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Tcl_ChannelSeekProc

Tcl_CreateChannel(3)        Tcl Library Procedures        Tcl_CreateChannel(3)



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NAME
       Tcl_CreateChannel,    Tcl_GetChannelInstanceData,   Tcl_GetChannelType,
       Tcl_GetChannelName,      Tcl_GetChannelHandle,      Tcl_GetChannelMode,
       Tcl_GetChannelBufferSize,  Tcl_SetChannelBufferSize, Tcl_NotifyChannel,
       Tcl_BadChannelOption, Tcl_ChannelName, Tcl_ChannelVersion, Tcl_Channel-
       BlockModeProc,  Tcl_ChannelCloseProc,  Tcl_ChannelClose2Proc, Tcl_Chan-
       nelInputProc,  Tcl_ChannelOutputProc,  Tcl_ChannelSeekProc,   Tcl_Chan-
       nelSetOptionProc,    Tcl_ChannelGetOptionProc,    Tcl_ChannelWatchProc,
       Tcl_ChannelGetHandleProc, Tcl_ChannelFlushProc, Tcl_ChannelHandlerProc,
       - procedures for creating and manipulating channels

SYNOPSIS
       #include <tcl.h>

       Tcl_Channel
       Tcl_CreateChannel(typePtr, channelName, instanceData, mask)

       ClientData
       Tcl_GetChannelInstanceData(channel)

       Tcl_ChannelType *
       Tcl_GetChannelType(channel)

       char *
       Tcl_GetChannelName(channel)

       int
       Tcl_GetChannelHandle(channel, direction, handlePtr)

       int
       Tcl_GetChannelBufferSize(channel)

       Tcl_SetChannelBufferSize(channel, size)

       Tcl_NotifyChannel(channel, mask)

       int
       Tcl_BadChannelOption(interp, optionName, optionList)

       char *                                                                  |
       Tcl_ChannelName(typePtr)                                                |

       Tcl_ChannelTypeVersion                                                  |
       Tcl_ChannelVersion(typePtr)                                             |

       Tcl_DriverBlockModeProc *                                               |
       Tcl_ChannelBlockModeProc(typePtr)                                       |

       Tcl_DriverCloseProc *                                                   |
       Tcl_ChannelCloseProc(typePtr)                                           |

       Tcl_DriverClose2Proc *                                                  |
       Tcl_ChannelClose2Proc(typePtr)                                          |

       Tcl_DriverInputProc *                                                   |
       Tcl_ChannelInputProc(typePtr)                                           |

       Tcl_DriverOutputProc *                                                  |
       Tcl_ChannelOutputProc(typePtr)                                          |

       Tcl_DriverSeekProc *                                                    |
       Tcl_ChannelSeekProc(typePtr)                                            |

       Tcl_DriverSetOptionProc *                                               |
       Tcl_ChannelSetOptionProc(typePtr)                                       |

       Tcl_DriverGetOptionProc *                                               |
       Tcl_ChannelGetOptionProc(typePtr)                                       |

       Tcl_DriverWatchProc *                                                   |
       Tcl_ChannelWatchProc(typePtr)                                           |

       Tcl_DriverGetHandleProc *                                               |
       Tcl_ChannelGetHandleProc(typePtr)                                       |

       Tcl_DriverFlushProc *                                                   |
       Tcl_ChannelFlushProc(typePtr)                                           |

       Tcl_DriverHandlerProc *                                                 |
       Tcl_ChannelHandlerProc(typePtr)                                         |


ARGUMENTS
       Tcl_ChannelType      *typePtr       (in)      Points   to  a  structure
                                                     containing the  addresses
                                                     of procedures that can be
                                                     called to perform I/O and
                                                     other  functions  on  the
                                                     channel.

       char                 *channelName   (in)      The name of this channel,
                                                     such  as  file3; must not
                                                     be in use  by  any  other
                                                     channel.  Can be NULL, in
                                                     which case the channel is
                                                     created without a name.

       ClientData           instanceData   (in)      Arbitrary  one-word value
                                                     to  be  associated   with
                                                     this channel.  This value
                                                     is passed  to  procedures
                                                     in  typePtr when they are
                                                     invoked.

       int                  mask           (in)      OR-ed   combination    of
                                                     TCL_READABLE          and
                                                     TCL_WRITABLE to  indicate
                                                     whether   a   channel  is
                                                     readable and writable.

       Tcl_Channel          channel        (in)      The  channel  to  operate
                                                     on.

       int                  direction      (in)      TCL_READABLE   means  the
                                                     input handle  is  wanted;
                                                     TCL_WRITABLE   means  the
                                                     output handle is  wanted.

       ClientData           *handlePtr     (out)     Points  to  the  location
                                                     where the desired OS-spe-
                                                     cific  handle  should  be
                                                     stored.

       Tcl_EolTranslation   transMode      (in)      The translation mode; one
                                                     of      the     constants
                                                     TCL_TRANSLATE_AUTO,
                                                     TCL_TRANSLATE_CR,
                                                     TCL_TRANSLATE_LF      and
                                                     TCL_TRANSLATE_CRLF.

       int                  size           (in)      The  size,  in  bytes, of
                                                     buffers  to  allocate  in
                                                     this channel.

       int                  mask           (in)      An  OR-ed  combination of
                                                     TCL_READABLE,
                                                     TCL_WRITABLE          and
                                                     TCL_EXCEPTION that  indi-
                                                     cates  events  that  have
                                                     occurred on this channel.

       Tcl_Interp           *interp        (in)      Current interpreter. (can
                                                     be NULL)

       char                 *optionName    (in)      Name   of   the   invalid
                                                     option.

       char                 *optionList    (in)      Specific   options   list
                                                     (space  separated  words,
                                                     without "-") to append to
                                                     the   standard    generic
                                                     options   list.   Can  be
                                                     NULL for generic  options
                                                     error message only.

_________________________________________________________________


DESCRIPTION
       Tcl  uses  a  two-layered  channel  architecture. It provides a generic
       upper layer to enable C and Tcl programs to perform  input  and  output
       using  the  same APIs for a variety of files, devices, sockets etc. The
       generic C APIs are described in the manual entry for  Tcl_OpenFileChan-
       nel.

       The lower layer provides type-specific channel drivers for each type of
       device supported on each platform.  This manual entry describes  the  C
       APIs  used  to  communicate between the generic layer and the type-spe-
       cific channel drivers.  It also explains how new types of channels  can
       be added by providing new channel drivers.

       Channel  drivers consist of a number of components: First, each channel
       driver provides a  Tcl_ChannelType  structure  containing  pointers  to
       functions implementing the various operations used by the generic layer
       to communicate with the channel driver. The  Tcl_ChannelType  structure
       and  the  functions  referenced  by  it  are  described  in the section
       TCL_CHANNELTYPE, below.

       Second, channel  drivers  usually  provide  a  Tcl  command  to  create
       instances  of  that  type of channel. For example, the Tcl open command
       creates channels that use the file and command channel drivers, and the
       Tcl  socket  command  creates channels that use TCP sockets for network
       communication.

       Third, a channel driver optionally provides a C function to open  chan-
       nel  instances  of  that type. For example, Tcl_OpenFileChannel opens a
       channel that uses the file channel driver, and Tcl_OpenTcpClient  opens
       a channel that uses the TCP network protocol.  These creation functions
       typically use Tcl_CreateChannel internally to open the channel.

       To add a new type of channel you must implement a C API or a  Tcl  com-
       mand  that  opens  a  channel by invoking Tcl_CreateChannel.  When your
       driver calls Tcl_CreateChannel it passes in a Tcl_ChannelType structure
       describing  the  driver's  I/O procedures.  The generic layer will then
       invoke the functions referenced in that structure to perform operations
       on the channel.

       Tcl_CreateChannel opens a new channel and associates the supplied type-
       Ptr and instanceData with it. The channel is opened in the  mode  indi-
       cated  by  mask.  For a discussion of channel drivers, their operations
       and the Tcl_ChannelType structure,  see  the  section  TCL_CHANNELTYPE,
       below.

       Tcl_GetChannelInstanceData  returns  the  instance data associated with
       the channel in channel. This is the same as the  instanceData  argument
       in the call to Tcl_CreateChannel that created this channel.

       Tcl_GetChannelType  returns  a pointer to the Tcl_ChannelType structure
       used by the channel in the channel argument. This is the  same  as  the
       typePtr  argument  in  the  call to Tcl_CreateChannel that created this
       channel.

       Tcl_GetChannelName returns a string containing the name associated with
       the  channel,  or NULL if the channelName argument to Tcl_CreateChannel
       was NULL.

       Tcl_GetChannelHandle places the OS-specific  device  handle  associated
       with  channel for the given direction in the location specified by han-
       dlePtr and returns TCL_OK.  If the channel does not have a device  han-
       dle  for  the  specified direction, then TCL_ERROR is returned instead.
       Different channel drivers will return different types of handle.  Refer
       to  the manual entries for each driver to determine what type of handle
       is returned.

       Tcl_GetChannelMode returns an OR-ed  combination  of  TCL_READABLE  and
       TCL_WRITABLE, indicating whether the channel is open for input and out-
       put.

        Tcl_GetChannelBufferSize returns the size, in bytes, of buffers  allo-
       cated  to  store input or output in chan. If the value was not set by a
       previous call to Tcl_SetChannelBufferSize, described  below,  then  the
       default value of 4096 is returned.

       Tcl_SetChannelBufferSize  sets the size, in bytes, of buffers that will
       be allocated in subsequent operations on the channel to store input  or
       output. The size argument should be between ten and one million, allow-
       ing buffers of ten bytes to one million bytes. If size is outside  this
       range, Tcl_SetChannelBufferSize sets the buffer size to 4096.

       Tcl_NotifyChannel  is  called  by  a  channel driver to indicate to the
       generic layer that the events specified by mask have  occurred  on  the
       channel.   Channel  drivers  are responsible for invoking this function
       whenever the channel handlers need to be called for the  channel.   See
       WATCHPROC below for more details.

       Tcl_BadChannelOption  is  called from driver specific set or get option
       procs to generate a complete error message.


TCL_CHANNELTYPE
       A channel driver provides a  Tcl_ChannelType  structure  that  contains
       pointers  to functions that implement the various operations on a chan-
       nel; these operations are invoked as needed by the generic layer.   The
       structure  was versioned starting in Tcl 8.3.2/8.4 to correct a problem
       with stacked channel drivers.  See the OLD_CHANNEL  section  below  for
       details about the old structure.

       The Tcl_ChannelType structure contains the following fields:
              typedef struct Tcl_ChannelType {
                char *typeName;
                Tcl_ChannelTypeVersion version;
                Tcl_DriverCloseProc *closeProc;
                Tcl_DriverInputProc *inputProc;
                Tcl_DriverOutputProc *outputProc;
                Tcl_DriverSeekProc *seekProc;
                Tcl_DriverSetOptionProc *setOptionProc;
                Tcl_DriverGetOptionProc *getOptionProc;
                Tcl_DriverWatchProc *watchProc;
                Tcl_DriverGetHandleProc *getHandleProc;
                Tcl_DriverClose2Proc *close2Proc;
                Tcl_DriverBlockModeProc *blockModeProc;
                Tcl_DriverFlushProc *flushProc;
                Tcl_DriverHandlerProc *handlerProc;
              } Tcl_ChannelType;

       The driver must provide implementations for all functions except block-
       ModeProc, seekProc, setOptionProc, getOptionProc, and close2Proc, which
       may  be specified as NULL.  Other functions that can not be implemented
       for this type of device should return EINVAL when invoked  to  indicate
       that they are not implemented, except in the case of flushProc and han-
       dlerProc, which should specified as NULL if not otherwise defined.

       The user should  only  use  the  above  structure  for  Tcl_ChannelType |
       instantiation.  When referencing fields in a Tcl_ChannelType structure, |
       the following functions should be used to obtain the values:  Tcl_Chan- |
       nelName,   Tcl_ChannelVersion,  Tcl_ChannelBlockModeProc,  Tcl_Channel- |
       CloseProc, Tcl_ChannelClose2Proc, Tcl_ChannelInputProc, Tcl_ChannelOut- |
       putProc,  Tcl_ChannelSeekProc, Tcl_ChannelSetOptionProc, Tcl_ChannelGe- |
       tOptionProc, Tcl_ChannelWatchProc, Tcl_ChannelGetHandleProc,  Tcl_Chan- |
       nelFlushProc, or Tcl_ChannelHandlerProc.                                |

       The change to the structures was made in such a way that standard chan- |
       nel types are binary  compatible.   However,  channel  types  that  use |
       stacked  channels (ie: TLS, Trf) have new versions to correspond to the |
       above change since the previous code for stacked channels had problems.


TYPENAME
       The  typeName  field  contains a null-terminated string that identifies
       the type of the device  implemented  by  this  driver,  e.g.   file  or
       socket.

       This  value  can  be  retrieved  with  Tcl_ChannelName, which returns a |
       pointer to the string.


VERSION                                                                        |
       The version field should be set to TCL_CHANNEL_VERSION_2.  If it is not |
       set  to  this value TCL_CHANNEL_VERSION_2, then this Tcl_ChannelType is |
       assumed to have the older structure.  See OLD_CHANNEL for more details. |
       While  Tcl  will  recognize and function with either structure, stacked |
       channels must be of the newer style to function correctly.              |

       This value can be  retrieved  with  Tcl_ChannelVersion,  which  returns |
       either TCL_CHANNEL_VERSION_2 or TCL_CHANNEL_VERSION_1.


BLOCKMODEPROC
       The  blockModeProc  field  contains the address of a function called by
       the generic layer to set blocking and nonblocking mode on  the  device.
       BlockModeProc should match the following prototype:

              typedef int Tcl_DriverBlockModeProc(
                ClientData instanceData,
                int mode);

       The  instanceData  is the same as the value passed to Tcl_CreateChannel
       when  this  channel  was  created.   The  mode   argument   is   either
       TCL_MODE_BLOCKING or TCL_MODE_NONBLOCKING to set the device into block-
       ing or nonblocking mode. The function should return zero if the  opera-
       tion  was  successful,  or  a nonzero POSIX error code if the operation
       failed.

       If the operation is successful, the function can  modify  the  supplied
       instanceData to record that the channel entered blocking or nonblocking
       mode and to implement the blocking or nonblocking behavior.   For  some
       device  types, the blocking and nonblocking behavior can be implemented
       by the underlying operating system; for other device types, the  behav-
       ior must be emulated in the channel driver.

       This  value  can  be  retrieved  with  Tcl_ChannelBlockModeProc,  which |
       returns a pointer to the function.


CLOSEPROC AND CLOSE2PROC
       The closeProc field contains the address of a function  called  by  the
       generic  layer  to clean up driver-related information when the channel
       is closed. CloseProc must match the following prototype:

              typedef int Tcl_DriverCloseProc(
                ClientData instanceData,
                Tcl_Interp *interp);

       The instanceData argument is the same as the value provided to Tcl_Cre-
       ateChannel  when  the  channel was created. The function should release
       any storage maintained by the channel  driver  for  this  channel,  and
       close  the  input  and output devices encapsulated by this channel. All
       queued output will have been flushed to the device before this function
       is  called,  and  no  further driver operations will be invoked on this
       instance after calling the closeProc. If the close  operation  is  suc-
       cessful, the procedure should return zero; otherwise it should return a
       nonzero POSIX error code. In addition, if an error occurs and interp is
       not  NULL,  the  procedure  should store an error message in the inter-
       preter's result.

       Alternatively, channels that support closing the read and  write  sides
       independently may set closeProc to TCL_CLOSE2PROC and set close2Proc to
       the address of a function that matches the following prototype:

              typedef int Tcl_DriverClose2Proc(
                ClientData instanceData,
                Tcl_Interp *interp,
                int flags);

       The close2Proc will be called with flags set to an OR'ed combination of
       TCL_CLOSE_READ  or  TCL_CLOSE_WRITE  to indicate that the driver should
       close the read and/or write side of the channel.   The  channel  driver
       may  be  invoked  to perform additional operations on the channel after
       close2Proc is called to close one or both sides  of  the  channel.   If
       flags  is  0  (zero), the driver should close the channel in the manner
       described above for closeProc.  No further operations will  be  invoked
       on this instance after close2Proc is called with all flags cleared.  In
       all cases, the close2Proc function should  return  zero  if  the  close
       operation  was  successful;  otherwise it should return a nonzero POSIX
       error code. In addition, if an error occurs and interp is not NULL, the
       procedure should store an error message in the interpreter's result.

       These  value can be retrieved with Tcl_ChannelCloseProc or Tcl_Channel- |
       Close2Proc, which returns a pointer to the respective function.


INPUTPROC
       The inputProc field contains the address of a function  called  by  the
       generic  layer  to read data from the file or device and store it in an
       internal buffer. InputProc must match the following prototype:

              typedef int Tcl_DriverInputProc(
                ClientData instanceData,
                char *buf,
                int bufSize,
                int *errorCodePtr);

       InstanceData is the same as the value passed to Tcl_CreateChannel  when
       the  channel was created.  The buf argument points to an array of bytes
       in which to store input from the device, and the bufSize argument indi-
       cates how many bytes are available at buf.

       The errorCodePtr argument points to an integer variable provided by the
       generic layer. If an error occurs, the function should set the variable
       to a POSIX error code that identifies the error that occurred.

       The function should read data from the input device encapsulated by the
       channel and store it at buf.  On success, the function should return  a
       nonnegative  integer indicating how many bytes were read from the input
       device and stored at buf. On error, the function should return  -1.  If
       an  error  occurs  after  some data has been read from the device, that
       data is lost.

       If inputProc can determine that the input device has some  data  avail-
       able  but  less  than  requested  by the bufSize argument, the function
       should only attempt to read as much data as  is  available  and  return
       without  blocking. If the input device has no data available whatsoever
       and the channel is in nonblocking mode, the function should  return  an
       EAGAIN  error. If the input device has no data available whatsoever and
       the channel is in blocking mode, the  function  should  block  for  the
       shortest possible time until at least one byte of data can be read from
       the device; then, it should return as much data as it can read  without
       blocking.

       This  value can be retrieved with Tcl_ChannelInputProc, which returns a |
       pointer to the function.


OUTPUTPROC
       The outputProc field contains the address of a function called  by  the
       generic  layer  to  transfer data from an internal buffer to the output
       device.  OutputProc must match the following prototype:

              typedef int Tcl_DriverOutputProc(
                ClientData instanceData,
                char *buf,
                int toWrite,
                int *errorCodePtr);

       InstanceData is the same as the value passed to Tcl_CreateChannel  when
       the channel was created. The buf argument contains an array of bytes to
       be written to the device, and the toWrite argument indicates  how  many
       bytes are to be written from the buf argument.

       The errorCodePtr argument points to an integer variable provided by the
       generic layer. If an error occurs, the function should set  this  vari-
       able to a POSIX error code that identifies the error.

       The function should write the data at buf to the output device encapsu-
       lated by the channel. On success, the function should return a nonnega-
       tive  integer  indicating  how  many  bytes  were written to the output
       device.  The return value is normally the same as toWrite, but  may  be
       less  in some cases such as if the output operation is interrupted by a
       signal. If an error occurs the function should return -1.  In  case  of
       error, some data may have been written to the device.

       If the channel is nonblocking and the output device is unable to absorb
       any data whatsoever, the function should return -1 with an EAGAIN error
       without writing any data.

       This value can be retrieved with Tcl_ChannelOutputProc, which returns a |
       pointer to the function.


SEEKPROC
       The seekProc field contains the address of a  function  called  by  the
       generic  layer  to  move  the access point at which subsequent input or
       output operations will be applied. SeekProc must  match  the  following
       prototype:

              typedef int Tcl_DriverSeekProc(
                ClientData instanceData,
                long offset,
                int seekMode,
                int *errorCodePtr);

       The instanceData argument is the same as the value given to Tcl_Create-
       Channel when this channel was created.  Offset and  seekMode  have  the
       same  meaning  as  for  the Tcl_Seek procedure (described in the manual
       entry for Tcl_OpenFileChannel).

       The errorCodePtr argument points to an integer variable provided by the
       generic  layer for returning errno values from the function.  The func-
       tion should set this variable to a POSIX error code if an error occurs.
       The function should store an EINVAL error code if the channel type does
       not implement seeking.

       The return value is the new access point or -1 in case of error. If  an
       error occurred, the function should not move the access point.

       This  value  can be retrieved with Tcl_ChannelSeekProc, which returns a |
       pointer to the function.


SETOPTIONPROC
       The setOptionProc field contains the address of a  function  called  by
       the  generic  layer to set a channel type specific option on a channel.
       setOptionProc must match the following prototype:

              typedef int Tcl_DriverSetOptionProc(
                ClientData instanceData,
                Tcl_Interp *interp,
                char *optionName,
                char *optionValue);

       optionName is the name of an option to set, and optionValue is the  new
       value for that option, as a string. The instanceData is the same as the
       value given to Tcl_CreateChannel when this  channel  was  created.  The
       function should do whatever channel type specific action is required to
       implement the new value of the option.

       Some options are handled by the generic code and this function is never
       called to set them, e.g. -blockmode. Other options are specific to each
       channel type and the setOptionProc procedure of the channel driver will
       get  called  to  implement  them.  The setOptionProc field can be NULL,
       which indicates that  this  channel  type  supports  no  type  specific
       options.

       If  the  option  value  is  successfully modified to the new value, the
       function returns TCL_OK.  It  should  call  Tcl_BadChannelOption  which
       itself returns TCL_ERROR if the optionName is unrecognized.  If option-
       Value specifies a value for the option that is not supported  or  if  a
       system call error occurs, the function should leave an error message in
       the result field of interp if interp is not NULL. The  function  should
       also call Tcl_SetErrno to store an appropriate POSIX error code.

       This  value  can  be  retrieved  with  Tcl_ChannelSetOptionProc,  which |
       returns a pointer to the function.


GETOPTIONPROC
       The getOptionProc field contains the address of a  function  called  by
       the generic layer to get the value of a channel type specific option on
       a channel. getOptionProc must match the following prototype:

              typedef int Tcl_DriverGetOptionProc(
                ClientData instanceData,
                Tcl_Interp *interp,
                char *optionName,
                Tcl_DString *dsPtr);

       OptionName is the name of an option supported by this type of  channel.
       If  the option name is not NULL, the function stores its current value,
       as a string, in the Tcl dynamic string dsPtr.  If optionName  is  NULL,
       the  function  stores  in  dsPtr  an  alternating list of all supported
       options and their current values.  On  success,  the  function  returns
       TCL_OK.   It  should  call  Tcl_BadChannelOption  which  itself returns
       TCL_ERROR if the optionName is unrecognized. If  a  system  call  error
       occurs,  the function should leave an error message in the result field
       of interp if  interp  is  not  NULL.  The  function  should  also  call
       Tcl_SetErrno to store an appropriate POSIX error code.

       Some options are handled by the generic code and this function is never
       called to retrieve their value, e.g. -blockmode. Other options are spe-
       cific to each channel type and the getOptionProc procedure of the chan-
       nel driver will get called to implement them. The  getOptionProc  field
       can  be  NULL,  which indicates that this channel type supports no type
       specific options.

       This  value  can  be  retrieved  with  Tcl_ChannelGetOptionProc,  which |
       returns a pointer to the function.


WATCHPROC
       The  watchProc  field  contains the address of a function called by the
       generic layer to initialize the event notification mechanism to  notice
       events of interest on this channel.  WatchProc should match the follow-
       ing prototype:

              typedef void Tcl_DriverWatchProc(
                ClientData instanceData,
                int mask);

       The instanceData is the same as the value passed  to  Tcl_CreateChannel
       when  this  channel was created. The mask argument is an OR-ed combina-
       tion of TCL_READABLE,  TCL_WRITABLE  and  TCL_EXCEPTION;  it  indicates
       events the caller is interested in noticing on this channel.

       The  function  should  initialize  device  type  specific mechanisms to
       notice when an event of interest is present on the channel.   When  one
       or  more  of  the  designated events occurs on the channel, the channel
       driver is responsible  for  calling  Tcl_NotifyChannel  to  inform  the
       generic  channel  module.   The  driver  should take care not to starve
       other channel drivers or sources of callbacks by  invoking  Tcl_Notify-
       Channel too frequently.  Fairness can be insured by using the Tcl event
       queue to allow the channel event to be scheduled in sequence with other
       events.   See  the  description of Tcl_QueueEvent for details on how to
       queue an event.

       This value can be retrieved with Tcl_ChannelWatchProc, which returns  a |
       pointer to the function.


GETHANDLEPROC
       The  getHandleProc  field  contains the address of a function called by
       the generic layer to retrieve a device-specific handle from  the  chan-
       nel.  GetHandleProc should match the following prototype:

              typedef int Tcl_DriverGetHandleProc(
                ClientData instanceData,
                int direction,
                ClientData *handlePtr);

       InstanceData  is the same as the value passed to Tcl_CreateChannel when
       this channel was created. The direction argument is either TCL_READABLE
       to  retrieve the handle used for input, or TCL_WRITABLE to retrieve the
       handle used for output.

       If the channel implementation has device-specific handles, the function
       should  retrieve  the  appropriate  handle associated with the channel,
       according the direction argument.  The handle should be stored  in  the
       location  referred  to by handlePtr, and TCL_OK should be returned.  If
       the channel is not open for the specified direction, or if the  channel
       implementation  does not use device handles, the function should return
       TCL_ERROR.

       This  value  can  be  retrieved  with  Tcl_ChannelGetHandleProc,  which |
       returns a pointer to the function.


FLUSHPROC                                                                      |
       The flushProc field is currently reserved for future use.  It should be |
       set to NULL.  FlushProc should match the following prototype:           |

              typedef int Tcl_DriverFlushProc(                                 |
                ClientData instanceData);                                      |

       This value can be retrieved with Tcl_ChannelFlushProc, which returns  a |
       pointer to the function.                                                |


HANDLERPROC                                                                    |
       The  handlerProc field contains the address of a function called by the |
       generic layer to notify the channel that an event occured.   It  should |
       be  defined  for  stacked  channel  drivers that wish to be notified of |
       events that occur on the  underlying  (stacked)  channel.   HandlerProc |
       should match the following prototype:                                   |

              typedef int Tcl_DriverHandlerProc(                               |
                ClientData instanceData,                                       |
                int interestMask);                                             |

       InstanceData  is the same as the value passed to Tcl_CreateChannel when |
       this channel was created.  The interestMask is an OR-ed combination  of |
       TCL_READABLE  or  TCL_WRITABLE; it indicates what type of event occured |
       on this channel.                                                        |

       This value can be retrieved with Tcl_ChannelHandlerProc, which  returns |
       a pointer to the function.


TCL_BADCHANNELOPTION
       This  procedure generates a "bad option" error message in an (optional)
       interpreter.  It is used by channel  drivers  when  a  invalid  Set/Get
       option  is requested. Its purpose is to concatenate the generic options
       list to the specific ones and factorize the generic options error  mes-
       sage string.

       It always return TCL_ERROR

       An  error  message  is  generated in interp's result object to indicate
       that a command was invoked with the a bad option The  message  has  the
       form
                  bad option "blah": should be one of
                  <...generic options...>+<...specific options...>
              so you get for instance:
                  bad option "-blah": should be one of -blocking,
                  -buffering, -buffersize, -eofchar, -translation,
                  -peername, or -sockname
              when called with optionList="peername sockname"
       ``blah''  is  the  optionName  argument and ``<specific options>'' is a
       space separated list of specific option words.  The function takes good
       care  of inserting minus signs before each option, commas after, and an
       ``or'' before the last option.


OLD_CHANNEL
       The original (8.3.1 and below) Tcl_ChannelType structure  contains  the
       following fields:

              typedef struct Tcl_ChannelType {
                char *typeName;
                Tcl_DriverBlockModeProc *blockModeProc;
                Tcl_DriverCloseProc *closeProc;
                Tcl_DriverInputProc *inputProc;
                Tcl_DriverOutputProc *outputProc;
                Tcl_DriverSeekProc *seekProc;
                Tcl_DriverSetOptionProc *setOptionProc;
                Tcl_DriverGetOptionProc *getOptionProc;
                Tcl_DriverWatchProc *watchProc;
                Tcl_DriverGetHandleProc *getHandleProc;
                Tcl_DriverClose2Proc *close2Proc;
              } Tcl_ChannelType;

       It  is  still possible to create channel with the above structure.  The
       internal channel code will determine the version.  It is imperative  to
       use  the  new  Tcl_ChannelType  structure if you are creating a stacked
       channel driver, due to problems with the earlier stacked channel imple-
       mentation (in 8.2.0 to 8.3.1).


SEE ALSO
       Tcl_Close(3),          Tcl_OpenFileChannel(3),         Tcl_SetErrno(3),
       Tcl_QueueEvent(3), Tcl_StackChannel(3)


KEYWORDS
       blocking, channel driver, channel registration, channel type, nonblock-
       ing



Tcl                                   8.3                 Tcl_CreateChannel(3)