TCL_TRANSLATE_LF; /* == 'binary' */
    chanPtr->inEofChar           = pt->inEofChar;         /* Save */
    chanPtr->outEofChar          = pt->outEofChar;        /* Save */

    chanPtr->unreportedError     = pt->unreportedError;   /* Save */
    chanPtr->instanceData        = pt->instanceData;      /* Save */
    chanPtr->typePtr             = pt->typePtr;           /* Save */
    chanPtr->refCount            = 0;   /* None, as the structure is covered */
    chanPtr->closeCbPtr          = (CloseCallback*) NULL; /* TTO */

    chanPtr->outputStage         = (char*) NULL;
    chanPtr->curOutPtr           = pt->curOutPtr;    /* Save */
    chanPtr->outQueueHead        = pt->outQueueHead; /* Save */
    chanPtr->outQueueTail        = pt->outQueueTail; /* Save */
    chanPtr->saveInBufPtr        = pt->saveInBufPtr; /* Save */
    chanPtr->inQueueHead         = pt->inQueueHead;  /* Save */
    chanPtr->inQueueTail         = pt->inQueueTail;  /* Save */

    chanPtr->chPtr               = (ChannelHandler *) NULL;  /* TTO */
    chanPtr->interestMask        = 0;
    chanPtr->nextChanPtr         = (Channel*) NULL;     /* Is not in list! */
    chanPtr->scriptRecordPtr     = (EventScriptRecord *) NULL; /* TTO */
    chanPtr->bufSize             = CHANNELBUFFER_DEFAULT_SIZE;
    chanPtr->timer               = (Tcl_TimerToken) NULL;      /* TTO */
    chanPtr->csPtr               = (CopyState*) NULL;          /* TTO */

    /*
     * Place new block at the head of a possibly existing list of previously
     * stacked channels, then do the missing initializations of translation
     * and buffer system.
     */

    chanPtr->supercedes          = pt->supercedes;

    Tcl_SetChannelOption (interp, (Tcl_Channel) chanPtr,
        "-translation", "binary");
    Tcl_SetChannelOption (interp, (Tcl_Channel) chanPtr,
        "-buffering",   "none");

    /*
     * Save accomplished, now reinitialize the (old) structure for the
     * transformation.
     *
     * - The information about encoding and eol-translation is taken
     *   without change.  There is no need to fiddle with
     *   refCount et. al.
     *
     * Don't forget to use the same blocking mode as the old channel.
     */

    pt->flags               = mask | (chanPtr->flags & CHANNEL_NONBLOCKING);

    /*
     * EDITORS NOTE:  all the lines with "take it as is" should get
     * deleted once this code has been debugged.
     */

    /* pt->encoding,            take it as is */
    /* pt->inputEncodingState,  take it as is */
    /* pt->inputEncodingFlags,  take it as is */
    /* pt->outputEncodingState, take it as is */
    /* pt->outputEncodingFlags, take it as is */

    /* pt->inputTranslation,    take it as is */
    /* pt->outputTranslation,   take it as is */

    /*
     * No special EOF character, that condition is determined by the
     * old channel
     */

    pt->inEofChar           = 0;
    pt->outEofChar          = 0;

    pt->unreportedError     = 0; /* No errors yet */
    pt->instanceData        = instanceData; /* Transformation state */
    pt->typePtr             = typePtr;      /* Transformation type */
    /* pt->refCount,            take it as it is */
    /* pt->closeCbPtr,          take it as it is */

    /* pt->outputStage,         take it as it is */
    pt->curOutPtr           = (ChannelBuffer *) NULL;
    pt->outQueueHead        = (ChannelBuffer *) NULL;
    pt->outQueueTail        = (ChannelBuffer *) NULL;
    pt->saveInBufPtr        = (ChannelBuffer *) NULL;
    pt->inQueueHead         = (ChannelBuffer *) NULL;
    pt->inQueueTail         = (ChannelBuffer *) NULL;

    /* pt->chPtr,               take it as it is */
    /* pt->interestMask,        take it as it is */
    /* pt->nextChanPtr,         take it as it is */
    /* pt->scriptRecordPtr,     take it as it is */
    pt->bufSize             = CHANNELBUFFER_DEFAULT_SIZE;
    /* pt->timer,               take it as it is */
    /* pt->csPtr,               take it as it is */

    /*
     * Have the transformation reference the new structure containing
     * the saved channel.
     */

    pt->supercedes          = chanPtr;

    /*
     * Don't forget to reinitialize the output buffer used for encodings.
     */

    if ((chanPtr->encoding != NULL) && (chanPtr->flags & TCL_WRITABLE)) {
        chanPtr->outputStage = (char *)
            ckalloc((unsigned) (chanPtr->bufSize + 2));
    }

    /*
     * Event handling: If the information in the old channel shows
     * that there was interest in some events call the 'WatchProc'
     * of the transformation to establish the proper connection
     * between them.
     */

    if (interest) {
        (pt->typePtr->watchProc) (pt->instanceData, interest);
    }

    /*
     * The superceded channel is effectively unregistered
     * We cannot decrement its reference count because that
     * can cause it to get garbage collected out from under us.
     * Don't add the following code:
     *
     * chanPtr->supercedes->refCount --;
     */

    return (Tcl_Channel) chanPtr;
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_UnstackChannel --
 *
 *      Unstacks an entry in the hash table for a Tcl_Channel
 *      record. This is the reverse to 'Tcl_StackChannel'.
 *      The old, superceded channel is uncovered and re-registered
 *      in the appropriate data structures.
 *
 * Results:
 *      Returns the old Tcl_Channel, i.e. the one which was stacked over.
 *
 * Side effects:
 *      See above.
 *
 *----------------------------------------------------------------------
 */

void
Tcl_UnstackChannel (interp, chan)
    Tcl_Interp* interp; /* The interpreter we are working in */
    Tcl_Channel chan;   /* The channel to unstack */
{
    ThreadSpecificData *tsdPtr = TCL_TSD_INIT(&dataKey);
    Channel* chanPtr = (Channel*) chan;

    if (chanPtr->supercedes != (Channel*) NULL) {
        /*
         * Instead of manipulating the per-thread / per-interp list/hashtable
         * of registered channels we wind down the state of the transformation,
         * and then restore the state of underlying channel into the old
         * structure.
         */

        Tcl_DString       dsTrans; /* storage to save option information */
        Tcl_DString       dsBuf;   /* storage to save option information */
        Channel           top;     /* Save area for current transformation */
        Channel*          chanDownPtr = chanPtr->supercedes;
        int               interest;     /* interest mask of transformation
                                         * before destruct. */
        int               saveInputEncodingFlags;  /* Save area for encoding */
        int               saveOutputEncodingFlags; /* related information */
        Tcl_EncodingState saveInputEncodingState;
        Tcl_EncodingState saveOutputEncodingState;
        Tcl_Encoding      saveEncoding;

        /*
         * Event handling: Disallow the delivery of events from the
         * old, now uncovered channel to the transformation.
         *
         * This is done before everything else to avoid problems
         * after our heavy-duty shuffling of pointers around.
         */

        interest = chanPtr->interestMask;
        (chanPtr->typePtr->watchProc) (chanPtr->instanceData, 0);

        /* 1. Swap the information in the top channel (the transformation)
         *    and the channel below, with some exceptions. This additionally
         *    cuts the top channel out of the chain. Without the latter
         *    a Tcl_Close on the transformation would be impossible, as that
         *    procedure will free the structure, making 'top' unusable.
         *
         * chanPtr     -> top channel, transformation.
         * chanDownPtr -> channel immediately below the transformation.
         */

        memcpy ((void*) &top,        (void*) chanPtr,     sizeof (Channel));
        memcpy ((void*) chanPtr,     (void*) chanDownPtr, sizeof (Channel));
        top.supercedes = (Channel*) NULL;
        memcpy ((void*) chanDownPtr, (void*) &top,        sizeof (Channel));

        /* Now:
         * chanPtr     -> channel immediately below the transformation, now top
         * chanDownPtr -> transformation, cut loose.
         *
         * Handle the exceptions mentioned above, i.e. move the information
         * from the transformation into the new top, and reinitialize it to
         * safe values in the transformation.
         */

        chanPtr->refCount        = chanDownPtr->refCount;
        chanPtr->closeCbPtr      = chanDownPtr->closeCbPtr;
        chanPtr->chPtr           = chanDownPtr->chPtr;
        chanPtr->nextChanPtr     = chanDownPtr->nextChanPtr;
        chanPtr->scriptRecordPtr = chanDownPtr->scriptRecordPtr;
        chanPtr->timer           = chanDownPtr->timer;
        chanPtr->csPtr           = chanDownPtr->csPtr;

        chanDownPtr->refCount        = 0;
        chanDownPtr->closeCbPtr      = (CloseCallback*) NULL;
        chanDownPtr->chPtr           = (ChannelHandler*) NULL;
        chanDownPtr->nextChanPtr     = (Channel*) NULL;
        chanDownPtr->scriptRecordPtr = (EventScriptRecord*) NULL;
        chanDownPtr->timer           = (Tcl_TimerToken) NULL;
        chanDownPtr->csPtr           = (CopyState*) NULL;

        /* The now uncovered channel still has encoding and eol-translation
         * deactivated, i.e. switched to 'binary'. *Don't* touch this until
         * after the transformation is closed for good, as it may write
         * information into it during that (-> flushing of data waiting in
         * internal buffers!) and rely on these settings. Thanks to Matt
         * Newman <matt@sensus.org> for finding this goof.
         *
         * But we also have to protect the state of the encoding from removal
         * during the close. So we save it in some local variables.
         * Additionally the current value of the options is lost after we
         * close, we have to save them now.
         */

        saveEncoding            = chanDownPtr->encoding;
        saveInputEncodingState  = chanDownPtr->inputEncodingState;
        saveInputEncodingFlags  = chanDownPtr->inputEncodingFlags;
        saveOutputEncodingState = chanDownPtr->outputEncodingState;
        saveOutputEncodingFlags = chanDownPtr->outputEncodingFlags;

        Tcl_DStringInit (&dsTrans);
        Tcl_GetChannelOption (interp, (Tcl_Channel) chanDownPtr,
                "-translation", &dsTrans);

        Tcl_DStringInit (&dsBuf);
        Tcl_GetChannelOption (interp, (Tcl_Channel) chanDownPtr,
                "-buffering", &dsBuf);

        /*
         * Prevent the accidential removal of the encoding during
         * the destruction of the transformation channel.
         */

        chanDownPtr->encoding            = (Tcl_Encoding) NULL;
        chanDownPtr->inputEncodingState  = (Tcl_EncodingState) NULL;
        chanDownPtr->inputEncodingFlags  = TCL_ENCODING_START;
        chanDownPtr->outputEncodingState = (Tcl_EncodingState) NULL;
        chanDownPtr->outputEncodingFlags = TCL_ENCODING_START;

        /*
         * A little trick: Add the transformation structure to the
         * per-thread list of existing channels (which it never were
         * part of so far), or Tcl_Close/FlushChannel will panic
         * ("damaged channel list").
         *
         * Afterward do a regular close upon the transformation.
         * This may cause flushing of data into the old channel (if the
         * transformation remembered its own channel in itself).
         *
         * We know that its refCount dropped to 0.
         */

        chanDownPtr->nextChanPtr = tsdPtr->firstChanPtr;
        tsdPtr->firstChanPtr     = chanDownPtr;

        Tcl_Close (interp, (Tcl_Channel)chanDownPtr);

        /*
         * Now it is possible to wind down the transformation (in 'top'),
         * especially to copy the current encoding and translation control
         * information down.
         */
        
        /*
         * Move the currently active encoding from the save area
         * to the now uncovered channel. We assume here that this
         * channel uses 'encoding binary' (==> encoding == NULL, etc.
         * This allows us to simply copy the pointers without having to
         * think about refcounts and deallocation of the old encoding.
         *
         * And don't forget to reenable the EOL-translation used by the
         * transformation. Using a DString to do this *is* a bit awkward,
         * but still the best way to handle the complexities here, like
         * flag manipulation and event system.
         */

        chanPtr->encoding            = saveEncoding;
        chanPtr->inputEncodingState  = saveInputEncodingState;
        chanPtr->inputEncodingFlags  = saveInputEncodingFlags;
        chanPtr->outputEncodingState = saveOutputEncodingState;
        chanPtr->outputEncodingFlags = saveOutputEncodingFlags;

        Tcl_SetChannelOption (interp, (Tcl_Channel) chanPtr,
                "-translation", dsTrans.string);

        Tcl_SetChannelOption (interp, (Tcl_Channel) chanPtr,
                "-buffering", dsBuf.string);

        Tcl_DStringFree (&dsTrans);
        Tcl_DStringFree (&dsBuf);

        /*
         * Event handling: If the information from the now destroyed
         * transformation shows that there was interest in some events
         * call the 'WatchProc' of the now uncovered channel to renew
         * that interest with underlying channels or the driver.
         */

        if (interest) {
            chanPtr->interestMask = 0;
            (chanPtr->typePtr->watchProc) (chanPtr->instanceData,
                interest);
            chanPtr->interestMask = interest;
        }

    } else {
        /* This channel does not cover another one.
         * Simply do a close, if necessary.
         */

        if (chanPtr->refCount == 0) {
            Tcl_Close (interp, chan);
        }
    }
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_GetStackedChannel --
 *
 *      Determines wether the specified channel is stacked upon another.
 *
 * Results:
 *      NULL if the channel is not stacked upon another one, or a reference
 *      to the channel it is stacked upon. This reference can be used in
 *      queries, but modification is not allowed.
 *
 * Side effects:
 *      None.
 *
 *----------------------------------------------------------------------
 */

Tcl_Channel
Tcl_GetStackedChannel(chan)
    Tcl_Channel chan;
{
  Channel* chanPtr = (Channel*) chan;
  return (Tcl_Channel) chanPtr->supercedes;
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_GetChannelMode --
 *
 *      Computes a mask indicating whether the channel is open for
 *      reading and writing.
 *
 * Results:
 *      An OR-ed combination of TCL_READABLE and TCL_WRITABLE.
 *
 * Side effects:
 *      None.
 *
 *----------------------------------------------------------------------
 */

int
Tcl_GetChannelMode(chan)
    Tcl_Channel chan;           /* The channel for which the mode is
                                 * being computed. */
{
    Channel *chanPtr;           /* The actual channel. */

    chanPtr = (Channel *) chan;
    return (chanPtr->flags & (TCL_READABLE | TCL_WRITABLE));
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_GetChannelName --
 *
 *      Returns the string identifying the channel name.
 *
 * Results:
 *      The string containing the channel name. This memory is
 *      owned by the generic layer and should not be modified by
 *      the caller.
 *
 * Side effects:
 *      None.
 *
 *----------------------------------------------------------------------
 */

char *
Tcl_GetChannelName(chan)
    Tcl_Channel chan;           /* The channel for which to return the name. */
{
    Channel *chanPtr;           /* The actual channel. */

    chanPtr = (Channel *) chan;
    return chanPtr->channelName;
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_GetChannelType --
 *
 *      Given a channel structure, returns the channel type structure.
 *
 * Results:
 *      Returns a pointer to the channel type structure.
 *
 * Side effects:
 *      None.
 *
 *----------------------------------------------------------------------
 */

Tcl_ChannelType *
Tcl_GetChannelType(chan)
    Tcl_Channel chan;           /* The channel to return type for. */
{
    Channel *chanPtr;           /* The actual channel. */

    chanPtr = (Channel *) chan;
    return chanPtr->typePtr;
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_GetChannelHandle --
 *
 *      Returns an OS handle associated with a channel.
 *
 * Results:
 *      Returns TCL_OK and places the handle in handlePtr, or returns
 *      TCL_ERROR on failure.
 *
 * Side effects:
 *      None.
 *
 *----------------------------------------------------------------------
 */

int
Tcl_GetChannelHandle(chan, direction, handlePtr)
    Tcl_Channel chan;           /* The channel to get file from. */
    int direction;              /* TCL_WRITABLE or TCL_READABLE. */
    ClientData *handlePtr;      /* Where to store handle */
{
    Channel *chanPtr;           /* The actual channel. */
    ClientData handle;
    int result;

    chanPtr = (Channel *) chan;
    result = (chanPtr->typePtr->getHandleProc)(chanPtr->instanceData,
            direction, &handle);
    if (handlePtr) {
        *handlePtr = handle;
    }
    return result;
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_GetChannelInstanceData --
 *
 *      Returns the client data associated with a channel.
 *
 * Results:
 *      The client data.
 *
 * Side effects:
 *      None.
 *
 *----------------------------------------------------------------------
 */

ClientData
Tcl_GetChannelInstanceData(chan)
    Tcl_Channel chan;           /* Channel for which to return client data. */
{
    Channel *chanPtr;           /* The actual channel. */

    chanPtr = (Channel *) chan;
    return chanPtr->instanceData;
}

/*
 *---------------------------------------------------------------------------
 *
 * AllocChannelBuffer --
 *
 *      A channel buffer has BUFFER_PADDING bytes extra at beginning to
 *      hold any bytes of a native-encoding character that got split by
 *      the end of the previous buffer and need to be moved to the
 *      beginning of the next buffer to make a contiguous string so it
 *      can be converted to UTF-8.
 *
 *      A channel buffer has BUFFER_PADDING bytes extra at the end to
 *      hold any bytes of a native-encoding character (generated from a
 *      UTF-8 character) that overflow past the end of the buffer and
 *      need to be moved to the next buffer.
 *
 * Results:
 *      A newly allocated channel buffer.
 *
 * Side effects:
 *      None.
 *
 *---------------------------------------------------------------------------
 */

static ChannelBuffer *
AllocChannelBuffer(length)
    int length;                 /* Desired length of channel buffer. */
{
    ChannelBuffer *bufPtr;
    int n;

    n = length + CHANNELBUFFER_HEADER_SIZE + BUFFER_PADDING + BUFFER_PADDING;
    bufPtr = (ChannelBuffer *) ckalloc((unsigned) n);
    bufPtr->nextAdded   = BUFFER_PADDING;
    bufPtr->nextRemoved = BUFFER_PADDING;
    bufPtr->bufLength   = length + BUFFER_PADDING;
    bufPtr->nextPtr     = (ChannelBuffer *) NULL;
    return bufPtr;
}

/*
 *----------------------------------------------------------------------
 *
 * RecycleBuffer --
 *
 *      Helper function to recycle input and output buffers. Ensures
 *      that two input buffers are saved (one in the input queue and
 *      another in the saveInBufPtr field) and that curOutPtr is set
 *      to a buffer. Only if these conditions are met is the buffer
 *      freed to the OS.
 *
 * Results:
 *      None.
 *
 * Side effects:
 *      May free a buffer to the OS.
 *
 *----------------------------------------------------------------------
 */

static void
RecycleBuffer(chanPtr, bufPtr, mustDiscard)
    Channel *chanPtr;           /* Channel for which to recycle buffers. */
    ChannelBuffer *bufPtr;      /* The buffer to recycle. */
    int mustDiscard;            /* If nonzero, free the buffer to the
                                 * OS, always. */
{
    /*
     * Do we have to free the buffer to the OS?
     */

    if (mustDiscard) {
        ckfree((char *) bufPtr);
        return;
    }
    
    /*
     * Only save buffers for the input queue if the channel is readable.
     */
    
    if (chanPtr->flags & TCL_READABLE) {
        if (chanPtr->inQueueHead == (ChannelBuffer *) NULL) {
            chanPtr->inQueueHead = bufPtr;
            chanPtr->inQueueTail = bufPtr;
            goto keepit;
        }
        if (chanPtr->saveInBufPtr == (ChannelBuffer *) NULL) {
            chanPtr->saveInBufPtr = bufPtr;
            goto keepit;
        }
    }

    /*
     * Only save buffers for the output queue if the channel is writable.
     */

    if (chanPtr->flags & TCL_WRITABLE) {
        if (chanPtr->curOutPtr == (ChannelBuffer *) NULL) {
            chanPtr->curOutPtr = bufPtr;
            goto keepit;
        }
    }

    /*
     * If we reached this code we return the buffer to the OS.
     */

    ckfree((char *) bufPtr);
    return;

keepit:
    bufPtr->nextRemoved = BUFFER_PADDING;
    bufPtr->nextAdded = BUFFER_PADDING;
    bufPtr->nextPtr = (ChannelBuffer *) NULL;
}

/*
 *----------------------------------------------------------------------
 *
 * DiscardOutputQueued --
 *
 *      Discards all output queued in the output queue of a channel.
 *
 * Results:
 *      None.
 *
 * Side effects:
 *      Recycles buffers.
 *
 *----------------------------------------------------------------------
 */

static void
DiscardOutputQueued(chanPtr)
    Channel *chanPtr;           /* The channel for which to discard output. */
{
    ChannelBuffer *bufPtr;
    
    while (chanPtr->outQueueHead != (ChannelBuffer *) NULL) {
        bufPtr = chanPtr->outQueueHead;
        chanPtr->outQueueHead = bufPtr->nextPtr;
        RecycleBuffer(chanPtr, bufPtr, 0);
    }
    chanPtr->outQueueHead = (ChannelBuffer *) NULL;
    chanPtr->outQueueTail = (ChannelBuffer *) NULL;
}

/*
 *----------------------------------------------------------------------
 *
 * CheckForDeadChannel --
 *
 *      This function checks is a given channel is Dead.
 *      (A channel that has been closed but not yet deallocated.)
 *
 * Results:
 *      True (1) if channel is Dead, False (0) if channel is Ok
 *
 * Side effects:
 *      None
 *
 *----------------------------------------------------------------------
 */

static int
CheckForDeadChannel(interp, chanPtr)
    Tcl_Interp *interp;         /* For error reporting (can be NULL) */
    Channel    *chanPtr;        /* The channel to check. */
{
    if (chanPtr->flags & CHANNEL_DEAD) {
        Tcl_SetErrno(EINVAL);
        if (interp) {
            Tcl_AppendResult(interp,
                             "unable to access channel: invalid channel",
                             (char *) NULL);   
        }
        return 1;
    }
    return 0;
}

/*
 *----------------------------------------------------------------------
 *
 * FlushChannel --
 *
 *      This function flushes as much of the queued output as is possible
 *      now. If calledFromAsyncFlush is nonzero, it is being called in an
 *      event handler to flush channel output asynchronously.
 *
 * Results:
 *      0 if successful, else the error code that was returned by the
 *      channel type operation.
 *
 * Side effects:
 *      May produce output on a channel. May block indefinitely if the
 *      channel is synchronous. May schedule an async flush on the channel.
 *      May recycle memory for buffers in the output queue.
 *
 *----------------------------------------------------------------------
 */

static int
FlushChannel(interp, chanPtr, calledFromAsyncFlush)
    Tcl_Interp *interp;                 /* For error reporting during close. */
    Channel *chanPtr;                   /* The channel to flush on. */
    int calledFromAsyncFlush;           /* If nonzero then we are being
                                         * called from an asynchronous
                                         * flush callback. */
{
    ChannelBuffer *bufPtr;              /* Iterates over buffered output
                                         * queue. */
    int toWrite;                        /* Amount of output data in current
                                         * buffer available to be written. */
    int written;                        /* Amount of output data actually
                                         * written in current round. */
    int errorCode = 0;                  /* Stores POSIX error codes from
                                         * channel driver operations. */
    int wroteSome = 0;                  /* Set to one if any data was
                                         * written to the driver. */

    /*
     * Prevent writing on a dead channel -- a channel that has been closed
     * but not yet deallocated. This can occur if the exit handler for the
     * channel deallocation runs before all channels are deregistered in
     * all interpreters.
     */
    
    if (CheckForDeadChannel(interp,chanPtr)) return -1;
    
    /*
     * Loop over the queued buffers and attempt to flush as
     * much as possible of the queued output to the channel.
     */

    while (1) {

        /*
         * If the queue is empty and there is a ready current buffer, OR if
         * the current buffer is full, then move the current buffer to the
         * queue.
         */
        
        if (((chanPtr->curOutPtr != (ChannelBuffer *) NULL) &&
                (chanPtr->curOutPtr->nextAdded == chanPtr->curOutPtr->bufLength))
                || ((chanPtr->flags & BUFFER_READY) &&
                        (chanPtr->outQueueHead == (ChannelBuffer *) NULL))) {
            chanPtr->flags &= (~(BUFFER_READY));
            chanPtr->curOutPtr->nextPtr = (ChannelBuffer *) NULL;
            if (chanPtr->outQueueHead == (ChannelBuffer *) NULL) {
                chanPtr->outQueueHead = chanPtr->curOutPtr;
            } else {
                chanPtr->outQueueTail->nextPtr = chanPtr->curOutPtr;
            }
            chanPtr->outQueueTail = chanPtr->curOutPtr;
            chanPtr->curOutPtr = (ChannelBuffer *) NULL;
        }
        bufPtr = chanPtr->outQueueHead;

        /*
         * If we are not being called from an async flush and an async
         * flush is active, we just return without producing any output.
         */

        if ((!calledFromAsyncFlush) &&
                (chanPtr->flags & BG_FLUSH_SCHEDULED)) {
            return 0;
        }

        /*
         * If the output queue is still empty, break out of the while loop.
         */

        if (bufPtr == (ChannelBuffer *) NULL) {
            break;      /* Out of the "while (1)". */
        }

        /*
         * Produce the output on the channel.
         */
        
        toWrite = bufPtr->nextAdded - bufPtr->nextRemoved;
        written = (chanPtr->typePtr->outputProc) (chanPtr->instanceData,
                (char *) bufPtr->buf + bufPtr->nextRemoved, toWrite,
                &errorCode);
            
        /*
         * If the write failed completely attempt to start the asynchronous
         * flush mechanism and break out of this loop - do not attempt to
         * write any more output at this time.
         */

        if (written < 0) {
            
            /*
             * If the last attempt to write was interrupted, simply retry.
             */
            
            if (errorCode == EINTR) {
                errorCode = 0;
                continue;
            }

            /*
             * If the channel is non-blocking and we would have blocked,
             * start a background flushing handler and break out of the loop.
             */

            if ((errorCode == EWOULDBLOCK) || (errorCode == EAGAIN)) {
                /*
                 * This used to check for CHANNEL_NONBLOCKING, and panic
                 * if the channel was blocking.  However, it appears
                 * that setting stdin to -blocking 0 has some effect on
                 * the stdout when it's a tty channel (dup'ed underneath)
                 */
                if (!(chanPtr->flags & BG_FLUSH_SCHEDULED)) {
                    chanPtr->flags |= BG_FLUSH_SCHEDULED;
                    UpdateInterest(chanPtr);
                }
                errorCode = 0;
                break;
            }

            /*
             * Decide whether to report the error upwards or defer it.
             */

            if (calledFromAsyncFlush) {
                if (chanPtr->unreportedError == 0) {
                    chanPtr->unreportedError = errorCode;
                }
            } else {
                Tcl_SetErrno(errorCode);
                if (interp != NULL) {
                    Tcl_SetResult(interp,
                            Tcl_PosixError(interp), TCL_VOLATILE);
                }
            }

            /*
             * When we get an error we throw away all the output
             * currently queued.
             */

            DiscardOutputQueued(chanPtr);
            continue;
        } else {
            wroteSome = 1;
        }

        bufPtr->nextRemoved += written;

        /*
         * If this buffer is now empty, recycle it.
         */

        if (bufPtr->nextRemoved == bufPtr->nextAdded) {
            chanPtr->outQueueHead = bufPtr->nextPtr;
            if (chanPtr->outQueueHead == (ChannelBuffer *) NULL) {
                chanPtr->outQueueTail = (ChannelBuffer *) NULL;
            }
            RecycleBuffer(chanPtr, bufPtr, 0);
        }
    }   /* Closes "while (1)". */

    /*
     * If we wrote some data while flushing in the background, we are done.
     * We can't finish the background flush until we run out of data and
     * the channel becomes writable again.  This ensures that all of the
     * pending data has been flushed at the system level.
     */

    if (chanPtr->flags & BG_FLUSH_SCHEDULED) {
        if (wroteSome) {
            return errorCode;
        } else if (chanPtr->outQueueHead == (ChannelBuffer *) NULL) {
            chanPtr->flags &= (~(BG_FLUSH_SCHEDULED));
            (chanPtr->typePtr->watchProc)(chanPtr->instanceData,
                    chanPtr->interestMask);
        }
    }

    /*
     * If the channel is flagged as closed, delete it when the refCount
     * drops to zero, the output queue is empty and there is no output
     * in the current output buffer.
     */

    if ((chanPtr->flags & CHANNEL_CLOSED) && (chanPtr->refCount <= 0) &&
            (chanPtr->outQueueHead == (ChannelBuffer *) NULL) &&
            ((chanPtr->curOutPtr == (ChannelBuffer *) NULL) ||
                    (chanPtr->curOutPtr->nextAdded ==
                            chanPtr->curOutPtr->nextRemoved))) {
        return CloseChannel(interp, chanPtr, errorCode);
    }
    return errorCode;
}

/*
 *----------------------------------------------------------------------
 *
 * CloseChannel --
 *
 *      Utility procedure to close a channel and free its associated
 *      resources.
 *
 * Results:
 *      0 on success or a POSIX error code if the operation failed.
 *
 * Side effects:
 *      May close the actual channel; may free memory.
 *
 *----------------------------------------------------------------------
 */

static int
CloseChannel(interp, chanPtr, errorCode)
    Tcl_Interp *interp;                 /* For error reporting. */
    Channel *chanPtr;                   /* The channel to close. */
    int errorCode;                      /* Status of operation so far. */
{
    int result = 0;                     /* Of calling driver close
                                         * operation. */
    Channel *prevChanPtr;               /* Preceding channel in list of
                                         * all channels - used to splice a
                                         * channel out of the list on close. */
    ThreadSpecificData *tsdPtr = TCL_TSD_INIT(&dataKey);

    if (chanPtr == NULL) {
        return result;
    }
    
    /*
     * No more input can be consumed so discard any leftover input.
     */

    DiscardInputQueued(chanPtr, 1);

    /*
     * Discard a leftover buffer in the current output buffer field.
     */

    if (chanPtr->curOutPtr != (ChannelBuffer *) NULL) {
        ckfree((char *) chanPtr->curOutPtr);
        chanPtr->curOutPtr = (ChannelBuffer *) NULL;
    }
    
    /*
     * The caller guarantees that there are no more buffers
     * queued for output.
     */

    if (chanPtr->outQueueHead != (ChannelBuffer *) NULL) {
        panic("TclFlush, closed channel: queued output left");
    }

    /*
     * If the EOF character is set in the channel, append that to the
     * output device.
     */

    if ((chanPtr->outEofChar != 0) && (chanPtr->flags & TCL_WRITABLE)) {
        int dummy;
        char c;

        c = (char) chanPtr->outEofChar;
        (chanPtr->typePtr->outputProc) (chanPtr->instanceData, &c, 1, &dummy);
    }

    /*
     * Remove TCL_READABLE and TCL_WRITABLE from chanPtr->flags, so
     * that close callbacks can not do input or output (assuming they
     * squirreled the channel away in their clientData). This also
     * prevents infinite loops if the callback calls any C API that
     * could call FlushChannel.
     */

    chanPtr->flags &= (~(TCL_READABLE|TCL_WRITABLE));
        
    /*
     * Splice this channel out of the list of all channels.
     */

    if (chanPtr == tsdPtr->firstChanPtr) {
        tsdPtr->firstChanPtr = chanPtr->nextChanPtr;
    } else {
        for (prevChanPtr = tsdPtr->firstChanPtr;
                 (prevChanPtr != (Channel *) NULL) &&
                     (prevChanPtr->nextChanPtr != chanPtr);
                 prevChanPtr = prevChanPtr->nextChanPtr) {
            /* Empty loop body. */
        }
        if (prevChanPtr == (Channel *) NULL) {
            panic("FlushChannel: damaged channel list");
        }
        prevChanPtr->nextChanPtr = chanPtr->nextChanPtr;
    }

    /*
     * Close and free the channel driver state.
     */
            
    if (chanPtr->typePtr->closeProc != TCL_CLOSE2PROC) {
        result = (chanPtr->typePtr->closeProc)(chanPtr->instanceData, interp);
    } else {
        result = (chanPtr->typePtr->close2Proc)(chanPtr->instanceData, interp,
                0);
    }
    
    if (chanPtr->channelName != (char *) NULL) {
        ckfree(chanPtr->channelName);
    }
    Tcl_FreeEncoding(chanPtr->encoding);
    if (chanPtr->outputStage != NULL) {
        ckfree((char *) chanPtr->outputStage);
    }
    
    /*
     * If we are being called synchronously, report either
     * any latent error on the channel or the current error.
     */
        
    if (chanPtr->unreportedError != 0) {
        errorCode = chanPtr->unreportedError;
    }
    if (errorCode == 0) {
        errorCode = result;
        if (errorCode != 0) {
            Tcl_SetErrno(errorCode);
        }
    }

    /* Andreas Kupries <a.kupries@westend.com>, 12/13/1998
     * "Trf-Patch for filtering channels"
     *
     * This is the change to 'CloseChannel'.
     *
     * Explanation
     *          Closing a filtering channel closes the one it
     *          superceded too. This basically ripples through
     *          the whole chain of filters until it reaches
     *          the underlying normal channel.
     *
     *          This is done by reintegrating the superceded
     *          channel into the (thread) global list of open
     *          channels and then invoking a regular close.
     *          There is no need to handle the complexities of
     *          this process by ourselves.
     *
     *          *Note*
     *          This has to be done after the call to the
     *          'closeProc' of the filtering channel to allow
     *          that one to flush internal buffers into
     *          the underlying channel.
     */

    if (chanPtr->supercedes != (Channel*) NULL) {
        /*
         * Insert the channel we were stacked upon back into
         * the list of open channels, then do a regular close.
         */

        chanPtr->supercedes->nextChanPtr = tsdPtr->firstChanPtr;
        tsdPtr->firstChanPtr             = chanPtr->supercedes;
        chanPtr->supercedes->refCount --; /* is deregistered */
        Tcl_Close (interp, (Tcl_Channel) chanPtr->supercedes);
    }

    /*
     * Cancel any outstanding timer.
     */

    Tcl_DeleteTimerHandler(chanPtr->timer);

    /*
     * Mark the channel as deleted by clearing the type structure.
     */

    chanPtr->typePtr = NULL;

    Tcl_EventuallyFree((ClientData) chanPtr, TCL_DYNAMIC);

    return errorCode;
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_Close --
 *
 *      Closes a channel.
 *
 * Results:
 *      A standard Tcl result.
 *
 * Side effects:
 *      Closes the channel if this is the last reference.
 *
 * NOTE:
 *      Tcl_Close removes the channel as far as the user is concerned.
 *      However, it may continue to exist for a while longer if it has
 *      a background flush scheduled. The device itself is eventually
 *      closed and the channel record removed, in CloseChannel, above.
 *
 *----------------------------------------------------------------------
 */

        /* ARGSUSED */
int
Tcl_Close(interp, chan)
    Tcl_Interp *interp;                 /* Interpreter for errors. */
    Tcl_Channel chan;                   /* The channel being closed. Must
                                         * not be referenced in any
                                         * interpreter. */
{
    ChannelHandler *chPtr, *chNext;     /* Iterate over channel handlers. */
    CloseCallback *cbPtr;               /* Iterate over close callbacks
                                         * for this channel. */
    EventScriptRecord *ePtr, *eNextPtr; /* Iterate over eventscript records. */
    Channel *chanPtr;                   /* The real IO channel. */
    int result;                         /* Of calling FlushChannel. */
    ThreadSpecificData *tsdPtr = TCL_TSD_INIT(&dataKey);
    NextChannelHandler *nhPtr;

    if (chan == (Tcl_Channel) NULL) {
        return TCL_OK;
    }
    
    /*
     * Perform special handling for standard channels being closed. If the
     * refCount is now 1 it means that the last reference to the standard
     * channel is being explicitly closed, so bump the refCount down
     * artificially to 0. This will ensure that the channel is actually
     * closed, below. Also set the static pointer to NULL for the channel.
     */

    CheckForStdChannelsBeingClosed(chan);

    chanPtr = (Channel *) chan;
    if (chanPtr->refCount > 0) {
        panic("called Tcl_Close on channel with refCount > 0");
    }

    /*
     * Remove any references to channel handlers for this channel that
     * may be about to be invoked.
     */

    for (nhPtr = tsdPtr->nestedHandlerPtr;
             nhPtr != (NextChannelHandler *) NULL;
             nhPtr = nhPtr->nestedHandlerPtr) {
        if (nhPtr->nextHandlerPtr &&
                (nhPtr->nextHandlerPtr->chanPtr == chanPtr)) {
            nhPtr->nextHandlerPtr = NULL;
        }
    }

    /*
     * Remove all the channel handler records attached to the channel
     * itself.
     */
        
    for (chPtr = chanPtr->chPtr;
             chPtr != (ChannelHandler *) NULL;
             chPtr = chNext) {
        chNext = chPtr->nextPtr;
        ckfree((char *) chPtr);
    }
    chanPtr->chPtr = (ChannelHandler *) NULL;
    
    
    /*
     * Cancel any pending copy operation.
     */

    StopCopy(chanPtr->csPtr);

    /*
     * Must set the interest mask now to 0, otherwise infinite loops
     * will occur if Tcl_DoOneEvent is called before the channel is
     * finally deleted in FlushChannel. This can happen if the channel
     * has a background flush active.
     */
        
    chanPtr->interestMask = 0;
    
    /*
     * Remove any EventScript records for this channel.
     */

    for (ePtr = chanPtr->scriptRecordPtr;
             ePtr != (EventScriptRecord *) NULL;
             ePtr = eNextPtr) {
        eNextPtr = ePtr->nextPtr;
        Tcl_DecrRefCount(ePtr->scriptPtr);
        ckfree((char *) ePtr);
    }
    chanPtr->scriptRecordPtr = (EventScriptRecord *) NULL;
        
    /*
     * Invoke the registered close callbacks and delete their records.
     */

    while (chanPtr->closeCbPtr != (CloseCallback *) NULL) {
        cbPtr = chanPtr->closeCbPtr;
        chanPtr->closeCbPtr = cbPtr->nextPtr;
        (cbPtr->proc) (cbPtr->clientData);
        ckfree((char *) cbPtr);
    }

    /*
     * Ensure that the last output buffer will be flushed.
     */
    
    if ((chanPtr->curOutPtr != (ChannelBuffer *) NULL) &&
           (chanPtr->curOutPtr->nextAdded > chanPtr->curOutPtr->nextRemoved)) {
        chanPtr->flags |= BUFFER_READY;
    }

    /*
     * If this channel supports it, close the read side, since we don't need it
     * anymore and this will help avoid deadlocks on some channel types.
     */

    if (chanPtr->typePtr->closeProc == TCL_CLOSE2PROC) {
        result = (chanPtr->typePtr->close2Proc)(chanPtr->instanceData, interp,
                TCL_CLOSE_READ);
    } else {
        result = 0;
    }

    /*
     * The call to FlushChannel will flush any queued output and invoke
     * the close function of the channel driver, or it will set up the
     * channel to be flushed and closed asynchronously.
     */

    chanPtr->flags |= CHANNEL_CLOSED;
    if ((FlushChannel(interp, chanPtr, 0) != 0) || (result != 0)) {
        return TCL_ERROR;
    }
    return TCL_OK;
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_Write --
 *
 *      Puts a sequence of bytes into an output buffer, may queue the
 *      buffer for output if it gets full, and also remembers whether the
 *      current buffer is ready e.g. if it contains a newline and we are in
 *      line buffering mode.
 *
 * Results:
 *      The number of bytes written or -1 in case of error. If -1,
 *      Tcl_GetErrno will return the error code.
 *
 * Side effects:
 *      May buffer up output and may cause output to be produced on the
 *      channel.
 *
 *----------------------------------------------------------------------
 */

int
Tcl_Write(chan, src, srcLen)
    Tcl_Channel chan;                   /* The channel to buffer output for. */
    char *src;                          /* Data to queue in output buffer. */
    int srcLen;                         /* Length of data in bytes, or < 0 for
                                         * strlen(). */
{
    Channel *chanPtr;

    chanPtr = (Channel *) chan;
    if (CheckChannelErrors(chanPtr, TCL_WRITABLE) != 0) {
        return -1;
    }
    if (srcLen < 0) {
        srcLen = strlen(src);
    }
    return DoWrite(chanPtr, src, srcLen);
}

/*
 *---------------------------------------------------------------------------
 *
 * Tcl_WriteChars --
 *
 *      Takes a sequence of UTF-8 characters and converts them for output
 *      using the channel's current encoding, may queue the buffer for
 *      output if it gets full, and also remembers whether the current
 *      buffer is ready e.g. if it contains a newline and we are in
 *      line buffering mode.
 *
 * Results:
 *      The number of bytes written or -1 in case of error. If -1,
 *      Tcl_GetErrno will return the error code.
 *
 * Side effects:
 *      May buffer up output and may cause output to be produced on the
 *      channel.
 *
 *----------------------------------------------------------------------
 */

int
Tcl_WriteChars(chan, src, len)
    Tcl_Channel chan;           /* The channel to buffer output for. */
    CONST char *src;            /* UTF-8 characters to queue in output buffer. */
    int len;                    /* Length of string in bytes, or < 0 for 
                                 * strlen(). */
{
    Channel *chanPtr;

    chanPtr = (Channel *) chan;
    if (CheckChannelErrors(chanPtr, TCL_WRITABLE) != 0) {
        return -1;
    }
    if (len < 0) {
        len = strlen(src);
    }
    if (chanPtr->encoding == NULL) {
        /*
         * Inefficient way to convert UTF-8 to byte-array, but the  
         * code parallels the way it is done for objects.
         */

        Tcl_Obj *objPtr;
        int result;

        objPtr = Tcl_NewStringObj(src, len);
        src = (char *) Tcl_GetByteArrayFromObj(objPtr, &len);
        result = WriteBytes(chanPtr, src, len);
        Tcl_DecrRefCount(objPtr);
        return result;
    }
    return WriteChars(chanPtr, src, len);
}

/*
 *---------------------------------------------------------------------------
 *
 * Tcl_WriteObj --
 *
 *      Takes the Tcl object and queues its contents for output.  If the 
 *      encoding of the channel is NULL, takes the byte-array representation 
 *      of the object and queues those bytes for output.  Otherwise, takes 
 *      the characters in the UTF-8 (string) representation of the object 
 *      and converts them for output using the channel's current encoding.  
 *      May flush internal buffers to output if one becomes full or is ready 
 *      for some other reason, e.g. if it contains a newline and the channel 
 *      is in line buffering mode.
 *
 * Results:
 *      The number of bytes written or -1 in case of error. If -1, 
 *      Tcl_GetErrno() will return the error code.
 *
 * Side effects:
 *      May buffer up output and may cause output to be produced on the
 *      channel.
 *
 *----------------------------------------------------------------------
 */

int
Tcl_WriteObj(chan, objPtr)
    Tcl_Channel chan;           /* The channel to buffer output for. */
    Tcl_Obj *objPtr;            /* The object to write. */
{
    Channel *chanPtr;
    char *src;
    int srcLen;

    chanPtr = (Channel *) chan;
    if (CheckChannelErrors(chanPtr, TCL_WRITABLE) != 0) {
        return -1;
    }
    if (chanPtr->encoding == NULL) {
        src = (char *) Tcl_GetByteArrayFromObj(objPtr, &srcLen);
        return WriteBytes(chanPtr, src, srcLen);
    } else {
        src = Tcl_GetStringFromObj(objPtr, &srcLen);
        return WriteChars(chanPtr, src, srcLen);
    }
}

/*
 *----------------------------------------------------------------------
 *
 * WriteBytes --
 *
 *      Write a sequence of bytes into an output buffer, may queue the
 *      buffer for output if it gets full, and also remembers whether the
 *      current buffer is ready e.g. if it contains a newline and we are in
 *      line buffering mode.
 *
 * Results:
 *      The number of bytes written or -1 in case of error. If -1,
 *      Tcl_GetErrno will return the error code.
 *
 * Side effects:
 *      May buffer up output and may cause output to be produced on the
 *      channel.
 *
 *----------------------------------------------------------------------
 */

static int
WriteBytes(chanPtr, src, srcLen)
    Channel *chanPtr;           /* The channel to buffer output for. */
    CONST char *src;            /* Bytes to write. */
    int srcLen;                 /* Number of bytes to write. */
{
    ChannelBuffer *bufPtr;
    char *dst;
    int dstLen, dstMax, sawLF, savedLF, total, toWrite;
    
    total = 0;
    sawLF = 0;
    savedLF = 0;

    /*
     * Loop over all bytes in src, storing them in output buffer with
     * proper EOL translation.
     */

    while (srcLen + savedLF > 0) {
        bufPtr = chanPtr->curOutPtr;
        if (bufPtr == NULL) {
            bufPtr = AllocChannelBuffer(chanPtr->bufSize);
            chanPtr->curOutPtr  = bufPtr;
        }
        dst = bufPtr->buf + bufPtr->nextAdded;
        dstMax = bufPtr->bufLength - bufPtr->nextAdded;
        dstLen = dstMax;

        toWrite = dstLen;
        if (toWrite > srcLen) {
            toWrite = srcLen;
        }

        if (savedLF) {
            /*
             * A '\n' was left over from last call to TranslateOutputEOL()
             * and we need to store it in this buffer.  If the channel is
             * line-based, we will need to flush it.
             */

            *dst++ = '\n';
            dstLen--;
            sawLF++;
        }
        sawLF += TranslateOutputEOL(chanPtr, dst, src, &dstLen, &toWrite);
        dstLen += savedLF;
        savedLF = 0;

        if (dstLen > dstMax) {
            savedLF = 1;
            dstLen = dstMax;
        }
        bufPtr->nextAdded += dstLen;
        if (CheckFlush(chanPtr, bufPtr, sawLF) != 0) {
            return -1;
        }
        total += dstLen;
        src += toWrite;
        srcLen -= toWrite;
        sawLF = 0;
    }
    return total;
}

/*
 *----------------------------------------------------------------------
 *
 * WriteChars --
 *
 *      Convert UTF-8 bytes to the channel's external encoding and
 *      write the produced bytes into an output buffer, may queue the 
 *      buffer for output if it gets full, and also remembers whether the
 *      current buffer is ready e.g. if it contains a newline and we are in
 *      line buffering mode.
 *
 * Results:
 *      The number of bytes written or -1 in case of error. If -1,
 *      Tcl_GetErrno will return the error code.
 *
 * Side effects:
 *      May buffer up output and may cause output to be produced on the
 *      channel.
 *
 *----------------------------------------------------------------------
 */

static int
WriteChars(chanPtr, src, srcLen)
    Channel *chanPtr;           /* The channel to buffer output for. */
    CONST char *src;            /* UTF-8 string to write. */
    int srcLen;                 /* Length of UTF-8 string in bytes. */
{
    ChannelBuffer *bufPtr;
    char *dst, *stage;
    int saved, savedLF, sawLF, total, toWrite, flags;
    int dstWrote, dstLen, stageLen, stageMax, stageRead;
    Tcl_Encoding encoding;
    char safe[BUFFER_PADDING];
    
    total = 0;
    sawLF = 0;
    savedLF = 0;
    saved = 0;
    encoding = chanPtr->encoding;

    /*
     * Loop over all UTF-8 characters in src, storing them in staging buffer
     * with proper EOL translation.
     */

    while (srcLen + savedLF > 0) {
        stage = chanPtr->outputStage;
        stageMax = chanPtr->bufSize;
        stageLen = stageMax;

        toWrite = stageLen;
        if (toWrite > srcLen) {
            toWrite = srcLen;
        }

        if (savedLF) {
            /*
             * A '\n' was left over from last call to TranslateOutputEOL()
             * and we need to store it in the staging buffer.  If the
             * channel is line-based, we will need to flush the output
             * buffer (after translating the staging buffer).
             */
            
            *stage++ = '\n';
            stageLen--;
            sawLF++;
        }
        sawLF += TranslateOutputEOL(chanPtr, stage, src, &stageLen, &toWrite);

        stage -= savedLF;
        stageLen += savedLF;
        savedLF = 0;

        if (stageLen > stageMax) {
            savedLF = 1;
            stageLen = stageMax;
        }
        src += toWrite;
        srcLen -= toWrite;

        flags = chanPtr->outputEncodingFlags;
        if (srcLen == 0) {
            flags |= TCL_ENCODING_END;
        }

        /*
         * Loop over all UTF-8 characters in staging buffer, converting them
         * to external encoding, storing them in output buffer.
         */

        while (stageLen + saved > 0) {
            bufPtr = chanPtr->curOutPtr;
            if (bufPtr == NULL) {
                bufPtr = AllocChannelBuffer(chanPtr->bufSize);
                chanPtr->curOutPtr = bufPtr;
            }
            dst = bufPtr->buf + bufPtr->nextAdded;
            dstLen = bufPtr->bufLength - bufPtr->nextAdded;

            if (saved != 0) {
                /*
                 * Here's some translated bytes left over from the last
                 * buffer that we need to stick at the beginning of this
                 * buffer.
                 */
                 
                memcpy((VOID *) dst, (VOID *) safe, (size_t) saved);
                bufPtr->nextAdded += saved;
                dst += saved;
                dstLen -= saved;
                saved = 0;
            }

            Tcl_UtfToExternal(NULL, encoding, stage, stageLen, flags,
                    &chanPtr->outputEncodingState, dst,
                    dstLen + BUFFER_PADDING, &stageRead, &dstWrote, NULL);
            if (stageRead + dstWrote == 0) {
                /*
                 * We have an incomplete UTF-8 character at the end of the
                 * staging buffer.  It will get moved to the beginning of the
                 * staging buffer followed by more bytes from src.
                 */

                src -= stageLen;
                srcLen += stageLen;
                stageLen = 0;
                savedLF = 0;
                break;
            }
            bufPtr->nextAdded += dstWrote;
            if (bufPtr->nextAdded > bufPtr->bufLength) {
                /*
                 * When translating from UTF-8 to external encoding, we
                 * allowed the translation to produce a character that
                 * crossed the end of the output buffer, so that we would
                 * get a completely full buffer before flushing it.  The
                 * extra bytes will be moved to the beginning of the next
                 * buffer.
                 */

                saved = bufPtr->nextAdded - bufPtr->bufLength;
                memcpy((VOID *) safe, (VOID *) (dst + dstLen), (size_t) saved);
                bufPtr->nextAdded = bufPtr->bufLength;
            }
            if (CheckFlush(chanPtr, bufPtr, sawLF) != 0) {
                return -1;
            }

            total += dstWrote;
            stage += stageRead;
            stageLen -= stageRead;
            sawLF = 0;
        }
    }
    return total;
}

/*
 *---------------------------------------------------------------------------
 *
 * TranslateOutputEOL --
 *
 *      Helper function for WriteBytes() and WriteChars().  Converts the
 *      '\n' characters in the source buffer into the appropriate EOL
 *      form specified by the output translation mode.
 *
 *      EOL translation stops either when the source buffer is empty
 *      or the output buffer is full.
 *
 *      When converting to CRLF mode and there is only 1 byte left in
 *      the output buffer, this routine stores the '\r' in the last
 *      byte and then stores the '\n' in the byte just past the end of the 
 *      buffer.  The caller is responsible for passing in a buffer that
 *      is large enough to hold the extra byte.
 *
 * Results:
 *      The return value is 1 if a '\n' was translated from the source
 *      buffer, or 0 otherwise -- this can be used by the caller to
 *      decide to flush a line-based channel even though the channel
 *      buffer is not full.
 *
 *      *dstLenPtr is filled with how many bytes of the output buffer
 *      were used.  As mentioned above, this can be one more that
 *      the output buffer's specified length if a CRLF was stored.
 *
 *      *srcLenPtr is filled with how many bytes of the source buffer
 *      were consumed.  
 *
 * Side effects:
 *      It may be obvious, but bears mentioning that when converting
 *      in CRLF mode (which requires two bytes of storage in the output
 *      buffer), the number of bytes consumed from the source buffer
 *      will be less than the number of bytes stored in the output buffer.
 *
 *---------------------------------------------------------------------------
 */

static int
TranslateOutputEOL(chanPtr, dst, src, dstLenPtr, srcLenPtr)
    Channel *chanPtr;           /* Channel being read, for translation and
                                 * buffering modes. */
    char *dst;                  /* Output buffer filled with UTF-8 chars by
                                 * applying appropriate EOL translation to
                                 * source characters. */
    CONST char *src;            /* Source UTF-8 characters. */
    int *dstLenPtr;             /* On entry, the maximum length of output
                                 * buffer in bytes.  On exit, the number of
                                 * bytes actually used in output buffer. */
    int *srcLenPtr;             /* On entry, the length of source buffer.
                                 * On exit, the number of bytes read from
                                 * the source buffer. */
{
    char *dstEnd;
    int srcLen, newlineFound;
    
    newlineFound = 0;
    srcLen = *srcLenPtr;

    switch (chanPtr->outputTranslation) {
        case TCL_TRANSLATE_LF: {
            for (dstEnd = dst + srcLen; dst < dstEnd; ) {
                if (*src == '\n') {
                    newlineFound = 1;
                }
                *dst++ = *src++;
            }
            *dstLenPtr = srcLen;
            break;
        }
        case TCL_TRANSLATE_CR: {
            for (dstEnd = dst + srcLen; dst < dstEnd;) {
                if (*src == '\n') {
                    *dst++ = '\r';
                    newlineFound = 1;
                    src++;
                } else {
                    *dst++ = *src++;
                }
            }
            *dstLenPtr = srcLen;
            break;
        }
        case TCL_TRANSLATE_CRLF: {
            /*
             * Since this causes the number of bytes to grow, we
             * start off trying to put 'srcLen' bytes into the
             * output buffer, but allow it to store more bytes, as
             * long as there's still source bytes and room in the
             * output buffer.
             */

            char *dstStart, *dstMax;
            CONST char *srcStart;
            
            dstStart = dst;
            dstMax = dst + *dstLenPtr;

            srcStart = src;
            
            if (srcLen < *dstLenPtr) {
                dstEnd = dst + srcLen;
            } else {
                dstEnd = dst + *dstLenPtr;
            }
            while (dst < dstEnd) {
                if (*src == '\n') {
                    if (dstEnd < dstMax) {
                        dstEnd++;
                    }
                    *dst++ = '\r';
                    newlineFound = 1;
                }
                *dst++ = *src++;
            }
            *srcLenPtr = src - srcStart;
            *dstLenPtr = dst - dstStart;
            break;
        }
        default: {
            break;
        }
    }
    return newlineFound;
}

/*
 *---------------------------------------------------------------------------
 *
 * CheckFlush --
 *
 *      Helper function for WriteBytes() and WriteChars().  If the
 *      channel buffer is ready to be flushed, flush it.
 *
 * Results:
 *      The return value is -1 if there was a problem flushing the
 *      channel buffer, or 0 otherwise.
 *
 * Side effects:
 *      The buffer will be recycled if it is flushed.
 *
 *---------------------------------------------------------------------------
 */

static int
CheckFlush(chanPtr, bufPtr, newlineFlag)
    Channel *chanPtr;           /* Channel being read, for buffering mode. */
    ChannelBuffer *bufPtr;      /* Channel buffer to possibly flush. */
    int newlineFlag;            /* Non-zero if a the channel buffer
                                 * contains a newline. */
{
    /*
     * The current buffer is ready for output:
     * 1. if it is full.
     * 2. if it contains a newline and this channel is line-buffered.
     * 3. if it contains any output and this channel is unbuffered.
     */

    if ((chanPtr->flags & BUFFER_READY) == 0) {
        if (bufPtr->nextAdded == bufPtr->bufLength) {
            chanPtr->flags |= BUFFER_READY;
        } else if (chanPtr->flags & CHANNEL_LINEBUFFERED) {
            if (newlineFlag != 0) {
                chanPtr->flags |= BUFFER_READY;
            }
        } else if (chanPtr->flags & CHANNEL_UNBUFFERED) {
            chanPtr->flags |= BUFFER_READY;
        }
    }
    if (chanPtr->flags & BUFFER_READY) {
        if (FlushChannel(NULL, chanPtr, 0) != 0) {
            return -1;
        }
    }
    return 0;
}

/*
 *---------------------------------------------------------------------------
 *
 * Tcl_Gets --
 *
 *      Reads a complete line of input from the channel into a Tcl_DString.
 *
 * Results:
 *      Length of line read (in characters) or -1 if error, EOF, or blocked.
 *      If -1, use Tcl_GetErrno() to retrieve the POSIX error code for the
 *      error or condition that occurred.
 *
 * Side effects:
 *      May flush output on the channel.  May cause input to be consumed
 *      from the channel.
 *
 *---------------------------------------------------------------------------
 */

int
Tcl_Gets(chan, lineRead)
    Tcl_Channel chan;           /* Channel from which to read. */
    Tcl_DString *lineRead;      /* The line read will be appended to this
                                 * DString as UTF-8 characters.  The caller
                                 * must have initialized it and is responsible
                                 * for managing the storage. */
{
    Tcl_Obj *objPtr;
    int charsStored, length;
    char *string;

    objPtr = Tcl_NewObj();
    charsStored = Tcl_GetsObj(chan, objPtr);
    if (charsStored > 0) {
        string = Tcl_GetStringFromObj(objPtr, &length);
        Tcl_DStringAppend(lineRead, string, length);
    }
    Tcl_DecrRefCount(objPtr);
    return charsStored;
}

/*
 *---------------------------------------------------------------------------
 *
 * Tcl_GetsObj --
 *
 *      Accumulate input from the input channel until end-of-line or
 *      end-of-file has been seen.  Bytes read from the input channel
 *      are converted to UTF-8 using the encoding specified by the
 *      channel.
 *
 * Results:
 *      Number of characters accumulated in the object or -1 if error,
 *      blocked, or EOF.  If -1, use Tcl_GetErrno() to retrieve the
 *      POSIX error code for the error or condition that occurred.
 *
 * Side effects:
 *      Consumes input from the channel.
 *
 *      On reading EOF, leave channel pointing at EOF char.
 *      On reading EOL, leave channel pointing after EOL, but don't
 *      return EOL in dst buffer.
 *
 *---------------------------------------------------------------------------
 */

int
Tcl_GetsObj(chan, objPtr)
    Tcl_Channel chan;           /* Channel from which to read. */
    Tcl_Obj *objPtr;            /* The line read will be appended to this
                                 * object as UTF-8 characters. */
{
    GetsState gs;
    Channel *chanPtr;
    int inEofChar, skip, copiedTotal;
    ChannelBuffer *bufPtr;
    Tcl_Encoding encoding;
    char *dst, *dstEnd, *eol, *eof;
    Tcl_EncodingState oldState;
    int oldLength, oldFlags, oldRemoved;

    chanPtr = (Channel *) chan;
    if (CheckChannelErrors(chanPtr, TCL_READABLE) != 0) {
        copiedTotal = -1;
        goto done;
    }

    bufPtr = chanPtr->inQueueHead;
    encoding = chanPtr->encoding;

    /*
     * Preserved so we can restore the channel's state in case we don't
     * find a newline in the available input.
     */

    Tcl_GetStringFromObj(objPtr, &oldLength);
    oldFlags = chanPtr->inputEncodingFlags;
    oldState = chanPtr->inputEncodingState;
    oldRemoved = BUFFER_PADDING;
    if (bufPtr != NULL) {
        oldRemoved = bufPtr->nextRemoved;
    }

    /*
     * If there is no encoding, use "iso8859-1" -- Tcl_GetsObj() doesn't
     * produce ByteArray objects.  To avoid circularity problems,
     * "iso8859-1" is builtin to Tcl.
     */

    if (encoding == NULL) {
        encoding = Tcl_GetEncoding(NULL, "iso8859-1");
    }

    /*
     * Object used by FilterInputBytes to keep track of how much data has
     * been consumed from the channel buffers.
     */

    gs.objPtr           = objPtr;
    gs.dstPtr           = &dst;
    gs.encoding         = encoding;
    gs.bufPtr           = bufPtr;
    gs.state            = oldState;
    gs.rawRead          = 0;
    gs.bytesWrote       = 0;
    gs.charsWrote       = 0;
    gs.totalChars       = 0;

    dst = objPtr->bytes + oldLength;
    dstEnd = dst;

    skip = 0;
    eof = NULL;
    inEofChar = chanPtr->inEofChar;

    while (1) {
        if (dst >= dstEnd) {
            if (FilterInputBytes(chanPtr, &gs) != 0) {
                goto restore;
            }
            dstEnd = dst + gs.bytesWrote;
        }
        
        /*
         * Remember if EOF char is seen, then look for EOL anyhow, because
         * the EOL might be before the EOF char.
         */

        if (inEofChar != '\0') {
            for (eol = dst; eol < dstEnd; eol++) {
                if (*eol == inEofChar) {
                    dstEnd = eol;
                    eof = eol;
                    break;
                }
            }
        }

        /*
         * On EOL, leave current file position pointing after the EOL, but
         * don't store the EOL in the output string.
         */

        eol = dst;
        switch (chanPtr->inputTranslation) {
            case TCL_TRANSLATE_LF: {
                for (eol = dst; eol < dstEnd; eol++) {
                    if (*eol == '\n') {
                        skip = 1;
                        goto goteol;
                    }
                }
                break;
            }
            case TCL_TRANSLATE_CR: {
                for (eol = dst; eol < dstEnd; eol++) {
                    if (*eol == '\r') {
                        skip = 1;
                        goto goteol;
                    }
                }
                break;
            }
            case TCL_TRANSLATE_CRLF: {
                for (eol = dst; eol < dstEnd; eol++) {
                    if (*eol == '\r') {
                        eol++;
                        if (eol >= dstEnd) {
                            int offset;
                            
                            offset = eol - objPtr->bytes;
                            dst = dstEnd;
                            if (FilterInputBytes(chanPtr, &gs) != 0) {
                                goto restore;
                            }
                            dstEnd = dst + gs.bytesWrote;
                            eol = objPtr->bytes + offset;
                            if (eol >= dstEnd) {
                                skip = 0;
                                goto goteol;
                            }
                        }
                        if (*eol == '\n') {
                            eol--;
                            skip = 2;
                            goto goteol;
                        }
                    }
                }
                break;
            }
            case TCL_TRANSLATE_AUTO: {
                skip = 1;
                if (chanPtr->flags & INPUT_SAW_CR) {
                    chanPtr->flags &= ~INPUT_SAW_CR;
                    if (*eol == '\n') {
                        /*
                         * Skip the raw bytes that make up the '\n'.
                         */

                        char tmp[1 + TCL_UTF_MAX];
                        int rawRead;

                        bufPtr = gs.bufPtr;
                        Tcl_ExternalToUtf(NULL, gs.encoding,
                                bufPtr->buf + bufPtr->nextRemoved,
                                gs.rawRead, chanPtr->inputEncodingFlags,
                                &gs.state, tmp, 1 + TCL_UTF_MAX, &rawRead,
                                NULL, NULL);
                        bufPtr->nextRemoved += rawRead;
                        gs.rawRead -= rawRead;
                        gs.bytesWrote--;
                        gs.charsWrote--;
                        memmove(dst, dst + 1, (size_t) (dstEnd - dst));
                        dstEnd--;
                    }
                }
                for (eol = dst; eol < dstEnd; eol++) {
                    if (*eol == '\r') {
                        eol++;
                        if (eol == dstEnd) {
                            /*
                             * If buffer ended on \r, peek ahead to see if a
                             * \n is available.
                             */

                            int offset;
                            
                            offset = eol - objPtr->bytes;
                            dst = dstEnd;
                            PeekAhead(chanPtr, &dstEnd, &gs);
                            eol = objPtr->bytes + offset;
                            if (eol >= dstEnd) {
                                eol--;
                                chanPtr->flags |= INPUT_SAW_CR;
                                goto goteol;
                            }
                        }
                        if (*eol == '\n') {
                            skip++;
                        }
                        eol--;
                        goto goteol;
                    } else if (*eol == '\n') {
                        goto goteol;
                    }
                }
            }
        }
        if (eof != NULL) {
            /*
             * EOF character was seen.  On EOF, leave current file position
             * pointing at the EOF character, but don't store the EOF
             * character in the output string.
             */

            dstEnd = eof;
            chanPtr->flags |= (CHANNEL_EOF | CHANNEL_STICKY_EOF);
            chanPtr->inputEncodingFlags |= TCL_ENCODING_END;
        }
        if (chanPtr->flags & CHANNEL_EOF) {
            skip = 0;
            eol = dstEnd;
            if (eol == objPtr->bytes) {
                /*
                 * If we didn't produce any bytes before encountering EOF,
                 * caller needs to see -1.
                 */

                Tcl_SetObjLength(objPtr, 0);
                CommonGetsCleanup(chanPtr, encoding);
                copiedTotal = -1;
                goto done;
            }
            goto goteol;
        }
        dst = dstEnd;
    }

    /*
     * Found EOL or EOF, but the output buffer may now contain too many
     * UTF-8 characters.  We need to know how many raw bytes correspond to
     * the number of UTF-8 characters we want, plus how many raw bytes
     * correspond to the character(s) making up EOL (if any), so we can
     * remove the correct number of bytes from the channel buffer.
     */
     
    goteol:
    bufPtr = gs.bufPtr;
    chanPtr->inputEncodingState = gs.state;
    Tcl_ExternalToUtf(NULL, gs.encoding, bufPtr->buf + bufPtr->nextRemoved,
            gs.rawRead, chanPtr->inputEncodingFlags,
            &chanPtr->inputEncodingState, dst, eol - dst + skip + TCL_UTF_MAX,
            &gs.rawRead, NULL, &gs.charsWrote);
    bufPtr->nextRemoved += gs.rawRead;

    /*
     * Recycle all the emptied buffers.
     */

    Tcl_SetObjLength(objPtr, eol - objPtr->bytes);
    CommonGetsCleanup(chanPtr, encoding);
    chanPtr->flags &= ~CHANNEL_BLOCKED;
    copiedTotal = gs.totalChars + gs.charsWrote - skip;
    goto done;

    /*
     * Couldn't get a complete line.  This only happens if we get a error
     * reading from the channel or we are non-blocking and there wasn't
     * an EOL or EOF in the data available.
     */

    restore:
    bufPtr = chanPtr->inQueueHead;
    bufPtr->nextRemoved = oldRemoved;

    for (bufPtr = bufPtr->nextPtr; bufPtr != NULL; bufPtr = bufPtr->nextPtr) {
        bufPtr->nextRemoved = BUFFER_PADDING;
    }
    CommonGetsCleanup(chanPtr, encoding);

    chanPtr->inputEncodingState = oldState;
    chanPtr->inputEncodingFlags = oldFlags;
    Tcl_SetObjLength(objPtr, oldLength);

    /*
     * We didn't get a complete line so we need to indicate to UpdateInterest
     * that the gets blocked.  It will wait for more data instead of firing
     * a timer, avoiding a busy wait.  This is where we are assuming that the
     * next operation is a gets.  No more file events will be delivered on 
     * this channel until new data arrives or some operation is performed
     * on the channel (e.g. gets, read, fconfigure) that changes the blocking
     * state.  Note that this means a file event will not be delivered even
     * though a read would be able to consume the buffered data.
     */

    chanPtr->flags |= CHANNEL_NEED_MORE_DATA;
    copiedTotal = -1;

    done:
    /*
     * Update the notifier state so we don't block while there is still
     * data in the buffers.
     */

    UpdateInterest(chanPtr);
    return copiedTotal;
}

/*
 *---------------------------------------------------------------------------
 *
 * FilterInputBytes --
 *
 *      Helper function for Tcl_GetsObj.  Produces UTF-8 characters from
 *      raw bytes read from the channel.  
 *
 *      Consumes available bytes from channel buffers.  When channel
 *      buffers are exhausted, reads more bytes from channel device into
 *      a new channel buffer.  It is the caller's responsibility to
 *      free the channel buffers that have been exhausted.
 *
 * Results:
 *      The return value is -1 if there was an error reading from the
 *      channel, 0 otherwise.
 *
 * Side effects:
 *      Status object keeps track of how much data from channel buffers
 *      has been consumed and where UTF-8 bytes should be stored.
 *
 *---------------------------------------------------------------------------
 */
 
static int
FilterInputBytes(chanPtr, gsPtr)
    Channel *chanPtr;           /* Channel to read. */
    GetsState *gsPtr;           /* Current state of gets operation. */
{
    ChannelBuffer *bufPtr;
    char *raw, *rawStart, *rawEnd;
    char *dst;
    int offset, toRead, dstNeeded, spaceLeft, result, rawLen, length;
    Tcl_Obj *objPtr;
#define ENCODING_LINESIZE   30  /* Lower bound on how many bytes to convert
                                 * at a time.  Since we don't know a priori
                                 * how many bytes of storage this many source
                                 * bytes will use, we actually need at least
                                 * ENCODING_LINESIZE * TCL_MAX_UTF bytes of
                                 * room. */

    objPtr = gsPtr->objPtr;

    /*
     * Subtract the number of bytes that were removed from channel buffer
     * during last call.
     */

    bufPtr = gsPtr->bufPtr;
    if (bufPtr != NULL) {
        bufPtr->nextRemoved += gsPtr->rawRead;
        if (bufPtr->nextRemoved >= bufPtr->nextAdded) {
            bufPtr = bufPtr->nextPtr;
        }
    }
    gsPtr->totalChars += gsPtr->charsWrote;

    if ((bufPtr == NULL) || (bufPtr->nextAdded == BUFFER_PADDING)) {
        /*
         * All channel buffers were exhausted and the caller still hasn't
         * seen EOL.  Need to read more bytes from the channel device.
         * Side effect is to allocate another channel buffer.
         */
         
        read:
        if (chanPtr->flags & CHANNEL_BLOCKED) {
            if (chanPtr->flags & CHANNEL_NONBLOCKING) {
                gsPtr->charsWrote = 0;
                gsPtr->rawRead = 0;
                return -1;
            }
            chanPtr->flags &= ~CHANNEL_BLOCKED;
        }
        if (GetInput(chanPtr) != 0) {
            gsPtr->charsWrote = 0;
            gsPtr->rawRead = 0;
            return -1;
        }
        bufPtr = chanPtr->inQueueTail;
        gsPtr->bufPtr = bufPtr;
    }

    /*
     * Convert some of the bytes from the channel buffer to UTF-8.  Space in
     * objPtr's string rep is used to hold the UTF-8 characters.  Grow the
     * string rep if we need more space.
     */

    rawStart = bufPtr->buf + bufPtr->nextRemoved;
    raw = rawStart;
    rawEnd = bufPtr->buf + bufPtr->nextAdded;
    rawLen = rawEnd - rawStart;

    dst = *gsPtr->dstPtr;
    offset = dst - objPtr->bytes;
    toRead = ENCODING_LINESIZE;
    if (toRead > rawLen) {
        toRead = rawLen;
    }
    dstNeeded = toRead * TCL_UTF_MAX + 1;
    spaceLeft = objPtr->length - offset - TCL_UTF_MAX - 1;
    if (dstNeeded > spaceLeft) {
        length = offset * 2;
        if (offset < dstNeeded) {
            length = offset + dstNeeded;
        }
        length += TCL_UTF_MAX + 1;
        Tcl_SetObjLength(objPtr, length);
        spaceLeft = length - offset;
        dst = objPtr->bytes + offset;
        *gsPtr->dstPtr = dst;
    }
    gsPtr->state = chanPtr->inputEncodingState;
    result = Tcl_ExternalToUtf(NULL, gsPtr->encoding, raw, rawLen,
            chanPtr->inputEncodingFlags, &chanPtr->inputEncodingState,
            dst, spaceLeft, &gsPtr->rawRead, &gsPtr->bytesWrote,
            &gsPtr->charsWrote); 
    if (result == TCL_CONVERT_MULTIBYTE) {
        /*
         * The last few bytes in this channel buffer were the start of a
         * multibyte sequence.  If this buffer was full, then move them to
         * the next buffer so the bytes will be contiguous.  
         */

        ChannelBuffer *nextPtr;
        int extra;
        
        nextPtr = bufPtr->nextPtr;
        if (bufPtr->nextAdded < bufPtr->bufLength) {
            if (gsPtr->rawRead > 0) {
                /*
                 * Some raw bytes were converted to UTF-8.  Fall through,
                 * returning those UTF-8 characters because a EOL might be
                 * present in them.
                 */
            } else if (chanPtr->flags & CHANNEL_EOF) {
                /*
                 * There was a partial character followed by EOF on the
                 * device.  Fall through, returning that nothing was found.
                 */

                 bufPtr->nextRemoved = bufPtr->nextAdded;
            } else {
                /*
                 * There are no more cached raw bytes left.  See if we can
                 * get some more.
                 */

                goto read;
            }
        } else {
            if (nextPtr == NULL) {
                nextPtr = AllocChannelBuffer(chanPtr->bufSize);
                bufPtr->nextPtr = nextPtr;
                chanPtr->inQueueTail = nextPtr;
            }
            extra = rawLen - gsPtr->rawRead;
            memcpy((VOID *) (nextPtr->buf + BUFFER_PADDING - extra),
                    (VOID *) (raw + gsPtr->rawRead), (size_t) extra);
            nextPtr->nextRemoved -= extra;
            bufPtr->nextAdded -= extra;
        }
    }

    gsPtr->bufPtr = bufPtr;
    return 0;
}

/*
 *---------------------------------------------------------------------------
 *
 * PeekAhead --
 *
 *      Helper function used by Tcl_GetsObj().  Called when we've seen a
 *      \r at the end of the UTF-8 string and want to look ahead one
 *      character to see if it is a \n.
 *
 * Results:
 *      *gsPtr->dstPtr is filled with a pointer to the start of the range of
 *      UTF-8 characters that were found by peeking and *dstEndPtr is filled
 *      with a pointer to the bytes just after the end of the range.
 *
 * Side effects:
 *      If no more raw bytes were available in one of the channel buffers,
 *      tries to perform a non-blocking read to get more bytes from the
 *      channel device.
 *
 *---------------------------------------------------------------------------
 */

static void
PeekAhead(chanPtr, dstEndPtr, gsPtr)
    Channel *chanPtr;           /* The channel to read. */
    char **dstEndPtr;           /* Filled with pointer to end of new range
                                 * of UTF-8 characters. */
    GetsState *gsPtr;           /* Current state of gets operation. */
{
    ChannelBuffer *bufPtr;
    Tcl_DriverBlockModeProc *blockModeProc;
    int bytesLeft;

    bufPtr = gsPtr->bufPtr;

    /*
     * If there's any more raw input that's still buffered, we'll peek into
     * that.  Otherwise, only get more data from the channel driver if it
     * looks like there might actually be more data.  The assumption is that
     * if the channel buffer is filled right up to the end, then there
     * might be more data to read.
     */

    blockModeProc = NULL;
    if (bufPtr->nextPtr == NULL) {
        bytesLeft = bufPtr->nextAdded - (bufPtr->nextRemoved + gsPtr->rawRead);
        if (bytesLeft == 0) {
            if (bufPtr->nextAdded < bufPtr->bufLength) {
                /*
                 * Don't peek ahead if last read was short read.
                 */
                 
                goto cleanup;
            }
            if ((chanPtr->flags & CHANNEL_NONBLOCKING) == 0) {
                blockModeProc = chanPtr->typePtr->blockModeProc;
                if (blockModeProc == NULL) {
                    /*
                     * Don't peek ahead if cannot set non-blocking mode.